THE SEDIMENTARY RECORD OF FLOODING IN THE MOHAWK RIVER DRAINAGE BASIN PRESERVED IN FLOODPLAIN LAKES

Any attempt to attribute changes in the frequency and magnitude of floods to human induced global warming requires knowledge of the natural variability of the hydrologic system prior to ~CE (common era) 1800. Continuous records of flooding are preserved in the sediment of ox bow lakes and avulsed channels, and this study focused on such records from the Mohawk and Schoharie River Valleys in eastern NY State.

The bedrock underlying the Mohawk River drainage basin varies considerably: the northern part of the drainage basin is underlain mainly by gneiss, the central part by calcareous shale and dolostone, and the southern part by carbonates and Paleozoic red beds of the Catskill Mountains. Collin’s Pond (42°50’N; 73°57’W; 64 m asl) is a small (0.25 km²), shallow (z_max=8.5 m), eutrophic pond on the floodplain of the Mohawk River near Scotia, New York. Collin’s Pond has accumulated sediment at a high rate (~7 mm yr^-1 for the last 1000 years). An ~7.5-meter-long sediment core from Collins Pond reveals numerous discrete laminae 0.1-10 cm thick of pink-colored sediment with a mineral magnetic signature similar to that of modern alluvium derived from the Paleozoic red beds of the Catskill region. Young’s Pond (42°52’N; 74°16’W; 116 m asl) is a very small (0.03 km²), very shallow (z_max=1 m), eutrophic ox-bow lake on the floodplain of the Schoharie River ~7 km north of Burtonsville, New York. An ~ 3.5 meter-long core from this lake reveals a rich stratigraphy of flood events spanning the past 400 years; the average sedimentation rate is high, ~9 mm yr^-1. Flood waters of the Mohawk and Schoharie Rivers in response to Hurricane Irene on 29-30 August, 2011 inundated both Collins Pond and Young’s Lake depositing a discrete layer of pink sediment 0.2-0.5 cm thick throughout the lake basins. The red-color intensity of the sediment cores was measured continuously with sub-millimeter resolution revealing dozens of probable flood events over the past millennium, many of which correlate with Hurricane landings in the mid Atlantic and New England states.