THE IMPACT OF BRIDGES AND URBAN DEVELOPMENT ON STREAM GEOMORPHOLOGY

This project investigated how watershed land use and road crossings impact stream morphology in northern New Jersey. The ability of watershed land use changes and road crossings (e.g., bridges and culverts) to have geological, environmental, hydrological, and ecological impacts is understood at broad scales, but the individual response of riparian systems to these external forces is important to document. This project, as part of an NSF-funded Research Experience for Undergraduates (REU) program, analyzed channel morphology at 33 sites in three different watersheds: the Flatbrook, Wallkill and Rockaway. These watersheds were selected because their rivers have similar watershed characteristics in terms of slope, area, and bedrock lithology but vary in land use. The watersheds include highly forested (Flatbrook), agricultural (Wallkill) and urbanized (Rockaway) land uses. The study also analyzed the effects that bridges have on local river conditions by measuring stream channel constriction and channel changes upstream and downstream of bridges, as road crossings can negatively impact stream morphology and fragment aquatic habitats. 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. RGA scores increased with increasing 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 Rockaway watershed baseflow slightly decreased while surface runoff increased as urbanization increased. Ultimately, this information could be used to make recommendations about watershed and river management, as well as for designing road crossings with minimal impact on stream morphology.