REGULATED RIVERS: RESEARCH & MANAGEMENT, VOL. II, 105-119 (1995)
ECOLOGICAL CONNECTIVITY IN ALLUVIAL RIVER
ECOSYSTEMS AND ITS DISRUPTION BY FLOW REGULATION
J. V. WARD*
Department of Biology, Colorado State University, Fort Collins, CO 80523 USA
and
J. A. STANFORD
Flathead Lake Biological Station, The University of Montana, Polson, MT 59860 USA
ABSTRACT
The dynamic nature of alluvial floodplain rivers is a function of flow and sediment regimes interacting with the physiographic features and vegetation cover of the landscape. During seasonal inundation, the flood pulse forms a 'moving
littoral' that traverses the plain, increasing productivity and enhancing connectivity. The range of spatio-temporal connectivity between different biotopes, coupled with variable levels of natural disturbance, determine successional patterns
and habitat heterogeneity that are responsible for maintaining the ecological integrity of floodplain river systems. Flow
regulation by dams, often compounded by other modifications such as levee construction, normally results in reduced
connectivity and altered successional trajectories in downstream reaches. Flood peaks are typically reduced by river regulation, which reduces the frequency and extent of floodplain inundation. A reduction in channel-forming flows reduces
channel migration, an important phenomenon in maintaining high levels of habitat diversity across floodplains. The seasonal timing of floods may be shifted by flow regulation, with major ramifications for aquatic and terrestrial biota. Truncation of sediment transport may result in channel degradation for many kilometres downstream from a dam. Deepening
of the channel lowers the water-table, which affects riparian vegetation dynamics and reduces the effective base level of
tributaries, which results in rejuvenation and erosion. Ecological integrity in floodplain rivers is based in part on a diversity of water bodies with differing degrees of connectivity with the main river channel. Collectively, these water bodies
occupy a wide range of successional stages, thereby forming a mosaic of habitat patches across the floodplain, This diversity is maintained by a balance between the trend toward terrestrialization and flow disturbances that renew connectivity
and reset successional sequences. To counter the influence of river regulation, restoration efforts should focus on reestablishing dynamic connectivity between the channel and floodplain water bodies.
KEY WORDS
alluvial rivers;
ecological connectivity;
floodplains;
flow regulation;
habitat heterogeneity;
succession
INTRODUCTION
The ecological integrity of alluvial floodplain rivers is a function of ecological connectivity operating across a
range of spatio-temporal scales (Antipa, 1928; Botnariuc, 1967; Amoros and Roux, 1988; Ward, 1989a;
Stanford and Ward, 1993). Connectivity in this context refers mainly to exchange pathways of water,
resources and organisms between the channel, the aquifer and the floodplain, although interactions with
adjacent uplands must also be considered. The premise of this paper is that high connectivity, and associated
habitat heterogeneity, productivity and biodiversity of floodplain river ecosystems, is maintained by natural
disturbance (i.e. the flood regime and its ramifications). Anthropogenic alterations of floodplain rivers often
disrupt the intensity, frequency and timing of the natural disturbance regime that is responsible for maintaining the ecological integrity of these dynamic systems.
This synthesis paper is presented in three parts: the first examines how connectivity operates across a range
*Present address: Department of Limnology, EA WAG/ETH, Uberlandstrasse 133, CH-8600 Diibendorf, Switzerland.
CCC 0886-9375/95/050105-15
© 1995 by John Wiley & Sons, Ltd.
Received 15 July 1994
Accepted 10 February 1995
,
106
J. V. WARD AND J. A. STANFORD
of scales to maintain the ecological integrity of floodplain river systems through dynamic interactions related
to hydrology, geomorphology and successional phenomena; the second analyses how river regulation, by
reducing connectivity, alters hydrological and geomorphic processes and successional trajectories; and the
final part addresses ways to re-establish the connectivity/ecological integrity of regulated floodplain river systems.
CONNECTIVITY AND ECOLOGICAL INTEGRITY
Floodplain rivers are dynamic systems, the persistence of which depends on interactions between subsystems. Floodplain ecosystems are also disturbance-dependent, relying on the kinetic energy of flooding (fluvial dynamics) to maintain connectivity. As stated by Pinay eta/. (1990) in reference to plant communities,
'subsystem instability maintains the metastability of the whole riparian ecotone.'
Hydrology and geomorphology
Hydrological and geomorphic conditions interact to determine pattern and process across a range of
scales. At small spatial scales, water movement patterns during seasonal flooding, for example, produce
patches of aerobic and anaerobic soils on the floodplain which differ in nutrient and decomposition
dynamics. Alluvial forest zonation patterns provide an example of moderate scale spatio-temporal phenomena as they represent successional stages structured by the lateral migration of the river channel across the
floodplain. Large-scale topographic features of floodplains, such as terraces, were formed by processes
related to glaciation/deglaciation events, sea-level changes, tectonic uplift and other long-term phenomena.
General features offringing floodplains. The valley of an alluvial river typically consists of the following
topographic features (Leopold et a/., 1964; Welcomme, 1979, 1985; Mitsch and Gosselink, 1993): (1)
hillslopes forming the sides of the valley, (2) one or more levels of terraces, remnants of abandoned
floodplains at higher elevations than the (3) active flood plain (inundated annually), (4) natural levees
that form along the river channel from sediment deposited as the water velocity abruptly decreases when
the river overtops its banks, (5) meander scrolls, the ridge and swale topography on the floodplain
surface, that form a series of progressively older levees (ridges) of abandoned meander bends and the
depressions (swales) between them (levees, and therefore ridges, are usually the highest points on the
active floodplain and are composed of coarse, well-drained soils), (6) natural drainage channels that
breach the levee, forming connections between the floodplain and the river channel and (7) permanent
and temporary flocdplain water bodies in various stages of hydrarch succession. During an annual flood
of normal amplitude, only the tops of the highest levees and ridges are above water; the remainder of the
active floodplain and the river channel are inundated by a physically continuous aquatic milieu.
Although fully cognizant of the variability in the physical environment of riverine landscapes (Church, 1992),
in this paper we emphasize the common characteristics of fringing floodplain systems. For example, Wolman
and Leopold (1957) reported that the floodplains of rivers flowing through diverse physiographic and climatic
regions are typically inundated by channel overspill about once each year. Yet we recognize that an assumption
of annual inundation for undisturbed floodplain rivers masks important variability between systems. Detailed
analysis of differences across latitudinal gradients are also beyond the scope of this paper. The role of ice action,
an important agent in northern rivers (e.g. Prowse, 1994), is not treated herein. Most information on the effects
of flow regulation on downstream reaches is based on studies conducted at temperate latitudes, with very little
data on tropical river systems (Ward and Stanford, 1993). It is not known whether the consequences of regulation on tropical rivers will, in fact, be the same as for rivers in the temperate zone.
The flood pulse. To emphasise the primary role of hydrology in structuring the floodplain biota, Junket a/.
