Lake Champlain: Partnerships and Research in the New Millennium

Abstract

Using side-scan sonar and sub-bottom profiles, a 40 m diameter, similar to4 m deep lakebed pockmark was investigated in Burlington Bay, Lake Champlain, Vermont. Five piston cores (2 inside, 1 on the rim and 2 outside of the pockmark) were collected and analyzed for magnetic susceptibility, physical properties and Pb-210 and Cs-137 concentrations. A yearlong subsurface mooring, equipped with an Acoustic Doppler Current Profiler (ADCP) and vertical temperature chain, was placed in the deepest section of the pockmark (27.7 m). A second temperature chain was placed along the pockmark floor and over its rim. At the base of the mooring, 35 mm stereo cameras and a Savonius rotor current meter (similar to 0.3 m above the lake floor) were installed. Bottom photographs show episodic events of suspended sediment. Temperature data show the presence of anomalously cold temperature water near the rim of the pockmark during stratified periods and anomalously warmer water temperatures during isothermal winter type conditions. Coarser grain size and bulk density occur inside the pockmark with an associated decrease in water content in comparison to sediment outside the pockmark. Radionuclide results show no detectable net accumulation of sediment within the rim of the pockmark, slight accumulation in the deepest part of the pockmark, and only a few tenths of a mm/yr accumulation outside. This pockmark was most likely formed by groundwater seepage and is subsequently modified by continuing groundwater seepage and bottom-flowing currents. Taylor column and frictional boundary layer dynamics are believed to play a role in the circulation within this feature.