"While details of the fish kill are still being analyzed, it appears the event was associated with concrete work being performed in building and securing rockery walls along Larimer County Road 43 and replacement of the nearby Storm Mountain Road Bridge which spans the lower North Fork, as part of the massive redesign of County Road 43 and the adjacent North Fork Big Thompson. The Storm Mountain Bridge is located approximately 0.4 miles upstream of the confluence at Drake, Colorado.
Using electro fishing, a non lethal industry standard method for estimating fish populations in rivers and streams in conjunction with citizen reports in the days immediately following the kill, CPW was able to pinpoint where fish were dying and locate the lower extent of the kill. The extent of the loss extended 8.3 miles below the confluence to Loveland’s water treatment facility where sentinel fish kept in tanks to monitor river water quality died. CPW did not find any sick or dead fish further downstream. By comparing CPW historic sites surveyed during the Fall of 2015 with post-incident surveys CPW provided a statistically accurate estimate of number of fish killed.
Ben Swigle, CPW’s aquatic biologist for the Big Thompson drainage, concluded sections of the Big Thompson River between Drake and Estes Park were not impacted and “that healthy populations of both native and sportfish species in the upper sections will partially serve to repopulate sections of river compromised as part of this unfortunate event.” The 0.4 miles of the North Fork likely suffered a complete loss, whereas and the main stem Big Thompson from Drake downstream to the Loveland facility had suffered an estimated 52 percent loss. It is estimated that total loss was in excess of 5,600 fish