(1989) 'postulate that if no organic material except living animals were exchanged between floodplain and
channel, no qualitative and, at most, limited quantitative changes would occur in the floodplain.' Their
'flood pulse concept' provides a theoretical framework for analysing the adaptive strategies employed by
organisms to exploit the wet and dry phases that alternate in a predictable annual pattern on the
floodplains of large rivers. On small rivers, or in the upper reaches of large rivers, floodplains are not
inundated for a sufficient length of time and the periodicity of flooding is too irregular for the evolution
of adaptive strategies that are tightly coupled to the flood regime (Junk et a/., 1989).
CONNECTIVITY IN ALLUVIAL RIVERS
107
ALLUVIAL
ADJACENT
UPLANDS
FOREST
CAK:R'(
TERRESTRIAL
FAUNA
AQUATICffERRESTRIAL TRANSITION ZONE
AQUATIC
FAUNA
RIVER
QiANNa
R.OODPI..AIN
WATER BODIES
Figure I. Movement patterns of terrestrial and aquatic animals between the aquatic/terrestrial transition zone (floodplain surface) and
other habitats during dry and wet phases of the hydrological regime
The flood pulse concept refers to the floodplain surface as the "aquatic/terrestrial transition zone' (ATTZ) to
emphasize the importance of alternating dry and wet phases in enhancing biotic diversity and productivity.
Inundation of the floodplain results in a 'moving littoral' that traverses the ATTZ from the channel to
the upland transition, thereby creating a dynamic edge effect. Alternating dry and wet phases increase
organic matter decomposition and nutrient cycling and shorten periods of stagnation, compared with conditions in permanently inundated habitats.
The rich resources of the ATTZ are utilized by terrestrial animals during the dry phase and by aquatic
animals during the wet phase (Figure 1). During the wet phase some terrestrial animals migrate to the
uplands, whereas others remain on the floodplain. Those that remain are adapted to tolerate extended periods of inundation or move into the alluvial forest canopy to escape the rising water. For example, in central
Amazonian floodplains, which are inundated for five to seven months of the year, terrestrial invertebrates
evolved migratory and non-migratory survival strategies (Adis, 1992). Some species migrate horizontally
ahead of the high water line, others migrate vertically to the forest canopy and yet others engage in migratory
flights to upland forests. Most non-migrators survive the aquatic phase as dormant stages, although a few
species have developed plastron respiration, enabling them to remain active under water.
Kafue Flats, a large floodplain of the Kafue River in Zambia, is flooded to a depth of up to 5 m for several
months each year (Sheppe and Osborne, 1971 ). As water recedes, terrrestrial mammals move in from the
surrounding woodlands to graze on the productive grasslands that cover the floodplain during the dry phase.
The Kafue lechwe (Kobus leche kafuensis), an ungulate endemic to the Flats, provides an example of a large
terrestrial mammal that is dependent on and adapted to the floodplain (Chabwela and Ellenbroek, 1990).
The grazing regime of the lechwe, the most abundant large mammal of the Flats, is closely tied to the flood
cycle and is thought to be an important structuring agent for the development of plant communities on the
floodplain. The complicated lekking behaviour of the lechwe is also tied to the hydroperiod.
Aquatic animals colonize the ATTZ during the wet phase to take advantage of the high productivity and
diverse habitat conditions. Movements of fishes associated with floodplain rivers exhibit highly diverse patterns, many facets of which are poorly understood (reviewed by Welcomme, 1985). Nonetheless, most fish
species are in one of the following three main categories, based on movement patterns: (1) those that
108
J. V. WARD AND J. A. STANFORD
complete all life stages within the river channel; (2) those that reside in the river channel during the dry season and on the floodplain surface (ATTZ) during the wet phase (flood-dependent fishes); and (3) those that
reside in len tic water bodies of the floodplain during the dry phase and on the ATTZ during the wet phase.
Few fishes associated with floodplain rivers use main channel habitats exclusively (Junket a!., 1989) and this
first category will not be considered further.
Flood-dependent fishes are highly adapted to exploit the predictable flood pulse, using the river channel as
a dry season/winter refuge and for longitudinal migration (potamodromy). Adaptations are most pronounced in the tropics, partly because the flood pulse of most large temperate rivers has been modified
by river engineering works. Maturation of reproductive products is synchronized with the flooding regime,
which ensures that suitable spawning conditions are present and that adequate food and shelter are available
for young fishes (Heeg and Breen, 1982). Plant detritus and dung that accumulated during the dry phase and
the influx of riverborne nutrients provide immediate sources of organic matter and stimulate aquatic productivity. As the 'moving littoral' traverses the floodplain, cover for young fishes is provided by recently submerged terrestrial vegetation and the rapidly developing aquatic macrophytes. Shallow water habitats
with high temperatures optimize the growth rates of young fishes while reducing predation by large fishes.
The cover from inundated terrestrial and developing aquatic plants provides protection from terrestrial predators. The forest canopy contributes an array of organic detritus (leaves, fruits, seeds) and organisms
(mainly terrestrial arthropods) to the inundated floodplain (Goulding, 1980). There is evidence for a coevolutionary relationship whereby the trees provide food for the fishes and rely on them for seed dispersal.
At the end of the wet phase, juvenile and adult fishes return to the river channel in a predictable sequence
based on body size, age class, food habits and species-specific differences in low oxygen tolerance. Heavy
predation losses of prey species and juveniles occur at the mouths of drainage channels where predators congregate. Flood-dependent fishes are not adapted to the stagnant conditions that prevail in many floodplain
water bodies during the dry phase. Those that fail to migrate to the river channel at the end of the wet phase
perish.
Certain fishes, however, move from the A TTZ to standing water bodies of the floodplain as water levels
subside, being adapted to survive deoxygenation and, in some instance, desiccation during the dry phase
(Welcomme, 1985; Lowe-McConnell, 1987). Because of the relationships between temperature and metabolic rate/oxygen solubility, the most dramatic adaptations have evolved in tropical waters.
Many taxa of fishes have independently evolved adaptations for air breathing (Welcomme, 1985). Some
species, such as lung fishes, are obligate air breathers that must surface at frequent intervals. Many additional fishes from numerous families are able to meet respiratory needs by extracting oxygen from the surface
microlayer, which remains well oxygenated during periods of hypoxia (Kramer and McClure, 1982). Species
able to inhabit low oxygen waters are subjected to reduced interspecific competition and predation, although
frequent surface visits increase their vulnerability to aerial predators. Some air-breathing fishes are capable
of limited terrestrial locomotion, allowing them to colonize isolated water bodies and to emigrate from habitats undergoing desiccation (Kramer eta!., 1978; Seghers, 1978).
Very few fishes are specially adapted to endure habitat desiccation. Several species can survive for short
periods in damp mud or sand, but only African lungfishes (Protopterus spp.), which secrete a slime cocoon,
can aestivate as adults within a dry substratum (Welcomme, 1985). 'Annual fishes', known among the cyprinodonts of Africa and South America, maintain permanent populations in temporary ponds (Simpson,
1979; Lowe-McConnell, 1987). The life cycle is completed during the wet phase and they survive the dry
phase as drought-resistant eggs. Because diapause can occur in three stages of embryogenesis, different combinations of diapause result in several developmental series, thereby ensuring that some individuals survive
should precipitation temporarily fill the pond before the extended wet phase (Wourms, 1972).
