More than 5,600 fish were killed in the Big Thompson River in early March. Colorado Parks and Wildlife thinks the die-off is connected to concrete chemicals entering the north fork of the Big Thompson at this construction site near Drake, Colorado.

 

(Grace Hood/CPR News)

A biological mystery landed at the doorstep of Colorado Parks and Wildlife officials this March. Why did more than 5,600 fish die in the Big Thompson River in northern Colorado?

On the Lower North Fork of the Big Thompson River, 100 percent of the fish died. About 50 percent of fish died in an eight-mile stretch of the lower river between Drake and Loveland. 

Larry Rogstad, area widlife manager for Colorado Parks and Wildlife, said there was no clear-cut answer back when people noticed the dead fish two months ago. After painstaking research by his team, they say a cement pour appears to be the culprit. And a berm set up by construction contractors failed to catch it.

"Somehow during the process at this location there was a batch of chemical that went into the water and caused a pH jump, and that’s what caused the tragedy," said Rogstad.

Colorado Parks and Wildlife’s Larry Rogstad is overseeing an investigation in the Big Thompson Canyon where 5,600 fish died in the river.

(Grace Hood/CPR News)

The fish kill started at a bridge replacement site on County Road 43 near Drake. American Civil Constructors (ACC) was doing cement work there. 

"We had done 17 other abutments prior in the same manner without issue," said ACC Project Manager Travis Madsen. He explained that crews have been in the area since 2014. The goal all along has been to rework stream channels and improve the Larimer County road—both of which were severely damaged by the floods. 

"That's what was a bit of an anomaly for everybody involved," Madsen said.  "We didn’t know exactly what happened because we hadn’t done anything different than what had been done 17 times previous."

The results of the investigation, which began as a criminal probe, and penalties are expected in the coming months. The fine could be as high as $200,000, although Rogstad says it's no longer a criminal investigation. The goal now is to recover the loss.

But river watchers say the fish kill draws attention to a larger issue: Fish counts and habitat along the Big Thompson are far from restored after the 2013 floods.  

A Slow Recovery Post Flood

The last three years haven’t been kind to the Big Thompson River. In 2013, a torrent of flood water left tons of debris, household items like refrigerators and tree limbs lodged in the stream. Riparian areas and the riverbanks were also decimated—the floods swept away willows on the streambanks.

"The Thompson River has had a lot of hard knocks lately," said Will Huett, president of the Rocky Mountain Flycasters—a Colorado chapter of Trout Unlimited. "Some flood and some people errors that have created some real problems that we need to overcome and prevent in the future."

Trout Unlimited was critical of the fish kill. But Huett also said he understands the larger complexity and fine intricacies involved in construction work and river restoration. 

 "We can plant all the willows we want but they won’t grow any faster than nature will let them grow."

Rocky Mountain Flycasters has worked on projects to restore the river since the 2013 floods. The work is about to get more complicated this year.  The Colorado Department of Transportation has launched a three-year project to rebuild U.S. Highway 34 between Loveland and Estes Park. The idea is to make the road more resilient to floods in future decades. 

Rebuilding A Healthier River

Drive up the Big Thompson Canyon today and you see signs of change. There are piles of tree branches and logs throughout the canyon that will eventually be added back in to the river.

Parks and Wildlife’s Larry Rogstad said that when the tree limbs are added to the river, they will attract bugs, and bugs attract more fish. In places like The Narrows, the river needs to be enhanced from what's essentially a ditch, to a multi-stage channel with different stream flows for healthier fish habitat.

In places like Narrows Park, the river needs to be enhanced from what's essentially become a ditch post flood to a multi-stage channel for healthier fish habitat.

(Grace Hood/CPR News)

One of the biggest changes this year will happen at a place known as the Horseshoe, near the site of the Big Thompson Indian Village Store. The goal is to widen the channel of the river and re-reroute the road. 

Rodstad said road planners have noticed that both the 1976 Big Thompson Flood and the 2013 event decimated this tight turn in the canyon.

"You have an outside curve on the river there. And so when the river comes up, it just historically washes this road out totally," he said.

The road re-route will require building two bridges and blasting significant chunks of rock from the mountainside. It won’t be without its own challenges. Right now CDOT is working with dozens of partners on the project to review any wildlife and aquatic concerns. 

For fish, Rogstad says the laundry list of concerns includes making sure trout can move up and down stream during construction. What happens if blasted rock falls into the stream? What piece of heavy equipment to you use to retrieve it?

"Maybe it’s using a long reach excavator instead of putting a bulldozer in the creek itself. All those things have to be thought about, discussed, and cussed, and finally negotiated out," he said.

The clock is ticking to determine exactly what the details are. CDOT expects work to begin this summer.