FLOOD STRATIGRAPHY AT THE MARGIN OF A DELTA FRONT IN A MODERN RESERVOIR: MORRIS POND, NY

We hypothesize that for rivers debouching into standing bodies of water, grain size should vary directly with discharge, and thus, grain size variations in sedimentary cores could serve as a proxy flood record. To test this hypothesis, we investigate a small historic reservoir on Morris Brook in upstate New York (42.5096° N, 75.2899° W). The reservoir has existed at least since the 1940s. Morris Brook drains an area of about 20 km², where land use consists of a mix of agricultural pastures and forests. The stream is predominantly alluvial, but cuts a bedrock (Devonian sandstone) gorge a few hundred meters above the reservoir. A 4-5 m masonry dam impounds the reservoir, and an active delta has prograded into the reservoir. Sediments trapped in the reservoir comprise muddy bottomsets with gravel foresets and topsets. Aerial photographs dating to 1960 permit a reconstruction of the delta front over time, and indicate a significant increase in the growth rate of the delta since 2000, from roughly 0.2 m²/day to 1.7 m²/day. Floods in 2006 and 2011 contributed substantially to this increase. We delve into the sedimentary record using sediment cores to identify horizons associated with the recent floods, and to extend the record into the past. We have sampled the bottomsets just offshore of the delta front, and we have moved into a more distal portion of the delta as well to document spatial variability of grain size in the cores. In addition to fining upward sequences which we interpret as flood events, we find packets bounded by leaf layers in the distal delta, which we interpret as annual(?) markers. Given the large number of reservoirs and lakes in central New York, the effort could open up a significant library of flood records for the Anthropocene and beyond. Delta fronts, however, are challenging to core, and the record is always in danger of being reworked as thalwegs wander.