Indicators of Stream and Riparian Zone Condition In Urbanizing Inner Coastal Plain Watersheds, North Carolina

The SE U.S. is experiencing rapid population growth and expanding urban land use. Recent work suggests that streams respond differently to urban land use depending on their physiographic setting. Little work has documented stream response to urbanization in the southeastern Coastal Plain. The objective was to identify indicators to quantify urbanization effects on channel and riparian zone condition along Inner Coastal Plain streams. Six sites in the Greenville, NC, area represented a rural to urban land use gradient as quantified by a range of catchment total impervious area (TIA: 3.8 - 36.7%; Catchment area:1.19 - 3.46 km2). In addition, 20 rural (<10% TIA) and 20 urban (>10% TIA) stream reaches were studied to identify indicators related to urban stormwater effects on channel-riparian zone connection, bank stability, and sediment regime. Total impervious area (TIA, as a percent of watershed area) frequently served as an indicator of stream and floodplain condition. Oxygen-18 showed promise for quantification of TIA effects on storm peak flows. Responses to increased urban runoff included channel widening, incision, and grain size increases in proportion to the percentage of TIA. Stream channel alteration was greatest directly downstream of stormwater culverts. Urban streams shifted from baseflow to stormwater dominated discharge, proportional to TIA. Vertical distance from bankfull stage to the floodplain surface was found to be a measure of the degree of stream channel-riparian zone connection. This indicator was supported by the relationship between riparian ground-water depth and catchment TIA. As TIA and stormwater runoff increased, the degree of stream channel incision and widening increased and riparian groundwater tables declined. This has resulted in a shift to drier urban riparian zones. Channel incision ratio was found to be a reliable indicator of “riparian hydrologic drought”. These indicators can be used to identify restoration sites and to help guide restoration efforts.