Thousands of fish die in Colorado, amid flood recovery projects
This story first appeared on High Country News.


In early March, a resident of the small Colorado towns of Drake and Glen Haven — situated within northern Colorado’s Big Thompson River Canyon — reported noticing funky gray water in a side creek of the river and a murder of crows picking at a few dead fish. A few days later, March 7, a large plume of more cloudy water ran down the Big Thompson, leaving behind a massive fish kill. Colorado Parks and Wildlife officials now confirm that more than 5,600 fish, mostly rainbow and brown trout, died in the Big Thompson and its North Fork, and are blaming concrete from a bridge reconstruction project, part of the state’s massive recovery and reconstruction effort following the devastating September 2013 floods.

The die-off is alarming news for the Big Thompson, a popular fly fishing river among tourists and locals, which formerly generated an annual $4.3 million for the region. Larry Rogstad, Colorado Parks and Wildlife Area Manager, says the “iconic” fishery is also important as one of the only rivers in Colorado with wild rainbow trout free of whirling disease. The 2013 floods had already knocked back the river’s fish populations, and Rogstad estimates the recent incident killed more than half of the estimated fish within an eight-mile-long downstream stretch of river.

Cars drive on County Road 43, in Larimer County, Colorado, where a road reconstruction project to ameliorate some of the damage done in the 2013 floods is underway. The project involves rock blasting, rebuilding bridges and embankment armoring, and then restoring the Big Thompson River where it’s been impacted by construction. Some of the bridge-building may have contributed to a recent fish die-off.
Larimer Country Road 43 Public Infrastructure Project

Running from Rocky Mountain National Park and the tourist town of Estes Park to the city of Loveland, the tightly constrained Big Thompson Canyon includes the river, a state highway, summer homes, tourist stops, and campgrounds, all vulnerable to floods. The notorious 1976 flood killed 145 people, destroyed more than 400 homes and caused $40 million-plus in damages. The 2013 flood wasn’t as deadly but still catastrophic, as rising waters annihilated a county road along the North Fork of the Big Thompson, which provides access for Drake and Glen Haven residents. Locals needed helicopter evacuations to escape the disaster.

In the aftermath of the floods, the state, the Federal Highway Administration, and other partners are still rebuilding, including reconstructing roads within the Big Thompson Canyon. Along the side canyon county road, American Civil Constructors (ACC), a contractor for the highway administration’s Central Federal Lands Highway Division, is building 11 bridge crossings. In the narrow canyon, that means running heavy equipment directly in river channels and pouring concrete abutments for the bridges. Crews are also pouring concrete and grouting rock into riverbanks to stabilize channels in some areas to withstand future flooding.

An excavator cleans up rubble from rock blasting as part of the reconstruction of County Road 43, which runs along the Big Thompson River.

Eight previous bridge installations went smoothly, says Travis Madsen, an ACC project manager. But something went wrong in March at the site where the North Fork meets the main river. According to the state, “it appears the event was associated with concrete work” building grouted rock walls and replacing a nearby road bridge. Madsen says that the site geology may have played a role.

Concrete can include toxic compounds and is very alkaline — which can be lethal to fish. High alkalinity was documented in the Big Thompson downstream of the bridge construction for eight river miles to a Loveland water-treatment plant in the days following the apparent concrete spill. Since then, water-quality levels have returned to normal, and officials continue to monitor any effects.

Jeff Crane, a river hydrologist and restoration expert, who is consulting with the highway administration and state on the recovery effort, including the North Fork county road construction, says he’s surprised at the magnitude of the fish kill. But he adds that it’s also important to recognize the complexity and ambition behind recovery projects aiming to improve rivers’ natural functions and flood resiliency.

For instance, the North Fork reconstruction project has flipped the former alignment of the road and river channel, so the road is now snugged next to canyon rock walls. The previously straightened river now bends and courses through the middle of the canyon, while several of the new bridges are replacing buried culverts that typically get blocked or exceed capacity during flooding. “We’re actually ‘building’ a whole new river,” says Crane, a proponent of “natural channel design” that mimics natural landforms and uses less grouted rock, or riprap, than conventional flood-protection measures. Despite the fish kill, the local restoration should improve fish and aquatic habitat and reduce flooding damage in the long run, Crane says.

For now, state wildlife officials will continue to monitor the Big Thompson. River managers had just begun to see a rebound in fish populations since the floods, a trend that should still continue for several years, Rogstad says, although the kill-off is “definitely a significant setback in the recovery of the fishery.” American Civil Constructors will also likely have to pay some fines. State laws prohibit unlawful fish kills and the company may even have to pay a $35 fine for each fish killed. “It’s more of a fluke issue with a devastating impact,” Rogstad says. “We’d like to see that money go back into the river, in terms of habitat and riparian stability to maintain a long-term, more resilient river and ultimately to allow for a better fishery.”

High Country NewsHigh Country News is a nonprofit news organization that covers the important issues that define the American West. Subscribe, get the enewsletter, and follow HCN on Facebook and Twitter.

Josh Zaffos is an HCN correspondent based in Fort Collins, Colorado.

Photo credit: Larimer Country Road 43 Public Infrastructure Project