HOW LARGE WERE FLOODS CAUSED BY 19TH AND 20TH CENTURY SPLASH DAMS IN THE UPPER HUDSON RIVER WATERSHED, NEW YORK?

Logging practices from the early 1800s to the mid-1900s in the Adirondack Mountains, New York, used dams, commonly referred to as splash dams, to store and release massive pulses of water to transport logs down remote headwater streams. These dams released water up to several times per year during the spring snowmelt season. The repeated discharges from the dam releases, combined with the annual snow melt discharges, likely resulted in geomorphic and ecologic changes that may have persisted decades after the use of splash dams was discontinued. Presently, it is unknown how the splash dam discharges compare to the natural magnitude and frequency of flooding for Adirondack streams.

In order to estimate the discharge from splash dams, we surveyed five streams that were impacted by these flow alterations. We surveyed between 12 and 20 cross-sections downstream of the dam location. High water marks were identified by old rip rap along meander banks, man-made boulder levees along straight channels, and locations of abandoned floodplains. By using these field markers, we reconstructed the channel cross sectional areas and the water surface slopes. The reconstructed high-water slopes were similar to the present day water-surface slope. We estimated a range of Manning’s roughness coefficients (Barnes, 1967; Arcement and Schneider, 1989) and used Manning’s equation in order to estimate the ‘paleoflood’ discharge, assuming minimal channel bed erosion.

We used equations of flood magnitude and frequency for the Adirondack region (Lumia, Freehafer, and Smith, 2006) to compare the estimated splash dam discharges to the natural flooding regime. Preliminary results suggest that the dam release discharges are stream dependent. Two streams have estimated discharges that range between ~2-year and ~10-year floods. The other three streams have estimated discharges that are at least 15-year floods and range to > 100-year flood. These differences cannot be explained by drainage basin size; the smallest and largest basins have the highest return intervals. More likely, the splash dam discharges relate to dam size. If these estimates are correct, splash dams released floods that were consistently greater than the mean annual flood several times per year.