Often vocal and occasionally strident - even those who agree with her wince at times - van Rossum has a strict ethic: "We will not make the needs of the river and its communities subservient to the needs or goals of someone or something else."
That includes big government, big pharma, big power, big oil and gas - some of the biggest interests in the country.
The network challenged the deepening of the river channel and lost. But van Rossum sometimes wins. And sometimes wins big.
The network helped scuttle a proposal to release treated VX nerve agent into the river. Another action prompted federal standards to prevent fish from being sucked into industrial cooling water intakes. A suit against the U.S. Environmental Protection Agency resulted in pollution thresholds and allocations that ensure water quality far beyond the Delaware.
Robert Tudor, deputy executive director of the Delaware River Basin Commission, which oversees the watershed, credits a network legal petition with leading to special protections for much of the river above Trenton.
Years later, they are part of the debate over natural gas development in the watershed, where a moratorium is still in place.
The network fought parts of Pennsylvania's oil and natural gas act. In December, the state Supreme Court reinstated municipal authority over gas drilling locations and reaffirmed fundamental rights to a clean environment.
"They have made so much of a difference," said James May, codirector of Widener University's Environmental Law Center. "What a dividend to the future, and it didn't cost the taxpayer a dime."
Of course, the tab for opponents is hefty.
Travis Windle, a spokesman for Pennsylvania's Marcellus Shale Coalition, an industry group, declined to talk about van Rossum's network, saying a better Earth Day story would discuss the benefits of natural gas. Eventually, he said "a small group of activists" was wrongly disputing the facts.
Van Rossum, 48, says injustice - to the river and its communities - is her motivation. A Radnor High School graduate, she is the daughter of university professors; her father was a biochemist, her mother a mathematician. She and her second husband, David Wood, a teacher at the high school, have six children combined.
The Delaware Riverkeeper Network began in 1988, soon after Robert F. Kennedy Jr. founded Hudson Riverkeeper.
There are now 218 keepers - of bays, rivers, sounds - in 24 countries, said Marc Yaggi, executive director of the Waterkeeper Alliance. He credited successes of the Delaware River group for inspiring "hundreds of advocates . . . to be the eyes and ears and voice of their waterway."
Van Rossum joined in 1994 and was named riverkeeper two years later. She had been working in the Widener University law clinic, and May recommended her. "She's smart, she's passionate. She's a difference-maker," he said.
Petite and blue-eyed, she's also been called a cross between Tinkerbell and Jesse Ventura.
When she took over the organization, fewer than 100 of its 3,000 members paid dues. So she decided people didn't need to pay "to have a voice in the river"; now, writing a letter can serve as dues. The network has 14,000 members; its budget doubled, to $2 million, from 2009 to 2012.
Andrew Johnson is a senior program officer at the William Penn Foundation, which contributes an average $250,000 a year. He said the foundation has "consistently been impressed" by the organization's "traction at the grassroots level."
Other groups share a primary interest in the river; some focus on science, others on cooperation. "Maya has different approaches . . . that's good," said Jennifer Adkins, executive director of the Partnership for the Delaware Estuary.
The Riverkeeper Network, like others, does habitat restoration and water testing. What sets it apart is the focus on litigation.
"There's only so much you can do with advocacy," van Rossum said. When the law is ignored or misinterpreted, "if you don't have the ability to go to court, you're stuck. Simply because you don't have the resources, you lose. And I guarantee you, industries and the agencies do have the resources."
In 2002, the network was involved in fewer than a dozen legal actions. At the end of 2013, more than three dozen were in process or being seriously considered - involving pipelines, water withdrawals, septic system regulations, wetlands encroachment, storm water permits.
Its legal staff grew from none to four - not counting van Rossum, who has a law degree and a master of law in corporate finance (and is a member of the bar in New Jersey, Pennsylvania, and the District of Columbia).
Forget that "at your peril," said Chris Day, an EPA attorney in Philadelphia, who has argued both for and against her. "Yes, she can push your limits sometimes, but she does know how to make a constructive settlement."
Inevitably, van Rossum is accused of blocking economic development.
She insists she's not against projects per se. It's about how they're done - whether they'll flood communities or kill fish.
"People of my ilk," she said, "are the best advocates of economic development, because we're insisting on healthy economic development that benefits everyone, not damaging economic development that benefits . . . the corporate presidents and the polluters."
Not long ago, after a river basin commission meeting in Bucks County, van Rossum excused herself from a conversation and headed for the riverbank.
She often does, dipping her hand in the water to reflect. If the river could speak, what would it say?
In the final analysis, she said, her work is "about giving the river a voice."