Watershed Improvement District

Lake Facts

Lake Barcroft is a manmade lake formed as a reservoir from the stored water of the intersection of Holmes Run and Tripps Run.

The original dam that created the reservoir was completed in 1915 by the Alexandria Water Company.  That dam was 400 feet wide with a spillway at the top, 205 feet above mean sea level.  The reservoir of 520 million gallons and formed a lake of 115 acres.  In 1942, five-foot-high wooden gates were installed at the top of the dam and expanded the lake to 800 million gallons and 135 acres, near its present size.

In the early 1950’s, the reservoir, dam and 680 acres surrounding the lake were purchased by a Boston developer.  The lake ceased serving as a water supply reservoir and was converted to recreational use.

The Watershed Improvement District (WID) was formed in 1973 in the wake of the destruction of the Lake by the failure of the dam as a result of tropical Storm Agnes in 1972. Today, the WID functions as a State Agency to maintain and operate the dam, manage water quality of the Lake, manage the discharges from the dam and maintain and improve the environmental integrity of the waste shed above the Lake.

By law, the WID must maintain the water level of the Lake.  The dam is designed and operated to maintain a nearly constant water level.  The Lake collects runoff from fourteen and one-half square miles of Fairfax County and the City of Falls Church.  The rain water arrives from Tripps Run and Upper Holmes Run into the lake.  The dam is not designed for nor is it operated in a flood control capacity.  For whatever volume of water that flows into the Lake from Upper Holmes and Tripps Runs, an equal volume must flow out into Lower Holmes Run.  The goal is to maintain the Lake water level between 208.5 to 209 feet above mean sea level. 

Controlling the water level is a 12 foot high by 151 foot wide bascule gate that is continuously managed by a computer that responds to inflows of storm water.  As the water enters, the gate is automatically opened only by as much is necessary to pass an equal volume of water downstream.  The inflow and outflow are balanced and the reservoir level is maintained within the target range.  The gate is operated by four hydraulic rams powered by computer controlled electric pumps.  There is diesel powered generators as first back up and gasoline powered pumps if all else fails.

Because the Lake is a free flowing body of water, it supports a wide variety of plant and animal life.  There are over 22 varieties of fish known to inhabit the lake.  The extent and variety of both terrestrial and aquatic life attest to the overall health of the Lake.  Although testing of the water quality has been done, it is not done periodically unless there is some indicated reason.  The Lake is considered safe at most times for recreational activities.  After a major storm, runoff could make the Lake undesirable for swimming.  Although the Lake is safe for most activities, some people could be sensitive to dissolved impurities in the water.  The water is not considered to be drinkable.  The fish can be eaten sparingly however there may be heavy metals in the water that would make eating the fish regularly undesirable.  Most of the dissolved elements in the Lake come from highway run off and are associated with automobile use.