In Rock Creek, Engineering a Path for Fish to Spawn
Thursday, November 10, 2005
When migratory fish return to the fresh waters of Rock Creek Park next spring, they'll be able to do something that they haven't done in more than 100 years: swim upstream to spawn.
At least that's the hope of park rangers, engineers and designers who are trying to restore a fish commute that's been blocked. Earlier this year, rangers and volunteers used buckets to carry fish upstream to dramatize their plight.
Come spawning season, there will be highly engineered passageways that include boulder step pools and a fish ladder. Park visitors might not notice all the changes, but officials hope that the fish will as they trek from the Potomac River through Northwest Washington and into Montgomery County.
The $2.5 million project is scheduled to be completed by late winter. It is one of the environmental mitigation programs paid for by the Maryland Department of Transportation and the Federal Highway Administration to compensate for resources damaged or lost during the replacement of the Woodrow Wilson Bridge. Blueback herring, white perch and striped bass are among the species expected to return to the creek from saltwater habitats for spawning north of the city.
Nature lovers can barely wait.
"There will be an increase in biodiversity for Rock Creek and the ecosystem," said Bill Yeaman, a Park Service ranger. "It will bring back some of the natural resources that we've lost, and it will improve the health of the fish population and the entire ecosystem. The long-term benefit is going to be important to watch and study. The spawning will be an amazing and exciting event for people to see."
In late 2003, engineers and designers began focusing on eight areas in the creek in the District where fish had difficulty navigating upstream. Officials from the D.C. Department of Parks and Recreation, the National Park Service, the Maryland State Highway Administration and the Virginia Department of Transportation were involved in the project.
Nearly 16 miles of habitat had become blocked because of six exposed sewer lines, two concrete roadways and one man-made dam. For herring, which are relatively weak swimmers, the sewage lines, fords and Peirce Mill Dam were essentially stop signs. And unlike salmon, herring can't jump.
What they can do is take off in short bursts in areas of high-velocity current. But for them to do that, the water level has to obscure obstructions.
To hide the sewage lines, which are near the Sherrill Drive crossing, engineers positioned boulders to control the flow and depth of the water. Fish will be able to dive upstream and then rest in calmer waters just beyond the passageway. The elevation of the rocks had to be precise, as did the slope of the stones. The aim was to divert the water's natural flow through a man-made design.
"We tried to mimic conditions in nature so the fish will perceive it as part of the environment, so it just looks like a pile of rocks across the water," said Timothy J. Morris, an environmental mitigation manager with Potomac Crossing Consultants. "But it's highly designed, and everything was pre-determined and measured. We want to make it as easy for the fish as possible." Potomac Crossing Consultants is a consortium of companies formed to work on environmental projects related to the Woodrow Wilson Bridge reconstruction.
Engineers also had to contend with a historic ford. Rather than removing it, they broke it up and built it lower so fish could pass over it.
One mitigation effort that will be much more noticeable to visitors is the fish ladder at Peirce Mill Dam. Duncan Kerr, a project engineer with Potomac Crossing Consultants, said the ramp-like structure will have slats that will slow the water coming off the dam.
It will be more difficult for the fish to swim up the ladder than for them to navigate upstream at other points because of the turbulent waters, Kerr said. But there will be small areas where the fish can rest before continuing up the ramp. The concrete structure will be about 120 feet long and four feet wide.
Park officials expect tens of thousands of fish to make their way next spring to Rock Creek's waters to spawn. Over the past five years, D.C. Fisheries and Wildlife Division authorities have been "imprinting" fish so they will naturally return to Rock Creek for spawning season. In that process, fish were taken upstream, where they spawned. Hundreds of thousands of their offspring could be drawn back because of the imprinting process.
"There are chemicals and patterns in the water that will tell the fish, 'This is where I was born,' " said Ricardo A. Gonzalez, a stream specialist with the URS Corp. "We have to wait and see how many come back."
The ecological effects of the large population of spawning fish upstream aren't known, Gonzalez said. But he said there will be clear benefits to migratory fish and also to predators such as herons, osprey and turtles. The long-term effectiveness of the new fish paths won't be known for several years, but project managers will begin monitoring the structures and possibly tagging fish in the spring.
"We can't control the fish," Morris, the environmental mitigation manager, said. "But what we can do is build structures so fish can get through and try to restore the creek to what it was historically."