Upper, Middle, and Lower Basins: Understanding Lake Anna’s Watershed and Effect of Runoff

October 25, 2024
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Basin Map Courtesy of Randy Thomas.

Consensus seems to agree that the Upper part of Lake Anna is defined as the area above the Splits. This border is from just below Rose Valley to the State Park. The Middle is from the Splits to around the Power Plant, and the Lower is below the Power Plant.

The Watershed (see map) is the drainage basin that feeds the lake. It’s 342 square miles, with the largest portion feeding the Upper lake and the smaller portion the Middle and Lower parts of the lake.

The Lower portion is about 30 square miles and directly feeds about 2/3rds of the lake. The land use in the Lower portion is mostly homes with very little farmland. There are no major rivers, but many small streams and creeks. Runoff from these areas is usually not very muddy and short-lived. This is why the Middle and Lower portions tend to remain clear throughout the year.

The Upper portion is about 312 square miles (about 10 times the Middle and Lower basins’ drainage) and directly feeds about 1/3rd of the lake. Eventually, this water works its way into the Middle and Lower basins. This area encompasses much of Orange and the northwest portion of Louisa counties. The land use is mostly farmland, and there is one river, creek, and run that feed this area. Almost all of the three water sources start in Orange County, from several smaller streams and creeks. The North Anna River, which also gains water from northwest Louisa County, flows into the southern branch of the Splits. Pamunkey Creek feeds the middle branch of the Splits, and Terry’s Run feeds the northern branch of the Splits.

Since most of this area is farmland, when there is heavy rainfall, the runoff becomes loaded with silt, especially during the winter months, when there are very few crops growing to hold the soil in place. This, along with the size of the basin, is the reason why the area above the Splits tends to be muddier. When we get a big rainfall over the Upper basin, we can sometimes see very muddy water working slowly down the lake over several days. This mudline tends to end around the Splits, where the water flow slows, the lake broadens, and depth increases, which allows the silt to settle out before moving further down the lake.

Lake Anna “Full Pool” is 250 feet above mean sea level at the dam. It is measured in tenths of feet, with measurements taken Monday through Friday mornings. This is the preferred level of the lake for optimal use for recreation and Dominion Energy. When the lake levels exceed 250 feet above MSL, Dominion can open between 1 and 3 gates to let out water. At 265 feet above MSL, water would top the dam. Lower levels cause some restrictions in water release, but there are regulations in place to maintain a specific flow rate of the North Anna River below the dam.


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Basin Map Courtesy of Randy Thomas.

Consensus seems to agree that the Upper part of Lake Anna is defined as the area above the Splits. This border is from just below Rose Valley to the State Park. The Middle is from the Splits to around the Power Plant, and the Lower is below the Power Plant.

The Watershed (see map) is the drainage basin that feeds the lake. It’s 342 square miles, with the largest portion feeding the Upper lake and the smaller portion the Middle and Lower parts of the lake.

The Lower portion is about 30 square miles and directly feeds about 2/3rds of the lake. The land use in the Lower portion is mostly homes with very little farmland. There are no major rivers, but many small streams and creeks. Runoff from these areas is usually not very muddy and short-lived. This is why the Middle and Lower portions tend to remain clear throughout the year.

The Upper portion is about 312 square miles (about 10 times the Middle and Lower basins’ drainage) and directly feeds about 1/3rd of the lake. Eventually, this water works its way into the Middle and Lower basins. This area encompasses much of Orange and the northwest portion of Louisa counties. The land use is mostly farmland, and there is one river, creek, and run that feed this area. Almost all of the three water sources start in Orange County, from several smaller streams and creeks. The North Anna River, which also gains water from northwest Louisa County, flows into the southern branch of the Splits. Pamunkey Creek feeds the middle branch of the Splits, and Terry’s Run feeds the northern branch of the Splits.

Since most of this area is farmland, when there is heavy rainfall, the runoff becomes loaded with silt, especially during the winter months, when there are very few crops growing to hold the soil in place. This, along with the size of the basin, is the reason why the area above the Splits tends to be muddier. When we get a big rainfall over the Upper basin, we can sometimes see very muddy water working slowly down the lake over several days. This mudline tends to end around the Splits, where the water flow slows, the lake broadens, and depth increases, which allows the silt to settle out before moving further down the lake.

Lake Anna “Full Pool” is 250 feet above mean sea level at the dam. It is measured in tenths of feet, with measurements taken Monday through Friday mornings. This is the preferred level of the lake for optimal use for recreation and Dominion Energy. When the lake levels exceed 250 feet above MSL, Dominion can open between 1 and 3 gates to let out water. At 265 feet above MSL, water would top the dam. Lower levels cause some restrictions in water release, but there are regulations in place to maintain a specific flow rate of the North Anna River below the dam.


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