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 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.
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.