NORTH MAYFAIR — The stuff that comes out of the Chicago River can be pretty revealing.

Each year, on Chicago River Day, volunteers across the city fish garbage and miscellany out of the murky waters of the city's river branches.

A common find at Gompers Park Saturday - the 21st Chicago River Day - was underpants.

"We found a whole outfit," said Zosia, 11, a student at Chicago Grammar School, who volunteered with schoolmates Alice, 11, and Aveiyah, 13.

"There was a jacket, some jeans, three pairs of socks, some underwear, a used diaper," Aveiyah said.

You've been warned: It gets more unseemly than used diapers.

"We found a whole plate of food with rice and chicken and a dead bird," Aveiyah said.

Half of a plastic Adirondack chair sits with other garbage pulled from the Chicago River by volunteers on Saturday at Gompers Park. View Full Caption DNAinfo/Kyla Gardner

"It was gross," the girls agreed, wrinkling their noses.

About 50 volunteers laid mulch, tore down dead vegetation and picked up trash at the park on the Northwest Side from 9 a.m. to noon.

"You might find sleeping bags, tires...laundry baskets," said Larry Kaplan, a member of the board of directors for Friends of the Chicago River, which hosted the event. "Use your imagination."

Veteran volunteer Greg Pranski, 66, doesn't have the luxury of only imagining what could be in the river's murky waters.

"There's always the unusual things that you wish weren't there," he said. "Sometimes you find things you really don't want to see."

Pranski declined to name the more grotesque things he's seen floating in the water, but said he found tennis balls, golf balls, a shopping cart, and, of course, underwear, on Saturday.

Volunteers also discovered a mattress, another common underwater item from year to year.

"Talk about getting it out," Pranski said. "Have you ever tried to move a wet mattress?"

Huge items, waterlogged and bogged down by silt, often prove a weighty task requiring multiple volunteers to drag them out.

A utility sink — the two-basin kind you'd find in a someone's unfinished basement — had to be rolled out of the water one year, said Tony Watrobinski, 66, of the North Mayfair Improvement Association.

"You name it, we find it," said Ron Valvick, 52, who is often knee-deep in the river as part of the team hauling out the heaviest finds.

The Portage Park resident has found bicycles, "parts of grills in trees," and what volunteers believed to be a "half" couch until they tried to pull it from the water.

"It turned out to be a long couch," Valvick said.

In 2012, there was the infamous "tiny, evil chair."

"We pulled this really nice executive office chair out of the Chicago River," Watrobinski said. The men used a grappling hook and rope to it get out the chair, which was half-floating because of its plushy cushions.

But you never keep anything you find in the Chicago River, Valvick advised, even if it's nice, like some of the bicycle tire rims he's found.

"Who knows what other little diseases are on them?" he said.

The lush executive office chair would go no further than the river's edge.

"Once we got it halfway up on the bank, we sliced it up," Watrobinski said.

When Watrobinski tells the story of the evil chair, people wonder, who would do something like that?

"I said, well, you know, you get drunk and you do things," he said.

Lazy litterers, strong winds and a nearby homeless population were also offered up as suspects for the river's overwhelming amounts of garbage.

Saturday, Watrobinski estimated the crew hauled in about 1,000 pounds of refuse.

Walking along the freshly-mulched path, he pointed to a West Foster Avenue overpass, from where he imagines someone hurled the chair into the river.

Nearly-record-breaking flooding in April caused some complications for this year's river cleanup. A fence separating the river from the rest of Gompers Park was full of leaves and trash from the overflow.

Watrobinski stopped to poke the brush through with the end of a broom.

"That river, when it gets mean and nasty, it's mean and nasty," he said.

But still — Watrobinski is a Friend of the Chicago River.

'What makes someone want to pick up someone else's garbage?" he said, walking with his broom down the path, still working, even after an exhausting, mattress-filled morning. "We want our Gompers Park to look beautiful."