Floodplain water bodies
Alluvial floodplains contain a variety of lotic and len tic biotopes, including the river and its side channels,
emergent springbrooks, tributary streams and abandoned channel segments (Figure 2). The main river and
its side channels are designated 'eupotamon'; 'parapotamon' refers to dead arms that retain a connection to
the active channel only at their downstream end; 'plesiopotamon' are former braided segments that became
CONNECTIVITY IN ALLUVIAL RIVERS
109
PLESJOPOTAMON
Figure 2. Types of water bodies, based on structural and functional attributes, associated with alluvial floodplains (see text)
disconnected from the river channel; 'palaeopotamon' are former meander bends that became disconnected.
This terminology was initially developed for the French Rhone in recognition of ecological differences
between floodplain water bodies based on attributes such as connectivity, successional trajectory and community structure (Amoros eta/., 1982; Castella eta/., 1984, Copp, 1989). Floodplain water bodies are also
differentially influenced by groundwater dynamics, which is reflected by hypogean faunal assemblages
(Gibert eta/., 1981; Marmonier eta/., 1992; Stanford and Ward, 1993; Ward and Palmer, 1994), although
the present paper focuses on the epigean realm.
Floodplain water bodies may be arrayed along a gradient of connectivity with the main channel or thalweg
(Figure 3). Side arms of the eupotamon, which are connected with the main channel at both ends, are true
UPLANDS------------------1
.._z
zo
o-1-
~~
~0~
>~
wa:
u~
-w
t:a:
Za:
ww
~ .....
PALEOPOTAMON
PLESIOPOTAMON
PARAPOTAMON
EUPOTAMON
(side arms)
~-----------------THALWEG
CONNECTIVITY WITH CHANNEL
low
high
Figure 3. Relative connectivity of floodplain water bodies with the main river channel
110
J. V. WARD AND J. A. STANFORD
lotic segments with higher connectivity than the parapotamon, which lack unidirectional current, being connected only at the downstream end. Plesiopotamon water bodies have greater connectivity with the active
channel than the palaeopotamon (oxbow lakes). Plesiopotamon are smaller, shallower habitats that rapidly
undergo terrestrialization and generally occur near the active channel, whereas the palaeopotamon are larger, deeper and longer lived aquatic habitats that may be situated a great distance from the active channel.
Besides distance, levees, which do not usually form on braided segments, reduce the frequency of flooding of
the palaeopotamon, making them more isolated from floodplain inundation than other water bodies. As the
water level rises during the flood pulse, the upstream ends of parapotamon habitats are reconnected with the
active channel. With additional flood height the plesiopotamon resume a running water character. At the
height of the flood all water bodies, including the palaeopotamon, are inundated. As described by Lewis
eta/. (1990) for the Orinoco River system, 'adjacent floodplain basins that are separate during the dry season
become essentially uniform and contiguous with respect to nutrient chemistry and biological properties during the season of inundation. However, following inundation, these adjacent systems reestablish their indivuality.'
Natural floodplains thus contain a broad diversity of len tic, lotic and semi-lotic habitats. Circulation patterns range from stagnant to strong unidirectional currents. Substrate conditions range from highly organic
mud to coarse, highly permeable mineral deposits. Aquatic macrophytes, absent from some biotopes, in
other situations form dense emergent stands, floating mats or submerged beds. The temperature regime exhibits marked changes between and within biotopes, greatly contributing to environmental heterogeneity
(Ward, 1985). Springbrooks, fed by groundwater, provide summer-cool and winter-warm conditions at
mid- and high latitudes and also have a longitudinal temperature gradient. Distinct spatial gradients may
also occur in parapotamon habitats which receive groundwater seepage at their upstream ends (Bouvet et
a/., 1985). The thalweg of the eupotamon may have a different temperature than its side channels and
even individual branches of a braided segment sometimes exhibit different thermal conditions. Tributaries
that originate in the uplands typically have different temperatures than other lotic segments of the floodplain. Different lentic water bodies have markedly different thermal conditions as a function of size/depth,
degree of shading and wind exposure. These are just a few examples of the spatia-temporal heterogeneity
that structures biotic communities at the floodplain scale.
The spatia-temporal heterogeneity of floodplain river systems is, therefore, responsible for a diverse array
of dynamic aquatic habitats. Habitat diversity is further enhanced by the different ages (stages of development) of the various types of water bodies. In this way, natural levels of disturbance maintain the ecological
integrity of alluvial river ecosystems.
Channel inputs
In addition to fluxes of water and sediment, natural alluvial rivers are characterized by massive exchanges
of organic and inorganic matter between the river channel and the floodplain. As a general rule, there is a net
import of dissolved inorganic compounds to the floodplain and a net export of particulate organic matter,
including living biomass elaborated on the floodplain (Gosselink eta/., 1990; Mitsch and Gosselink, 1993;
Sharitz and Mitsch, 1993). Exports of particulate organic matter to the channel include detritus (including
episodic pulses of large wood debris), plankton, benthos, fishes and floating islands (Sudds) with associated
flora and fauna (Welcomme, 1985).
Accumulations of large woody debris play a major part in structuring channel morphology (Keller and
Swanson, 1979; Triska, 1984). Debris jams, by altering local current patterns, may scour the bed, increase
channel width or facilitate meander cutoff. Deposition below debris jams may initiate the formation of
mid-channel bars and islands. In some instances, blockage of main channels results in the formation of riparian lakes (Triska, 1984). Submerged wood provides an important habitat for aquatic organisms. For example, in a coastal plains river, submerged wood (snags) constituted 4% of the habitat surface, but supported
the majority of invertebrate biomass (Benke eta/., 1985). Invertebrate production on snags provided a major
food source for riverine fishes.
Large contributions ofliving organisms from floodplains to river channels have been documented. During
a winter flood, Wainright eta/. (1992) estimated inputs of up to 4000kg bacteria Ch- 1 from the floodplain
CONNECTIVITY IN ALLUVIAL RIVERS
Ill
over a 50 km reach of a blackwater river. Bacteria elaborated on the floodplain are an important food
resource for filter feeders and deposit feeders in the river channel (Meyer, 1990).
Although only a small proportion of the phytoplankton production on the floodplain of the Orinoco
River is transferred to the river channel, this yield accounts for 37% of the annual transport of phytoplankton carbon (Lewis, 1988). Additional phytoplankton biomass was contributed from 'near-channel
stagnant or slow-flowing areas', but the amount was not separated from that transported by the main
channel.
Studies of a Venezuelan floodplain river showed how the annual hydrograph regulated which source areas
provided suitable habitat for zooplankton development and controlled the export of zooplankton from source
areas (Saunders and Lewis, 1988a). Zooplankton is abundant in the main channel during the dry phase when
side channels develop large populations and serve as source areas. As the water level rises, zooplankton is
flushed from the side channels. Following a brief pulse, populations remain low in the main channel until
the river inundates the floodplain. Zooplankton transport increases as floodplain water bodies containing dense
populations are inundated. The slow movement of water across the wide floodplain provides suitable conditions for the sustained development of zooplankton during the flood phase. The inundated floodplain continues
to export zooplankton to the river for several months, at a time when high velocity precludes zooplankton
reproduction in the main channel. This contrasts with another Venezuelan river that has a narrow floodplain
with shorter hydraulic residence time, where zooplankton populations are flushed from lentic water bodies at
the beginning of the flood phase and remain depleted during the period of inundation (Saunders and Lewis,
1988b). As the water recedes and the floodplain loses its connection with the river, zooplankton remains low
in the main channel until source areas are re-established in the side channels.
