Quaternary Science Reviews 30 (2011) 855e866
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Quaternary Science Reviews
journal homepage: www.elsevier.com/locate/quascirev
Reconciling arroyo cycle and paleoflood approaches to late Holocene alluvial
records in dryland streams
Jonathan E. Harvey*, Joel L. Pederson
Utah State University, Department of Geology, 4505 Old Main Hill, Logan, UT 84321, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 April 2010
Received in revised form
22 December 2010
Accepted 30 December 2010
Available online 12 February 2011
A century of research into the environmental records of alluvial-valley fills in drylands has led to new
theories about landscape response to climate change and cultural evolution over the Holocene. In
a largely separate line of inquiry, paleoflood hydrologists have scoured bedrock canyons for slackwater
deposits, extending the flood record and exploring their climatic significance. Both approaches rely on
the analysis and dating of late Holocene alluvium, sometimes along the same drainages, yet they differ in
the geomorphic setting in which they should be applied. Studies of arroyo cut-and-fill cycles are focused
on broad alluvial valleys, whereas paleoflood hydrology has been focused mainly in narrow bedrock
canyons.
With a focus on the southwestern U.S., we review and compare the fundamentals of these two
approaches and their paradigmatic disconnect, and then discuss potential linkages that could lead to
a more complete understanding of how dryland streams adjust to external stimuli. Recent regional
compilations provide insight into the broader relation between the two record types over entire physiographic provinces. Meanwhile, new tools such as OSL dating, short-lived isotopes, and improved
hydraulic modeling are paving the way for refinement and reconciliation of the two approaches within
individual drainages. The relation between past arroyo cut-and-fill cycles and paleofloods must be
thoroughly explored if we are to fully understand how drainages will respond to a changing climate over
the coming decades to centuries.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Arroyos
Paleofloods
Alluvial records
Drylands
1. Introduction
Holocene alluvial deposits stored along dryland streams provide
records of environmental change in their watersheds. Careful
interpretation of these alluvial archives can reveal past landscape
responses to changes in climate, hydrology, and cultural activity
(e.g. Bryan, 1925). Such records are especially pertinent because
they often record changes similar in scale to those predicted for the
upcoming century (e.g. Hughes and Diaz, 2008).
Most studies of alluvial deposits in drylands fall under one of
two paradigms. The first stems from a long history of study of
alluvial cut-and-fill cycles and the “arroyo problem” as reviewed
below. The second, younger approach is based on the analysis of
slackwater sediments to reconstruct paleoflood frequency and/or
* Corresponding author. Present address: UCSB Department of Earth Science,
1006 Webb Hall, Santa Barbara, CA 93106, USA. Tel.: þ1 805 893 7242; fax: þ1 805
893 2314.
E-mail addresses: [email protected] (J.E. Harvey), [email protected]
(J.L. Pederson).
0277-3791/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.quascirev.2010.12.025
magnitude (e.g. Patton et al., 1979; Baker, 2008). Although each
approach is recognized by its practitioners as applicable to Holocene alluvial records in particular settings, confusion remains about
their appropriate application and relation to one another along and
across drainages. In fact, these contrasting approaches may lead to
quite different interpretations of the same alluvial deposits, yet
they have the potential to be complementary rather than contradictory. Reconciliation of these distinct yet fundamentally related
approaches is essential for a complete and systematic understanding of sensitive dryland streams in a changing climate.
Geomorphic setting is the key factor distinguishing these two
scientific approaches and record types (Fig. 1). In the example of the
southwestern U.S., stream valleys range from broad, unconstricted
reaches a kilometer or more wide to bedrock slot canyons only
meters wide. Often, such variation in valley geometry can be seen
along individual drainages. Large volumes of valley-fill alluvium
can be stored along the broader reaches, where bedrock exerts little
to no control on channel form. Early workers in the southwestern
U.S. recognized that these valley-fills record episodes of aggradation and degradation on the scale of centuries or millennia, and that
these have key relations to paleoclimate and archeology (e.g.
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J.E. Harvey, J.L. Pederson / Quaternary Science Reviews 30 (2011) 855e866
Fig. 1. Contrasting geomorphic settings of alluvial records in drylands: (A) the arroyo cut-and-fill approach, typically applied in wider valleys with alluvial channel boundaries, and
(B) the paleoflood hydrology approach, typically applied in constricted bedrock canyons with immobile channel boundaries.
Huntington, 1914; Gregory, 1917; Bryan, 1922; Hack, 1942; Antevs,
1952). In stark contrast to broad, alluvial reaches, very little sediment can be stored in narrow bedrock canyons. The few places
where sediment can be accommodated (alcoves, confluences,
expansions, etc.) are utilized by paleoflood hydrologists to reconstruct individual flood events and Holocene-scale flood chronologies (e.g. Baker, 1977; Kochel and Baker, 1982; Ely, 1992). Thus, two
end-member paradigms now exist in the literature: reconstruction
of arroyo cut-and-fill histories from deposits in broad alluvial
valleys, and reconstructions of paleoflood frequency and magnitude from deposits stored in constricted bedrock canyons.
Despite the number of contributions demonstrating either
paradigm in the study of dryland alluvial records, the two
approaches remain poorly reconciled. For example, paleoflood
hydrologists often assume that channel boundaries are stable
through constricted bedrock canyons over millennial timescales
despite the pervasiveness of arroyo cutting and filling cycles in the
alluvial reaches of the same streams. Further, our experience is that
drainages vary continuously in valley geometry and often contain
laterally-transitional reaches with a more ambiguous affinity.
Workers in such cases must make the difficult determination of
whether the deposits therein record only episodes of large floods in
a stable channel or instead a more complex suite of aggradational
events relating to changes in sediment load and/or hydrology.
Finally, the temporal relation of deposition across these geomorphic settings is poorly understood; leaving a major gap in our
understanding of how different drainage components respond to
changing environmental factors (Hereford, 2002).
