The result is a churning mass of ugly fish, some of them as large as 40 to 50 pounds.
The most unforgettable element is the hollow, sucking sound created by hundreds of carp, O-shaped mouths with yellow lips wide open, at the water's surface in the horseshoe-shaped spillway.
It's a nightmarish sound that makes the whole experience more than a little surreal.
It's been a summer tradition at Pymatuning since the 1930s, and it thrives despite very little promotion. It gets one sentence in the Pennsylvania state park brochure.
Between 300,000 and 400,000 visitors a year come to feed the carp, says Pennsylvania state park manager Pete Houghton.
That makes Pymatuning one of the biggest tourist attractions in western Pennsylvania.
It's been said that the ducks walk across the backs of the hungry fish to compete for chunks of bread off the spillway at the lake's northeast corner.
In fact, the community of Linesville calls itself the place "Where the Ducks Walk on the Fish," although it's a sight rarely seen. You can buy T-shirts touting the Duck Walk or another reading "Carpe Feed'm."
Day-old white bread is available from a concessionaire at the spillway for $1. Many fish feeders arrive with dozens of loaves.
What you offer the fish doesn't seem to matter. They will eat just about anything: loaves, bagels, muffins, dinner rolls.
The carp, bottom dwellers that normally feed largely on zooplankton, gather at the spillway where the water from one section of the lake drops into another in Pennsylvania's Crawford County.
In 2008, Pennsylvania had proposed a ban on feeding bread to the carp, saying it was boosting phosphorus levels in the lake. It proposed feeding the carp healthier pellets instead. But a public uproar ended that possibility.
The horseshoe-shaped 17-mile-long lake with 77 miles of shoreline sits on the state border. The two Pymatuning State Parks - one in Pennsylvania, one in Ohio - together draw about 600,000 visitors a year and an estimated 1 million visitors cross the causeway that connects the two states.
The Ohio park on the lake's western shore covers 3,512 acres in Ashtabula County.
The Pennsylvania park covers 21,122, acres including its portion of the 17,088-acre lake.
Both parks cater to anglers, campers, and boaters; their pontoon boats are especially popular.
Walleye is the big attraction for anglers, with 50,000 a year being pulled from Pymatuning Reservoir. They average 16 inches and some are as large as 10 pounds. May and June are big angling months before the lake warms up.
Other fish are muskellunge, crappie, perch, bluegills, largemouth and smallmouth bass, and channel catfish.
Pennsylvania offers three marinas where float boats, motorboats, rowboats, canoes, and motors can be rented.
Pymatuning features 423 sites in the Jamestown and Linesville campgrounds. The Tuttle campground with 190 sites is closed because of budget cuts.
Campers can reserve spots in the Jamestown campgrounds. Call 1-888-7272.
Pennsylvania also offers 25 rental cabins and 11 picnic pavilions. Its four swimming beaches are open from Memorial Day through Labor Day.