Studies of the Danube demonstrate much higher zooplankton abundances in backwaters than in the main
channel (Vranovsky, 1974; Bothar, 1981). At times, zooplantkon densities may be orders of magnitude
higher in backwaters than density levels in the river. As flood waters rise, increasing the connectivity, plankton is exported to the river where it is used as food by a variety of consumers.
Benthic invertebrates are also flushed from productive floodplain water bodies during high water, thereby
re-establishing functional connectivity and supplying food to channel residents (Castella et a/., 1984; Eckblad eta/., 1984; Friedrich and Muller, 1984; Shaeffer and Nickum, 1986a; Cellot and Bournaud, 1987;
Ward, 1989b; Obrd1ik and Garcia-Lozano, 1992). Large numbers of invertebrates are translocated to the
channel in association with drifting plants (Bonetto, 1975; Welcomme, 1985). Both terrestrial and aquatic
animals inhabit floating plants (e.g. Pistia, Eichhornia) and floating islands (Sudds). Up to 300 animals
may occur on a single Pistia stratiotes plant (Rz6ska, 1974) and the mat in the centre of floating meadows
contains as many as 100000 animals per square metre (Junk, 1973).
In addition to passive transport, some invertebrates actively migrate between the river and adjacent water
bodies (Soderstrom, 1987). A well-documented example involves the mayfly Leptoph/ebia cupida, which
emerges from floodplain marshes in Canada and flies to the river to oviposit (Hayden and Clifford,
1974). Nymphs reside in the river during autumn and winter. The initial vernal rise in water level from snowmelt runoff initiates a day-active upstream migration of nymphs along the edges of the river, up drainage
channels and into floodplain marshes, where the life cycle is completed. The annual migratory pattern
enables nymphs to avoid severe winter conditions in the shallow marshes and adverse spring runoff conditions in the river.
The diversity of len tic and lotic biotopes of a floodplain river system (Figure 2) provides suitable conditions for a variety of benthic communities. The eupotamon of side channels, for example, is characrerized by
lotic benthos that differ from those of the main channel, from those of the parapotamon and from all other
floodplain water bodies. It is because of the association between biota and habitat conditions that macroinvertebrate communities may be used as 'describers' of various types of floodplain water bodies (Castella
eta/., 1984). This also means that invertebrates contributed to the river from the floodplain vary qualitatively
and quantitatively as a function of source (habitat type), thereby providing a diversity of organic resources as
temporally sequenced inputs along a continuum of connectivity.
The different types of floodplain water bodies are also characterized by specific assemblages of fishes
(Welcomme, 1985; Copp, 1989). During the dry phase fish biomass is higher in backwater habitats than
112
J. V. WARD AND J. A. STANFORD
in the main channel and the fish productivity of a given reach is positively correlated with the development of
backwater habitats (Shaeffer and Nickum, l986b; Amoros and Roux, 1988; Copp, 1989). Within the main
channel of the Mississippi River, the highest densities of young fishes (larvae and juveniles) occurred downstream from backwater connections (Shaeffer and Nickum, l986b ). Fish biomass elaborated on the floodplain during the wet phase-that is transferred to the river by the lateral migration of flood-dependent
species (as discussed in a previous section)-constitutes the major input of ichthyomass to the channel.
Floodplain forest succession
The important role of natural disturbance in maintaining ecological integrity at the floodplain scale is well
documented (e.g. Junket a/., 1989; Duncan, 1993). The high biotic diversity of the upper Amazon basin is
attributable to large-scale disturbance induced as river channels migrate back and forth across their floodplains, creating a mosaic of forest stands and successional stages (Salo et a/., 1986).
The floodplain of the Manu River in upper Amazonia (Peru) provides an excellent example of the role of
river channel migration in structuring alluvial forest succession (Terborgh and Petren, 1991). Channel
migration across the 6 km wide floodplain simultaneously deposits alluvium on the convex side of meander
bends and erodes the opposite bank. Erosion of the concave bank averaged 25m/year on one meander
bend, a migration rate of 2.5 km per century. As the forest growing on the concave bank is undercut by
erosion, primary succession is initiated on beaches (point bars) formed in annual increments on the inside
bends. During low water annual plants and the woody composite Tessaria integrifolia germinate on newly
formed beaches. Many Tessaria, which grows to heights of 2m or more during the first year, are carried
away by the next flood, but some stems are flattened and buried by the new layer of sediment. The second
year, after flood waters recede, the buried vegetative shoots attain heights of 3-4m. They are now at a
higher level further from the migrating river and can withstand the next flood. In three to five years the
mature stands of Tessaria, 8-10 m high, are invaded by the cane Gynerium, which eventually overtops Tessaria, resulting in its demise. Seedlings of many tree species become established in Gynerium stands, being
protected from washout by the dense cane stems which reduce current velocities during floods. Even-aged
stands of Cecropia, the fastest growing tree, attain heights of 20m or more, forming a cane-Cecropia association that is maintained for several years, but is eventually overtopped by slower growing trees. Reduced
light levels eliminate the cane; Cecropia only lives for a few decades and so is also eliminated from the
mixed forest association. A tall herb layer, dominated by He/iconia, forms a dense understorey that suppresses further recruitment of trees. Eventually, two slow-growing trees (Ficus and Cedrella) grows to
heights of 35-40 m, forming a late successional association that persists for about a century. Succession
to the mature floodplain forest phase takes from 300-500 years and is possible only on areas of the floodplain with lower than average rates of channel migration. The mature forest has high structural diversity
(five vertically superimposed strata, the tallest of which exceeds 50 m), high tree diversity(> 200 species per
hectare) and the highest diversity of birds and mammals. The meandering river is, therefore, an example of
a natural disturbance that creates multiple successional stages and a dynamic mosaic of habitat types
across the alluvial floodplain.
REGULA TED FLOODPLAIN RIVERS
Lost connectivity
Flow regulation by dams disrupts the downstream river system's natural disturbance regimes (Figure 4).
Reduction of flood peaks reduces the frequency, extent and duration of floodplain inundation. Reduction of
channel-forming flows reduces channel migration. Truncated sediment transport (sedimentation within the
impoundment) typically results in channel degradation below the dam and a concomitant lowering of the
water-table. The temporal pattern of flooding is also altered by regulation, one effect of which is to desynchronize annual flow and temperature regimes (Sparks eta/., 1990). These changes and others directly and
indirectly influence a myriad of dynamic factors that affect habitat heterogeneity and successional trajectories and, ultimately, the ecological integrity of floodplain rivers.
