Effects of urbanization on stream channel geometry in the Illinois River watershed in Northwest Arkansas
Shortcut URL: https://serc.carleton.edu/60231
Location
Continent: North America
Country: United States
State/Province:Arkansas
City/Town: Fayetteville
UTM coordinates and datum: none
Setting
Climate Setting: Humid
Tectonic setting: Passive Margin
Type: Process, Stratigraphy
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Fig. 2. Location of the measured headwater streams and the land use/land cover of the sub-catchments. Details
Description
The land-use changes associated with urbanization have a range of critical impacts on watersheds, stream geomorphology, and habitat. Much of the thinking about how streams respond to urbanization proliferating through the catchment follows a two step model. First, when construction begins, streams aggrade due to increased sediment delivery to channels from cleared land or active construction sites. Second, once construction ends the land surface is stabilized and sediment load is decreased, streams scour and erode. This thinking assumes that there is a finite end to development and that urbanization is completed at some point in time. On the contrary, urban development is an ongoing process that spreads throughout watersheds in a non-systematic spatial and temporal pattern. Development is often followed by re-development and new construction may take place within areas that were previously urbanized.
The main geomorphic effect of urban development on watersheds is the change in the infiltration capacity of the Earth's surface because of the increase of impervious surfaces such as parking lots, roads, and rooftops. In urban catchments, the lag time between peak precipitation and peak runoff is shortened because the flow of water in the catchment is not slowed by infiltration. Culverts, storm drains, and other artificial links in the stream network also increase the rate at which runoff enters a channel. Both of the principle controls on stream morphology, discharge and sediment supply are affected by urbanization. Surface runoff can erode sediment from open construction sites where vegetation has been removed, increasing the sediment load in streams. The changes in hydrology increase the in-stream potential for erosion and scour, thus increasing the sediment load contribution from the channel itself. Our research in the Illinois River Watershed in northwest Arkansas, demonstrates the two step model of stream response to urbanization is accurate, but inadequate in describing the full range of channel responses to changes in land use. Spatial variation in aggradation and scour tend to follow the pattern of urbanization in the Illinois River watershed as a result of local influence on discharge and sediment load. These findings have implications for regional stream restoration.
Until very recently, much of northwest Arkansas was covered by forest or agricultural land, primarily for cattle grazing. From 1999-2006, there was a 48% increase in population in the region resulting in an expansion of urban land use into areas that were previously agricultural or forested, creating impacts on stream morphology. The Illinois River watershed encompasses 4,300 km2 in Arkansas and Oklahoma (Fig. 1). The headwaters of the Illinois River are in the Springfield Plateau sub-province of the Ozark Plateaus, a heavily dissected, irregular, flattened dome in northwest Arkansas and southern Missouri. The Springfield Plateau rises from 300 to 450 m above sea level and is dominated by the Boone Formation, a cherty limestone of Mississippian age. The plateau surface is deeply incised by a dendritic drainage network of gravel-mantled bedrock streams.
In northwest Arkansas, we compared cross-sectional stream channel geometry and planform measurements in forested, agricultural, and urban headwater streams to investigate channel morphology in small catchments under different land uses (Fig. 2). We found that forested streams have lower slope and higher sinuosity than both agricultural and urban streams (Fig. 3). Bankfull area is higher in urban streams than forested and agricultural ones. While the unit stream power (the rate at which streams do work per units of channel width) in forested and agricultural streams are fairly similar, the urban streams have a stream power that is 4 times greater than forested streams (Fig. 3). In many urban streams, the channel has scoured to bedrock but this was not observed in forested or agricultural streams. In the urban streams, where the channel has eroded to bedrock, the stream power is distributed laterally, increasing bank erosion as reflected by bankfull width that is 25% higher than forested streams and bankfull area that is 85% larger than forested streams (Fig. 3).
The differences in channel morphology of headwater streams in the Illinois River watershed illustrate the changes in stream geometry that occur over time as land use progresses from forested to agricultural to urban (Fig. 4). Cross sections measured in forested streams tend to have a U-shape characterized by a small low flow channel surrounded by wide gravel deposits within the bankfull channel (Fig. 5). As disturbance increases, the gravel deposits are mobilized and shifted downstream. Agricultural streams have trapezoidal shape with a wider low flow channel and coarser grained bed materials (Fig. 4). Even in urban channels that have not eroded to bedrock, the urban channel geometries tend to be rectangular in shape. In urban channels with the greatest anthropogenic disturbance, the gravels are evacuated and little to no new gravel is washed into the stream. Where the streams erode to the underlying bedrock, the stream erosive power is distributed laterally, widening the channel through bank erosion and undercutting (Fig. 5).
The geomorphic changes have important implications for the ecological health of streams, because flora and fauna are adapted to the geomorphology of environment in which they live. Many changes in stream geomorphology associated with urbanization have impacts on stream ecology. In northwest Arkansas, stream reaches with more gravel have higher biodiversity of benthic species than reaches without gravel. Gravel provides benthic species with refugia (places to hide) from predators and from the disturbance of high flows. Where hydrologic changes result in eroding gravel from riffles, as observed in urbanized streams in the Illinois River watershed, the habitat is often altered to such a degree that habitat for these organisms is lost.
