Urbanization and Adjustment of
Ephemeral Stream Channels
Anne Chin* and Kenneth J. Gregory**
*Department of Geography, Texas A&M University
**Department of Geography, University of Southampton
Comparatively few studies of stream channel adjustment following urbanization have been undertaken in dryland
environments. In the new master planned community of Fountain Hills, a residential area near Phoenix, Arizona developed since the 1970s, surveys in 1987 and 2001 of ephemeral wash channels show that they are larger than comparable channels in humid areas, reflecting the effects of rare but substantial floods. Morphological adjustment is spatially varied and is influenced by wide road crossings that are responsible for fragmentation of the adjusting channels
into segments. By 2001, these segments are characterized by scour immediately downstream of a crossing and a relatively high width-depth ratio farther downstream before the next road crossing. Such spatially distributed responses
have caused management problems unique to arid environments, so that, although road drainage was originally allowed to flow into the washes at the crossings, the stormwater network has now been augmented to improve drainage
and to inhibit scour at the crossings. In maintaining such washes, consideration of channel adjustments as a result of
urbanization could form the basis for an approach comparable to restoration methods in more humid areas. Key
Words: ephemeral channels, hydrological consequences of urbanization, stormwater drainage, stream channel adjustment.
M
orphological adjustments of river channels
consequent upon land-use changes have now
been investigated in a range of different environments, with the most emphasis on humid temperate
areas. Since Wolman (1967) first illustrated how human
activity causes changes in runoff and sediment transport
leading to adjustments in stream channel dimensions,
river channel adjustments have been found to result
from dams and river regulation, land-use changes, channelization, bridge effects, and urbanization. Research focused on the reasons for the change (Gregory 1987a), on
what is changed (Gregory 1987b), and on the extent of
the change, especially how far downstream such changes
are identifiable. Urbanization affects river channels by
increasing runoff so that channels tend to enlarge over
time. Channels may initially aggrade during building activity due to increased sediment supply to urban stream
channels (Wolman 1967; Wolman and Schick 1967).
After urbanization is complete and sediment sources
have been reduced, channel erosion and channel enlargement may become pronounced. Since such changes
are one reason for channelization of urban streams
(Brookes 1988, 172; Brookes and Shields 1996), channel
impacts often extend downstream of the urbanized area
(Gregory, Davis, and Downs 1992).
Three issues of increasing interest are associated with
channel adjustments downstream of urban areas. First,
stream channels are more sensitive to the downstream
impact of urban influence in some environments than in
others (Downs and Gregory 1995); little is known of the
sensitivity of channels in drylands. Second, enlarged
channels downstream of urban areas are not always
problematic because they can accommodate increased
flows more readily, a matter not fully explored since
being identified by Hirsch (1977). Third, public perception of stream channels may affect decision making
despite little understanding of what is natural (Gregory
and Davis 1993; Graf 1996). For example, allegations of
channel and floodplain aggradation on the Snowy River
flood plain in Victoria (Brizga and Finlayson 1994) are
not supported by compelling scientific evidence; oral
traditions have been adopted despite contradictory
information on environmental history (Finlayson and
Brizga 1995). Widespread perception of aggradation
along the Herbert River, Queensland is similarly not supported by historical accounts, by gauging station data, or
by cross sections (Ladson and Tilleard 1999). Along the
Missouri River in Montana, landowners believe that a
dam has initiated bank erosion, but geomorphological
evidence indicates otherwise (Darby and Thorne 2000).
Because similar nuances exist in urban areas, perception
of urban stream channels may be relevant to river management (Tapsell 1995). Thorne, Hey, and Newson
(1977) give comparatively little consideration to the
problems of urban streams, but an example in Sydney,
Australia, shows the important role that geomorphologists can play in selecting stormwater management strategies (Warner 2000).
Annals of the Association of American Geographers, 91(4), 2001, pp. 595–608
© 2001 by Association of American Geographers
Published by Blackwell Publishers, 350 Main Street, Malden, MA 02148, and 108 Cowley Road, Oxford, OX4 1JF, UK.
596
Chin and Gregory
Studies of stream channel adjustment should ideally
be undertaken in the same basin before, during, and after
the process of urbanization (see, e.g., Leopold 1973), but
data are seldom available in sufficient detail before the
effects of urbanization are evident. Therefore, the spacetime substitution approach suggested by Wolman (1967)
was used to compare channels affected by urbanization
with others not affected in Pennsylvania (Hammer 1972)
and in the U.K. (Gregory and Park 1976; Gregory 1987a)
and linked to flood plain expansion in the environs of
Denver (Graf 1975). In detailed investigations of an urban basin in New South Wales, studied by Gregory
(1977), Neller (1988) showed that bank erosion was 3.6
times and knickpoint retreat 2.4 times greater in an urban channel than in a nearby rural channel. In the U.K.,
Roberts (1989) further found that mean enlargement
ratios are generally explicable in terms of simple parameters such as percentage urbanized or sewered, and that relaxation times for most changes are relatively short for
many British rivers. These studies demonstrated that, as
a result of urbanization, stream channel capacity can increase up to three times and occasionally up to as much
as fifteen times (Gregory 1987b).