113
CONNECTIVITY IN ALLUVIAL RIVERS
ECOLOGICAL INTEGRITY REDUCED
Figure 4. Some implications of major hydrological changes induced by flow regulation on downstream river-floodplain systems
/
rapid
/
ALLOGENIC
ffiXESSES
I PALEOPOTAMON I
AUTOGENIC
PROCESSES
PARAPOTAMON
~
Figure 5. Hydrarch succession of floodplain water bodies along two trajectories (see Figure 2)
114
J. V. WARD AND J. A. STANFORD
ACTIVE
MEANDERING
1-
-~
iD
~~
z_(j)
::Sffi
C..>
coo
g
LL
~
BRAIDE
REGULATED
0
low
CHANNEL INSTABILITY
(DISTURBANCE)
high
Figure 6. Channel instability perceived as a disturbance gradient related to floodplain habitat diversity
In much of the world anthropogenic impacts have severed major interactive pathways, thereby isolating
rivers from their floodplains and disrupting the dynamic patterns and processes that structure river-floodplain ecosystems. The downstream effects of flow regulation (Figure 4) are typically compounded by other
modifications (e.g. channelization, levee construction) that further reduce connectivity between the river
channel and the floodplain (Amoros eta/., 1982; Welcomme, 1985; Bravard eta/., 1986; Ward and Stanford,
1989; Brinson, 1990; Gosselink eta/., 1990; Junk and Welcomme, 1990; Petts, 1990; Sparks eta/., 1990;
Penka eta/., 1991; Stanford and Ward, 1992).
Altered successional pathways
In the context of river-floodpain ecosystems, 'succession' may be considered from two very different perspectives: (1) hydrarch succession of floodplain waterbodies; and (2) alluvial forest succession.
Hydrarch succession of floodplain water bodies (Figure 5) follows two distinct trajectories towards terrestrialization (Amoros eta/., 1982; 1987b; Copp, 1989). Those that form by the abandonment of braided segments
undergo relatively rapid succession dominated by allogenic processes (e.g. sedimentation), whereas those that
form by the abandonment of meander loops to form oxbow lakes (palaeopotamon) undergo much slower succession dominated by autogenic processes (e.g. eutrophication). Palaeopotamon habitats exhibit lower connectivity (Figure 3), are larger, deeper, older and more distant from the active channel, on average, than
plesiopotamon habitats. Terrestrialization of abandoned meanders requires centuries, in contrast with decades
for abandoned braids (Amoros eta/., 1987b). The autogenic pathway in Figure 5 applies to situations where
entire meander loops are abandoned to form oxbow lakes. Meandering rivers also abandon smaller aquatic
habitats that are dominated by allogenic processes. The predominance of allogenic or ·autogenic processes
determines the restoration potential of floodplain water bodies, as will be discussed subsequently.
-At the floodplain scale, the trend toward terrestrialization (hydrarch succession) is countered by the formation of new water bodies and by the rejuvenation of extant water bodies. Such high connectivity results in a
diversity of aquatic and semi-aquatic habitats that collectively encompass a wide range of successional stages.
Anthropogenic impacts that reduce fluvial dynamics generally accelerate terrestrialization and retard the formation and rejuvenation of floodplain water bodies (Figure 4). In a comparison of two floodplains of the
Danube, one disconnected from the channel and the other with connectivity largely intact, Loffier (1990) documented much greater diversities of macrophytes (60 versus 20 species), zoo benthos (35 versus 16 species of molluscs) and fishes (30 versus four species) on the unaltered site. Similar results were documented for two
CONNECTIVITY IN ALLUVIAL RIVERS
115
floodplains of the upper Rhine (active versus abandoned), where restricted fluvial dynamics on the abandoned
floodplain reduced habitat heterogeneity of aquatic biotopes, thereby altering community composition and
reducing diversity of macrozoobenthos (Obrdlik and Fuchs, 1991).
Flow regulation, by altering fluvial dynamics and sediment transport, induces major changes in alluvial
forest succession (Johnson eta/., 1976; Pautou and Decamps, 1985; Bren, 1988; Decamps eta/., 1988;
Johnson, 1994).
One mode of action occurs through the influence of regulation on channel migration (Figure 4). Channel
stability may be perceived as a disturbance gradient from stable reaches below dams (regulated) to highly
unstable reaches of severe braiding, both extremes exhibiting low levels of habitat diversity relative to
actively meandering rivers (Figure 6). The stable channels of regulated rivers truncate forest successions
so that the potential for high levels of habitat heterogeneity and biodiversity described in a previous section
is not realized.
Changes in the hydroperiod from the channel margins to the uplands, coupled with differences in soil type
and surface topography across the floodplain, contribute to spatial heterogeneity of alluvial forest composition
and successional stages. Flow regulation typically reduces the frequency and extent of floodplain inundation
and lowers the water-table, with major ramifications for floodplain vegetation. Productive pioneer species
tend to be replaced by less productive upland species (Decamps, 1984) that are able to invade the floodplain
under conditions of artificially enhanced environmental stability. In essence, the overall effect of regulation
on downstream river-floodplain ecosystems is to impose equilibrium conditions on non-equilibrium communities. It is not surprising that the communities that develop after regulation are poorly adapted to the
infrequent catastrophic flood resulting from 'flood control' measures.
Recruitment, establishment and survival of riparian tree species is closely tied to the hydrological regime.
This has been well documented for floodplain forest corridors along prairie rivers in North America dominated by the cottonwood-willow (Populus-Salix) community (Johnson et a/., 1976; Reily and Johnson,
1982; Bradley and Smith, 1986; Rood and Mahoney, 1990). Wind dispersal of seeds coincides with the recession of spring floods that form seedbeds on newly deposited alluvium of point bars. Seeds remain viable for
only a few weeks. Because seedlings are poor competitors, they are successful only on recently disturbed
microsites. On the river studied by Bradley and Smith (1986), successful establishment of seedlings (Populus
deltoides) was correlated 'with years when daily maximum flows during the period of seed dispersal (June 1 to
July 10) attain a stage equal to or greater than the 2-year return flood.' Once established, seedlings may be
uprooted by erosive flooding or killed by desiccation. Poplars are phreatophytes, the roots of which must
maintain contact with the water-table. As the elevation of the point bar is raised by sediment deposition
during annual floods, the surviving saplings are less likely to be adversely affected by scour. Suitable conditions for the long-term establishment of a new generation of trees does not occur annually, even in natural
rivers.
In floodplains downstream from dams, floods are reduced and may not coincide with the period of seed
dispersal. Reduced rates of channel migration, coupled with reduced flooding and sediment transport,
decrease the formation of new areas of bare substrate required as seedbeds. Lowered water-tables from channel degradation below dams may induce drought stress, killing seedlings and old trees and reducing the
growth rates of trees that survive. The pioneer cottonwood-willow forest is adversely affected, being gradually replaced by a late successional community dominated by green ash (Fraxinus pennsylvanica) that under
natural conditions occurs on terraces near the edge of the floodplain (Johnson et a/., 1976). Prairie rivers of
North America are highly regulated by dams and the pioneer floodplain forests are in decline. The natural
disturbance regime that maintains high habitat heterogeneity and biodiversity has been largely replaced by
more stable conditions induced by river regulation.