Here we review the body of work representing both endmember approaches to dryland alluvial records. We explore the
nature of the disconnect between these two paradigms and
possible linkages that could lead to a greater model of dryland
fluvial response to external forcing. Finally, we advocate additional
studies based on focused stratigraphy, sedimentology, geochronology, and modeling to clarify the relations between approaches
and record types within individual drainages. Though we focus on
the much-studied southwestern U.S., the issues we discuss are
relevant to drylands worldwide.
2. Holocene arroyo dynamics
2.1. Historic arroyo cutting
Geologists and archeologists have studied the alluvial records of
semiarid streams for over a century (Dodge, 1902). Much of this
work was motivated by a widespread episode of channel incision in
the American Southwest from w1880e1920 AD. Over these few
decades, drainages incised up to 30 m into their alluvium, leaving
former floodplains behind as terraces. Because many farms and
settlements had been built on these surfaces in the preceding
decades, erosion accompanying arroyo cutting resulted in
substantial property damage, agricultural losses, and abandonment
of entire communities (Bryan, 1929; Peterson, 1950; Cooke and
Reeves, 1976).
2.2. Mechanisms of arroyo formation
The causes of historic arroyo cutting have been the subject of
longstanding debate in the literature. A number of reviews discuss
the variety and evolution of hypothesized mechanisms over the
past century (e.g. Cooke and Reeves, 1976; Graf, 1983; Miller and
Kochel, 1999); and though we include a brief review below, we
defer to these works for a more exhaustive treatment of the issue.
2.2.1. Anthropogenic
Early workers suggested that land use changes during
Anglo-American settlement of the Southwest triggered arroyo
incision (e.g. Dodge, 1902; Huntington, 1914; Bailey, 1935). Specifically, the introduction of livestock grazing accompanying settlement
had disturbed vegetation and compacted sediment on hillslopes and
valley floors, hypothetically leading to increased storm runoff, more
erosive floods, and channel entrenchment (Bailey, 1935; Antevs,
1952). The construction of roads, irrigation ditches, and other
floodplain modifications have also been cited as potential anthropogenic influences on arroyo formation (Cooke and Reeves, 1976;
Webb, 1985). However, infilled paleoarroyos exposed in the walls
of modern cutbanks indicate that there have been several cycles of
alluviation and erosion prior to the influence of Anglo settlers
(Bryan, 1925; Peterson, 1950). Though attribution of incision to
anthropogenic forcings persisted in the literature for many decades,
it is now recognized that human activity alone cannot explain all
Holocene arroyo cut-and-fill cycles (Cooke and Reeves, 1976).
2.2.2. Climatic
Decades of stratigraphic and geochronologic study in the
southwestern U.S. have revealed broad regional correlations in the
timing of stream aggradation and degradation in the late Holocene
(Haynes, 1968; Knox, 1983; Karlstrom, 1988; Hereford, 2002). These
correlations have motivated research into the hydroclimatic
changes that could explain the regionally synchronous stream
J.E. Harvey, J.L. Pederson / Quaternary Science Reviews 30 (2011) 855e866
behavior during these cycles. At least four episodes representing
two cycles over the past millennium have been correlated across
several catchments in Arizona, New Mexico, and Utah: (1) a period
of aggradation before w1200 AD coincident with the rise of
Puebloan cultures; (2) prehistoric arroyo cutting around w1200 AD
that coincides roughly with the Puebloan abandonment; (3) an
episode of aggradation from w1300e1880 AD and (4) historic
arroyo cutting from w1880e1920 AD (Hack, 1942; Haynes, 1968;
Hall, 1977; Euler et al., 1979; Graf, 1987; Hereford, 2002;
Huckleberry and Duff, 2008) (Fig. 2A). Incipient refilling of the
historic arroyos began around 1940 AD in many southwestern
streams (Leopold, 1976; Hall, 1977; Hereford, 1984, 1986; Graf et al.,
1991). Though several episodes of arroyo cutting and filling prior to
1200 AD have been recognized (e.g. Waters and Haynes, 2001;
Mann and Meltzer, 2007; Harvey et al., in press), records of these
older events are more rare and generally lack the temporal resolution necessary for inter-drainage comparison.
Mechanisms linking decadal- to centennial-scale climate
changes to arroyo cut-and-fill cycles have long been debated. Some
early workers speculated that arroyo formation was simply associated with episodes of enhanced precipitation and a resulting
857
increase in runoff (Dutton, 1882; Huntington, 1914; Gregory and
Moore, 1931). Bryan (1925) and Hack (1942) suggested as an
alternative that it was a change to more arid conditions that
increased surface runoff by decreasing vegetation cover on hillslopes. In more recent decades, the increasing density of meteorological and streamflow data has fed a number of studies that
attempt to link decadal-scale changes in precipitation frequency
and intensity to floodplain growth in modern arroyos. Hereford
(1984, 1986), Balling and Wells (1990), and Graf et al. (1991) each
used such records to show that the arroyo refilling of the midtwentieth century generally took place during a time of drought
and reduced streamflow. This is in contrast to the historic arroyo
cutting of the preceding few decades, which was characterized by
more intense precipitation interspersed with drought (Hereford
and Webb, 1992).
