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© J. Miles Cary/Knoxville News Sentinel/APCrews using rakes and an industrial vacuum remove dead fish from Butterfly Lake in Knoxville, Tenn., on Monday. Some 10,000 bluegills died.
In lakes and rivers across parched areas of the U.S., heat and lower water levels are reducing oxygen levels -- and killing fish populations by the thousands.

At one lake in Delaware, up to 6,000 dead gizzard shad and 600 perch were found floating this week.

"Aggravating this summertime problem, increased temperatures lead to warmer water, which holds less dissolved oxygen," state natural resources spokesman John Clark told NBCPhiladelphia.com.

In South Carolina, some 500 fish died at Lake Hartwell.

"It started Sunday afternoon," local resident Brandi Pierce told NBC affiliate WYFF-TV. "We started seeing ten fish popping up out of the water. Then Monday, it was full."

Across South Dakota, fishermen have reported thousands of fish kills in multiple lakes and rivers.

And in Tennessee, a fish kill on Butterfly Lake left a horrid stench in one Knoxville neighborhood.

"It's really putrid," Paula Gumpman, president of the local neighborhood association, told the Knoxville News Sentinel. "It's like after a hurricane. Gooky and yucky."

Some 10,000 bluegills were thought to have died, and city workers were tasked with the cleanup even though the lake is on private property.

"It's a public health issue," said Public Service Director David Brace, "and it just smells real bad."