Pohick Lakes Rehabitation and Dredging
By Eric Yan
Pohick Creek watershed is one of the largest in Fairfax County and home to a number of outstanding lakes and parks. It is characterized primarily by residential land use and accompanying commercial industries. The largest of the watershed lakes, Burke Lake, was created for recreational purposes in the 1950s. The remaining six lakes (Mercer, Braddock, Barton, Huntsman, Woodglen and Royal) were created during the 1970s and early 1980s to control stormwater runoff and manage erosion. The lake dams have since helped mitigate downstream flood damages and provided improved water quality, wildlife habitat and recreational opportunities to the residents of Fairfax County. In more recent years, other best management practices and methods of controlling stormwater runoff have been adopted to protect Fairfax County streams like Pohick Creek. This in turn helps to protect the Potomac River and the Chesapeake Bay.
Planning for the Future
Lake Barton, Huntsman Lake, Woodglen Lake and Royal Lake have recently been included in a rehabilitation plan. Fairfax County and the USDA Natural Resources Conservation Service (NRCS) conducted studies that determined the need for improvement on the dam auxiliary spillways. There is no damage to the dams, but new government regulations and standards require them to be upgraded. Fairfax County is also repairing and replacing riser structures that control the flow of lake water into the streams below the dam. Underwater berms or ridges will be constructed near lake inflows in order to capture and hold sediment at a designated location. This will make future sediment management easier and prevent new sedimentation in the deeper lake areas. Furthermore, the rehabilitation will enhance recreational opportunities and extend the lifespan of the lakes.
Fairfax County, in partnership with NRCS and the Northern Virginia Soil and Water Conservation District (NVSWCD), has completed rehabilitation of three of the four lakes. Construction on the fourth facility, Huntsman Lake, will begin this winter. Fairfax County also initiated a dredging program to restore the sediment pool capacity of these lakes. This will allow for the continual intake of upstream waters and sediment without the lakes filling up with sediment. Dredging at Huntsman Lake will be completed together with the dam rehabilitation project. Dredging at Woodglen and Royal Lake will take place in 2014. The Lake Barton rehabilitation project and dredging has already been completed.
Eventually, even the deepest lakes will fill in with sediment. But in urbanizing areas like Fairfax County, lake sedimentation happens much more quickly. Roads, roofs and other impervious surfaces increase runoff, causing soil erosion from the land and stream banks. Sediment is picked up by runoff and washed downstream. When the water reaches a lake, it slows down and the sediment is deposited on the lake bottom. In order to keep the lake deep enough for recreational uses and flood control, some of the sediment needs to be removed periodically.
Sediment is also a big problem in the lower Potomac River and Chesapeake Bay. The Chesapeake Bay drains water from roughly 64,299 square miles upstream, including parts of six states and the District of Columbia. Eroded sediment covers habitat, clouds the water and carries nutrients with it that can cause other water quality issues. In lakes that are being dredged, sediment disturbance can result in a temporary and rapid release of nutrients into the water column. However, removing all of that phosphorous-rich lake sediment helps in the long run to decrease mass algal blooms and pollution from internal nutrient loading. Removing built-up lake sediment will not only increase pool capacity, but also remediate any trapped nutrients that could cause water quality problems.
How Dredging Works
Dredging is the process of removing lake, stream or ocean sediment. There exist two methods of dredging: hydraulic/suction and mechanical. Hydraulic dredging involves drawing sediment through a long tube - like a vacuum cleaner - from a small dredge barge. Mechanical dredging can be done with the lake full (wet) or drained (dry). A bucket dredger removes sediment from a small barge or the exposed dry lake bottom.
Dredged material is deposited in an on-shore disposal basin which can then be incorporated into the landscape or trucked off to a landfill. The four Pohick Creek watershed lakes in need of rehabilitation will undergo mechanical dredging. This method has been shown to be less expensive than hydraulic dredging but can cause more sediment disturbance. The disposed sediment will be used as topsoil for resale and landfill cover.
Thinking of the Critters
Taking sediment out of the lake will cause some disturbance, regardless of whether mechanical or hydraulic methods are used. Dragonfly larvae, snails, aquatic worms and other invertebrates are removed along with the sediment, and stirring up sand and silt affects water quality.
Drawing down water levels, needed to conduct dam maintenance, will also impact wildlife. These impacts are temporary but will be felt by fish, frogs, turtles, invertebrates and plant life. The recreational and aesthetic benefits for people will also be interrupted.