In addition to a changing hydrology, some have pointed to
changes in sediment supply from hillslopes and upper fluvial
systems as a primary driver of channel change. Graf (1987) studied
the patterns of sediment storage and evacuation for streams of
varying drainage area in the Colorado Plateau. He concluded that
the storage and transport of sediment are most sensitive to climatic
Fig. 2. Climatic and stratigraphic reconstructions for the southwestern U.S.: (A) Late Holocene arroyo cycles in the Colorado Plateau (generalized from Hereford, 2002); (B)
Frequency of dated paleoflood slackwater deposits in the southwestern U.S. (redrawn from Ely, 1997 e note abbreviated height of last column); (C) Percent of western U.S. area in
drought as derived from tree-rings (black line, smoothed over 60 yr). Two-tailed 95% bootstrap confidence intervals shown as dashed lines, mean percent area shown as gray line
(redrawn from Cook et al., 2004); (D) Columns: Frequency of El Niño events per 100-yr bin (modified from Moy et al., 2002). Line: Reconstructed Southern Oscillation Index (data
from Stahle et al., 1998); and (E) Cumulative probability density functions for radiocarbon-dated deposits in alluvial (dashed) and bedrock (solid) river reaches (redrawn from
Harden et al., 2010).
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and land use changes in local, lower-order streams (drainage areas
of 101e103 km2), whereas regional, higher-order streams
(103e104 km2) store and evacuate sediment in response to the
changing supply from these smaller local tributaries. This suggests
that sediment storage propagates down-system from local streams
toward regional streams over time, a finding supported by more
recent work in the Great Plains by Mandel (2008). This pattern
should be discernable chronostratigraphically as an inverse correlation in sediment age between upper alluvial valleys and downstream reaches.
Hereford (2002) suggested that the cooler, wetter climate of the
Little Ice Age increased freeze-thaw action on hillslopes, thereby
increasing sediment yield and promoting regional channel aggradation. A different link between hillslopes and valley floor alluviation is described by McAuliffe et al. (2006), who argue that major
hillslope-stripping events have occurred during transitions from
drought to episodes of increased precipitation. Further, Graf (1987),
Graf et al. (1991), and Pederson (2000) concluded that sediment
storage and mobilization within upper fluvial systems is a key
process effecting downstream changes in sediment load. Thus,
there is broad agreement that late Holocene cut-and-fill cycles
likely involve changes in both hydrology and upstream sediment
delivery.
2.2.3. Autogenic
Other workers have emphasized the role of autogenic adjustments in driving cycles of aggradation and erosion. They document
field examples and experimental studies wherein pulses of incision
followed by widening and alluviation migrate up-network in
response to the formation of local slope anomalies (Schumm and
Hadley, 1957; Schumm and Parker, 1973; Patton and Schumm,
1981). These anomalies represent local base level changes that do
not necessarily occur simultaneously across regional catchments.
For example, local oversteepening may result from deposition of
a tributary debris fan or deposition resulting from downstream
losses in sediment transport capacity due to bed infiltration. The
resulting oversteepened reaches are subject to higher shear
stresses and bed incision during large floods, sending upstream
transient waves of incision followed by widening and renewed
alluviation. The signature of such internal complex response is
a sequence of relatively small-scale, time-transgressive (younging
upstream) terraces (Schumm and Parker, 1973; Daniels, 2008).
Waters (1985) concluded that such “intrabasin geomorphic
parameters” were stronger influences than external climatic
drivers on the timing of alluviation and erosion for a network of
streams in southeastern Arizona. In a different example of internal
response, Patton and Boison (1986) showed that recent landslidegenerated debris overwhelms the fluvial system of Harris Wash,
a tributary of the Escalante River in southern Utah, causing the
spatial and temporal patterns of alluviation and incision to differ
from that seen in other regional streams. Despite these scattered
cases, complex response is unlikely to outweigh other allogenic
forcings such as climate and land use over late Holocene timescales
in the southwest U.S. (Daniels, 2008).
2.2.4. Working hypothesis
Though there may be no simple “smoking gun” to explain all late
Holocene arroyo cutting and filling cycles (Cooke and Reeves, 1976),
there has been a recent convergence of hypotheses in the literature.
Many authors now agree that a shift toward an episode of increased
flood frequency was an important factor in prehistoric and historic
arroyo cutting (Webb, 1985; Webb et al., 1991; Hereford, 2002;
Mann and Meltzer, 2007; Huckleberry and Duff, 2008). Others
point out that the effect of such a shift would be enhanced if the
watershed were already in a sensitive state due to extended
drought and/or land-use changes (Cooke and Reeves, 1976; Webb
et al., 1991; Miller and Kochel, 1999; Tucker et al., 2006). There is
less agreement on what specific climate shifts could have altered
hydrology in such a way as to cause synchronous incision of arroyos
over such a large region. For a more detailed treatment of that issue,
we point the reader to Webb (1985).
As reviewed by Hereford (2002), many streams in the Colorado
Plateau region saw peak streamflow around the time of historic
arroyo cutting. These observations are based on tree-ring derived
streamflow reconstructions (Hereford et al., 1996a), anecdotal
evidence, and in some cases, discharge reconstructions based on
flood-stage indicators. While this supports the working hypothesis,
performing a similar comparison of paleoclimatological and paleohydrological proxies with prehistoric arroyo cutting is more
difficult for several reasons. First is the rough temporal resolution of
climatic reconstructions covering the late Holocene (Fig. 2). Treering reconstructions are helpful in that they generally capture
variations in cool-season precipitation at an annual resolution.
However, their utility is limited by their insensitivity to precipitation intensity, which is arguably more important for flood
production. Further, large, destructive floods can occur during
extended droughts without affecting tree-ring growth, just as
moist intervals do not necessarily require anomalously large floods
(Pederson, 2000; Huckleberry and Duff, 2008).
The types of storms that produce extreme floods in the
southwestern U.S. include localized, diurnal thunderstorms associated with the summer monsoon; dissipating eastern Pacific
tropical cyclones; and slow-moving winter and spring frontal
systems (Ely, 1997). A general rule is that drainages are most
responsive to storms of a spatial scale similar to the size of the
drainage basin (Leopold, 1942). Therefore, monsoonal storms are
more likely to dominate flood hydrology in smaller headwater
drainages, while larger frontal systems and tropical cyclones are
more likely to trigger large floods in higher-order trunk drainages
(Baker, 1977; Graf, 1983; Webb, 1985).
