Several regional health, water and recreation officials are monitoring a potentially harmful algal bloom at Willow Creek Reservoir. Willow Creek is part of the Colorado-Big Thompson Project in Grand County and a source of water for several Front Range communities including Longmont.
Monitoring teams in late July detected the presence of blue-green algae (cyanobacteria), which can sometimes produce toxins (cyanotoxins) that can be harmful to humans and animals, according to a joint news release from several agencies including Northern Water. After the discovery, the Forest Service’s Arapaho National Forest placed restrictions on water recreation and posted signs informing the public of the issue.
Recent tests indicate the concentration of cyanotoxins in the two samples was nearly negligible. However, because of evidence of algae in other parts of the reservoir where the sampling has not occurred, water recreation remains restricted at the reservoir, the news release states.
Willow Creek is part of the collection system for the Colorado-Big Thompson Project, which gathers water in the headwaters of the Colorado River for delivery to cities, farms and industries in northeast Colorado.
Colorado-Big Thompson water is supplemental to Longmont’s water supply as the city gets most of its water from native basins in the region, Wes Lowrie, a water resources analyst with the city’s Water Resources Division, told the Leader in June 2020.
The Colorado-Big Thompson water, or C-BT water, provides an added buffer, if those basins, including Ralph Price and Union reservoirs, were insufficient in a dry year, Lowrie told the Leader.
C-BT water is collected at Lake Granby on the Western Slope and is distributed via Horsetooth Reservoir and Carter Lake on the Eastern Slope, Jeff Stahla, spokesman for Northern Water, told the Leader.
Longmont owns 12,800 of C-BT water, or roughly the equivalent of 12,800 acre-feet, the city’s Lowrie said. If Longmont were to receive 100% of its allotment, it would fill an empty Union Reservoir.
C-BT water rights holders include cities, towns, water districts and industries in eight counties, including Boulder County, individual farmers and more than 120 ditch, reservoir and irrigation companies serving thousands of farms and more than 640,000 acres. Longmont has been among them since Northern Water was formed to build the Colorado-Big Thompson Project in the late 1930s, Stahla told the Leader.
In the East Troublesome Fire of 2020, as much as 90 percent of the watershed that feeds into the Willow Creek Reservoir sustained damage, the news release states. This summer, the arrival of monsoon rains has pumped more nutrients from the burn scar to the reservoir, which helped support increased growth of all kinds of algae.
However, the vast majority of algae species are not harmful, the news release states.
