THE IMPACT OF LAND USE ON STREAM GEOMORPHOLOGY

The purpose of this Research Experience for Undergraduates (REU) project was to address how watershed land use and road crossings impact stream morphology. It is important to understand the ways in which agricultural and urban land uses affect rivers and riparian systems because their impacts have environmental and economic consequences. Road crossings (e.g., bridges and culverts) also can negatively impact stream morphology and fragment aquatic habitats, decreasing and disconnecting available habitat. This study analyzed the change in stream morphology at 33 sites in three different watersheds in northern New Jersey: the Flatbrook, Wallkill and Rockaway. These watersheds were selected because their rivers have similar watershed characteristics in terms of slope, size, and bedrock lithology but vary in land use. The watersheds include highly forested (Flatbrook), agricultural (Wallkill) and urbanized (Rockaway) land uses. At each site the Rapid Geomorphic Assessment (RGA) score was determined, and the river bed sediment size, velocity, bankfull width and slopes upstream and downstream of road crossings were also measured. The study also analyzed the effect bridges have on local river conditions by determining the relative amount that the bridges constricted the rivers. RGA scores increased with increasing upstream urban land use and decreased with increasing forested land cover, suggesting that urbanization decreases stream stability. Results show that bridges that influence the channel width by constricting or widening the channel affect the widths of the banks, size of streambed sediments and velocity of the water upstream and downstream of the crossing. The study also analyzed historic USGS stream gage data to determine changes in total annual baseflow and surface runoff. In the urban Rockaway watershed baseflow decreased while surface runoff increased as urbanization increased. Ultimately, this information can be used to make recommendations about watershed and river management, as well as designing road crossings with minimal impact on stream morphology. Rebuilding and redesigning bridges, buildings and structures by river banks is very costly and understanding the ways in which these developments affect and are in turn affected by streams can have high time, energy and cost returns.