Spatial patterns of adjustments have also been recognized. In Zimbabwe, reconstruction of the extensive
dambo system showed that change occurring during the
growth of Harare from 1891 to 1984 involved metamorphosis of the original channels, average channel widening of 1.7 times, and average bank erosion rates of 0.33 m
per year (Whitlow and Gregory 1989). In the Monks
Brook basin in southern England, channel enlargement
of up to 2.5 times was shown to be discontinuous and involved widening of up to 2.2 times, deepening of up to
0.4 times, or a combination of the two (Gregory, Davis,
and Downs 1992).
Generalizations from these studies are not easy to apply to dryland streams because ephemeral channels may
respond to urbanization differently from the way in
which perennial streams respond. Dryland environments
have ample sediment supplies from unvegetated hillslopes and unstable channel banks (Leopold and Miller
1956), leading to high suspended load (Reid and Frostick
1987) and especially bedload (Laronne and Reid 1993;
Reid and Laronne 1995) fluxes. The lack of coarse armor
layers (Laronne et al. 1994) and scour and fill during
flash floods (Leopold, Emmett, and Myrick 1966; Wertz
1966) further encourage high sediment transport rates.
Dryland environments also exhibit extreme spatial and
temporal variability in both precipitation input and response mechanisms. Geomorphic activity is discontinuous and is characterized by uneven movement and storage of sediments (Graf 1982, 1983a). Thus, the hydraulic
geometry of ephemeral channels is subject to rapid and irregular changes in both space and time (Rendell and
Alexander 1979), with periodic channel width oscillations resulting from transmission losses and tributary inputs (Thornes 1976, 1980).
Although extreme events may account for only 5 percent of total events, they can transport 65 percent of
total sediment in ephemeral streams (Garcia 1995).
Extreme events are therefore more geomorphologically
effective in arid streams, and adjustment processes and
recovery times are longer in arid than in humid regions
(Wolman and Gerson 1978). Channel morphology is
shaped by infrequent large floods and also by scour and
fill during smaller flows (Thornes 1977). Explanation for
ephemeral channel form is more complicated than
for perennial rivers, which have a more constant, timecontinuous mutual adjustment between form and process
(Graf 1983b; Rhoads 1988).
Given the particular dynamics of arid stream processes, urbanization impacts are likely to be less predictable and more variable than in humid-temperate streams.
Urbanization also changes frequency-magnitude relations by increasing runoff so that the magnitude of low
frequency events may be increased. Thus, urbanization
has the effect of increasing geomorphic effectiveness and
accentuating the role of extreme events in arid environments, leading to even longer recovery times and requiring more complex explanations. Although the
background to urbanizing streams in dryland environments has been outlined (Cooke et al. 1982) with representative examples from broad valley floors (e.g.,
Graf 2000), there remains a critical need for empirical
data (Tooth 2000) and more studies of urbanizing ephemeral channels.
This article examines the case of Fountain Hills, a
master planned community flanking the eastern foothills
of the McDowell Mountains in Arizona, as an example
of urban impacts on a representative ephemeral channel
in a dryland environment. Three specific questions are
addressed. First, how has urbanization affected stream
channels in Fountain Hills? Second, are stream channel
adjustments similar to those found in urbanized humid
temperate environments? Third, do particular management implications arise in the case of dryland urban river
channels?
Fountain Hills, Arizona
Fountain Hills has the advantage of being a completely new urban area with residential development superimposed rapidly upon the physical system (Figure 1).
Urbanization and Adjustment of Ephemeral Stream Channels
Before urbanization, a natural system of wash channels
delivered runoff from the McDowell Mountains eastward
to the Verde River. Six small dams were installed in the
headwaters of the ephemeral channels some twenty years
ago because of flash floods. Several flood control structures were subsequently emplaced. Development began
in 1970 with the construction of a fountain that was recorded in the Guinness Book of Records as the world’s
tallest (Figure 1). The fountain soared 170 m every hour
on the hour, and the area quickly attracted a year-round
population of 8,000, with 3,000 additional winter residents (Fountain Hills Chamber of Commerce 1985).
Fountain Hills was incorporated as a town in 1989, and
by 1999 the population had increased to 17,000. As
development expanded westward into the McDowell
Mountains, the mountain front rapidly became covered
with impervious surfaces. Fountain Hills is therefore an
ideal location for investigating the effects of urbanization
on stream channels, because these effects can be identified relatively easily.
