It was assumed that the reservoir would not have a significant impact on the upper Potomac. Surprisingly, today Maryland operates a trout culture immediately below the outlet structure of the dam where a considerable number of trout are raised in pens and where wild trout are beginning to flourish. Had reservoir designers anticipated the improvement in North Branch water quality, additional features could have been included which would benefit the river even more. One example is that the dam lacks the ability to aerate the water as it is released. Without aeration, water released from the dam still contains a significant level of nitrogen which has an adverse effect on aquatic life.
Sediment carried by tributaries creates other problems.
According to the most recent available estimates, nearly one million tons of sediment washed into the tidewater Potomac during 1989. This amount can vary, however, depending on flow and averaged 1.7 million tons from 1978 through 1986. Approximately two-thirds of this sediment stems from erosion of agricultural land and the rest from construction sites and other sources in the Washington metro area. Over time, the sediment from the Potomac and its tributaries can pile up at an astounding rate in downstream areas. Once bustling commercial ports such as Bladensburg and Port Tobacco are now useless for navigation.