The movement of a solute, as represented by a soluble fluorescent dye, was observed in the Potomac River estuary at Washington, D.C. The average net rate of downstream movement of the solute centroid was less than 0.6 mile per day. The movement of a solute is highly dependent on the nontidal inflow to the estuary. During the study, the average inflow was 900 cubic feet per second a very low value, equaled or exceeded 98 percent of the time. Using a storage equation, the average movement of a solute was estimated for nontidal inflow of 3,100 and 6,500 cubic feet per second; these inflows are equaled or exceeded 75 and 50 percent of the time, respectively. The study showed that tidal action was fairly efficient in dispersing the solute longitudinally. The solute, which was dumped 1,000 feet upstream from the 14th Street Bridge, was observed as far upstream as Roosevelt Island. A transient longitudinal dispersion coefficient at the end of 150 hours was determined to be 210 square feet per second. On the other hand, the lateral diffusion was a slow process and the lateral distribution of the solute was far from uniform at the end of 6½ days after the release.