Many workers have cited a centennial- to millennial-scale cycle
in the frequency of El Niño-Southern Oscillation (ENSO) events as
a potential driver of changes in the frequency and intensity of larger
fall and winter cyclonic storm systems in the southwestern U.S.
(Hereford and Webb, 1992; Ely, 1997; House and Hirschboeck, 1997;
Waters and Haynes, 2001; Hereford, 2002). In addition to causing
floods enhanced by saturated soils, these storms are also responsible for much of the winter snowpack that accumulates in higherelevation drainages (Cayan et al., 1999). Further, Webb and
Betancourt (1992) argue that El Niño years are related to more
frequent landfall of tropical cyclones from the eastern Pacific,
which have been known to produce floods-of-record in drainages
in the southwestern U.S. (Webb, 1985). While historic arroyo
cutting appears to be coincident with an episode of increased El
Niño activity (Fig. 2B and Hereford, 2002), it is difficult to reconstruct the record of earlier El Niño events at a resolution high
enough to confirm whether or not prehistoric arroyo cutting can be
related to anomalous ENSO behavior (Fig. 2C).
Recognizing the extreme precipitation intensities that occur
during late summer convective thunderstorms, Mann and Meltzer
(2007) tie their reconstructed histories of aggradation and incision
in New Mexico to changes in the strength of the North American
monsoon. In contrast to prehistoric variations in cool-season
precipitation, which can be reconstructed with tree-rings; shortduration, high-intensity monsoonal storms are very difficult to
reconstruct.
Limitations in length and resolution of existing paleoclimatic
reconstructions preclude confident determination of the drivers of
non-stationarity of large floods. Regardless, one could test the
hypothesis that arroyo cut-and-fill cycles are driven by variations in
J.E. Harvey, J.L. Pederson / Quaternary Science Reviews 30 (2011) 855e866
the frequency and magnitude of flood-producing storms by
comparing dated cut-and-fill cycles with historic and prehistoric
flood records from the same drainages (Webb, 1985; Webb et al.,
1991; Ely et al., 1996; Hereford, 1999).
3. Paleoflood hydrology
3.1. Background
The field of paleoflood hydrology is based on using depositional
and erosional evidence to reconstruct floods that occurred before
the instrumented record (Patton et al., 1979; Kochel and Baker,
1982). Like for arroyo cycles, there are existing reviews of paleoflood hydrology (Costa, 1987; Baker et al., 2002; Benito and
Thorndycraft, 2005; Baker, 2008). Early paleoflood studies were
focused on characterizing catastrophic flood events such as glacial
outburst floods (Dana, 1882; Bretz, 1923). Modern applications of
these methods began in earnest in the late 1970s, and include
extending the flood record to refine flood recurrence intervals (e.g.
Baker, 1977; Kochel and Baker, 1982; O’Connor et al., 1994) and
paleoflood reconstructions related to climatic trends (e.g. Ely, 1997;
Harden et al., 2010). Depositional evidence for paleofloods includes
slackwater sediments, perched woody debris, and silt lines plastered on bedrock walls. Erosional evidence includes scarring on
trees within the flood zone, abrasion of bedrock walls, and truncation of landforms such as debris fans.
3.2. Application in bedrock canyons
In the Southwest, paleoflood studies rely heavily on slackwater
sediments preserved in bedrock canyons. These sand and silt
deposits rapidly fall out of suspension in areas of flow separation
such as alcoves, backwaters upstream of severe constrictions, and
interior channel bends. Repeated deposition in these zones by
progressively larger floods produces series of distinct flood
deposits, constituting a paleoflood package (Baker et al., 1983;
Kochel and Baker, 1988; Benito, 2003; Benito and Thorndycraft,
2005). The stratigraphy of these packages can then be described
and dated using some combination of radiocarbon, optically stimulated luminescence, short-lived isotopes, association with cultural
materials, and dendrochronology. Workers are often interested in
determining the discharge of specific paleofloods, for which they
rely on careful identification and surveying of paleostage indicators
such as silt lines and slackwater deposit surfaces. Water-surface
profiles reconstructed from these stage indicators are then input
into a hydraulic modeling package such as HEC-RAS to estimate
paleoflood discharges (e.g. Webb and Jarrett, 2002). Approaches
utilizing the above methods have produced paleoflood records for
dozens of rivers worldwide; each constructed with the goal of
better assessing the magnitudes of floods a stream is capable of
producing as well as their non-stationarity in recent millennia.
A critical factor in the calculation of paleodischarges is knowledge of channel geometry at the time of deposition (Patton et al.,
1979). Most workers make the assumption that the modern
channel geometry is a reasonable approximation (Baker et al., 1983;
Webb et al., 1991). This assumption would certainly be invalid in
those stream reaches undergoing arroyo cutting and filling cycles,
where grade changes on the order of 10 s of meters in decades have
been recorded. Only a few researchers have attempted to correct for
changes in stream grade and/or channel geometry since deposition
using geologic evidence (e.g. Webb et al., 2002). Indeed, minimizing
this uncertainty is one reason paleoflood hydrologists have focused
their efforts on constricted bedrock canyons with relatively stable
lateral channel boundaries. However, the thickness of the alluvial
veneer is rarely known, and some have noted the possibility of
859
episodic channel aggradation and sediment storage in bedrock
canyons during the late Holocene (e.g. Tinkler and Wohl, 1998;
Webb et al., 2002; Harvey et al., in press).
3.3. Non-stationarity of dated flood deposits
The southwestern U.S. benefits from an abundance of intact
slackwater deposits in bedrock canyons, resulting in a large body of
work on paleoflood frequency and magnitude. Regional compilations of this work suggest large floods tend to cluster into particular
episodes during the Holocene (Ely, 1997; Harden et al., 2010;
Fig. 2B, E). These workers have interpreted this record to suggest
that episodes of increased flood frequency generally occur during
cooler, wetter periods such as the Little Ice Age (w1500e1900 AD).
