STREAM CHANNEL DYNAMICS IN THE LITTLE BEAVER CREEK WATERSHED, SOUTHERN LANCASTER COUNTY

Stream channels in Lancaster County are complex systems that are 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. In addition, many stream reaches in Lancaster County were straightened or aligned in the early 1900’s and were pushed further out of equilibrium. Aligned stream reaches reacted by increasing their sinuosity and length and decreasing their gradients (for example: Fishing Creek). This process is clearly visible in reaches that flow through agricultural areas which allows for the stream channels to be mapped over time. The shape of stream reaches can be qualitatively assessed in maps dating back to the 1860’s and can be accurately mapped from aerial photos dating since the late 1930’s and early 1940’s. This data provides information about changing channel morphology which in turn provides information about stream dynamics and landscape evolution.

One question is whether aligned streams have returned to more equilibrium conditions by increasing their sinuosity and reducing their gradients over the last 100 years. Little Beaver Creek located just south of Strasburg in Lancaster County is one location where this might be assessed. Comparisons can be made between reaches with different land uses and between reaches that were not aligned and reaches that were aligned. The watershed covers an area of 13.3 miles² (34.5 km²) and is partly structurally controlled by NNE-SSW trending folds and thrust sheets of Cambrian-Ordovician carbonate and clastic rocks while the headwaters to the SW originate in pre-Cambrian Basement rocks of Mine Ridge. The topography is hilly, ranging in elevation from ~920 ft (285 m) in the headwaters to ~300 ft (32.8 m) where Beaver Creek discharges into Pequea Creek. Seven stream reaches were mapped for several time periods between 1940 and 2019. Sinuosity was generally much lower in 1940 (ranged from 1.05 to 1.35) compared to 2019 (ranged from 1.10 to 1.90) with most reaches showing a large increase in sinuosity (between 105% and 166%). Aligned reaches showed the greatest increase in sinuosity.