Staff test Stagecoach Reservoir weekly for toxic algae blooms

Algae has not turned toxic this season but precautions are advised

On Thursday, blue-green algae growth is visible at Harding Cove on the north side of Stagecoach Reservoir. State park staff test for possible toxic algae blooms every week, but so far this year, the levels have not reached toxic.
Craig Preston/Courtesy photo

Every week since early July, Stagecoach State Park Manager Craig Preston has performed field testing to make sure the blue-green algae that appears annually at Stagecoach Reservoir has not bloomed to toxic levels.

Once a month, Preston sends a water sample to the U.S. Environmental Protection Agency for more intensive testing.

So far this year, the algae has not multiplied rapidly and turned toxic, Preston said, yet he encourages reservoir users to take precautions such as rinsing pets, humans and fish after contact with the water. Dogs should not be allowed to drink the water where blue-green algae is visible.

Stagecoach Reservoir currently is on the “caution” list along with 14 other Colorado water bodies, according to the Colorado Department of Public Health and Environment.

Steamboat and Pearl lakes have not experienced elevated algae levels this summer, said Julie Arington, park manager. Steamboat Lake was closed for two weeks for swimming for humans and pets in mid-August 2020 due to blue-green algae blooms.

Before Stagecoach Reservoir was completed in 1989, planners warned that algae blooms fed by nutrients could be a long-term issue to contend with, said Preston, park manager since 2006. The highest levels of algae are often found in four reservoir coves, including Morrison, Pike, Haybro and Keystone coves.

The park staff receive multiple calls a week from people asking about the algae, which may move daily, Preston said, and visitors at the park often ask staff if it is safe to swim.

CDPHE/Courtesy image

One organization working on the algae concern is the Upper Yampa River Watershed group. Coordinator Lyn Halliday, an environmental scientist, said the group recently established an algae project stakeholder committee to oversee a study of nutrient loading and water quality analysis in the upper Yampa River watershed and Stagecoach Reservoir.

This month, the group received three responses to a request for proposals to study the Bear River subbasin upstream from Stagecoach Reservoir. The study will include water quality sampling and subbasin modeling to identify and quantify nutrient-loading sources, Halliday said.

The goal is to learn where excess nutrients are coming from with many possibilities of non-point source pollution. Sources could come from upstream soils, geology, atmospheric conditions, land use, agriculture, forestry, old mining, developed areas or towns, Halliday said.

After the subbasin modeling study is completed by spring 2024, then the hard work of devising solutions begins, which could range from education to funding for improved practices or even structural controls, Halliday said.

“Prevention in these cases is always less expensive than remediation,” Halliday said.

A photo taken in July shows blue-green algae along the shoreline of Stagecoach Reservoir adjacent to the marina dock.
Craig Preston/Courtesy image

Last week, Halliday and Preston used specialized equipment lowered from a boat to sample the nutrient-rich sediment at the bottom of the reservoir to see how those conditions play into the algae problems.

The CDPHE has maintained a voluntary reporting dashboard of testing and algae levels since 2019. Division Director Nicole Rowan with the CDPHE Water Quality Control Division said water bodies in Colorado experienced 23 reported toxic algae blooms from June through this week.

“As temperatures increase, we would expect this to result in an increase in water temperature that could create more algae blooms in the future,” Rowan said.

Colorado Parks and Wildlife, which manages state parks, notes algae are an important part of the aquatic food system, but some types of blue-green algae are capable of producing toxins that may cause negative health impacts for humans and pets at elevated concentrations.

At Stagecoach State Park, visitors are advised to avoid areas of scum whether swimming, wading or boating and to clean and rinse fish well and discard fish guts in the trash. Visitors should keep dogs leashed around water with elevated algae and not let dogs lick or ingest dried scum along the shoreline. If dogs are exposed to the algae, they should not be allowed to lick their fur or paws, and owners should rinse dogs thoroughly with potable water while wearing gloves if possible.

The elevated algae levels at Stagecoach Reservoir typically start in June and last through September. Algae caution information signs were posted at the park in 2021.

“As in many bodies of water in Colorado and around the country, cyanobacteria (blue-green algae) is known to be present in Stagecoach Reservoir,” the signs read. “Although many species of cyanobacteria are harmless, some can produce toxins. The production of these toxins is unpredictable.”

Stagecoach staff have been monitoring for cyanobacteria and testing for toxins since 2015, Preston said. Most tests come back negative but occasionally in past years have showed positive for low levels of toxins.

Symptoms of toxic algae exposure include skin irritation, fever, diarrhea, headache and muscle and joint pain, according to the CDPHE. More information about toxic algae is available at

Toxic algae can naturally dissipate with cooler temperatures, wind events or storms that mix the water, according to the CDPHE. Citizens can help reduce the risk of toxic algae and improve Colorado water quality by reducing the use of fertilizers that can run off during rains, avoiding using de-icers especially with nitrogen or urea, and picking up pet waste.

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