One of Rock Creek’s tributaries near 30th Street NW at Rock Creek Park on Dec. 13, 2018. The Audubon Naturalist Society has released a report that found the health of Rock Creek’s D.C. tributaries to be compromised by pollution. (Astrid Riecken/For The Washington Post)

On a misty December morning, two women in high rubber boots stood in a babbling stream in Northwest Washington, gushing over bugs.

“I think we have a lot of Hydropsychidae in this net; do you want to pull it out?” said Eliza Cava, director of conservation for the Audubon Naturalist Society.

Her colleague, Cathy Wiss, coordinator of the organization’s water quality monitoring program, sifted through wet, brown leaves and pointed at a wriggling black insect: the larva of a common variety of caddis fly. It was a good sign, but not wholly unexpected — they thrive better than some of their cousins in polluted water.

“What we have lost is some of the more sensitive, more intolerant caddis flies in D.C., and mayflies,” Wiss explained.

In the metaphorical swamp that is Washington, “intolerant” and “sensitive” are not generally seen as positive, but in a city built across what was once actual swampland, fragile organisms are signs of aquatic health.

Assisted by several dozen volunteers, ANS has been monitoring the tributaries that feed Rock Creek for over a quarter-century and giving the raw data to government agencies. This month, for the first time, the organization published a report that is available to the public, “Stream Health at Select Tributaries in Rock Creek in Washington, DC,” assessing the condition of Northwest Washington’s waterways over the past nine years by looking at the organisms living in them.

Fingernet caddisfly larvae taken from a Rock Creek tributary await examination by Cathy Wiss, a water quality specialist with the Audubon Naturalist Society in Maryland and the District. (Astrid Riecken/For The Washington Post)

Volunteers return four times a year to three streams that feed Rock Creek: Pinehurst Branch, near Chevy Chase; Melvin Hazen Run, near Tilden Street NW; and Normanstone Run, near Cleveland Park and Embassy Row.

In the report, all three received scores of “poor” health, with Melvin Hazen in “clear decline,” showing a loss of diversity since 2010.

Ten families of mayflies, stoneflies, beetles, dragonflies and others have gone missing in recent years. And the presence of algae and E. coli indicates possible leaks from old sewage pipes near the creeks. Occasionally, rushes of “gray” water — which is frothy and stinky — appear in the creeks, indicating sewage leaks whose sources have to be tracked down with city agencies and the National Park Service.

“It could be inside a facility,” said Wiss, adding that a leak came from the Naval Observatory’s sanitary sewer a few years ago.

The report also noted some positive indicators, such as an increase in the numbers of ­intolerant-to-pollution dobsonflies, along with rare sightings of eel (once an essential food source for Native Americans in the area) and salamander.

Eliza Cava, director of conservation with the Audubon Naturalist Society, shows how to catch small organisms such as larvae in one of the streams that flow into Rock Creek in Washington. (Astrid Riecken/For The Washington Post)

“Eels are a good sign because they’re a host for the early life stages of Eastern elliptio mussels, and that’s something that we hope will come back,” Wiss said. “Mussels help filter the water. . . . It is exciting, because we used to have them, and getting them back is great.”

A century ago, residents swam in and ate fish from Rock Creek’s tributaries. But early cars also drove through them, contributing to an onslaught of pollution that has continued. These days, debris from storm runoff and misdirected sewage are problems, as is construction that changes the flow of groundwater and rainwater. Road salt is also toxic to fauna in the freshwater streams.

Like many urban waterways, those in the District share space with people who may not understand all they contain, Cava said.

“So many people use the streams that run through the city, especially Rock Creek Park, as an amenity, a beautiful place to walk and run and play with their kids,” she said. “We want people to know that there’s life in these streams. . . . We share the water with them. They’re our neighbors, and this is their home.”

As the streams have become more polluted, only the hardiest and most adaptable organisms have continued to thrive. Today, the streams examined in the report have less diversity than in the past. “But there is life there,” Cava said. “If it were a total sewer, it would be hard for them to survive.”

Cava blasted a Trump administration proposal this month to sharply limit federal regulation of pollution in wetlands and tributaries that run into the nation’s largest rivers, a policy that could affect urban streams such as the ones cited in the ANS report.

“We already have endured a tremendous loss of those types of ecosystems,” Cava said. “They were covered with pavement, buildings and houses, and that’s a big part of why urban streams, like the ones we study in D.C., are in overall poor condition today. Those impacts in most of D.C. happened before we had the laws in place that today should help protect wetlands and small streams.”

In recent years, the District has worked to improve water quality, including using road construction methods that are sensitive to the streams, and installing rain gardens along stream banks to restore groundwater flow that has been lost to urban construction and erosion.

It is unclear whether the streams will one day be clean enough for people to wade through or fish in again; for now, people and dogs are encouraged to avoid them after a storm.

But just being out there testing the water offers a way to educate residents. Mike Kolian, an environmental scientist with the Environmental Protection Agency who has been a volunteer water monitor with ANS for two decades, said passersby often stop to ask what he and his team are doing with their nets in the middle of a stream. “We get a chance to explain what we’re doing. . . . We have a table set up with microscopes” so passersby can get a close look at the water’s inhabitants, said Kolian, a D.C. resident.

Emily Boyer, an upper-school science teacher at Sidwell Friends School, takes her AP environmental science class to monitor the creek water for ANS. When the students hear that their findings will be reported to city agencies that might base regulatory decisions on them, they take the work more seriously, she said.

“There’s definitely a reaction of, ‘Oh, wow, we’d better not mess that up.’ ”