SHORELINE EROSION ALONG LAKE ARTHUR IN MORAINE STATE PARK, BUTLER COUNTY, PENNSYLVANIA

In 2006, Moraine State Park asked the Pennsylvania Geological Survey to undertake a shoreline erosion study. The horsepower limit for motor boats had been increased in 2003 from 10 to 20 horsepower. With no baseline study available to quantify the rate of shoreline erosion occurring during the 33 years of the 10 horsepower limit, an evaluation of the increase in erosion caused by the horsepower limit increase is not possible.

Lake Arthur is a smaller re-creation of a Pleistocene glacier-dammed lake. Near the location of the former glacier dam at the western end of the lake, the shoreline is composed predominantly of unconsolidated glaciolacustrine and glaciodeltaic sediments, in the clay to fine sand range. Further east, beyond the glacial sediments, the shoreline is bedrock of the Pennsylvanian Allegheny Formation. Much of the shoreline is shale, but in some places, sandstone. In addition to the effects of the shoreline geology on erosion, the western end of the lake is much wider, and the shoreline there is more exposed to prevailing westerly winds than in the eastern two-thirds. The largest erosional bluffs occur in this area.

Eight sites were selected for erosion monitoring, based on location relative to the prevailing winds and monitoring the erosion in different geologic materials. A landmark was selected as a reference point at each monitoring station. Twice per year, in the spring, shortly after the lake is raised to the summer pool, and in the fall, just before the lake is lowered to its winter pool, bluff recession is measured. In addition to the measurement, the face of the bluff is painted to determine at the next measurement period if there is undercutting of the bluff, without recession of the edge of the bluff.

There has been little recession of the edges of the bluffs in the last 4 years. Preliminary observations and data suggest that recession occurs by undercutting for a number of years until collapse occurs, and the edge of the bluff retreat occurs in large discrete blocks. The retreat of the fine, unconsolidated glacial sediment seems to be by the undercutting at lake level, accompanied by the downward movement of large blocks intact, whereas the retreat in bedrock areas appears to be by erosion of the face of the bluff, until sufficient undercutting occurs to allow the collapse of the edge of the bluff.