But Kreeger, who was out on the river in a boat, got hot. Putting on her mask and snorkel, she slipped into the river and swam through the murky water toward the bottom.
Suddenly, she saw them. The riverbed was studded with mussels. They weren't the edible kind, but it was better still - a seven-species mother lode including two species thought to be locally extinct. One, the tidewater mucket, hasn't been seen in this area for more than half a century. The discovery bodes well for the mussels and the river itself.
"I stayed underwater for quite a while, sort of not believing my eyes," she said. Then she got busy. She had a mesh bag, and between gulps of air, she began stuffing it with specimens.
Many species are difficult to differentiate, so they were sent to a U.S. Geological Survey lab, where recently their identities were confirmed.
Kreeger won't be more specific about where the mussels are because she wants to protect them. Conceivably, they could influence development in that part of the river.
Historically, at least a dozen species of freshwater mussels were known in this region. Their folksy common names - alewife floater and squawfoot, for instance - hint at their importance to earlier cultures.
But in a decade of searching, Kreeger has been hard pressed to find more than one species in smaller streams.
In only two Pennsylvania streams south of the Schuylkill - the Brandywine and Ridley Creeks - has she found any at all.
"All of a sudden, it's far more interesting than we could have imagined," said Roger Thomas, an Academy of Natural Sciences fisheries biologist who also was in the boat that day.
Nationally, nearly three-quarters of the nation's 300 native freshwater mussel species are in decline, said Catherine Gatenby, manager of the U.S. Fish and Wildlife Service's White Sulphur Springs National Fish Hatchery in West Virginia, where mussel research is being conducted. About 70 species are listed by the service as endangered.
Scientists say they are one of the most imperiled taxonomic groups in the nation.
Mussels are important because they filter sediment, nutrients, and other contaminants, making conditions better for fish and aquatic life.
A 2006 study of a mussel bed in a six-mile stretch of Brandywine Creek showed that the animals there - half a million, not a large number when it comes to mussels - removed 25 tons of suspended solids a year.
Freshwater mussels also are good indicators of problems within a stream.
Mussels can live to 80 years or more. But once embedded in a streambed they can't move, which isn't exactly an ideal survival strategy. If the females were to simply release their larvae, the current would carry the young downstream. Eventually, the population would disappear.
Mussels get around the problem by using eels and fish to hitch rides upstream.
They lure the fish with body parts that resemble small fish. When a big fish comes close - in search of dinner - the female releases larvae, which then attach themselves to the fish.
Some fish will swim downstream. But others will swim upstream, repopulating it with mussels when the larvae mature and drop off.
All this is put in jeopardy when a stream is dammed and fish movements are restricted. Or when habitat or water-quality degradation affects the fish population. Many mussels depend on a specific host fish species.
"They are an ideal sentinel organism, reflecting generalized water quality within a watershed," Thomas said.
Which makes it all the more strange that the mussels were found in such an urban spot.
"It's paradoxical that within footsteps of Philadelphia, this urban center for 400 years, you have the last remaining biodiversity of this species in the whole basin," Kreeger said. "You would expect them to be hanging out in streams and pristine areas. But they're gone from the streams."
Stranger still, the mussel bed she found is apparently not a remnant population, merely hanging on. There were both young animals and some as old as 80, a sign of vibrancy.
"They've been there all along, weathering all the difficult issues on the Delaware," Kreeger said, adding that "it may mean the river is not as bad as some people think it is" along that stretch.
The find is important for another reason.
Since 2007, scientists have been developing a mussel reintroduction program, funded by ConocoPhillips, the National Fish and Wildlife Foundation, and the Pennsylvania Coastal Resources Management Program.
Steven Hughes, associate professor and director of aquaculture at Cheyney University, and a small core of students have been able to breed the region's one remaining common mussel species, the eastern elliptio, in a lab. It's the only species still commonly found in this region.
They collect larvae from adult females with a syringe, then mix it with water in a bucket that's holding an American eel or lake trout.
They've been able to get the larvae to attach and grow for several weeks. But once the larvae change into tiny mussels and drop off, "they just seem to wane," Hughes said.
This year, he plans to tweak the water chemistry and feed different kinds of algae.
The Delaware River find potentially adds six more species that can be propagated at Cheyney.
Meanwhile, scientists have been testing streams to see where conditions are most hospitable. They put adult mussels into cages and sink them in the water for a year.
If they die or lose fitness, that waterway would not be targeted for restoration. So far, Chester Creek and the White Clay Creek are emerging as good candidates.
But mussels also can be the driver for cleaning up a whole system.
"If you want them, it forces you to do everything else, to take out the dams and improve the water quality and the habitat," Kreeger said.
In the end, the mussels will help finish the job.
The newly discovered mussels may have one final benefit. They may solve a persistent mystery.
Scientists have long known that nutrients washing downstream in the Delaware River seem to lessen in the area below Trenton. But why?
An academy researcher assessed wetlands fringing the river to see if they were absorbing the nutrients, but that couldn't account for the volume that went missing.
If subsequent research shows that the newly discovered mussel beds are large enough, Kreeger and others think they just might be the ones responsible for cleaning up this stretch of the river.
She hopes to be back out in the water come summer.
Contact staff writer Sandy Bauers at 215-854-5147 or firstname.lastname@example.org Visit her blog at http://go.philly.com/greenspace