A LATE HOLOCENE RECORD OF MOHAWK RIVER FLOODING PRESERVED IN A SEDIMENT CORE FROM COLLIN'S POND IN SCOTIA, NEW YORK

Collin's Pond (42°50'N; 73°57'W; 64 m asl) is a small (0.25 km2), shallow (zmax=8.5 m), eutrophic pond on the floodplain of the Mohawk River near Scotia, New York. The small drainage basin of Collin's Pond is similar in size to the lake itself, yet Collin's Pond has accumulated sediment at a remarkable rate (~4 mm yr-1 for the last 2000 years). The sedimentary record indicates that repeated flooding of the Mohawk River has led to increased sediment inputs to the Pond. Cores contain discrete laminae 0.1 to 10 cm-thick of normally graded medium sand to silt that are intercalated with massive, organic-rich sediment. Many of these laminae possess erosional basal contacts, and some contain rip-up clasts of fine-grained organic sediment. These characteristics suggest that many clastic layers were deposited by density-driven undercurrents during flooding of the Mohawk River. The bottom of the core contains wood fragments that are overlain by a layer of coarse sand, which marks the formation of Collin's Pond (~2000 yr BP). Several of the clastic strata contain silt to clay laminae with a distinctive reddish color. The most likely source area for these sediments is the Paleozoic red beds of the Catskill Mountain region of the upper Schoharie Valley, which is one of two principal tributaries to Mohawk River. If so, then these pink laminae record discrete flooding events that were driven by high precipitation and/or snow melt in the Catskill Mountain region that were not felt to the same degree in the northern part of the Mohawk River drainage basin. The frequency of flood lamiae decreases in the upper 4m of core, and this may reflect a decrease in flood frequency of the Mohawk River, a lateral migration of the River away from Collin's Pond, vertical incision of the River, or some combination of these three factors. The upper 1.5 meters of core records a pronounced increase in organic carbon content (from 2-10%), which likely reflects cultural eutrophication of Collin's Pond, and construction of a levee between the Mohawk River and the Pond that has reduced clastic sediment input from the Mohawk River.