Tracer study of mixing and transport in the upper Hudson River with multiple dams

In October 2001, ∼0.2 mol of SF₆ was injected into the upper Hudson River, a modified natural channel with multiple dams, at Ft. Edward, N.Y. The tracer was monitored for 7 days as it moved ∼50 km downriver. The longitudinal evolution of the tracer distribution was used to estimate one-dimensional advection (9.0±0.2 km d ^-1) and dispersion (17.3±4.0 m² s ^-1) along the river axis. Comparison of these results to tracer studies on channels without dams suggests that dams reduce longitudinal dispersion below the value expected in a natural channel with the same discharge. SF₆ loss through air-water gas exchange along the river and at two dams (10.7 m combined height) was estimated by observing decay in peak concentration. Losses at dams (approximately 50% per dam) were dominant. The estimated gas exchange at dams was compared to a simple model adapted from those available in literature. Small amounts of tracer were trapped in a canal segment (∼5 km long) that parallels the river, where advection and dispersion were sharply reduced.