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Environmental Characterization of Blue Mesa Reservoir and Potential Causes of and Management Strategies for Harmful Algal Blooms, 1970 through 2023, Curecanti National Recreation Area, Colorado

Scientific Investigations Report 2025-5109
Prepared in cooperation with the National Park Service, Colorado River Water Conservation District, Upper Gunnison River Water Conservancy District, Gunnison County, Project 7 Water Authority, and Uncompahgre Valley Water Users Association
By: , , , , , , , , and 
https://doi.org/10.3133/sir20255109

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Abstract

Blue Mesa Reservoir, in the Curecanti National Recreation Area, is the largest storage reservoir in Colorado and consists of three distinct basins: Iola (the shallowest), Cebolla, and Sapinero. After algal toxins were first documented in Iola basin in 2018, the U.S. Geological Survey began a study in cooperation with the National Park Service, Colorado River Water Conservation District, Upper Gunnison River Water Conservancy District, Gunnison County, Project 7 Water Authority, and Uncompahgre Valley Water Users Association to better understand occurrence of toxic cyanobacteria harmful algal blooms (cyanoHABs) and identify possible causal mechanisms to potentially inform management strategies.

Toxic cyanoHABS occurred when the algal toxin microcystin exceeded a concentration of 8 micrograms per liter primarily in Iola basin in 2018 and 2020–22, years having some of the lowest reservoir water-level elevations (reservoir levels) since 1984. The toxic cyanoHABs started in mid-September and continued through the fall months. Algal abundance was greatest in Iola basin compared to Cebolla and Sapinero basins, with Aphanizomenon, a toxin-producing cyanobacterium, being the most abundant. During blooms, enhanced algal photosynthesis caused elevated pH and dissolved oxygen concentrations especially in Iola basin. Continuous monitor data in Iola basin indicated peaks in phycocyanin fluorescence, pH, and dissolved oxygen concentration that preceded the onset of toxic cyanoHABs by about 2 weeks potentially indicating a useful early warning monitoring strategy for future response to toxic cyanoHABs. Long-term trends showed increases in mean air and surface-water temperatures and chlorophyll-a concentrations in the reservoir but no change in nutrient inputs from major tributaries. In Iola basin, reservoir level was positively correlated with Secchi disk depth and inversely correlated with total phosphorus concentration. Because of its shallow depth, the effect of low reservoir levels may disproportionately affect Iola basin compared to other basins, resulting in algal blooms and toxin production especially at reservoir levels below about 7,470 feet above North American Vertical Datum of 1988. Elevated phosphorus at low reservoir level likely was primarily phosphorus contained in algal tissue.

This report indicates that the main driver for recent toxic cyanoHABs in Iola basin is low reservoir level that likely causes favorable conditions (shallow and warm) for algal growth and increased recruitment of algae from bottom sediments such as during wind-driven turbulence. Control of external nutrients to the reservoir is unlikely to help control algal blooms because Aphanizomenon fixes nitrogen from the atmosphere, and there is an abundant geogenic source of phosphorus. Maintenance of reservoir levels greater than about 7,470 feet might help minimize the occurrence of toxic cyanoHABs. Additional data could help better understand how the timing and duration of reservoir levels below 7,470 feet contribute to toxic cyanoHABs.

Plain Language Summary

Blue Mesa Reservoir (Blue Mesa), in the Curecanti National Recreation Area, is the largest storage reservoir in Colorado and consists of three distinct basins: Iola (the shallowest), Cebolla, and Sapinero. After algal toxins were first documented in Iola basin in 2018, the U.S. Geological Survey began a study to better understand occurrence of toxic harmful algal blooms (HABs) and identify possible causal mechanisms to potentially inform management strategies. Harmful algal blooms occurred in Blue Mesa when concentration of a toxic substance produced by dying algae was greater than health advisory levels, prompting no contact warnings for humans and their pets in Blue Mesa. This condition occurred starting in September and lasted as late as early November in Iola basin in 2018 and 2020–22. These years had some of the lowest recorded reservoir water-level elevations since 1984. Iola basin had the greatest amount of algae compared to Cebolla and Sapinero Basins, and a type of algae that could produce toxins was the most abundant. Multiple climate and water-quality indicators were examined in the reservoir and its tributaries to determine the causes of toxic HABs in Blue Mesa. The results indicate that the main cause for recent toxic HABs in Iola basin may be low reservoir level that likely causes favorable conditions (shallow and warm) for algal growth and increased release of algae from bottom sediments, for example, during wind-driven turbulence. Control of external nutrients to the reservoir is unlikely to help control algal blooms because the toxin-producing algae can use nitrogen from the atmosphere, and there are abundant geologic sources of phosphorus providing that nutrient to Blue Mesa. Maintenance of reservoir water-level elevation greater than about 7,470 feet might help minimize the occurrence of toxic HABs in Blue Mesa.

Suggested Citation

Walton-Day, K., Day, N.K., Mast, M.A., Gidley, R.G., Gohring, E.J., King, T.V., Day, W.C., Gibney, N.D., and Bauch, N.J., 2025, Environmental characterization of Blue Mesa Reservoir and potential causes of and management strategies for harmful algal blooms, 1970 through 2023, Curecanti National Recreation Area, Colorado: U.S. Geological Survey Scientific Investigations Report 2025–5109, 64 p., https://doi.org/10.3133/sir20255109.

ISSN: 2328-0328 (online)

Study Area

Table of Contents

  • Acknowledgments
  • Plain Language Summary
  • Abstract
  • Introduction
  • Study Methods
  • Environmental Characterization of the Blue Mesa Reservoir System
  • Potential Causes of Harmful Algal Blooms
  • Potential Management Strategies for Cyanobacteria Harmful Algal Blooms
  • Summary
  • References Cited
  • Appendix 1. Tables Presenting Collection Schedules, Analytical Methods, and Water-Quality Constituents used during the Study
  • Appendix 2. Quality-Assurance Results for Field Blanks and Replicate Samples
  • Appendix 3. Supplemental Data Collection
Publication type Report
Publication Subtype USGS Numbered Series
Title Environmental characterization of Blue Mesa Reservoir and potential causes of and management strategies for harmful algal blooms, 1970 through 2023, Curecanti National Recreation Area, Colorado
Series title Scientific Investigations Report
Series number 2025-5109
DOI 10.3133/sir20255109
Publication Date December 17, 2025
Year Published 2025
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Colorado Water Science Center
Description Report: ix, 64 p.; 8 Linked Appendix Tables; Data Release; Dataset
Country United States
State Colorado
Other Geospatial Blue Mesa Reservoir, Curecanti National Recreation Area
Online Only (Y/N) Y
Additional Online Files (Y/N) Y
Additional publication details