Understanding the geomorphologic characteristics of streams prior to urbanization can help us find the best ways to manage urban streams. Where stream restoration efforts are undertaken, understanding the conditions prior to disturbance is critical to repairing habitat which is often a primary goal of such projects. Sustainable development efforts can minimize the impacts to streams by taking into consideration the natural parameters governing stream morphology and creating strategies to incorporate these considerations into the design of urban landscapes.
The main geomorphic effect of urban development on watersheds is the change in the infiltration capacity of the Earth's surface because of the increase of impervious surfaces such as parking lots, roads, and rooftops. In urban catchments, the lag time between peak precipitation and peak runoff is shortened because the flow of water in the catchment is not slowed by infiltration. Culverts, storm drains, and other artificial links in the stream network also increase the rate at which runoff enters a channel. Both of the principle controls on stream morphology, discharge and sediment supply are affected by urbanization. Surface runoff can erode sediment from open construction sites where vegetation has been removed, increasing the sediment load in streams. The changes in hydrology increase the in-stream potential for erosion and scour, thus increasing the sediment load contribution from the channel itself. Our research in the Illinois River Watershed in northwest Arkansas, demonstrates the two step model of stream response to urbanization is accurate, but inadequate in describing the full range of channel responses to changes in land use. Spatial variation in aggradation and scour tend to follow the pattern of urbanization in the Illinois River watershed as a result of local influence on discharge and sediment load. These findings have implications for regional stream restoration.
Until very recently, much of northwest Arkansas was covered by forest or agricultural land, primarily for cattle grazing. From 1999-2006, there was a 48% increase in population in the region resulting in an expansion of urban land use into areas that were previously agricultural or forested, creating impacts on stream morphology. The Illinois River watershed encompasses 4,300 km2 in Arkansas and Oklahoma (Fig. 1). The headwaters of the Illinois River are in the Springfield Plateau sub-province of the Ozark Plateaus, a heavily dissected, irregular, flattened dome in northwest Arkansas and southern Missouri. The Springfield Plateau rises from 300 to 450 m above sea level and is dominated by the Boone Formation, a cherty limestone of Mississippian age. The plateau surface is deeply incised by a dendritic drainage network of gravel-mantled bedrock streams.
In northwest Arkansas, we compared cross-sectional stream channel geometry and planform measurements in forested, agricultural, and urban headwater streams to investigate channel morphology in small catchments under different land uses (Fig. 2). We found that forested streams have lower slope and higher sinuosity than both agricultural and urban streams (Fig. 3). Bankfull area is higher in urban streams than forested and agricultural ones. While the unit stream power (the rate at which streams do work per units of channel width) in forested and agricultural streams are fairly similar, the urban streams have a stream power that is 4 times greater than forested streams (Fig. 3). In many urban streams, the channel has scoured to bedrock but this was not observed in forested or agricultural streams. In the urban streams, where the channel has eroded to bedrock, the stream power is distributed laterally, increasing bank erosion as reflected by bankfull width that is 25% higher than forested streams and bankfull area that is 85% larger than forested streams (Fig. 3).
The differences in channel morphology of headwater streams in the Illinois River watershed illustrate the changes in stream geometry that occur over time as land use progresses from forested to agricultural to urban (Fig. 4). Cross sections measured in forested streams tend to have a U-shape characterized by a small low flow channel surrounded by wide gravel deposits within the bankfull channel (Fig. 5). As disturbance increases, the gravel deposits are mobilized and shifted downstream. Agricultural streams have trapezoidal shape with a wider low flow channel and coarser grained bed materials (Fig. 4). Even in urban channels that have not eroded to bedrock, the urban channel geometries tend to be rectangular in shape. In urban channels with the greatest anthropogenic disturbance, the gravels are evacuated and little to no new gravel is washed into the stream. Where the streams erode to the underlying bedrock, the stream erosive power is distributed laterally, widening the channel through bank erosion and undercutting (Fig. 5).
The geomorphic changes have important implications for the ecological health of streams, because flora and fauna are adapted to the geomorphology of environment in which they live. Many changes in stream geomorphology associated with urbanization have impacts on stream ecology. In northwest Arkansas, stream reaches with more gravel have higher biodiversity of benthic species than reaches without gravel. Gravel provides benthic species with refugia (places to hide) from predators and from the disturbance of high flows. Where hydrologic changes result in eroding gravel from riffles, as observed in urbanized streams in the Illinois River watershed, the habitat is often altered to such a degree that habitat for these organisms is lost.
Understanding the geomorphologic characteristics of streams prior to urbanization can help us find the best ways to manage urban streams. Where stream restoration efforts are undertaken, understanding the conditions prior to disturbance is critical to repairing habitat which is often a primary goal of such projects. Sustainable development efforts can minimize the impacts to streams by taking into consideration the natural parameters governing stream morphology and creating strategies to incorporate these considerations into the design of urban landscapes.
Associated References
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