ENVIRONMENTAL BASELINE STUDY FOR MANAGED SEDIMENT RELEASE FROM THE LOWER RESERVOIR, ONEONTA CREEK, ONEONTA, NY

The Lower Reservoir in Oneonta, NY is one of the city’s main sources of drinking water. Storage capacity of the reservoir has decreased significantly over the years due to the increasing sediment infill behind the dam. The city management is considering flushing sediment downstream to restore the storage capacity of the reservoir. Limited research has been done to assess the impact of dam removal or reservoir flushing on downstream ecosystems and water quality. The goals of this study are to generate a comprehensive baseline data set for the conditions of the stream prior to sediment release, and predict the potential impacts associated with this practice.

Sedimentology, geomorphology, water chemistry, and biology of the stream were analyzed at sample sites above and below the reservoir using standard methods. Water chemistry and stream biology were assessed every 2-3 weeks to count for seasonal variations.

Pebble counts reaffirmed that the streambed and bar deposits contain primarily small boulders to fine gravels. During dry conditions, there is little to no aggradation of sediments. After heavy rain or snowmelt, discharge is so high that any accumulated fine sediments are carried out to the Susquehanna River.

Electrical conductivity and total suspended solid concentrations increase downstream during both low and high flow conditions, which is likely attributed to the increased anthropogenic influences downstream and input from drainage ditch runoff. Nitrate concentration is 0.26 mg/L in the reservoir, drops to zero directly below the dam, then increases to 0.64 mg/L farther downstream. The pH remains consistently around 6.40 at all sites.

Preliminary analysis of aquatic insect taxa data suggests greater species richness above the reservoir with 7 orders and 28 different families identified, and only 5 orders and 18 families downstream. The decreased stream flow below the reservoir and the increased anthropogenic influence at downstream sites potentially cause this trend.

The Lower Reservoir represents a common problem of many small reservoirs around the country. Abruptly flushing large amounts of sediment accumulated behind the dams could cause an unexpected response of the river ecosystem. Thus, long-term studies are needed to understand the potential consequences of this reservoir management practice.