WHAT ARE THE HALLMARKS OF FLOODS IN LACUSTRINE DELTAS IN CENTRAL NEW YORK?

In central New York sedimentary deposits in historic reservoirs are under-exploited. These deposits could reveal a significant record of storm and flood events. Our project begins the work of linking flooding events to the sedimentology and stratigraphy as recorded in deltas in modern and historic lakes and reservoirs. We hypothesize that larger floods transport larger particles to a given location on the delta. Thus, we target grain size changes in sediment cores as a proxy for flood events.

The selection of suitable coring sites is driven by considerations of the nature of sediment delivered to deltas in local lakes and reservoirs. During large runoff events, gravel, sometimes up to boulder size, is transported over the proximal portion of the delta. Given the challenge of coring through gravel bars, the distal portion of the deltas is targeted, in hope that grain size changes in the mud to coarse sand range could be detected.

Several methods of determining grain size have been developed for this project. One method creates sediment peels using glue and acetate sheets. These will serve as permanent records of the cores. Viewed with a binocular microscope, grain size can be visually estimated by comparison to a calibration chart. The use of peels helps us to initially identify layers with larger grain sizes. We also employ sediment smears for quick characterization of grain size throughout the core. Finally, 1 cm extracts of the core samples are sieved for the following size ranges: less than 55 μm; 55 to 110 μm; 110 to 300 μm; 300 to 500 μm; and greater than 500 μm. These sizes span the general size range in cores sampled. Sections of core with greater percentages of particles greater 300 μm are diagnostic of more vigorous stream flows.

We have sampled deltas in one large and one small breached historic reservoir. The small mill pond deposit shows some seasonality, as evidenced by leaf layers ~1 cm thick intercalated with muddy layers 5-10 cm thick. We will probe the mud layers to examine grain size changes indicative of multiple flow events. Flood records from nearby stream gages will be compared to grain size changes in the sediment cores. If larger runoff events fail to mark the sedimentary record, focus will turn to processes that blur the signal such as bioturbation, or from a lack of grain size sorting over the length scale of a reservoir delta.