Many local anglers enjoy fishing these Fairfax County lakes, but they'll have to go elsewhere to fish. Before the water level drawdowns, the fish living in the lakes will be temporarily moved to the other lakes not in the rehabilitation plans - Lake Mercer and Lake Braddock. The fish will be captured by electroshocking and netting. This procedure uses a slight electric shock to temporarily stun the fish for a couple of minutes after which they return to their natural state. It is a commonly used technique by fisheries biologists and when done correctly does not result in any permanent harm to the fish. The fish will be returned to their home lakes after the rehabilitation projects and dredging are complete.
Insects, plants and other bottom-dwelling life on the lower trophic levels are essential for fish food, habitat and overall ecosystem diversity. If these communities can bounce back after the lake dredging, the returning fish will survive. Research on how dredging affects ecosystems over time can help us understand what to expect in the Fairfax County lakes.
Scientists have found that immediately after dredging, invertebrate communities' species density and diversity will drop. Invertebrates depend on microhabitats: rocks, vegetation, leaf packs and woody debris, all of which can be negatively impacted by dredging. Invertebrate numbers will probably remain low for the first few months post-dredging, but studies show that they can recover in approximately one year's time. Tough pioneer species will colonize in the beginning and a more complex invertebrate community will develop over time to include sensitive and predatory species. For insects and other invertebrates, spring and summer are generally times for larval emergence and adult breeding.
Frogs, aquatic turtles and other larger organisms rely on invertebrates and the lake sediment for food and habitat. Both need an aquatic environment to rehydrate their skin and for breeding from spring to summer. Frogs lay their eggs in the water and the tadpoles hatch within one month. Turtles lay their eggs on land in summer and fall and the hatchlings emerge in spring. In winter, both frogs and turtles hibernate in the lake sediment.
As part of the rehabilitation project, new habitat structures will be put in place to assist in recovery at each lake. The shorelines will be integrated with biologs (dead tree trunks laid to slow inflowing water), riprap rocks, wetland plantings and woody vegetation to help prevent erosion and buffer the lakes from runoff. These structures will also provide habitat for aquatic and terrestrial organisms to feed, hide, breed and nest.
A Fresh Start
Spring may be the optimal time for a fresh start to a disturbed community. The different lifecycles of lake invertebrates, frogs and aquatic turtles will dictate exactly how each organism will respond to the lake dredging project. Because adult breeding and the emergence of young often occurs in the spring with warmer weather, it is best if dredging projects can be completed during the fall and winter. Any season with have some impact on pond fauna. The entire process of dredging can take 6-7 months to complete, so it may be impossible to avoid overlapping with one or more important stages of these organisms' lifecycles.
In the spring following dredging, the pre-existing seed banks, organisms coming out of hibernation and emerging larvae will be greatly reduced or gone, so new life in highly disturbed areas will rely on the colonization of organisms through species dispersal. Virginia resides in the Atlantic flyway for migratory waterfowl, which will often stop in spring to rest and recover before continuing to their breeding and nesting grounds farther north. Waterfowl can unintentionally carry fish, frog and invertebrate eggs on their wings, depositing them at stopover points. Other flying and terrestrial species will attempt to expand their habitat ranges in search of potential mates and breeding areas. The dredged lakes will offer these organisms a clean slate. Once a sustainable invertebrate community has been established, the fish kept at Lake Mercer or Lake Braddock can be moved back into their home lakes.
Lake rehabilitation and dredging have tremendous benefits in terms of flood control and natural resource management. There will of course be temporary disturbances for long-term interests, but proper timing of these procedures will allow for optimization of ecosystem recovery.
When the rehabilitation and dredging projects are completed, the sediment pool capacity and water quality will be improved. Over time, the organismal community will also recover. The improvements will make long-term maintenance easier, less invasive and less costly while keeping the Pohick lakes beautiful and healthy.
To learn more about the rehabilitation projects, see:
- USDA Natural Resources Conservation Service Project Overview
- Slideshow: Green Breakfast on Pohick Dam Projects
- Learn More About the Dam Rehabilitation (Previous Conservation Currents article)
Eric Yan is a recent graduate from Allegheny College in Pennsylvania. He grew up in Oakton, VA.
Image credits: Fairfax County Department of Public Works and Environmental Services