In contrast, overall drier and warmer episodes like the preceding
Medieval Warm Period (w1100e1400 AD) are poorly represented
in the flood record. Ely (1997) suggested that these centennial-scale
climatic variations are related to shifts in the frequency of El Niño
events, which have been statistically linked to cooler, wetter
conditions in the U.S. Southwest (Cayan et al., 1999). However,
dependable millennial-scale El Niño reconstructions remain
elusive, and this proposed correlation is not yet tested. In fact, local
paleoflood records for several rivers in the Colorado Plateau argue
instead that the LIA was not characterized by frequent, large floods
(Webb, 1985; Webb et al., 1991; Webb and Jarrett, 2002).
Indeed, the source of non-stationarity in regional flood
frequency records is not simple or clear. In addition to any primary
climate drivers, preservation bias and resolution issues obscure the
record. Regarding preservation, there is a significant skew toward
younger ages that is difficult to de-trend. Harden et al.’s (2010)
analysis includes streams ranging from the large trunk drainage
of the Colorado River that heads in the Rocky Mountains to lowerorder streams of the Colorado Plateau, to streams that flow
primarily through the Sonoran desert in southern Arizona, to
streams in southwest Texas. Such a blending of drainage locations,
sizes, and hypsometries can be problematic for two reasons: (1) The
effectiveness of any given storm at producing a flood is dependent
on drainage area. Smaller drainages may experience their flood-ofrecord during localized monsoonal thunderstorms, while the same
storm may have little or no geomorphic effect on a trunk drainage
downstream (Graf, 1983; Knox, 1983). The inverse is also possible
for broader regional storms such as stalled winter frontal systems
(House and Hirschboeck, 1997). (2) Climate changes that could
control the frequency of large floods may be manifest differently
Table 1
Examples of drainages featuring both arroyo cycle and paleoflood hydrology
research in the southwestern U.S.
Drainage
Alluvial cycles
Paleoflood
Kanab Creek, UT/AZ
Webb et al. (1991),
Ely (1992)
Webb et al. (2002)
Little Colorado River, AZ
San Juan River NM/UT
Virgin River, UT
Smith (1990);
Summa (2009)
Hereford (1986);
Hereford (2002)
Webb (1985); Patton
and Boison (1986)
Hereford (2002);
Harvey et al. (in press)
Hereford (1984)
Hall (1977)
Hereford et al. (1996a)
Colorado River, AZ
Verde River, AZ
Hereford et al. (1996b)
Johnson et al. (1997)
Gila River, AZ
Waters and
Ravesloot (2000)
Paria River, UT/AZ
Escalante River, UT
Buckskin Wash, UT/AZ
Webb (1985)
Ely (1992); Harvey et al.
(in press)
Ely (1997)
Orchard (2001)
Ely (1992); Enzel et al.
(1994)
O’Connor et al. (1994)
Ely and Baker (1985);
House et al. (2002)
Ely (1992)
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J.E. Harvey, J.L. Pederson / Quaternary Science Reviews 30 (2011) 855e866
across physiographic provinces. For example, streams draining the
uplands of the Colorado Plateau would likely respond differently to
an increase in winter snowpack than streams in the low deserts of
southern Arizona. Though these complications make them difficult
to interpret, such regional reconstructions suggest that flood
frequency and magnitude have seen dramatic variability in the
Holocene.
4. Exploring the disconnect between paradigms
4.1. Introduction
Though the study of arroyo cut-and-fill histories and the
construction of paleoflood chronologies generally have been
separate lines of inquiry, they are inherently related. Both rely upon
the analysis and dating of Holocene alluvial deposits. Additionally,
since many dryland streams flow for only short periods following
storm events, the study of dryland alluvial deposits is in most cases
the study of flood deposits (Graf, 1983; Tooth, 2000). Herein lies an
important distinction e paleoflood hydrology focuses on large,
unusual floods, whereas arroyo stratigraphies may be composed of
deposits from any range of flow magnitudes. Finally, the two
approaches have been applied in neighboring or even the same
reaches of a single drainage without much regard to the other. The
following discussion reviews some of these cases, as well as
previous attempts to link the two record types from the scale of
a single watershed to the entire southwestern U.S.
4.2. Settings between end-members
There are many drainages that contain both end-member valley
geometries shown in Fig. 1. In some cases the transition between
them is abrupt and obvious, while in others it is more gradual and
diffuse. These more ambiguous, transitional reaches present
a challenge, as it may not be clear whether perched alluvial
deposits therein were emplaced by large floods or are remnants
from an episode of aggradation. At what point in such a transition
zone do the cycles of aggradation and degradation that characterize
alluvial reaches give way to a stable canyon reach that preserves
only larger floods? It is possible that hydrologic modeling could
help define the geometric transition between these two endmember reach types.
The Colorado River is a classic example of a drainage that
features varying valley geometries along its length, as well as
deposits that have been interpreted as both paleoflood archives and
fill terraces. O’Connor et al. (1994) interpreted a sequence of
deposits at Axehandle Alcove in Marble Canyon as a sequence of
paleoflood deposits interbedded with locally-derived sediment and
cultural material. They use the surface of the deposits as paleostage
indicators to calculate discharges, assuming that the channel
geometry in 1990 AD reflects the geometry at the time of
emplacement. They acknowledge that this assumption is their
largest source of uncertainty, and suggest that their reconstructed
discharges should be interpreted as minima due to probable
channel scour during floods. They do not, however address the
possibility of longer-term grade changes over the >4000 yr-long
record.