A very different response was documented for the Platte River, however, where flow regulation was
accompanied by woodland expansion (Johnson, 1994). River regulation transformed the Platte River
from a wide braided system largely devoid of woody vegetation to a single thread channel with a dense alluvial forest corridor (Schumm, 1985). Johnson (1994) attributed the divergent response exhibited by the Platte
River to its geomorphic type (braided). In contrast with the situation described for meandering prairie rivers,
'successful recruitment and expansion of Populus and Salix in braided rivers are not positively associated
116
J. V. WARD AND J. A. STANFORD
with peak-flow events but with periods of low flow' (Johnson, 1994). Flow regulation, by reducing spring
flows, provided suitable conditions for seedling establishment across much of the braided channel.
RE-ESTABLISHING CONNECTIVITY
The preceding material consists of selected examples of the important part played by natural disturbance/
connectivity in maintaining the ecological integrity of river-floodplain ecosystems and how river regulation
may diminish that integrity. In the context of alluvial rivers, ecological integrity may be equated with the
diversity of successional stages of the aquatic and riparian biotopes on the floodplain. According to this perspective, sound ecosystem management is predicated on (1) comprehensive understanding of natural systems, (2) maintaining or re-establishing natural interactive pathways and (3) reconstituting natural
disturbance regimes.
It is necessary to distinguish between different levels of restoration potential or 'degrees of reversibility'
(sensu Amoros eta/., 1987a). For example, rejuvenation of a senescent palaeopotamon habitat, which developed slowly via mainly autogenic processes, would involve a massive effort compared with rejuvenation of a
plesiopotamon habitat which developed by allogenic processes (see Figure 5). Relatively minor excavation
may rejuvenate the plesiopotamon by reconnecting it to the flushing action of high flows.
Re-establishing connectivity between river channels and their floodplains is slowly being recognized as a
viable management strategy. In the upper Rhone, parapotamon water bodies have been reconnected at the
upstream ends, reconstituting their previous eupotamon stage; alluvial plugs have been removed from p1esiopotamon habitats, changing them back to parapotamon (Bravard eta/., 1992).0n the upper Rhine, gravel
is added to the channel to compensate for truncation of sediment transport by upstream impoundments, in
an effort to reduce bed degradation and maintain groundwater levels (Dister eta/., 1990). Because the Rhine
has been isolated from its floodplain, major cities along the river are no longer protected from the 200-year
flood as they were in 1955. For the purposes of flood control, it is proposed to restore quasi-natural dynamics
and reconnect the near-natural floodplains of the Rhine. Purchase of alluvial floodplains along the Charles
River, Massachusetts has been deemed the most effective and economical means of flood control to protect
the city of Boston (Mitsch and Gosselink, 1993). Schiemer and Waidbacher (1992) have made recommendations for restoring connectivity between the channel and floodplain water bodies in anticipation of establishing an 'Alluvial Zone National Park' along a reach of the Austrian Danube.
Because fluvial disturbance/connectivity are the critical attributes that maintain the ecological integrity of
floodplain rivers, they should serve as the focus of restoration efforts. Resource managers must become 'conservators of ecological connectivity' (Stanford and Ward, 1992).
ACKNOWLEDGEMENTS
I thank the conference organizers for the invitation to write this synthesis paper and for providing travel
funds. Mrs Nadine Kuehl typed the manuscript.
REFERENCES
Adis, J. 1992. 'How to survive six months in a flooded soil: strategies in Chilopoda and Symphyla from central Amazonian floodplains',
Stud. Neutrop. Fauna Environ., 21, 117-129.
Amoros, C. and Roux, A. L. 1988. 'Interactions between water bodies within the floodplains oflarge rivers: function and development of
connectivity' in Schreiber, K.-F. (Ed.), Connectivity in Landscape Ecology. Milnstersche Georg. Arb., 29, 125-130.
Amoros, C., Richardot-Coulet, M., and Patou, G. 1982. 'Les "Ensembles Fonctionelles": des entites ecologiques qui traduisent !'evolution de l'hydrosysteme en integrant Ia geormorphologie et l'anthropisation (exemple du Haut-Rhone fran~is)', Rev. Geogr. Lyon, 51,
49-62.
Amoros, C., Rostan, J.-C., Pautou, G., and Bravard, J.-P. 1987a. 'The reversible process concept applied to the environmental management oflarge river systems', Environ. Manage., 11, 607-617.
Amoros, C., Roux, A. L., Reygrobellet, J. L., Bravard, J.P., and Pautou, G. 1987b. 'A method for applied ecological studies of fluvial
hydrosystems', Regul. Riv., 1, 17-36.
Antipa, G. 1928. 'Die biologischen Grundlagen und der Mechanismums der Fischproduktion in den Gewassem der unteren Donau'.
Bull. Sect. Sci. Acad. Roumaine, 11, 1-20.
CONNECTIVITY IN ALLUVIAL RIVERS
117
Benke, A. C., Henry, R. L. III, Gillespie, D. M., and Hunter, R. J. 1985. 'Importance of snag habitat for animal production in southeastern streams', Fisheries, 10, 8-13.
Bonetto, A. A. 1975. 'Hydrologic regime of the Parana River and its influence on ecosystems' in Hasler, A. D. (Ed.), Coupling Land and
Water Systems. Springer-Verlag, New York. pp. 175-197.
Bothar, A. 1981. 'Vergleichende untersuchung der Crustacea-Gemeinschaften in Nebenarm "Alte Donau" und im Hauptstrom (Stomkm 1481) (Danub. Hungar, XCIX)', Ann. Univ. Sci. Budapestinensis, Sect. Bioi. 22-23, 159-174.
Botnariuc, N. 1967. 'Some characteristic features of the floodplain ecosystems of the Danube', Hidrobiologia (Bucurestl), 8, 39-50.
Bouvet, Y., Pattee, E., and Meggouh, F. 1985. 'The contribution of backwaters to the ecology of fish populations in large rivers. Preliminary results on fish migrations within a side arm and from the side arm to the main channel of the Rhone', Verh. Int. Verein.
Limnol., 22, 2576-2580.
Bradley, C. E. and Smith, D. G. 1986. 'Plains cottonwood recruitment and survival on a prairie meandering river floodplain, Milk River,
southern Alberta and northern Montana', Can. J. Bot., 64, 1433-1442.
Braum, E. and Junk, W. F. 1982. 'Morphological adaptations of two Amazonian characoids (Pisces) for surviving in oxygen deficient
waters, Int. Rev. Ges. Hydrobio/., 61, 968-886.
Bravard, J.-P., Amoros, C., and Pautou, G. 1986. 'Impact of civil engineering works on the successions of communities in a fluvial system', Oikos, 47, 92-lll.
Bravard, J.P., Roux, A. K., Amoros, C., and Reygrobellet, J. L. 1992. 'The Rhone River: a large alluvial temperate river' in Calow, P.
and Petts, G. E. (Eds). The Rivers Handbook. Hydrological and Ecological Principles. Blackwell, Oxford. pp. 426-447.
Bren, L. J. 1988. 'Effects of river regulation on flooding of a riparian red gum forest on the River Murray, Australia', Reg. Riv., 2,
65-77.
Brinson, M. M. 1990. 'Riverine forests' in Lugo, A. E., Brinson, M., and Brown, S. (Eds), Forested Wetlands. Elsevier, Amsterdam. pp.
87-141.
