Evaluating the impacts of impoundment on sediment transport using short-lived fallout radionuclides

Dams greatly influence water and sediment discharge regimes and can have significant impacts on channel morphology and sediment storage. By using the short-lived fallout radionuclide ⁷Be (t _1/2 = 53.4 days) as a tracer of fine (∼0.25-2 mm) bed material load transport, we capture the sedimentological and geomorphic impacts of the Union Village Dam, located on the Ompompanoosuc River, in eastern Vermont, USA. We measured ⁷Be activities in approximately monthly samples from streambed sediments in a regulated stream and an unregulated control stream. In the regulated stream our sampling spanned an array of management conditions during the annual transition from flood control in the winter and early spring to run-of-the-river operation from late spring to autumn. Because sediment stored behind the dam during the winter quickly became depleted in ⁷Be activity, it became possible to track this plug of "dead" sediment as it moved downstream. Measured average sediment transport velocities (30-80 m day ^-1) exceed those typically reported for bulk bed-load transport and are remarkably constant across varied flow regimes, possibly due to corresponding changes in the bed sand fraction. Results also show that the length scale of the downstream impact of this dam management (winter pool and summer run-of-the river with minimal sediment trapping efficiency) on sediment transport can be short (∼1 km); beyond this distance the sediment trapped by the dam is replaced by new sediment from point bars, tributaries and other downstream sources. The benthic community structure indicates significantly greater abundance of caddisflies downstream of the dam due primarily to a lack of bed disturbance following impoundment.