You’ve heard of wetlands, but have you ever heard of a hemi-marsh?

A hemi-marsh is a shallow wetland with water levels that change seasonally. It’s not a lake. The ideal hemi-marsh — “hemi” means “half” — is 50 percent water and 50 percent plants, a combination that provides biodiversity, a healthful balance for a wide variety of local native plants and animals.

Huntley Meadows Park, which has the largest nontidal wetlands in Northern Virginia, just completed a project to bring back the hemi-marsh ecosystem that beavers created there in the 1970s.

Why did nature need help?

There are several reasons. Beavers, often called “nature’s engineers,” build dams, which raise water levels and form ponds. This higher water allows the beavers to float the logs and branches they need to construct their lodges. The deeper water also protects their families from predators. In the process, beavers provide welcoming habitats for other wildlife.

Over the years, this huge natural area south of Alexandria became surrounded by roads and houses. That development caused tons of silt (small particles of rock and soil) to flow into the wetlands, reducing the water’s depth by one-third, destroying habitats and encouraging silt-loving plants such as cattails to take over, crowding out other plants.

Beavers are nomadic. That means they don’t stay in one place forever, especially when new food sources are needed. So, when beaver dams are abandoned and begin to fall apart, the ponds behind them start to dry up. Over time, a once-thriving hemi-marsh becomes a “dry marsh.” It’s a natural cycle that wetlands go through. If left alone, they eventually become meadows, then forests, and the biodiversity of the area changes.

For example, if a pond’s fish and crayfish don’t have enough water, they can’t survive, so birds and ducks that eat them will go elsewhere. If cattails take over, animals that depend on other plants will leave.

Huntley Meadows is an important site for environmental education because it contains one of the only large wetlands left in our area. If allowed to follow the natural cycle, it would be decades before a hemi-marsh and its biodiversity returned, said Kevin Munroe, the manager of Huntley Meadows.

After years of research and public meetings, Fairfax County officials decided to take a lesson from the beavers by creating a permanent hemi-marsh to give naturalists, researchers, students and others a chance to experience this unusual balance of nature.

How was the hemi-marsh restored?

A year ago, workers began building a 600-foot-long berm (a mound of earth) over a wall of vinyl panels; the berm and wall are designed to act like a beaver’s dam and to last for decades with little maintenance. (You can see the berm from the park’s observation tower by looking to your left.)

The project includes a device that allows park staff to raise or lower water levels in the hemi-marsh, in case the beavers move away or extreme weather disrupts wetland plant communities.

In the fall and winter, water floods into the park’s nearby forest, extending the wetland’s boundaries and providing aquatic life with deep pools that won’t freeze. Starting this month, the water level will gradually be lowered to encourage native plants to sprout during warmer weather, Munroe said.

You might be tempted to walk across the berm, but don’t. Native plants are being grown there to stabilize the soil and help the berm blend in with the park’s natural features. If people walk across it, all that hard work will be destroyed.

How did the project turn out?

A month after the restoration was completed, changes are already being noticed.

As Fiona Dreesbach, 9, and her mom walked along the half-mile boardwalk through the wetland recently, they watched a female hooded merganser and her nine fluffy ducklings paddling about in the flooded forest. Freshly gnawed trees and a well-maintained lodge made of packed mud and sticks provided evidence of an active beaver family.

Fiona searched for aquatic insects that were making small ripples in the water. Red-winged blackbirds flew across her path while a female goose in a nest gave promise of chicks in the near future.

Munroe said he has already heard more callings of southern leopard frogs than before the restoration was done.

It “may mean more eggs and tadpoles, as their singing is part of courtship,” he said. He also said that “ducks that we have not seen for years are all back because of deeper water.” A variety of turtles and birds also have returned.

Beavers, muskrats, crayfish, rough green and ribbon snakes are among the year-round residents at Huntley Meadows. Listen for some splashing in the water; it might be a playful river otter.

This spring and summer, look for dragonflies, black ducks, green tree frogs and birds such as bitterns, rails and green herons. When fall and winter come, watch for dabbling and diving ducks.

It will take years for Huntley Meadows’ hemi-marsh to fully return, but watching it happen will be amazing. “Every time you visit, you see something new,” Fiona said.

Visit Huntley Meadows Park

Huntley Meadows Park (3701 Lockheed Boulevard, Alexandria, Virginia) will host two family-friendly special events this month. For more information, a parent can call 703-768-2525 or go to

Wetlands Awareness Day

Learn about wetlands conservation. Look at birds through a spotting scope and meet a raptor up close. Games, a cake walk and face-painting.

When: Sunday, May 4, from noon to 4 p.m., rain or shine.

How much: Free, with a fee for some activities.

Restored Wetlands Project Grand Opening

Meet at the visitor center for a ribbon-cutting ceremony and behind-the-scenes tours of the project.

When: May 10 from 10 a.m. to noon.

How much: Free.

— Ann Cameron Siegal