Fountain Hills offers an especially timely test case for
assessing urbanization impacts in an arid environment
because of its topographic location. Most urban development in drylands has been confined to broad valley
floors, flood plains, or alluvial fans (Graf 1988), but
Fountain Hills is situated on a desert piedmont, an environment characterized by complex fluvial processes and
flood hazards (Rhoads 1986). Steep upland hillslopes and
high erodibility of such terrain often cause localized erosion and sedimentation (Schick 1979, 1995). A dearth
of information exists on urban impacts on stream channels in these settings, in part because streams are seldom
gauged, and hydrologic and geomorphic data are scarce
(Rhoads 1986). Flooding and associated problems are
therefore particularly difficult to plan and to manage in
these environments. However, because urban areas are
rapidly encroaching upon mountain fronts in response to
population growth (e.g., Albuquerque, New Mexico;
Eilat, Israel), increased knowledge of how these geomorphologically sensitive dryland environments are affected
by urbanization is critical for effective planning and
management.
Three planning decisions early in the development of
Fountain Hills had a direct impact on hydrologic and
geomorphic processes. First, stream channels in Fountain
Hills were identified in development plans as primary
mechanisms to remove storm waters. The Town of Fountain Hills’ general plan (Town of Fountain Hills 1993)
stated that “[t]he major washes generally traverse Fountain Hills in a northwest to southeast direction. These
washes are generally utilized as conveyors of storm drainage from developed and undeveloped land.” No other
597
reference was made to the natural or urban storm water
drainage in that plan.
Second, because of market-driven forces that place
high value on homes with views near washes, disturbance of the washes is prohibited through town subdivision ordinance regulations and rigorous streambed alteration specifications (Valder 2000). Thus, cutting and
filling within ten feet of washes require approval of the
town council and a vote of the people, there are hillside
preservation regulations that minimize grading activity
on sideslopes of washes, and the Town of Fountain Hills
requires that any drainage parcel having greater than 100
cfs of flow in a 100-year flood be placed in a separate tract
of land. Therefore, although urbanization has caused significant changes in the physical landscape, the original
washes, including their alignments, have generally remained intact.
Third, as in many desert cities, such as Tucson, Arizona (Resnick, DeCook, and Phillips 1983) and Eilat, Israel (Schick 1995; Schick, Grodek, and Wolman 1999),
rather than investment being made in costly but infrequently needed storm sewers, streets in Fountain Hills
were built to serve as storm drainages. Many cross and
slope towards channels in such a way that surface runoff
is naturally directed there. Thus, as observed by local residents, not only do roads increase runoff by providing impervious surfaces, they also deliver large amounts of
storm water into stream channels during heavy rains.
Roads in Fountain Hills therefore form an integral part of
the urban drainage network. Because road surfaces tighten
the connection between the drained areas and the watercourses removing runoff in Fountain Hills, decreased lag
times and increased flood peaks are to be expected (see,
e.g., Dunne and Leopold 1978), as well as direct channel
responses to the hydrological changes.
Methods
A representative channel flowing through Fountain
Hills (Tulip/Legend Wash) was selected in which to examine the adjustment of ephemeral channels to urbanization. Using space-time substitution techniques (e.g.,
Gregory 1987b), a relationship was first established between channel dimensions and drainage area in the upper part of the basin, where urbanization had not yet occurred. The logic for developing these relationships was
that natural channel dimensions at any point reflect the
discharge delivered to the site in relation to sediment
and local characteristics. Therefore, as channel dimensions usually increase with increased discharge downstream, and as drainage area is commonly used as an in-
598
Chin and Gregory
Figure 1. (top) Aerial photograph of Fountain Hills taken 29 January 1999. Photograph has approximate same scale as Figure 2 and shows an
area 5.6 4.6 km. Note the fountain in the lower right corner, where development began in 1970. Photo from Landiscor Aerial Information.
Figure 2. (bottom) Study sites for 1987 survey. The study channel is at the center of Figure 1. Fountain in lower right corner provides locational reference.
Urbanization and Adjustment of Ephemeral Stream Channels
dex of spatial location as well as a surrogate for discharge,
one can expect a direct relation between channel dimensions and drainage area. Data were also collected from
downstream sites in the urbanized areas where roads
cross channels with increasing frequency. Downstream
deviations from the extrapolated upstream relations were
then interpreted in the context of urbanization effects.
Whereas this method is relatively simple to apply in humid regions, in dryland areas such as Fountain Hills, the
possibility of downstream transmission losses could have
a complicating effect on the relationship between channel dimensions and drainage area (Leopold and Miller
1956).
