Satellite Imagery Uncovers the Mystery Behind Harmful Algal Blooms at Colorado's Blue Mesa Reservoir

Drought Hammers Blue Mesa, Setting the Stage for an Algal Crisis

The summers of 2021 and 2022 were brutal for Blue Mesa Reservoir in Colorado. A severe drought gripping much of the American West, combined with emergency water releases, pushed the reservoir to its lowest level since 1984. Boat ramps and marinas were forced to close, the ruins of a long-submerged ghost town re-emerged from the mud, and portions of the water turned an eerie green — choked with toxic cyanobacteria blooms.

Scientists from the U.S. Geological Survey (USGS) and the National Park Service (NPS) have now analyzed decades of data from Blue Mesa Reservoir to establish the link between low water levels, elevated water temperatures, and harmful algal bloom (HAB) events.

"When water levels drop below 7,470 feet and water temperatures exceed approximately 19.5°C (67.1°F), bloom frequency increases markedly," said USGS research hydrologist Tyler King. Such low-water conditions are not unusual — they have recurred at the reservoir every few years in recent decades.

The Toxic Threat of Cyanobacteria

While trace amounts of cyanobacteria are always present in the reservoir, the problem emerges when certain species proliferate en masse. Species such as Aphanizomenon, Dolichospermum, and Woronichinia thrive particularly well in warm, stagnant water. When they do, they release microcystin — a toxin capable of causing skin and eye irritation, respiratory problems, and liver damage. Children and pets are especially vulnerable due to their smaller body mass and proportionally higher water intake.

Satellite Data: A 'Time Machine' for Scientists

King and his team analyzed both field water samples and satellite observations from the European Space Agency's Sentinel-2 mission and NASA/USGS Landsat satellites. Sentinel-2's sensors can detect chlorophyll — the photosynthetic pigment — making it particularly effective for mapping bloom locations. Landsat sensors were used to track long-term water temperature changes.

The NPS and USGS formally launched the research program in 2021 after receiving anecdotal reports and water sample results indicating unusually high cyanobacteria concentrations. Scientists not only collected new samples but also worked backward through historical records and satellite imagery — a process King described as a "time machine" that allowed researchers to examine reservoir conditions before systematic water sampling began. The dataset includes satellite chlorophyll records dating back to 2016, water temperature data from 2000, and on-site water-level measurements stretching back to the 1970s.

Blooms Typically Originate in the Iola Basin

Satellite data revealed that blooms typically begin in the Iola Basin — the easternmost section of the reservoir, where the Gunnison River enters and the water is shallowest. Imagery occasionally captured blooms spreading westward into other parts of the reservoir, sometimes extending across roughly two-thirds of its length. However, toxin concentrations exceeding health advisory thresholds were generally confined to the Iola Basin.

2026: Warning Bells Ring Again

King noted that the same conditions responsible for the 2021–2022 crisis are re-emerging in 2026. Drought has again spread across much of the western United States, mountain snowpack is sparse, and Blue Mesa's water level remains critically low. As of June 27, 2026, the reservoir held approximately 43% of its normal storage for that date — the lowest on record for that day in the past 30 years. The U.S. Bureau of Reclamation projects water levels will continue to decline through October.

Should another bloom occur in 2026, researchers expect satellites to play a key monitoring role. The team currently uses USGS's WaterMAP (Water Monitoring Above the Planet) tool to track potential bloom conditions within hours of a satellite overpass. NASA's STREAM (Satellite-based Tool for Rapid Evaluation of Aquatic Environments) program also integrates Landsat and Sentinel-2 data to map potential bloom areas within hours of each overpass. Meanwhile, the multi-agency CyAN (Cyanobacteria Assessment Network) program collects daily satellite data to monitor bloom conditions across large bodies of water.

"It's remarkable to be able to track the effects of microorganisms — invisible to the naked eye — from nearly 500 miles (approximately 800 kilometers) above Earth," King said. He was quick to emphasize, however, that field sampling and direct toxin testing remain indispensable. "Satellites are not the final word," he added. "They can tell us where problems might be developing, but toxins often don't appear until late in a bloom's lifecycle."

Satellite observation data can help managers determine where to deploy personnel to collect water samples for detailed bloom toxicity analysis, making it a valuable complement to ground-level monitoring.

NASA Earth Observatory images by Michala Garrison, using USGS Landsat data. Photographs by Katie Walton-Day (USGS) and Nicole Gibney (NPS). Article by Adam Voiland.

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