Castella, E., Richardot-Coulot, M., Roux, C., and Richoux, P. 1984. 'Macroinvertebrates as "describers" of morphological and hydrological types of aquatic ecosystems abandoned by the RhOne River', Hydrobiologia, 119, 219-225.
Cellot, B. and Bournaud, M. 1987. 'Dynamique spatio-temporelle des deplacements de macroinvertebres dan une grande riviere', Can. J.
Zoo/., 66, 352-363.
Chabwela, H. N. and Ellenbroek, G. A. 1990. 'The impact of hydroelectric developments on the Lechwe and its feeding grounds at
Kafue Flats, Zambia' in Whigham, D. F., Good, R. E., and Kvet, J. (Eds), Wetland Ecology and Management: Case Studies. Kluwer,
Dordrecht. pp. 95-101.
Church, M. 1992. 'Channel morphology and typology' in Calow, P. and Petts, G. E. (Eds), The Rivers Handbook. Hydrological and Ecological Principles. Blackwell, Oxford. pp. 126-143.
Copp, G. H. 1989. 'The habitat diversity and fish reproductive function of floodplain ecosystems', Environ. Bioi. Fish., 26, 1-27.
Cuffney, T. F. 1988. 'Input, movement and exchange of organic matter within a subtropical coastal blackwater river-floodplain system',
Freshwater Bioi., 19, 305-320.
Decamps, H. 1984. 'Biology of regulated rivers in France' in Lillehammer, A. and Saltveit, S. J. (Eds), Regulated Rivers. Univ. Oslo
Press, Oslo. pp. 495-514.
Decamps, H., Fortune, M., Gazelle, F., and Pautou, G. 1988. 'Historical influence of man on the riparian dynamics of a fluvial landscape', Landscape Eco/., 1, 163-173.
Dister, E., Gomer, D., Obrdlik, P., Petermann, P., and Schneider, E. 1990. 'Water managemement and ecological perspectives of the
upper Rhine's floodplains,' Regul. Riv. 5, 1-15.
Duncan, R. P. 1993. 'Flood disturbance and the coexistence of species in a lowland podocarp forest, South Westland, New Zealand', J.
Ecol., 81, 403-416.
Eckblad, J. W ., Volden, C. S., and Weilgart, L. S. 1984. 'Allochthonous drift from backwaters to the main channel of the Mississippi
River', Am. Midland Nat., 111, 16-22.
Friedrich, G. and Muller, D. 1984. 'Rhine' in Whitton, B. A. (Ed), Ecology of European Rivers. Blackwell, Oxford. pp. 265-315.
Gibert, J ., Ginet, R., Mathieu, J ., and Reygrobellet, J. L. 1981. 'Structure et fonctionnement des ecosystemes du Haut-Rhone fran~ais, 9.
Analyse des peuplements de deux stations phreatiques alimentant des bras morts', Int. J. Speteo/., 11, 141-158.
Gosselink, J. G., Lee, L. C., and Muir, T. A. (Eds) 1990. Ecological Processes and Cumulative Impacts: Illustrated by Bottomland Hardwood Wetland Ecosystems. Lewis, Chelsea, Michigan. 708 pp.
Goulding, M. 1980. The Fishes and the Forest. Univ. California Press, Berkeley.
Hayden, W. and Clifford, H. F. 1974. 'Seasonal movements of the mayfly Leptoph/ebia cupida (Say), in a brown-water stream of Alberta,
Canada', Am. Midland Nat., 91, 90-102.
Heeg, J. and Breen, C. M. 1982. 'Man and the Pongolo Floodplain', South African National Scientific Programmes Report, 56, CSIR,
Pretoria, South Africa.
Johnson, W. C. 1994. 'Woodland expansion in the Platte River, Nebraska: patterns and causes', Eco/. Monogr., 64, 45-84.
Johnson, W. C., Burgess, R. L., and Keammerer, W. R. 1976. 'Forest overstory vegetation and environment on the Missouri River
.
floodplain in North Dakota', Eco/. Monogr., 46, 59-84.
Junk, W. 1973. 'Investigations on the ecology and production biology of the floating meadows (Paspalo echinoch/oetum) on the middle
Amazon, part 2, the aquatic fauna in the root zone of floating vegetation', Amazoniana, 4, 9-102.
Junk, W. J. and Welcomme, R. L. 1990. 'Floodplains' in Patten, B. C. (Ed.), Wetlands and Shallow Continental Water Bodies. Vol. I.
SPB Academic, The Hague. pp. 491-524.
Junk, W. J., Bayley, P. B., and Sparks, R. E. 1989. 'The flood pulse concept in river-floodplain systems', Can. Spec. Pub/. Fish. Aquat.
Sci., 106,110-127.
Keller, E. A. and Swanson, F. J. 1979. 'Effects of large organic material on channel form and fluvial processes', Earth Surf Process., 4,
361-380.
Kramer, D. L. and McClure, M. 1982. 'Aquatic surface respiration, a widespread adaptation to hypoxia in tropical freshwater fishes',
Environ. Bioi. Fish., 1, 47-55.
J
118
J. V. WARD AND J. A. STANFORD
Kramer, D. L., Lindsey, C. C., Moodie, G. E. E., and Stevens, E. D. 1978. 'The fishes and the aquatic environment of the central Amazon basin, with particular reference to respiratory patterns', Can. J. Zoo/., 56, 717-729.
Leopold, L. B., Wolman, M.G., and Miller, J.P. 1964. Fluvial Processes in Geomorphology. Freeman, San Francisco. pp. 552.
Lewis, W. M. Jr 1988. 'Primary production in the Orinoco River', Ecology, 69, 679-692.
Lewis, W. M. Jr, Weibezahn, F. H., Saunders, J. F., and Hamilton, S. K. 1990. 'The Orinoco River as an ecological system', lnterciencia,
15, 346-357.
Loftier, H. 1990. 'Danube backwaters and their response to anthropogenic alteration' in Whigham, D. F., Good, R. E., and Kvet, J.
(Eds), Wetland Ecology and Management: Case Studies. Kluwer, Dordrecht. pp. 127-130.
Lowe-McConnell, R. H. 1987. Ecological Studies in Tropical Fish Communities. Cambridge Univ. Press, Cambridge.
Marmonier, P., Dole-Oiivier, M.-J., and Creuze des Chatelliers, M. 1992. 'Spatial distribution of interstitial assemblages in the floodplain of the Rhone River,' Regul. Riv., 7, 75-82.
Meyer, J. L. 1990. 'A blackwater perspective on riverine ecosystems', BioScience, 40,643-651.
Mitsch, W. J. and Gosselink, J. G. 1993. Wetlands. Van Nostrand Reinhold, New York. pp. 722.
Obrdlik, P. and Fuchs, U. 1991. 'Surface water connection and the macrozoobenthos of two typers of floodplains on the upper Rhine,'
Regul. Riv., 6, 279-288.
Obrdlik, P. and Garcia-Lozano, L-C. 1992. 'Spatio-temporal distribution of macrozoobenthos abundance in the Upper Rhine alluvial
floodplain', Arch. Hydrobiol., 124, 205-224.
Pautou, G. and Decamps, H. 1985. 'Ecological interactions between the alluvial forests and hydrology of the upper Rhone', Arch.
