IDENTIFYING SEASONAL DRAWDOWN EVENTS OF LAKE MERRIWEATHER: IMPLICATIONS FOR WATER QUALITY IN THE LITTLE CALFPASTURE AND MAURY RIVERS

Goshen Dam forms 425-acre Lake Merriweather on the Little Calfpasture River near Goshen, Virginia, just upstream of where it joins the Calfpasture River to form the Maury River. The Little Calfpasture River’s 215km² watershed is otherwise comprised of forested land with some agricultural land predominately used as pastures. Following periods of recreational use during the summer, the lake level is lowered several feet, allowing much of the sediment that has been trapped in the impoundment to be exposed along its shallowly-sloping banks. This exposed sediment is washed into the water draining from the lake during rainstorms, subsequently increasing the turbidity of the water in the Little Calfpasture River and ultimately the Maury River. When the lake is full, water collected upstream and downstream of the impoundment is almost identical with regard to both a variety of dissolved solids as well as suspended sediment. It is when the water level is lowered and a rainstorm occurs that turbidity reaches problematic levels downstream from the dam. While these high turbidity events are only temporary, there remain persisting observed changes in the quality of the benthic habitat and in 1996 the EPA listed this reach of the Little Calfpasture River as impaired for failing to meet the designated use for the aquatic life general benthic standard. To determine the frequency and timing of these drawdown events, models built using USGS stream gauge data for areas both upstream and downstream of the lake were combined with local precipitation data and 38 incongruities were identified between upstream and downstream flows between 2000 and 2013. These drawdown events can be described as times when the upstream water discharge significantly does not match the discharge downstream. While the reason for some of these 38 events is unclear, 14 can be explained by rainfall distribution and 14 can not. These events appear to represent the times when the dam operators are draining water from the lake, exposing the sediment. Understanding the history and future of these drawdown events can be used more effectively understand the consequences these events have on turbidity and sediment loading, and ultimately could help to inform efforts to restore the benthic habitat to a good state of health and reduce the number and severity of high turbidity events.