Lincoln, Neb. — Thousands of fish are dying in the Midwest as the hot, dry summer dries up rivers and causes water temperatures to climb in some spots to nearly 100 degrees.

About 40,000 shovelnose sturgeon were killed in Iowa last week as water temperatures reached 97 degrees. Nebraska fishery officials said they’ve seen thousands of dead sturgeon, catfish, carp, and other species in the Lower Platte River, including the endangered pallid sturgeon. And biologists in Illinois said the hot weather has killed tens of thousands of large- and smallmouth bass and channel catfish and is threatening the population of the greater redhorse fish, a state-endangered species.

So many fish died in one Illinois lake that the carcasses clogged an intake screen near a power plant, lowering water levels to the point that the station had to shut down one of its generators.

“It’s something I’ve never seen in my career, and I’ve been here for more than 17 years,” said Mark Flammang, a fisheries biologist with the Iowa Department of Natural Resources. “I think what we’re mainly dealing with here are the extremely low flows and this unparalleled heat.”

The fish are victims of one of the driest and warmest summers in history. The federal U.S. Drought Monitor shows nearly two-thirds of the lower 48 states are experiencing some form of drought, and the Department of Agriculture has declared more than half of the nation’s counties — nearly 1,600 in 32 states — as natural disaster areas. More than 3,000 heat records were broken over the last month.

Iowa DNR officials said the sturgeon found dead in the Des Moines River were worth nearly $10 million, a high value based in part on their highly sought eggs, which are used for caviar. The fish are valued at more than $110 a pound.

Gavin Gibbons, a spokesman for the National Fisheries Institute, said the sturgeon kills don’t appear to have reduced the supply enough to hurt regional caviar suppliers.

Flammang said weekend rain improved some of Iowa’s rivers and lakes, but temperatures were rising again and straining a sturgeon population that develops health problems when water temperatures climb into the 80s.

“Those fish have been in these rivers for thousands of thousands of years, and they’re accustomed to all sorts of weather conditions,” he said. “But sometimes, you have conditions occur that are outside their realm of tolerance.”

In Illinois, heat and lack of rain has dried up a large swath of Aux Sable Creek, the state’s largest habitat for the endangered greater redhorse, a large bottom-feeding fish, said Dan Stephenson, a biologist with the Illinois Department of Natural Resources.

“We’re talking hundreds of thousands (killed), maybe millions by now,” Stephenson said. “If you’re only talking about game fish, it’s probably in the thousands. But for all fish, it’s probably in the millions if you look statewide.”