The small stream analyzed begins in a natural portion
of the foothills and flows eastward into and through the
central urban area. Twenty-two cross sections were selected for initial analysis during the fall of 1987 (Figure
2). The eleven upstream sites drained an area that was
entirely undeveloped except for two small cul-de-sacs
that overlook the basin. The next four sites occupied a
portion of the basin that could be considered partly urban, with two road crossings and some development on
one side of the channel. The lower seven cross sections
were located in an area completely surrounded by urban
development, including five street crossings and a school
parking lot.
Channel dimensions were measured for each cross
section, and the drainage area was determined from
1:24,000 topographic maps. Channel dimensions were
measured initially at seventeen sites by placing a tape
horizontally across the channel and measuring the vertical distance to the channel bed at breaks in slope. This
procedure yielded data for channel width, depth, and
capacity. Channel capacity was not always easy to determine, but trashline evidence, vegetation distribution
(particularly small shrubs), signs of bank erosion, and
breaks in slope were employed to determine channel limits as consistently as possible between sites. The surveys
were extended to five additional sites downstream after
preliminary data analysis showed considerably less scatter in the width variable than in depth and capacity. It
was decided that width would be emphasized and measured at these sites because it was relatively easy to
measure (Reinfelds 1997), because width is the most consistent channel variable correlated with flow parameters
(Dunne and Leopold 1978), and because erosion and
depositional responses to floods in ephemeral streams are
best reflected in width changes, whereas depth has little
meaning in the context of infrequent flows (Thornes
1976, 1977). Channel width, average depth, and capacity were thus related to basin size and degree of urbanization. Revisits to the field sites in 1999, 2000, and 2001
599
allowed opportunities to identify changes and management adaptations after twelve years of channel adjustment; these are evaluated in the context of planning in
arid environments.
Results
Upstream Channel Dimension and
Drainage Area Relations
Clearly defined relations between channel dimension
and drainage area were established for the upper, nonurban portion of the study basin in Fountain Hills. Data
from field measurements showed the expected downstream increase in channel width, depth, and capacity
with drainage area. Regression analysis produced power
function relationships (Table 1) where the exponent in
the equations represents the rate of increase in channel
dimension with drainage area. The exponent of 0.40 for
width and 0.18 for depth indicate that width increases
faster relative to depth downstream, which suggests an
adjustment of width-depth ratios with increasing basin
size. The more rapid increase of width compared to that
of depth conforms to the general model of downstream
hydraulic geometry for natural rivers (Leopold and Maddock 1953), although for ephemeral streams such models
are perhaps best viewed as representing large-scale system adjustments and may not necessarily show the effects of transient dryland processes (Leopold and Miller
1956; Thornes 1977; Graf 1982, 2000).
A comparison of the width and depth exponents for
Fountain Hills with published downstream hydraulic geometry exponents indicates that they are similar to those
of other ephemeral channels. The values for Fountain
Hills plot near other ephemeral streams in the triaxial diagram of Park (1977b), which showed that ephemeral
channels tend to have low width exponents (0.3), low
depth exponents (0.2), and high velocity exponents
(0.4–0.6). The low width and depth exponents for Fountain Hills also imply a greater downstream hydraulic adjustment in velocity, an adjustment shown to be characteristic of headwater mountain streams (Thornes 1970)
Table 1. Relation between Channel Dimension and
Drainage Area, Upstream Natural Basin
Equation
c 1.34 A
w 6.25 A
d 0.23 A
0.57
d
0.40
d
0.18
d
n
r
11
11
11
0.87
0.83
0.65
600
Chin and Gregory
in addition to ephemeral channels (Leopold and Miller
1956). The consistency of the data thus suggests that the
established relations between channel dimensions and
drainage area in Fountain Hills reflect downstream hydraulic changes, and that these changes are comparable
to those in other ephemeral and headwater systems.
Further comparison of the channel capacity exponent
for Fountain Hills shows that it is generally at the lower
end of the values reported for other humid temperate
streams. For example, although the capacity exponent of
0.57 for Fountain Hills is within the range of 0.33–1.02
reported for streams in Devon, U.K. (Park 1977a), capacity exponents of 1.00 for Burrator and 0.54 for Holsworthy (Park 1977a) were noted, as well as 0.62 for
Burn, 0.86 for Nidd, and 0.61 for Catterick (Gregory and
Park 1976). Similarly, the capacity exponent for Pennsylvania streams in the U.S. was determined to be 0.66
(Hammer 1972); the corresponding value for rural basins
in the tropics ranged from 0.50 to 0.90 (Odemerho 1984,
1992; Ebisemiju 1989). The lower capacity exponent indicates that channel size increases at a slower rate in the
arid channels of Fountain Hills than in those of humid
regions, reflecting a lower rate of discharge increase with
drainage area downstream (Wolman and Gerson 1978).
