Standing in the swift, ankle-deep current of the Tookany Creek on Monday afternoon, DaiJzanaee Martinez dipped a fine-meshed net into the water to gauge the creek's overall well-being.

She was delighted to find a juvenile mayfly, an insect that does not thrive in polluted water.

"It must be good water," said Martinez, 18, a senior at the Philadelphia High School for Creative and Performing Arts.

In some respects, yes, but the creek is one of hundreds in the Delaware River watershed that need care and monitoring. She joined three other Philadelphia high schoolers and the actor Mark Ruffalo at the Cheltenham site to learn about a small part of a vast project: protecting a river basin that provides drinking water for 15 million people.

Decades ago, pollution in the Delaware meant harmful chemicals flowing from a pipe. These days, the main threats are far more diffuse and difficult to control: manure, fertilizer, motor oil, and other agricultural and suburban contaminants that are washed into streams by rain.

Since 2014, the nonprofit William Penn Foundation has contributed more than $40 million to dozens of groups tasked with monitoring, restoring, and protecting an area that spans 13,000 square miles, starting in upstate New York.

One of the groups is the Tookany/Tacony-Frankford Watershed Partnership, whose volunteers and staff were on hand with foundation representatives and scientists from the Academy of Natural Sciences of Drexel University. Their goal was to explain the science to Ruffalo and the high school students.

Turns out the visitors already knew a thing or two.

When someone held up a slab of wet rock, Ruffalo noticed a cluster of glistening globules clinging to the surface, correctly identifying them as egg sacs from some aquatic creature.

Several years ago, Ruffalo founded his own group, Water Defense, in part because he was concerned about the impact of natural-gas drilling and fracking.

"What people really don't understand is that what's happening here is happening downstream," said Ruffalo, who has a house on the upper Delaware in upstate New York. "The systems that we live in are all interconnected."

The four students were participants in a program called Women in Natural Sciences, an enrichment program run by the Academy of Natural Sciences.

The students said they did not know in advance that Ruffalo was coming. With little prompting, they rattled off several of his movies, such as the various Avengers titles, in which Ruffalo plays the scientist who turns into the Hulk. Talk about being "green."

"I found it really inspiring that he's an activist," said Linda Gutierrez, 16, an 11th grader at Academy at Palumbo in South Philadelphia.

Ruffalo made a passing reference to Hollywood when someone showed him an insect called a caddisfly, a predator with strong, well-developed jaws.

"It's like my business," he quipped.

Though the mayfly in the stream was a good sign, the state of the Tookany Creek is not ideal, said Julie Slavet, executive director of the watershed partnership.

Two-thirds of its water should come from groundwater, and the rest from runoff. But because of all the paved surfaces along its length, "the percentages have been switched," she said. Heavy rains quickly turn the stream into a torrent, roiling with chemicals and sediment.

Across the Delaware basin, the U.S. Environmental Protection Agency has listed 12 percent of the watershed's 23,000 stream miles as "impaired."

So the foundation's funds are going toward land preservation and planting forested buffer zones, among other measures. Data collection is a big part of it, sampling the water chemistry and also scouting for creatures that serve as living barometers of stream health.

Scientists from the Academy of Natural Sciences and the Stroud Water Research Center measure water quality periodically at dozens of sites throughout the basin.

But there is only so much they can do, so they rely on helpers such as the Tookany/Tacony-Frankford group to cover a wider area, said Kathryn Christopher, a staff scientist at the academy.

"We can't be everywhere in the basin," she said. "We really value our partners."

Martinez was eager to help. Later she identified a creature called a midge, and immediately asked:

"What's the sensitivity for the midge?"

She was told the insect tolerates polluted water, and thus was not useful for indicating a healthy stream.

So it was back to the net, one dip in the water at a time.