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[NASA] Examining Algal Blooms in Blue Mesa

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  3. Examining Algal Blooms in Blue…
 


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November 15, 2017
November 17, 2021

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Cyanobacteria blooms turned Blue Mesa Reservoir green from September through November 2021, when water levels were among the lowest on record. The OLI (Operational Land Imager) on Landsat 8 captured this image (right) of a bloom on November 17, 2021, when the water was near its lowest level; the left image shows the same area on November 15, 2017, when water levels were closer to normal.
NASA Earth Observatory / Lauren Dauphin

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Cyanobacteria blooms turned Blue Mesa Reservoir green from September through November 2021, when water levels were among the lowest on record. The OLI (Operational Land Imager) on Landsat 8 captured this image (right) of a bloom on November 17, 2021, when the water was near its lowest level; the left image shows the same area on November 15, 2017, when water levels were closer to normal.
NASA Earth Observatory / Lauren Dauphin
November 15, 2017November 17, 2021

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Cyanobacteria blooms turned Blue Mesa Reservoir green from September through November 2021, when water levels were among the lowest on record. The OLI (Operational Land Imager) on Landsat 8 captured this image (right) of a bloom on November 17, 2021, when the water was near its lowest level; the left image shows the same area on November 15, 2017, when water levels were closer to normal.
NASA Earth Observatory / Lauren Dauphin
This is the hidden content, please
Cyanobacteria blooms turned Blue Mesa Reservoir green from September through November 2021, when water levels were among the lowest on record. The OLI (Operational Land Imager) on Landsat 8 captured this image (right) of a bloom on November 17, 2021, when the water was near its lowest level; the left image shows the same area on November 15, 2017, when water levels were closer to normal.
NASA Earth Observatory / Lauren Dauphin

November 15, 2017

November 17, 2021


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Image Details

Cyanobacteria blooms turned Blue Mesa Reservoir green from September through November 2021, when water levels were among the lowest on record. The
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(Operational Land Imager) on
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captured this image (right) of a bloom on November 17, 2021, when the water was near its lowest level; the left image shows the same area on November 15, 2017, when water levels were closer to normal.

The summers of 2021 and 2022 were tough seasons for Colorado’s Blue Mesa Reservoir.

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gripped much of the western U.S., prompting emergency water releases that brought the reservoir to its lowest level
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. Marinas and boat ramps closed, remnants of a
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emerged from the muck, and parts of the reservoir turned greenish and swirled with toxic
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blooms.

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conducted by scientists at the U.S. Geological Survey and the National Park Service analyzed decades of Blue Mesa Reservoir data and found a connection between low water levels, warm water temperatures, and harmful blooms.

“Algal blooms were more common when water levels were below 7,470 feet and water temperatures were above approximately 19.5 degrees Celsius (67.1 degrees Fahrenheit),” said Tyler King, a research hydrologist with U.S. Geological Survey. Water levels that low are relatively common and have occurred every few years in recent decades.  

While some cyanobacteria, also called blue-green algae, are always present in the reservoir in small numbers, problems occur when certain types proliferate. Aphanizomenon, Dolichospermum, and Woronichinia, for instance, thrive when the reservoir’s waters become warm and stagnant, releasing a toxin called

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that can cause skin and eye irritation, respiratory problems, and liver damage. Children and pets are particularly vulnerable to microcystin poisoning because of their size and tendency to ingest more water than adults.

King and colleagues analyzed in situ water samples and

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from the European Space Agency’s
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mission and the NASA/U.S. Geological Survey
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satellites. A Sentinel-2 sensor that detects the light-harvesting pigment
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was particularly useful for mapping the blooms, while Landsat sensors were used to map water temperatures over time.

The National Park Service and U.S. Geological Survey launched the project in 2021 after anecdotal reports and water sampling suggested elevated cyanobacteria concentrations, King said. The scientists collected water samples but also turned to historical records and

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—”like a time machine,” he said—to examine conditions before regular water sampling had begun. Their analysis included satellite records of chlorophyll levels that extended back to 2016 and temperature records that reached back to 2000. The research team also studied in situ data on water levels dating to the 1970s.

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A cyanobacteria bloom turned the water surface of Iola Basin green on September 8, 2021. Photo by Nicole Gibney/National Park Service.

The satellite data showed that blooms typically start in the eastern end of the reservoir, an area known as Iola Basin. The basin, where the Gunnison River flows into the reservoir, is the shallowest part of the reservoir. Occasionally, the satellite data showed, blooms spread westward into other parts of the reservoir, sometimes moving about two-thirds of the way across. However, concentrations of toxins rarely reached levels that posed health concerns beyond Iola Basin.

The same dynamics that caused challenges for Blue Mesa in 2021 and 2022 are present in 2026, said King. Drought again plagues much of the western U.S., the mountains hold

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, and water levels in Blue Mesa are
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. On June 27, 2026, the reservoir stored about
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of the water it typically does on that date, the lowest value observed for that day in the past 30 years. Water levels are expected to continue dropping until October, according to
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. 

If cyanobacteria blooms emerge in 2026, the researchers expect that satellites will help scientists track them. The researchers use the U.S. Geological Survey’s

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(Water Monitoring Above the Planet) tool to monitor for potential bloom conditions within hours of satellite overpasses. NASA’s
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(Satellite-based Tool for Rapid Evaluation of Aquatic Environments) project also uses data from Landsat and Sentinel-2 to map potential blooms within hours of a satellite overpass, and the multi-agency
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(Cyanobacteria Assessment Network) project collects daily data from other satellites to map blooms in larger water bodies.

“It’s amazing that we can use satellites to map the impacts of microscopic organisms from almost 500 miles away,” King said. Yet it will still be crucial to get people out on the water taking samples and directly testing for toxins, he emphasized. “The satellites aren’t definitive,” he added. “They can tell us where there might be a problem, but toxins often aren’t present until the later stages of a bloom.”

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Satellite observations can help managers decide where to send personnel to collect water samples for more detailed analysis of bloom toxicity. Photo by Katie Walton-Day/USGS.

NASA Earth Observatory images by Michala Garrison, using Landsat data from the 

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. Photos by Katie Walton-Day (USGS) and Nicole Gibney (NPS). Story by Adam Voiland.

Downloads

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November 15, 2017

JPEG (8.98 MB)

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November 17, 2021

JPEG (8.46 MB)

References & Resources

  • Aspen Journalism (2026, January 9)
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    . Accessed July 1, 2026.
  • Aspen Journalism (2021, September 4)
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    . Accessed July 1, 2026.
  • CPR News (2021, September 3)
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    . Accessed July 1, 2026.
  • The Colorado Sun (2023, June 16)
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    . Accessed July 1, 2026.
  • The Colorado Sun (2022, September 22)
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    . Accessed July 1, 2026.
  • Environmental Protection Agency (2026, January 5)
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    . Accessed May 28, 2026.
  • King, T. V.,  et al. (2025)
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    . JAWRA Journal of the American Water Resources Association, 61(4), e70038. 
  • National Park Service (2026)
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    . Accessed July 1, 2026.
  • U.S. Bureau of Reclamation (2026)
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    . Accessed July 1, 2026.
  • U.S. Bureau of Reclamation (2026)
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    . Accessed July 1, 2026.
  • University of Colorado Boulder (2026, January 26)
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    . Accessed May 28, 2026.
  • U.S. Drought Monitor (2026, May 28)
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    . Accessed July 1, 2026.
  • Walton-Day, K., et al. (2025)
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    . U.S. Geological Survey Scientific Investigations Report, 2025–5109.

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