This could be a result of infrequent and nonintegrated
flows (Leopold, Emmett, and Myrick 1966; Thornes
1977) and of losses of water by infiltration and evaporation (Keppel and Renard 1962) in addition to a greater
hydraulic adjustment in velocity (Leopold and Miller
1956). However, transmission losses are probably inconsequential given the small drainage areas and the sporadic occurrence of storm events (Tooth 2000).
Although the study channel in Fountain Hills enlarges at a slower rate downstream, the capacity constant
of 1.34 (Table 1) indicates that it is larger overall than
those of other areas. It is notable that, for a drainage area
of 1 km2, the channel capacity at Fountain Hills is more
than two times that for a comparable drainage area in
southern England (Gregory, Davis, and Downs 1992)
and up to four times the dimensions for other areas in the
U.K. (Gregory and Park 1976; Roberts 1989). The Fountain Hills channel is up to four times larger than those in
southeastern Australia for a drainage area of 1 km2 (Gregory 1977), but just half the capacity of channels that are
particularly sensitive to gullying in proximity to urban
areas in the same region of Australia (Neller 1988). That
this headwater wash has large capacities compared with
humid temperate areas must reflect the fact that it is affected by rare but very substantial flood runoff in a basin
with a high relief ratio. In 1999, for example, although
rainfall totaled only 4.5 in (114 mm) for the entire year,
records from the Town of Fountain Hills show that a
storm on August 27 produced nearly half an inch (11
mm) of rain in one hour. Similar high rainfall intensities
were encountered in 1998, when a series of thunderstorms delivered 3.75 in. (95 mm) of precipitation over
nine hours on 7 September, followed by another 1.10 in.
(28 mm) in three hours on 8 September. Although no
runoff records exist for Fountain Hills channels, analysis
of twenty-four years of flow record for a small desert
mountain stream near Phoenix showed that such intense
rainfall associated with summer monsoons generated the
largest floods, even though flow occurred only 0.05 percent of time (Rhoads 1990).
Downstream Changes in Channel Dimensions
The channel dimension-drainage area relations established for channels in Fountain Hills upstream of the urbanized area provide a basis for evaluating downstream
effects of urbanization. As shown on the width-basin
area graph (Figure 3a), downstream urban channels plot
both below and above the regression line extended from
the upstream relation. The absence of a consistent increase in channel capacity is similar to that identified by
Figure 3. Width and drainage area relation. (a) Prediction line
from upstream relation. Arrows indicate location of urban drainage
into channel, including two cul-de-sacs (at 0.1 km2) and a school
parking lot (at 1.5 km2). Relative length of arrows reflects relative
impervious areas. (b) Regression equations for natural and urban
channels.
Urbanization and Adjustment of Ephemeral Stream Channels
Gregory, Davis, and Downs (1992) for urban streams, although these data (Figure 3a) indicate that urban channels in Fountain Hills are both narrower and wider relative to their drainage areas than their natural upstream
counterparts. A distinct pattern is evident, in which the
narrower channels occupy the area immediately below
the first road crossings and the wider channels occur with
increasing drainage area downstream. These results suggest a contracting response at the first signs of urban activity, followed by widening farther downstream where
roads cross channels with higher frequency.
The magnitude of channel widening due to urbanization is quantified by comparing the field-measured widths
of the urban channels with the widths predicted from the
upstream relation based on drainage area (Table 1).
Such comparisons yield channel change ratios (Gregory
1987b) ranging from 0.61 to 2.02 (Table 2), with an average of 1.14, so that the downstream urban sites are up
to two times wider than the upstream natural channels.
However, as noted above, much of the channel widening
occurs at the urban sites farthest downstream. The suggested initial contracting response to urban activity is reflected in width ratios of less than 1.0 for five of the first
six downstream sites. The overall downstream widening
trend can be illustrated by calculating a new regression
line for the urban sites (Figure 3b). The higher slope represents a greater rate of increase in channel width relative to drainage area for urban channels, which indicates
a response to increased runoff draining from impervious
surfaces.
Downstream changes in depth and channel capacity
in the 1987 data do not show clear urban impacts (Figure
4). Measured depths and capacities are both higher
and lower than those predicted in the urban areas, and
the scatter of points is within the range of variance exhibited in the upstream areas. Nevertheless, on the
Figure 4. Channel capacity and depth versus drainage area.
whole, smaller channels in the area immediately below
the first road crossings are suggested by average depth
change and capacity change ratios of 0.78 and 0.70,
respectively (Table 3).