Representing the cut-and-fill approach, Hereford et al. (1996b)
studied the debris fans and alluvial terraces that line the
Colorado River at Nankoweap Rapids, w80 km downstream from
861
the site studied by O’Connor et al. (1994). They interpreted alluvial
deposits 5e17 m above the modern river as a series of three inset
aggradational terraces deposited between w800 BC and 1880 AD.
They associate deposition of the alluvial terraces with changes in
the grade of the river related to increased activity on nearby debris
fans. Interestingly, the dated alluvial sequences appear to correlate
to the aggradational sequences described elsewhere in the Colorado Plateau (Hereford, 2002). In a more recent study aimed at
addressing the confusion surrounding alluvial deposits along the
Colorado River, Tainer (2010) showed that Holocene alluvial
deposits in (constricted) Glen Canyon include strong evidence for
both changes in river grade as well as more recent paleoflood
deposits. This result suggests that despite the narrowness of the
canyon at the study site, bed aggradation and degradation has
taken place in recent millennia.
Clearly, the interpretations of the alluvial record in Grand
Canyon by Hereford et al. (1996b) and Tainer (2010) are remarkably
different than that of O’Connor et al. (1994). Considering the
overlapping ages and landscape positions of the studied deposits; it
is possible that the paleoflood sequence studied by O’Connor et al.
(1994) was a vertical cross-section through one or more of the
alluvial terraces described by Hereford et al. (1996b) and Tainer
(2010). Alternatively, the suite of inset fill terraces described by
Hereford et al. (1996b) could be flood packages formed while the
river underwent overall lowering of grade. A third solution is that
both approaches are valid, and terrace formation is associated with
localized aggradation around tributary debris fans. In any case, the
Colorado River is an example where it is not always clear whether
the Holocene alluvium should be interpreted as a stack of paleoflood deposits or as the result of a more complicated sequence of
changes in channel grade. We suggest that such ambiguous reaches
may be as common as those that are clearly recognizable as
belonging to either end-member stream geometry.
4.3. Unreconciled results from both approaches in single drainage
There are several examples where both approaches have been
demonstrated on different geomorphic reaches of the same
streams. In fact, a series of these cases in the southern Colorado
Plateau was the subject of an AMQUA field trip in 1996 (Ely et al.,
1996). Field trip participants visited several locations where the
alluvial deposits of both alluvial and constricted reaches had been
studied through separate lines of inquiry. For example, the Paria
River in southern Utah has been the subject of both upstream
arroyo stratigraphy (Hereford, 1986, 2002) and downstream paleoflood research (Ely, 1992; Webb et al., 2002). Additional examples
exist throughout the Southwest (Table 1), yet little work has been
done to reconcile these results and their distinct interpretations
regarding channel processes and controls. In most cases, such
reconciliation is precluded by inadequate temporal resolution, yet
workers sometimes do not acknowledge the results and implications of the alternative approach.
A worthwhile exercise for the cases mentioned in Table 1 would
be to compile and compare the chronostratigraphic results from
both approaches on a stream-by-stream basis, highlight the data
gaps that currently inhibit direct comparison, and attempt to fill
those gaps using all available techniques. The working hypothesis
described above in Section 2.2.4 and originally described by
Hereford (1999) predicts that, in a given drainage, the ages of
arroyo-fill deposits should be inversely correlated with paleoflood
Fig. 3. Potential linkages between upstream-alluvial reaches and downstream bedrock canyons. During arroyo cutting along the alluvial reach, three potential responses in
a bedrock canyon downstream are: (A) deposition of slackwater deposits with no change in grade (working hypothesis e see Section 2.2.4); (B) aggradation in response to upstream
incision; and (C) downcutting in kind with upstream reach, leaving behind former streambed deposits as a “false” paleoflood package. Profiles show stream grade through both
reaches before (solid) and after (dotted) arroyo cutting.
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J.E. Harvey, J.L. Pederson / Quaternary Science Reviews 30 (2011) 855e866
Fig. 4. Summary of alluvial geomorphic events in both the upstream-alluvial and downstream-constricted reaches of Buckskin Wash, Utah (U.S.) as revealed through OSL,
radiocarbon, dendrochronology, and short-lived isotopes (Harvey et al., in press). In this case study, constricted-reach deposits are inversely correlated with upstream valley-fills.
slackwater deposits (Fig. 3). As an example, in the alluvial reaches
of the Paria River basin, Hereford (1986, 2002) identified two major
arroyo-cutting events centered around 1300 AD and 1880 AD, each
bounded by longer-lasting aggradational episodes. Downstream, in
the narrows of Paria Canyon, Webb et al. (2002) identified 11
slackwater flood deposits. The highest deposit surface was determined to be historic (pre-1955 AD), and was linked to one of the
largest known historical floods (either in 1862, 1884, or 1909 AD).
The next highest deposits date to around w1420 AD and 1280 AD,
respectively. Thus, these highest three flood deposits appear to be
broadly correlated with upstream incision events, though the link is
not strong. Webb et al. (2002) also noted that several large historic
floods were not represented in the alluvial record of the bedrock
canyon at all. As with most of the other cases from Table 1, more
detailed, higher-resolution age control must be produced before
reconciliation of the records in the two end-member settings of the
Paria River is possible.
4.4. Regional-scale comparisons
Direct comparison of regional paleoflood compilations with the
record of arroyo dynamics is difficult, in part due to the contrasting
temporal resolution between them and uncertainties associated
with radiocarbon-based age control (Ely et al., 1996; Hereford,
2002; Huckleberry and Duff, 2008) (Fig. 4). Arroyo cut-and-fill
records are more numerous and supported by a close association
with archaeological studies, radiocarbon ages, and dendrochronology. The paleoflood record, on the other hand, is limited to
a smaller dataset. Additionally, the regional flood chronology constructed by Ely (1997) bins large flood events into 200-year intervals (Fig. 2B). We know, however, that arroyo headcuts can pass
through a drainage in a matter of a few years to decades (Gregory
and Moore, 1931; Webb, 1985). If the hydroclimatic conditions
associated with arroyo cutting occur over only years to decades,
they could be lost in 200-yr bins.
