Carelessly tossed plastic water bottles eventually go somewhere, and that somewhere might be a hidden cove off Plum Point on the Delaware River, a few miles north of Center City, that snags mighty amounts of trash as it flows downstream.

Enter Jay Kelly's class, determined not only to clean up the mess on the Jersey side of the Delaware, but also to log each and every one of the 7,917 pieces of debris found Saturday morning before the 60-mile bus ride back to Raritan Valley Community College.

The students' mission, in collaboration with the nonprofit Clean Ocean Action, was scientific: Where does all the trash come from? What types of debris are most common? The garbage not only is ugly, it also may be swallowed by animals, killing or injuring them. And some chemicals, scientists fear, can end up in the food chain, contaminating other creatures.

On Saturday, the class from Somerset County, N.J., found so many plastic water and soda bottles, it was impossible to take a step in a 75-foot length of the cove's shoreline without the crinkling, crunching sound of collapsing plastic. In all, the students collected nearly 4,000 plastic bottles.

But the bottles were just the start. Three dozen car and truck tires were wedged in the muck so densely they looked like headstones poking out of a graveyard. A rubber traffic cone stuck up like a warning. Two giant green plastic barrels, a playground slide, assorted combs, and a child's tricycle were among trash snarled in nearby brush.

Kelly, an associate professor in the school's science and engineering department, has brought his students to Taylor Wildlife Preserve in Burlington County the last two years to help clean up the riverbanks, just across the river from Maggie's Waterfront Cafe in Northeast Philadelphia.

And the problem isn't just at Plum Point. Kelly has surveyed 47 miles of the riverbank by canoe from Bordentown to West Deptford, taking measurements of debris fields he finds. He calculates about 13 percent of the shoreline is marred by trash. That's about six miles and 2.8 million pieces of trash.

"I study rare plant species in the summertime," Kelly said. "About five or six years ago, I went out to look at the tidal Delaware River. I immediately stumbled on these incredible areas of trash and was just shocked and appalled. It's a hidden problem."

Kelly's students spend hours categorizing each piece of junk as foam, plastic, metal, wood, rubber, cloth or paper. He coordinates the effort with the nonprofit Clean Ocean Action.

It's hard to reach a lot of the trash. Beaches might be rimmed by thick woods and muck, or they are on private property. Taylor Wildlife Preserve offers Kelly's class perfect access.

"The idea is to show students the issues here on the ground that you can't show in a classroom," Kelly said as the group of 40 environmental science students and volunteers from Clean Ocean Action walked to the banks.

They were guided to the site by Peter Taylor, whose family has farmed the property since the 1770s. The family donated an 89-acre easement of marsh, forest, and farmland along the river to the New Jersey Natural Land Trust as a wildlife preserve.

"All this debris is washed up from the Delaware River over the years," said Taylor, who helped with the cleanup. "All the plastic bottles and everything that floats comes in on high tide and then it stays."

Student Rebecca Boer, 20, said the amount of plastics made her stop and think.

"This is the only planet we have," she said as she stopped to grab a piece of trash and make sure it was logged in.

Nearby, Matt Zalewski, also 20, was repeating the same process.

"It's awful to think that everything we do has such an impact," he said of the carelessly discarded plastic bottles.

Clean Ocean Action's Mae Henry said the group covered about 500 feet of shoreline Saturday and logged 7,917 pieces of trash. About 90 percent of the debris was plastic. In the fall, the same group collected 5,305 pieces of trash at the preserve.

Even after 2½ hours of work, the group could only pick up most of the larger items. The plastics were placed in large bags provided by TerraCycle, a Trenton company that specializes in hard-to-recycle items, such as severely degraded plastics.

Rudy Sanfilippo, TerraCycle's manager of partnerships, said much of the plastic picked up in Cinnaminson could end up as packaging for new Procter & Gamble products.

Items that couldn't be recycled were placed in a dumpster provided by the New Jersey Lands Trust.

Left behind were the little bits and pieces that scientists call microplastics, left from decades of decomposing trash and small enough for animals to eat, Henry said.

"We see little fish eating plastics and bigger fish eating the little fish," she said. "So it ends up in the food chain. And the little plastics release toxins."

In just one nine-square-foot area, she said, the group found 152 drinking straws.

"A lot of people are not aware of the problem along the river," said Swarna Muthukrishnan, a scientist with Clean Ocean Action.  "If you got to the ocean beaches, it's really visible, so people take notice. But lots of these river shorelines don't get visited."

Kelly doesn't think people are hurling their unwanted water bottles directly into the river. Rather, he thinks that trash tossed along roadways gets into storm drainage systems that are then overwhelmed during heavy rain, or melt from snowstorms.

"It seems to me that most of this trash is coming from upstream during major storm events," Kelly said. Coves such as Plum Point catch and trap the debris.

He believes booms could be set up along the river above covers to funnel the debris into one area where it could be scooped out by heavy machinery. But, for now, he said, that's just a wish.

The more practical solution: Picking up and properly disposing of trash before it becomes a college class project.