STREAM DYNAMICS OVER TIME AT WALNUT RUN, LANCASTER COUNTY

Walnut Run watershed, located south of Strasburg in Lancaster County, PA, is characterized by its predominantly Amish agricultural land use. It covers an area of 7km² and is mostly underlain by Cambrian-Ordovician carbonates. Over the last several centuries it has responded to a variety of changes including evolving farming practices and climate, as well as increased deforestation and development. Many stream channels in Lancaster County were straightened during the early 1900s, which resulted in very low sinuosity and out of equilibrium conditions. It is unclear if this area experienced this phenomenon and whether the streams in the watershed are responding to this impact.

To better understand the current environmental conditions in the watershed, 6 stream reaches were mapped to document change over time. The stream reaches were selected based on their limited tree cover which allowed for accurate mapping of the stream channels. Each reach was mapped in ArcGIS Pro using aerial photographs for several time periods between 1940 to 2018. Older aerial photographs were georeferenced prior to digitizing. The reaches ranged in length from ~1000 ft (300m) to ~4000 ft (1200m). The stream gradients were determined using high-resolution DEMs. Stream gradients are low since the elevations in the watershed range from ~300 ft (91.44m) to 425 ft (129.54m). All reaches have experienced increases in stream length and changes in sinuosity since the 1940s. Some reaches did experience a reduction of ~0.01 in sinuosity in 2018 compared to 2012. Sinuosity ranged between 1.09 to 1.47 in 1940 and increased to a range of 1.12 to 1.67 in 2018. There are 3 reaches that show strong evidence of channel straightening where sinuosity increased an average of 0.231 between 1940 and 2018. These sinuosity changes could yield important information when considering stream restoration initiatives and whether this signifies that the stream is reaching equilibrium.