A newer method of synthesizing Holocene alluvial records over
entire regions at higher resolutions has been developed in recent
years (e.g. Macklin and Lewin, 2003; Lewin et al., 2005;
Thorndycraft and Benito, 2006; Benito et al., 2008; Harden et al.,
2010). Focused mainly on rivers throughout Europe, these studies
involve the construction of large databases of dated alluvial
deposits across regions. They first classify the dated units based on
their geomorphic and stratigraphic contexts. For example,
Thorndycraft and Benito (2006) break their dated deposits into the
following settings: alluvial overbank, flood basin, alluvial channel,
fluvio-torrential, and slackwater flood deposits, before converting
them into cumulative probability distribution functions (CPDFs)
and comparing them across geomorphic contexts and over time.
J.E. Harvey, J.L. Pederson / Quaternary Science Reviews 30 (2011) 855e866
863
Studies of this type have the potential to reveal the broader
adjustments of fluvial systems to anthropogenic and climate forcings over the mid-late Holocene as a function of geomorphic
setting.
Harden et al. (2010) builds upon Ely’s (1997) record by applying
this methodology to the abundant dated deposits in the southwestern US. They compiled a database of 724 radiocarbon dates
from 37 locations, making the important step of distinguishing
between “bedrock” and “alluvial” geomorphic settings. From this
database, they constructed CPDFs for alluvial deposits found in
either setting. The temporal pattern of these data illustrates a rough
inverse correlation between deposits in the two reach types, at
least toward the latter part of the record (Fig. 2E). Such a pattern
would appear to support the hypothesis that episodes of increased
flood frequency are associated with regional arroyo-cutting events.
Harden et al. (2010) suggest that alluvial reaches may not capture
the largest floods due to channel scour and enlargement during
such events. Alternatively, the more stable channel boundaries in
the bedrock reaches allow for more complete preservation of larger
floods (Baker, 1977). They conclude that episodes of frequent, large
floods as preserved in bedrock reaches correlate with cooler, wetter
periods; whereas flood deposits are more likely to be preserved in
alluvial reaches during cool, dry periods.
For the purposes of reconciling the temporal relations between
the two main types of dryland alluvial records, the Harden et al.
(2010) study is subject to the limitations described earlier in
section 3.3. Those issues could be addressed by comparing only
streams of a similar drainage area and physiography. Another
element of uncertainty associated with this method is the challenge
of distinguishing between climatically significant flood deposits
and those that record minor, less significant events. Since paleoflood magnitudes are calculated using flood-stage indicators,
factors that influence the apparent stage of a paleoflood such as
syn- or post-depositional incision or aggradation can have
a dramatic effect on resulting interpretations.
The preservation potential of a fluvial deposit is intricately tied
to the particular process and geometry in the reach of interest
(Lewin and Macklin, 2003). As an example, in the case of a channel
that spends hundreds of years migrating laterally across an alluvial
valley at a steady grade, most flow events will not be preserved. In
bedrock canyons, deposits are likely to be preserved only if a flood
event is large enough to passively overtop existing deposits in
a sheltered zone, yet is small enough not to scour out the entire
package. Effectively, however, a greater range of flow events is
likely to be preserved in alluvial reaches, due to overall increased
accommodation space along the drainage.
Despite existing limitations and uncertainties, regional paleoflood compilations can provide a helpful context in which to view
late Holocene arroyo cycles, especially once they begin to include
paleoflood deposits whose ages are constrained by other dating
techniques such as OSL, dendrochonology, and short-lived isotopes.
with radiocarbon dating, the instrumented record, and historic
photographs. He identified three episodes of arroyo formation:
w900 AD, 1600 AD, and w1909 AD. The most recent event could be
traced to a large flood observed by settlers that caused a knickpoint
to migrate several kilometers upstream. He notes the rough
correlation between clusters of large floods between
1200e900 yr BP and 600e400 yr BP with the development of the
first and second paleoarroyos, respectively, although this link is
limited by uncertainty in radiocarbon analysis.
Working on Harris Wash, a tributary in the Escalante River
basin, Patton and Boison (1986) similarly identified sandy flood
deposits that they temporally correlated with incision of valley-fills
upstream. They noted that the flood deposits they linked to
upstream incision were generally best-preserved in areas of flow
separation near bedrock walls, yet, the sedimentological characteristics that distinguish these deposits from the alluvial-valley fills
which they overlie were not described in detail. They suggest that
further identification and dating of these incision-related flood
packages could help refine the complex alluvial history of the
region.
Another interesting case is that of Kanab Creek, one of the
deepest and most dramatic arroyos in the Colorado Plateau. By
examining anecdotal evidence and flood-scarred trees, Webb et al.
(1991) document that the modern arroyo was cut by a series of
exceptionally large floods beginning the late 19th century. Despite
the magnitude of channel change caused by floods in the upper
alluvial reach, re-photography of several historical photos suggests
that there was very little channel scour or fill in the downstream
bedrock canyon. Though Smith (1990) and Summa (2009) have
investigated earlier arroyo cycles in Kanab Creek, contemporary
flood deposits have not been identified in the canyon downstream.
Following the lead of Webb (1985) and Webb et al. (1991),
Harvey et al. (in press) performed a similar study along Buckskin
Wash, a tributary to the Paria River in the Colorado Plateau that
features an alluvial reach draining into a slot canyon. They identify
four episodes of arroyo formation since w3 ka, with the last two
occurring at w1300 AD and w1880 AD (Fig. 4). The majority of the
slackwater deposits in the slot canyon downstream were deposited
between w1850 and 1950 AD, suggesting that the bedrock canyon
may record the same floods that caused arroyo cutting upstream.
