STREAM CHANNEL DYNAMICS IN THE FISHING CREEK WATERSHED, SOUTHERN LANCASTER COUNTY

Fishing Creek (Lancaster County, Pa) is a complex system that is responding to changes on various spatial and temporal scales. Changes included deforestation during the 1700’s, recent changes in farming practices, increasing development, large scale climate changes, invasive species, and human use pressures. Fishing Creek covers an area of 14.35 miles² (37.15 km²) and is within the HUC 12 020503061709 hydrologic unit. The topography is hilly, ranging in elevation from 935 ft (285 m) in the headwaters to 108 ft (32.8 m) where Fishing Creek discharges into the Susquehanna River. Most of the area includes soils from the Manor-Chester-Glenelg soil unit which reflects the variable topography and predominantly mica schist, quartzite and gneiss bedrock. The watershed is unique in Lancaster County in that it is relatively undeveloped with a substantial proportion of forest (27% based on LULC data independent of parcels) and several large preserves owned by the Lancaster Conservancy. While the impact of humans is increasing, it is much less than almost everywhere else in Lancaster County.

Many stream reaches within the Fishing Creek watershed were straightened or aligned in the early 1900’s and thus were forced further out of equilibrium. In this study we examined 6 stream reaches ranging in length from ~1000 ft (300m) to ~4000 ft (1200m). Stream reaches were mapped from aerial photos for 12 different time periods between 1940 and 2019. All the stream reaches had limited tree cover allowing for accurate mapping of the stream channel. Stream gradients were determined from high resolution DEMs. Straightening resulted in very low sinuosity in the 1940’s (typically close to 1.0). Since 1940, all reaches have increased in length and sinuosity, and decreased in gradient. By 2019, sinuosity increased to between 1.10 and 1.28. Gradients decreased from between 0.034 and 0.013 to 0.030 and 0.009. Changes seem to accelerate in the 1990’s, but some streams may have experienced an abrupt reduction in length and sinuosity in 2019. In one of the reaches the cause is clearly related to a stream restoration project, but the cause/s in the other reaches has yet to be determined. A big question is whether the results of this study indicate that the stream reaches are attaining equilibrium and changes in sinuosity are now more or less stable.