Tracer Study of Mixing and Transport in the Upper Hudson River with Multiple Dams

In October 2001, $\sim 0.2\mathrm{mol}$ of $S{F}_6$ 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 $\sim 50\mathrm{km}$ downriver. The longitudinal evolution of the tracer distribution was used to estimate one-dimensional advection $(9.0\pm 0.2\mathrm{km}{d}^{-1})$ and dispersion $(17.3\pm 4.0m^2{s}^{-1})$ 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. $S{F}_6$ loss through air–water gas exchange along the river and at two dams ($10.7m$ 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 ($\sim 5\mathrm{km}$ long) that parallels the river, where advection and dispersion were sharply reduced.