Pattern and Character of Channel Adjustments
When width, depth, and capacity changes are considered together in a downstream direction, channel reduction (change ratios less than unity) is evident within the
reach of 2 km downstream from the first road crossing
Table 3. Depth and Capacity Change Ratios
Table 2. Width Changes in Downstream Urban Sites
Site
a
b
c
d
e
f
g
h
i
j
k
Predicted
Width (m)
Observed
Width (m)
Change
Ratio*
2.08
4.56
5.07
5.17
5.48
7.72
7.79
7.90
8.24
8.71
11.86
1.92
2.78
4.54
5.15
4.47
6.53
8.90
6.70
13.40
17.60
22.00
0.92
0.61
0.90
1.00
0.82
0.85
1.14
0.85
1.63
2.02
1.86
* Observed value as ratio of predicted value.
601
Depth (m)
Predicted
Observed
a
b
c
d
e
f
0.14
0.20
0.21
0.21
0.22
0.25
0.07
0.15
0.25
0.11
0.22
0.15
0.54
0.78
1.20
0.52
1.02
0.61
0.78
a
b
c
d
e
f
0.28
0.86
0.99
1.02
1.01
1.81
0.14
0.43
1.14
0.56
0.99
1.00
0.51
0.50
1.14
0.54
0.98
0.55
0.70
Average
Capacity (m2)
Change
Ratio*
Site
Average
* Observed value as ratio of predicted value.
602
Chin and Gregory
Figure 5. Downstream pattern of width,
depth, and capacity changes and representative channel cross-section types from
1987 survey data. Arrows indicate downstream urban drainage, as in Figure 3a.
(Figure 5). Channel reduction may have been related to
sediment input from construction activity during the
early stages of channel adjustment in this newly urbanizing section of Fountain Hills, as has been reported elsewhere (see, e.g., Wolman and Schick 1967), or to the interruption of sediment transport by road crossings
(Hooke and Mant 2000), as was observed during and
after rainstorms. Even so, channel reduction occurred in
a situation where sediment input was high relative to
discharge, reflecting a desert environment that is characteristically transport-limited (Rendell and Alexander
1979; Reid and Laronne 1995), and where runoff from
road surfaces at the crossings was reaching the channel
before the main peak arrived. The 2001 measurements
(see below and Figure 7) indicate that the adjustments
in 1987 were the early stages of change. Taken together,
the moderate increase in road drainage in this partly urban area was apparently insufficient to offset high sediment loads in this reach of the channel up to 1987. On
the other hand, although depth and capacity data did
not extend beyond this area of contraction, increasing
width ratios of up to 2.02 were evident. Such channel
widening does not necessarily mean that the channel is
larger, because it could have resulted from deposition,
but width increases in ephemeral channels are indicative of erosional responses to floods (Thornes 1976,
1977). Enlargement is also consistent with the expected
response in this downstream reach completely surrounded
by urbanization. The increased runoff from substantially
more roads and impervious surfaces apparently exceeds
that required for transport of high sediment loads, possibly resulting in a flushing out of the system similar
to that described by Odemerho (1992) for a humid tropical stream.
Evaluation of downstream variations in channel
cross-sectional form, together with data on the extent
of impervious road surfaces draining to the channel, reveals greater details of the character of morphological
adjustments in Fountain Hills. The layout of this urban
residential community with high-cost properties includes wide roads that drain to the crossings. The pattern in the study area in 1987 featured culverts where
the roads crossed the stream channels, but there was no
road drainage provision along the roads. Measurements
were therefore made of all the impervious road areas
draining to the crossings. In 1987, these amounted to
6.25 percent of the drainage area (to site 16). It was
assumed that road runoff from occasional storms would
be conveyed down the road surface to the crossings,
where the runoff would enter the wash. The amount of
road surfaces therefore provided a means to evaluate
channel morphological adjustment according to degree
of urbanization.
When grouped according to degree of urbanization,
three characteristic channel types emerged that reflected
the nature of adjustment and the influence of road cross-
Urbanization and Adjustment of Ephemeral Stream Channels
ings (Figure 5). First, there was the natural channel in
the upper basin, with a typical wash morphology characterized by high roughness. Although 4,086 m2 of paved
surfaces were measured in two cul-de-sacs in the upper
basin, channel cross-sectional forms did not show identifiable urban impacts until roads began to cross channels.
Thus, in the partly urban area where two roads contribute urban drainage from 12,185 m2 of impervious surfaces, in 1987 the influence of the road drainage downstream of the crossings had produced a dissected channel
form with an asymmetrical v-shaped base (Figure 5).
Such incised channels result from local scouring by urban drainage below road crossings and have been described as being prevalent in ephemeral streams following small winter storms (Thornes 1977), where sediment
movement is impeded (Hooke and Mant 2000), and
where box culverts are used (Schick 1974). Farther
downstream, channels in the completely urbanized area
collected runoff from five paved roads and a school parking lot that totaled 28,712 m2. Some incision was observed here in 1987, but overall the wide channels in this
area were more reminiscent of arroyos where “subchannels” created by various flow frequencies were apparent
within the cross-section (Figure 5; Thornes 1977; Graf
1988). The channel morphology in these cross sections
showed the effects of larger floods whose magnitudes
were accentuated by increased urban runoff from paved
surfaces.
