BASELINE STUDY OF THE SEDIMENTOLOGY AND GEOMORPHOLOGY DOWNSTREAM FROM THE LOWER RESERVOIR ON ONEONTA CREEK NY PRIOR TO FLUSHING SEDIMENT

The reservoir for drinking water in Oneonta Creek has accumulated a significant amount of sediments over a 120-year period. As a means to recover lost reservoir space, the City of Oneonta, NY is considering the options of sediment flushing or dredging to increase volume. Because mechanical dredging is a costly endeavor, flushing currently provides the best option. Flushing is not a widely used method because of various documented limitations such as channel aggradation, habitat quality, fish kills, and contaminant transport. In this study, we present our baseline findings and observations of the geomorphology, water chemistry, and sediment deposition and transport along a portion of Oneonta Creek prior to reservoir flushing.

Comparisons of grain sizes before, during, and after flushing will be based on sediment samples collected at 10 gravel bars along Oneonta Creek. Grain size analysis is carried out using the Folks method looking at phi size 4.0 and finer. The streambed is comprised of imbricated cobbles to boulders with spaces filled by pebbles and fine- to coarse-sands and silts. The geology of the stream indicates large discharges events prior to the installation of the dam as evidenced by terraces and marginal debris flow levees. Recent flooding events (2006, 2011) have scoured the streambeds and banks of finer sediments.

Pre-flushing water analysis yielded an average turbidity of 11.0 – 30.0 NTU, conductivity of 80.6 – 110.7 us/cm, Ph of 7.12 – 7.3, dissolved oxygen of 7.84 - 8.40 mg/L, nitrates of 0.4 – 0.9 mg/L and phosphates 0.081 – 0.3 mg/ L. The ranges in values are dependent on water discharge events. Minimal attention has been focused on the patterns of sedimentation, transport and redeposition after a sediment release from small municipal reservoirs. Therefore this study provides baseline data for such patterns that may assist dam and resource managers in designing flow regimes below dams to minimize the impact of sediment release events.