Ever wonder how a lake appears out of nowhere? Today, we’re standing before Lake Needwood Dam. This structure created Lake Needwood. It is east of Rockville, Maryland. The dam tamed Rock Creek in 1965. Its mission was flood control and erosion reduction. Lake Needwood Dam helps the creek’s water quality. It acts as a retention basin. This traps sediment from stormwater runoff.
Lake Needwood Dam is an earthen embankment. It stands 65 feet high. Imagine it built on fractured rock. This rock formed where Boulder Gneiss meets Upper Pelitic Schist. Besides the spillway, there is an emergency spillway. It stretches 185 feet wide. It sits between the Westside and Rock Creek Trails. The Natural Resources Conservation Service designed this dam.
Construction finished in 1965. The Maryland-National Capital Park and Planning Commission owns the dam. During construction, workers only treated the right side with grout. This likely caused concentrated seepage. It happened on the left side during the 2006 Mid-Atlantic United States Flood. In 2007 and 2008, the dam had a 3-million-dollar repair. Workers installed a grout curtain to stop seepage. They also upgraded the outlet control tower.
Now I’ll tell you about the flood of 2006. Heavy rains caused evacuations downstream. People feared the dam’s structural integrity. About 2,400 people evacuated. This happened on June 28, 2006. They stayed in shelters at nearby high schools. The lake rose 25 feet above normal. Many roads closed because of flooding. Evacuees returned home on June 29, 2006. Officials estimated that a dam break could flood areas up to 19 feet.
Before 1990, the county dredged the lake regularly. This removed sediment. Dredging stopped for budget reasons. In 2008, the county prepared to dredge again in 2010-2011. The project improved water quality and boating. It also helped fishing.
In October 2010, the water level lowered about six feet. This dried the sediment for dredging. The north end deepened from 1-4 feet to 12-18 feet. The lake reopened in summer 2012.
You should also be aware of the lake’s water quality. In 2010, the water tested positive for microcystin. This toxin comes from cyanobacteria. It can harm the liver. The toxin appeared again in 2012-2017 and in 2020. Yellow signs warn people to keep pets away from the water. Cyanobacteria grow due to too many nutrients. These nutrients come from fertilizer and urban runoff. Higher water temperature and low circulation also contribute.
Lake Needwood Dam is more than just a structure. It is a protector. It prevents floods and soil erosion. It also improves water quality. Next time you see a lake, remember Lake Needwood Dam. Think about the engineering and effort that made it possible.