Bill Prater watched it coming for months. From his home a few hundred yards away, the past president of the Loveland Fishing Club saw Horseshoe Lake, a privately owned reservoir north of Denver, shrink through the summer into a patchwork of isolated pools and mud flats.
So when the fish began dying last weekend and floating through the inlet into neighbouring Boyd Lake, he was not surprised.
"The lake is essentially emptied," Prater told the Loveland Reporter-Herald. "And that's when the die-off really took hold."
Colorado Parks and Wildlife estimates thousands of fish died in Horseshoe before drifting into the Boyd Lake channel. Most were common carp, with smaller numbers of walleye, white bass, shad and catfish. Photographs of carcasses stacked along the shoreline spread quickly on social media, and drew a stream of visitors to a state park inlet that now smells of decomposition.
CPW has been careful about where the blame does and does not sit. Horseshoe Lake is private, so the agency neither manages nor stocks its fishery and did not investigate the conditions behind the die-off. Boyd Lake's own fishery, the agency says, is intact.
"These are not fish that were from Boyd Lake that had been living there before. They're all coming over from Horseshoe," CPW public information officer Kara Van Hoose told KDVR. "There's just dead fish that flowed into the inlet," she said. "There's not a lot you can really do."
Van Hoose also noted the species mix shaped the response. "Because it's more overwhelmingly common carp, it's a species that is not something that's really high on the CPW priority list, but certainly that we would still not want to see deceased." Crews salvaged surviving fish from the inlet and moved them to another water body that already held carp. The remaining carcasses will largely be left where they are.
The Greeley-Loveland Irrigation Company, which manages water levels across the system connecting the two lakes, said Horseshoe did not fill this year because of low snowpack, and that water was moved out of the reservoir after shareholders requested their deliveries. The company could not immediately be reached for comment by the Reporter-Herald.
The sharpest disagreement is over whether the fish could have been saved. AJ Macdonald, president of the Horseshoe Lake homeowners association board, provided an email the board sent to residents. According to that email, the board sought help after learning the irrigation company "intended to draw the lake to such a low level," approached both the ditch company and CPW, and concluded that "saving our fish was not a high priority." The board then went to the fish farm that had stocked the lake and drew up a plan to relocate the fish and restock the following year, applying to the state for the permit needed to do it legally.
"The owner of the fish farm was assured by the state that our permit would be approved last week and that they could start moving fish immediately," the email said. "No action has been taken as our permit has still not been issued. Unfortunately, now all our fish have died."
For residents, the water level was the story long before the fish were. Mike Davis, who has lived in northern Colorado for decades and whose in-laws bought a house nearby in 1978, watched the reservoir drop through the season.
"It just the level just kept going down further and further and further and further, and I was like, wow, this is really bad," Davis said. What stayed with him was what came next. "It got so low that I could see the fish just flopping around and they're out along in the mud, and they're just stuck there. They're dying, you know, and it just broke my heart to see that."
Davis is not disputing anyone's water rights. "I get that shareholders have a right to their water and they want to use it for irrigation. I get that, and I appreciate it," he said. "But we don't have to dump it out so fast, so quick, that it causes the loss of life of an animal."
The exact cause has not been formally determined, and Chris Myrick, a Colorado State University professor of fisheries and aquatic sciences, cautioned that confirming a fish kill normally requires temperature and dissolved-oxygen readings taken on site, plus testing for chemicals and toxic algae. He did not inspect Horseshoe Lake. But the physics of a shrinking, baking reservoir are not in dispute.
"The hotter the water is, the less oxygen it naturally holds, and, at the same time, the hotter the water is, the more oxygen fish use," Myrick said. "So it's a double whammy." Concentrate the fish into a fraction of the original volume and the maths gets worse fast. "You have a limited amount of water, limited amount of oxygen, fish that are using a lot of oxygen even when they're resting. Then you just run out of oxygen."
Myrick is not treating this as an isolated event. "I think this year it's been exacerbated by starting off with relatively low water levels in some of the reservoirs," he said. "And then high temperatures just make it worse." Fish kills and emergency salvages have been running well above normal across Colorado this summer.
The carcasses themselves pose little risk to Boyd Lake's surviving fish, according to Myrick - unless a toxin was involved, dead fish are usually left in place for scavengers, returning nutrients to the system. CPW still advises caution for anyone handling or harvesting fish in the area.
That has not made the inlet pleasant. Herons and gulls have moved in on the free food, and Prater says local fishing spots are showing signs of stressed fish.
"This might not be the end of it," he said.