Thousands of dead fish were found floating in the Cuyahoga River in Cleveland and lining the river banks, officials said. "I've seen large clumps of them floating down the river for the last three days," T.J. Towner, who works in the area of the city known as The Flats, said in a (Cleveland) Plain Dealer report Monday. The cause of the fish kill -- with many of the fish showing red bruise-like marks -- was not immediately known, but Northeast Ohio Regional Sewer District spokeswoman Jean Chapman said kills of Gizzard Shad are common this time of year. Among other things, fish may suffer thermal shock when they suddenly go from cold to warmer water, she said. Weather changes and warm effluents being released into the river can be factors. "It's naturally occurring at this time of year," Chapman said. "But we won't know for sure until someone looks at it." The state Environmental Protection Agency and the Department of Natural Resources were notified of the fish kill, the newspaper reported.