Summary of 1987 Data
Evidence collected in 1987 produces several important insights concerning ephemeral channels and urban impacts along mountain fronts. First, the original
ephemeral channels, or washes, in Fountain Hills are
larger in capacity than channels for comparable drainage
areas in humid regions, reflecting the geomorphological
impact of extreme events in arid environments. Second,
downstream changes in channel dimensions show urban
impacts characterized by width increases of up to two
times the natural channel, similar in amount to those in
humid rivers, but the detailed pattern and character of
morphological adjustment reflects processes associated
with aridity and the influence of desert road crossings.
Third, the major effect of urbanization in Fountain Hills
had been the introduction of wide roads that crossed
channels directly and served as conveyers of runoff from
paved surfaces, including roads, houses, and lots. Fourth,
in addition to increasing impervious surfaces, the repetitive road crossings had caused a fragmentation of the original system into segments. Because this type of road design
is typical for desert cities (Schick 1974, 1995), roads con-
603
stitute a substantial disturbance in a dynamic geomorphic
sense that differs from the way in which streams are impacted by urbanization in humid environments (Figure 6).
Changes, Problems, and Adaptations
after Twelve Years
Observations of the study channel after twelve years
of channel adjustment indicate a strong response to the
fragmentation of the channel system by road crossings.
Scour has accentuated the channels below road crossings
(Figure 6a, 6b), so that immediately downstream of a
crossing the channel has tended to become incised with
a low width-depth ratio, whereas farther downstream and
before the next road crossing the channel has a relatively
high width-depth ratio. In essence, the urban channel morphology has evolved in such a way that both channel
types identified in 1987 are present within a given channel segment. Thus, in ephemeral channel systems in
Fountain Hills, the impact of urban runoff on the channel system does not simply lead to increases in capacity
and width in the manner found to occur in humid areas.
Instead, there is a spatial pattern of influence on the morphology that arises because of the fragmentation of the
channel system, which is similar, but at a much smaller
scale, to the fragmentation of the river system due to the
interposition of dams and reservoirs (see, e.g., Graf 2001).
The pattern of adjustment above and below road
crossings was investigated by further field survey in
March 2001 to assess the extent to which it had occurred
more widely. The study channel investigated in 1987 is
part of a system of thirteen headwater washes that collectively compose what local planners regard as the northern wash system of the Fountain Hills area. Channel
characteristics at every location where the study wash intersects a road crossing were examined, as well as at each
intersection of all other washes in the system with a
crossing, providing fifty-four crossings for analysis. Although culverts had been installed at thirty-six crossings,
with infilling of scour holes having taken place in at least
two locations, there were eighteen sites where measurements of width could be made, and at ten of these, crosssectional area could be measured upstream and downstream of the road crossing.
By plotting channel width, capacity, and width-depth
ratio above a road crossing against the corresponding
down crossing value (Figure 7), it is possible to establish
the morphological differences that occur above and
below road crossings. These data show that, in all except one case (in which channelization had probably
been undertaken upstream), channels down-crossing are
deeper and narrower than those above the crossings,
604
Chin and Gregory
Figure 6. Fragmentation of original channel system by road crossings. (a) Typical road crosses channel directly and fragments channel into
segments. The McDowell Mountains are in background. (b) Morphological adjustment to repetitive road crossings has resulted in incised
channels downstream of crossings by 2001. (c) Road crossings interrupt sediment transport and create continuity problem. (d) Sediment pile
on road crossing awaiting removal following storm.
reflecting the pattern of accretion upstream and scour
downstream of the crossings (Figure 6b). Capacities
downstream are apparently much larger due to the effects
of scour which are most evident where the road lengths
draining to the crossing are greatest. These results clearly
demonstrate the significant influence of road crossings
on the pattern and character of channel adjustment and
show how channel adjustment had assumed a more
clearly developed pattern in the period 1987–2001.
Several management problems seem to have arisen in
Urbanization and Adjustment of Ephemeral Stream Channels
605
twice a year to remove undesirable desert vegetation
(e.g., salt cedar, desert broom) to reduce fire and flood
hazard. Furthermore, street cleaning is undertaken the
day after storms, in addition to the regular street-sweeping cycle, to remove sediment piles. Although in some
locations, such as pediment areas in Israel (see, e.g.,
Schick 1974), dip crossings have been employed to
prevent channel segmentation by road crossings, such
methods are not practicable in Fountain Hills in view of
the higher longitudinal slopes and the substantial
amount of drainage reaching the channel from the road
surfaces.