However, noting the complete absence of deposits from w1000 AD
to w1850 AD in the slot canyon, Harvey et al. argue that preservation of flood deposits in the slot canyon was temporarily
enhanced by extreme sediment loading resulting from the incision
of upstream valley-fills. This phenomenon would serve to exaggerate the frequency and magnitude of floods during arroyo cutting
and underestimate those that occurred during upstream aggradation. This has important implications for paleoflood studies, as the
site in Buckskin Wash has been referred to as a classic locality for
slackwater deposits in the southwest U.S. (Ely et al., 1996; Knox,
2000).
4.5. Single-drainage comparisons
5. Reconciliation
Few workers have addressed the chronostratigraphic relation
between record types in a single drainage. For his doctoral dissertation, Webb (1985) attempted to directly reconcile alluvial records
from both broad and constricted valley geometries within the
Escalante River drainage of south-central Utah. The Escalante has
a pattern common to the Colorado Plateau, consisting of an
upstream reach with an arroyo cut into a broad valley fill and
a downstream reach that is significantly constricted by bedrock
walls. Webb studied deposits in the upper alluvial reach to reconstruct its arroyo cut-and-fill history and in the constricted reach to
develop a paleoflood chronology, constraining ages of deposits
The studies summarized above are examples of how alluvial
records have been used to reconstruct the late Holocene behavior of
streams in the southwestern U.S. They also set the context for the
questions that remain in order to reconcile the different
approaches. Are constricted-reach channels truly stable over
Holocene time while neighboring alluvial reaches experience bed
elevation changes of tens of meters? How do alluvial records (and
assumed stream behaviors) geometrically transition from one
record type to the other along a single drainage? What are the
effects in both reach types of the largest floods vs. smaller, more
frequent events? Can we define the hydraulic and geometric
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J.E. Harvey, J.L. Pederson / Quaternary Science Reviews 30 (2011) 855e866
transition or threshold between record types and clarify their
distinctions?
We suggest that resolving these issues will require a series of
focused chronostratigraphic studies like Webb’s (1985) and Harvey
et al.’s (in press), wherein records from both end-member reach
types are studied and compared in detail within a single drainage.
Attention must also be paid to the transitional reaches that separate
end-members. In order to achieve the precise, high-resolution age
control necessary to compare the temporal pattern of deposition,
a suite of geochronologic methods must be employed. Newer tools
like optically-stimulated luminescence and short-lived isotopes can
augment AMS radiocarbon, dendrochronological, and archeological
methods; especially where datable organic or cultural material is
absent.
Equally important, more sedimentological detail is needed to
achieve a process-based understanding of deposition across the
two major reach types. There is a notable lack of such data in the
literature. For example, one can use the sedimentology of flood
deposits in bedrock canyons to distinguish between the scenarios
shown in Fig. 3. Typical slackwater deposits consist of silt- and
sand-size particles that fall rapidly out of suspension in areas of low
flow velocity. If coarser material and bedforms that resemble the
higher-velocity channel bed are preserved high in outcrop, it is
likely that significant changes in bed elevation have occurred.
Further, analyses of flood deposit composition and grain size
distributions could be helpful in correlating flood packages over
many kilometers, allowing exploration of the control of valley
geometry on flood deposit preservation.
Thanks to advances in hydraulic modeling techniques, exploration of the role that valley geometry plays in controlling longterm stream behavior in constricted vs. alluvial reaches is becoming
increasingly possible. New generations of two-dimensional
modeling packages may be better able to capture the complex
boundary conditions and across-channel variations in velocity or
shear stress than existing one-dimensional models (Miller and
Cluer, 1998). Such modeling may also allow exploration of the
transition zone between end-member geomorphic settings. If we
can quantify the thresholds in valley geometry that determine the
stability of channel geometry and control the preservation potential of events of various magnitudes, it could help guide those that
work in such settings toward the correct interpretation of the
alluvial deposits stored therein.
6. Conclusions
There is a notable disconnect between the approaches taken by
those studying arroyo cut-and-fill cycles in broad alluvial valleys
and those studying paleoflood records in bedrock canyons. While
each approach has been demonstrated dozens of times in endmember settings, little effort has been made to relate the two
record types in a given drainage. Similarly, little attention has been
paid to those reaches that lie morphometrically between the two
end-member geomorphic settings. However, the two seemingly
disparate datasets are fundamentally related in that they rely on
the analysis of late Holocene alluvial deposits, and are often
undertaken in different parts of the same streams. Further, reconciliation of the two approaches can serve as an important test of the
working hypothesis in the arroyo cut-and-fill literature: that
arroyos are cut during episodes of unusually frequent, large floods
tied to external climatic phenomena.
As these datasets continue to grow and new tools are employed,
it is becoming possible to combine them into a greater model of
fluvial response to environmental change in drylands. Regional
syntheses must be complemented by more focused, watershedscale studies. Every geochronological tool available must be
embraced in order to achieve sufficient temporal resolution to
compare records both along and across drainages. The value of such
studies would also benefit from greater sedimentological analysis
of alluvial deposits in either end-member setting as well as the
transitional zone between. Finally, improved numerical modeling
capabilities may permit quantitative exploration of the control that
reach geometry has on the preservation of alluvial deposits therein.
The collective results of these additional studies will help us
approach a more complete understanding of how dryland fluvial
systems have adjusted to past environmental changes and how
they might behave in the future.
Role of the funding source
This work was partially funded by a graduate student research
grant from the Geological Society of America, the Arthur D. Howard
award from the Quaternary Geology and Geomorphology division
of GSA, and student support from the Utah State University
Department of Geology.
Acknowledgments
Thanks to Richard Hereford, Varyl Thorndycraft, and James
Knox, whose thoughtful reviews improved this manuscript
considerably.
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