GEOMORPHIC ASSESSMENT OF SWARR RUN IN THE LITTLE CONESTOGA WATERSHED, LANCASTER, PA

The Little Conestoga Watershed (LCW), covering 65.5 mi2 in Lancaster County, PA, is a major contributor of nutrients and sediment to the Chesapeake Bay. This study, part of a KECK Geology Consortium sponsored project, examined the geomorphology of two reaches in the Swarr Run watershed (part of the LCW) to better understand the dynamics of the stream system. Swarr Run watershed includes residential, agricultural, and minor woodland land uses. A stream restoration project, sponsored by the LCW Alliance and funded by the State of PA, is scheduled to begin soon on a highly impacted reach of the stream.

A small tributary of Swarr Run (Millers Run) draining a residential area built before floodplain development and storm management regulations came into effect in the 1980’s, was studied to better understand the impact of residential development on stream morphology. Millers Run exemplifies the negative effects of poor residential development. The stream channel was straightened, most of the floodplain was developed for housing, and runoff is discharged directly into the stream. The stream is now out of equilibrium resulting in channel widening and extreme bank erosion. Where possible the channel is trying to reestablish its original meander pattern. Channel incision is limited by the armoring effect of large boulders which is enhancing lateral erosion. Local landowners are responding by rip rapping the channel banks, exacerbating erosion downstream.

A second study area, part of the main branch of Swarr Run, was heavily impacted by poor agricultural practices and channelization in the 1950’s. In part of this area, severe erosion resulting from uncontrolled access to the river by livestock and increased discharge, has resulted in an extremely wide and shallow channel. The stream is incising into sediments deposited in the channel since the stream was straightened, and eroding laterally into older floodplain sediments. Increased discharge and reduced sediment input from outside of the channel, is resulting in the remobilization of channel and floodplain sediments. Where livestock does not have access to the stream, the channel is narrower and the pool-riffle structure is different. This suggests that good agricultural practices can mitigate some of the negative impacts of residential development and channel modification.