Conclusion
Figure 7. Comparison of morphological characteristics upstream and
downstream of road crossings, from 2001 measurements. Dashed line
indicates equal upstream and downstream values. Capacity downstream of road crossings is affected by runoff from the road surfaces.
the twelve-year period since the 1987 surveys. First, runoff from road surfaces has accentuated scour immediately
downstream of the crossings (Figure 6b). Second, although the original intention was to allow drainage from
roads and paved areas to flow into the washes at the
crossings, sediment coming down the channel has not
always been effectively conveyed by the culverts under
the crossing (Figure 6c). Third, sediment washed from
the roadsides onto road surfaces has produced problems
on the road crossing (Figure 6d). In response to the local
problems, road drainage and other modifications, including wash infilling, have now been installed along
several of the roads in the major part of the urbanized
area: for example, along Boulder Avenue in the early
1990s and more recently on Golden Eagle Boulevard.
Although some developments were planned, these examples illustrate the way in which adaptation has had to
be made after urbanization in Fountain Hills, whereas in
many temperate urban areas, the road drainage system
would have been installed prior to the urban development (see, e.g., Gregory 1974). In addition, since 1997,
washes in Fountain Hills have been “cleaned” about
Analyses of morphologic data for ephemeral stream
channels in Fountain Hills, Arizona permit answers to
the three research questions posed in this article. First,
urbanization has impacted ephemeral stream channels
in Fountain Hills by increasing runoff from impervious
areas and by introducing roads that fragment the original
channel system, inducing identifiable channel changes.
Second, channel adjustments are not of the same character as those found in humid areas. Whereas urbanization usually leads to progressive downstream channel
enlargement in humid rivers, morphological adjustment
is spatially varied in Fountain Hills owing to the fragmentation of the channel by roads and to the dynamic
nature of arid streams, and adjustment has developed
progressively over the period up to 2001. Third, particular management implications can arise in the case of
dryland urban river channels, especially associated
with the movement of water and sediment over the
roads leading to the crossings. In Fountain Hills, however, although erosion problems were anticipated at
crossings where box culverts had been installed following the specifications in the Roadway Design Manual of
Maricopa County, drainage systems were not initially
installed for such roads, necessitating remedial modifications to improve drainage and to inhibit scour.
Fountain Hills is a prototype “ridge line” desert community, in which market-driven forces have placed high
values on homes on ridge lines. In the real-estate details
of desirable and high-cost properties in the Fountain
Hills area, reference is often made to the proximity of the
washes and to the views across them. However, although
the intention is to keep the washes unaltered and to
maintain them (Valder 2000), it is important to acknowledge the morphological sensitivities and changes
that will continue to occur as a result of the urbanization
process in such desert settings. At present, as the impacts
606
Chin and Gregory
are registered, adjustments have been made, but it could
be appropriate to be more proactive and to design washes
in a way that would anticipate likely channel adjustments. The geomorphologist could work with local officials to recognize system-wide effects and develop management schemes consistent with spatially distributed
response mechanisms. This would accord with the philosophy advocated in general for America’s watersheds
(National Research Council 1999), with the way in
which hydrogeomorphic considerations are now being
involved in planning stormwater management in central
Texas (Marsh and Marsh 1995), with recommendations
for an integrated approach in New Zealand (Cutler and
Simpson 1999), and with guidelines developed for restoring streams in cities (Riley 1998). It would also exemplify how physical geographers can become involved
in aspects of environmental design (Gregory 2000). Restoration of streams (Brookes and Shields 1996) and stream
condition (Ladson and White 2000) and naturalization
(Rhoads et al. 1999) have emerged as key issues in more
humid environments; further attention could be given in
respect of ephemeral channel systems.
Acknowledgments
At the Town of Fountain Hills, Jeffrey Valder (Community Development); David Stepanek (Public Works);
Randy Harrel, James Leubner, Thomas Ward, Pat Harvey, and Betty Brannon (Engineering) supplied valuable
information for this paper. Will Graf, Scott Lecce, and
Jonathan Phillips offered helpful comments on earlier
drafts. Daniel Harris, Deven Rohrer, Wei Tu, and Lei
Wang provided research and cartographic assistance.
Kenneth J. Gregory is grateful to the Leverhulme Trust
for provision of an Emeritus Fellowship. Improvements
were made following suggestions from three anonymous
referees, who are gratefully acknowledged, and the assistance of Bruce Rhoads was particularly helpful. This paper is dedicated to the memory of Melvin G. Marcus; it
was Mel who first suggested Fountain Hills as a case study
of urbanization in an arid environment.
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Correspondence: Department of Geography, Texas A&M University, College Station, TX 77843, e-mail: [email protected] (Chin); Department of Geography, University of Southampton, Southampton, SO17 1BJ, U.K., e-mail: [email protected] (Gregory).