FAYETTEVILLE GREEN LAKE AND ROUND LAKE: GEOPHYSICAL ANALYSIS AND PLUNGE POOL MODELLING

The vertical erosive power of a waterfall decreases with increasing pool depth and diameter. Along with sediment and water discharge, stream velocity, fall height, sediment grain size, and rock type, the geometry of the pool plays a first order control on its formation (Scheingross and Lamb, 2017). Green Lakes State Park in Fayetteville, New York contains two lakes, Green Lake and Round Lake, that are over 50 meters deep, less than 500 meters in diameter, have slopes in excess of 30° and occur within a glacial meltwater channel. They are generally thought to have formed by waterfall erosion draining a proglacial lake. Glacial reconstructions suggest that a readvancement moraine was deposited at the mouth of Green Lake. A borehole drilled in this area encountered 42 meters of lake marl and glacial sediment without penetrating bedrock (Coon, 1960), indicating a likely pool depth of less than 10 meters prior to sediment deposition. In the case of Round Lake, the outflow point lies in a narrow wooded wetland between the lakes. Cores collected here encountered lake marls with interbedded peats indicating past lake level alteration (Mullins et al., 2003). The deepest core reported refusal at 11.2 meters with indications of shale, common in the local bedrock. If this is the bedrock elevation then the depth of the Round Lake plunge pool would have been at least 40 meters.

A MATLAB modeling approach (Scheingross and Lamb, 2017) was used to test the theory that Round Lake formed by waterfall erosion. Most parameters were determined by observation or estimation based on modern and ancient analogues. The main parameter that needed to be determined was the pool depth. Three geophysical tools were used to confirm the borehole observations; ground penetrating radar (GPR), horizontal-vertical spectral ratio (HVSR) from passive seismic, and electrical resistivity tomography (ERT). With pool depth constrained, a range of geological plausible waterfall parameters were applied to test the waterfall formation hypothesis.