GEOMORPHIC AND HYDROLOGIC RESPONSES TO URBANIZATION IN NINE METROPOLITAN AREAS ACROSS THE U.S

From 2000–2004, the U.S. Geological Survey conducted a study of urbanization effects on stream ecosystems of 262 streams in nine metropolitan areas in various physiographic settings across the U.S. as part of the National Water-Quality Assessment Program. A rural to urban land-cover gradient approach was used. Landscape characteristics and measures of urbanization were examined at multiple spatial scales, including watershed, riparian buffer, and reach. Geomorphic and habitat characteristics included measures of channel geometry and hydraulics, channel-bottom substrate size, habitat complexity, and riparian/bank conditions, These characteristics were compared to several indicators of urbanization, other watershed land cover, physiography, and climate characteristics, and hydrologic-condition metrics. Hydrologic-condition metrics were based on one-year records of hourly changes in channel cross-sectional area from stage recorders and included descriptors of flashiness, frequency, and duration. Degree of historical channel modification (channelization, bank stabilization, and grade control) and alluvial/bedrock setting was also compiled.

Channel enlargement (channel widening or downcutting) was the most common geomorphic response to urbanization. The magnitude of the response depended on regional variations in climate, physiography, and historical water management (diversions, interbasin transfers, or storage) as well as local controls on erosion potential, such as grade control and bedrock. Channel shape and bottom substrate relations with urbanization were variable or lacking, likely because of temporal changes in the type, amount, and source of sediment loading associated with phases of urbanization. An increase in frequency of high flow events was the most common hydrologic response to urbanization. There were no national relations between urbanization and increasing or decreasing base flow. Results from this study emphasize the importance of including both a wide range of geologic and topographic variables at multiple scales and a working knowledge of historical channel modifications, in determining physical responses of streams to land-cover disturbance by urbanization.