Regular visitors to the 1,800-acre national park surrounding Rock Creek in Washington, DC, might be surprised to learn what’s living — and what’s struggling to live — just below the water’s surface.Volunteers for the Audubon Naturalist Society take samples each season of the aquatic life in three Rock Creek tributaries. This monitoring program goes a step further than most to identify organisms down to the taxonomic level of family and, in some cases, to the genus level. (Cathy Wiss)

For starters, American eels have been spotted in Rock Creek tributaries often enough — once in 2010 and three times last year — that the long, slithery sightings are no longer considered a fluke.

Their presence was the highlight of a 28-page report recently released by the Audubon Naturalist Society, whose staff members and volunteers have been counting species in three Rock Creek tributaries for nearly a decade.

“We’d been concerned about whether [eels] could make it up Rock Creek, but [there was] a lot of work done to remove dams and obstacles to fish passage,” said Cathy Wiss, coordinator of the ANS water quality monitoring program. She said the snakelike fish lives in freshwater but returns to the ocean to spawn. “They’re a good indicator species.”

The report also identifies a “surprising diversity of life” in the heavily urbanized streams, which include Melvin Hazen Run, Pinehurst Branch and Normanstone Run.

Like streams in many other cities — into which rain washes pollution from nearby parking lots, streets and rooftops — their water quality is considered “poor” or “very degraded.” But that doesn’t mean they’re lifeless.

Since 2011, the ANS and its teams of volunteers have gone out each season to net 20, 1-foot-square samples of what’s living in the streams, sorting them into buckets for identification according to the Maryland Biological Stream Survey’s protocol, then putting them back in the water.

Going a step beyond that protocol, ANS teams are trained to identify aquatic life down to the taxonomic level of family and, in some cases, they’re able to identify them to the genus level. For example, rather than counting the general number of mayflies, they can tally the number of “small minnow mayflies” (Baetidae family).

Eels have been making an appearance in Rock Creek tributaries over recent years. (Cathy Wiss)“Most other programs only go to the taxanomic level of order, so we have a much fuller picture of what is living in the streams,” said Wiss, who won the Maryland Water Monitoring Council’s Carl Weber Award in 2018 for her leadership of community monitoring efforts.

Over the last eight years, ANS volunteers, including students from a local high school, have found 33 taxa of benthic macroinvertebrates. These small backbone-less bugs, worms and crustaceans living on stream bottoms are large enough to see with the naked eye and are important foodstuffs for other aquatic creatures.

The volunteers love spotting something new or unusual. This last year, that included the American eels, one of them large enough that construction workers repairing a gas pipeline next to Normanstone Run noticed it this fall.

The eels (Anguilla rostrata) are a host species for the early life stage of Eastern elliptio mussels, without which the mussels cannot reproduce. Mussels are drawing more attention recently for their ability to filter pollution in freshwater streams.

Also in 2018, monitors found for the first time a two-tailed genus of small minnow flies (Baetidae) that they believed had not been seen in the District before. Over the last eight years, they noted short-legged striders (Veliidae) and backswimmers (Notonectidae) in the streams for the first time.

Dobsonflies, a pollution-intolerant insect that the ANS volunteers had spotted in Rock Creek and intermittently in one of the streams, has now been collected in the other two streams as well.

But all too often, volunteers observe species that appear to be dwindling or disappearing.

The ANS report includes a list of 10 aquatic families that had been found in Melvin Hazen Run before 2010 but have not been seen since. Melvin Hazen Run has long boasted the highest diversity of the three monitored streams, but now, according to the report, the stream is “in clear decline.”Rock Creek, a tributary of the Potomac River, flows through Rock Creek Park in the District of Columbia. (Dave Harp)

About 10 years ago, despite the impacts of an oil spill in 1990, there were still good signs from the stream: In 2006, its score exceeded the threshold for adequate diversity. In 2008, students from the Sidwell Friends School monitored the stream and found it to be steadily improving.

But construction projects in the late 1990s transformed a 1.6-acre wooded parcel in the stream’s watershed into homes, with more construction to follow, according to the ANS report. Over the last decade, monitors have photographed green algae covering the streambed, and the ANS recommends further study to identify sources of pollution to the still relatively healthy waterway.

“It’s a cautionary tale, looking over our historic data,” Wiss said. “We found that there were many [aquatic families] that we’re not finding any more, that have dropped out of the community within the last five to 10 years.”

Meanwhile, the two other streams included in the report remained stable or showed improved signs of diversity. Pinehurst Branch remained stable with high percentages of small minnow mayflies buoying its score. And Normanstone Run, where the proportion of mayflies and caddisflies has been increasing, posted some of its highest diversity scores yet over the last eight years.

All three streams monitored by the ANS are located in forested stream valley parks that help buffer the effects of the surrounding residential communities. But the streams still suffer eroded banks, sediment pollution and other impacts from their urban environs.

Still, ANS director of conservation Eliza Cava said that Rock Creek’s system is more diverse than many think it is.

“A lot of people in DC really enjoy the landscape and beauty of Rock Creek as a backdrop for recreation or meditation, but it’s sometimes hard to imagine there’s a lot of life in it,” she said, before listing some of her favorites: eels, salamanders, dragonflies, mussels. “There are all these animals that spend part or all of their life cycles in these streams with us.”

To learn how to become a water quality monitoring volunteer with the Audubon Naturalist Society, visit

This spring, water quality monitoring in the District will expand to include bacteria testing under a grant from the District Department of Energy and Environment. To learn more or for volunteer training, visit or