Hydrobiol., 104, 13-37.
Penka, M., Vyskot, M., Klimo, E., and Vasieek. 1991. Floodplain Forest Ecosystem. II. After Water Management Measures. Elsevier,
Oxford. 629 pp.
Petts, G. E. 1990. 'Forested river corridors: a lost resource' in Cosgrove, D. and Petts, G. (Eds), Water, Engineering and Landscape.
Bellhaven Press, London. pp. 12-34.
Pinay, G., Decamps, H., Chauvet, E., and Fustec, E. 1990. 'Functions of ecotones in fluvial systems' in Naiman, R. J. and Decamps, H.
(Eds), The Ecology and Management of Aquatic-Terrestrial Ecotones. Parthenon, Casterton Hall. pp. 141-169.
Prowse, T. D. 1994. 'Environmental significance of ice to streamflow in cold regions', Freshwater Bioi. ,32, 241-260.
Reily, P. W. and Johnson, W. C. 1982. 'The effects of altered hydrologic regime on tree growth along the Missouri River in North
Dakota', Can. J. Bot., 60, 2410-2423.
Rood, S. B. and Mahoney, J. M. 1990. 'Collapse of riparian poplar forests downstream from dams in western prairies: probable causes
and prospects for mitigration', Environ. Manage., 14,451-464.
Rzoska, J. 1974. 'The Upper Nile swamps, a tropical wetland study', Freshwater Bioi., 4, 1-30.
Salo, J., Kalliola, R., Hakkinen, 1., Makinen, Y., Niemela, P., Puhakka, M., and Coley, P. D. 1986. 'River dynamics and the diversity of
Amazon lowland forests', Nature, 322, 254-258.
Saunders, J. F. III and Lewis, W. M. Jr 1988a. 'Zooplankton abundance and transport in a tropical white-water river', Hydrobiologica,
162, 147-155.
Saunders, J. F. III and Lewis, W. M. Jr 1988b. 'Zooplankton abundance in the Caura River, Venezuela', Biotropica, 20, 206-214.
Schiemer, F. and Waidbacher, H. 1992. 'Strategies for conservation of a Danubian fish fauna' in Boon, P. J., Calow, P., and Petts, G. E.
(Eds),Rirer Conservation and Management. Wiley, Chichester. pp. 363-382.
Schumm, S. A. 1985. 'Patterns of alluvial rivers', Annu. Rev. Earth Planet. Sci., 13, 5-27.
Seghers, B. H. 1978. 'Feeding behavior and terrestrial locomotion in the cyprinodontid fish Rivulus hartii (Boulenger)', Verh.lnt. Verein.
Limnol., 20, 2055-2059.
Shaeffer, W. A. and Nickum, J. G. 1986a. 'Relative abundance ofmacroinvertebrates found in habitats associated with backwater area
confluences in Pool 13 of the Upper Mississippi River', Hydrobiologia, 136, 113-120.
Shaeffer, W. A. and Nickum, J. G. 1986b. 'Backwater areas as nursery habitats for fishes in pool 13 of the upper Mississippi',
Hydrobiologia, 136, 131-140.
Sharitz, R. R. and Mitsch, W. J. 1993. 'Southern floodplain forests' in Martin, W. H. Boyce, S. G., and Echternacht, A. C. (Eds), Biodiversity of the Southeastern United States. Wiley, New York. pp. 311-372.
Sheppe, W. and Osborne, T. 1971. 'Patterns of use of a flood plain by Zambian mammals', Ecol. Monogr., 41, 179-205.
Simpson, B. R. C. 1979. 'The phenology of annual killifishes' Symp. Zoot. Soc. London, 44, 243-261.
Soderstrom, 0. 1987. 'Upstream movements of invertebrates in running waters-a review', Arch. Hydrobiol. 111, 197-208.
Sparks, R. E., Bayley, P. B., Kohler, S. L., and Osborne, L. L. 1990. 'Disturbance and recovery of large floodplain rivers', Environ.
Manage., 14,699-709.
Stanford, J. A. and Ward, J. V. 1992. 'Management of aquatic resources in large catchments: recognizing interactions between ecosystem connectivity and environmental disturbance' in Naiman, R. J. (Ed.), Watershed Management. Springer-Verlag, New York. pp.
91-123.
Stanford, J. A. and Ward, J. V. 1993. 'An ecosystem perspective of alluvial rivers: connectivity and the hyporherc corridor', J. North Am.
Benthol. Soc., 12, 48-60.
Terborgh, J. and Petren, K. 1991. 'Development of habitat structure through succession in an Amazonian floodplain forest' in BelLS. S.,
McCoy, E. D., and Mushinsky, H. R. (Eds), Habitat Structure. Chapman and Hall, London. pp. 28-46.
Triska, F. J. 1984. 'Role of wood debris in modifying channel geomorphology and riparian areas of a large lowland river under pristine
conditions: a historical case study', Verh. Int. Verein. Limnol., 22, 1876-1892.
Vranovsky, M. 1974. 'Zur Kenntnis der Verteilung, Biomasse und Drift des Zooplanktons im tschechoslowakisch-ungarischen Donauabschnitt', Arch. Hydrobio/. Suppl. 44, 360-363.
Wainright, S.C., Couch, C. A., and Meyer, J. L. 1992. 'Fluxes of bacteria and organic matter into a blackwater river from river sediments and floodplain soils', Freshwater Bioi., 28, 37-48.
Ward, J. V. 1985. 'Thermal characteristics of running waters', Hydrobio/ogia, 125,31-46.
Ward, J. V. 1989a. 'The four-dimensional nature oflotic ecosystems', J. North Am. Benthol. Soc., 8, 2-8.
CONNECTIVITY IN ALLUVIAL RIVERS
119
Ward, J. V. 1989b. 'Riverine-wetland interactions' in Sharitz, R. R. and Gibbons, J. W. (Eds), Freshwater Wetlands and Wildlife. DOE
Symp. Ser. No. 61, Oak Ridge, Tennessee, 385-400.
Ward, J. V. and Palmer, M.A. 1994. 'Distribution patterns of interstitial freshwater meiofauna over a range of spatial scales, with
emphasis on alluvial river-aquifer systems', Hydrobiologia, 287, 147-156.
Ward, J. V. and Stanford, J. A. 1989. 'Riverine ecosystems: the influence of man on catchment dynamics and fish ecology', Can. Spec.
Pub/. Fish. Aquat. Sci. 106, 56-64.
Ward, J. V. and Stanford, J. A. 1993. 'Research needs in regulated river ecology', Reg. Riv. 8, 205-209.
Welcomme, R. L. 1979. Fisheries Ecology of Floodplain Rivers. Longman, London. 317 pp.
Welcomme, R. L. 1985. 'River fisheries', FAO Fish. Tech. Pap., 262, 1-330.
Wolman, M.G. and Leopold, L. B. 1957. 'River flood plains: some observations on their formation', US Geol. Surv. Prof Pap., 282-C,
87-109.
Wourms, J.P. 1972. 'The developmental biology of annual fishes. 3. Pre-embryonic diapause of variable duration in the eggs of annual
fishes', J. Exper. Zoo/., 182, 389-414.