Heat and Low Flow Are Stressing the Mancos River's Aquatic Community at Mesa Verde

The Mancos River. NPS.

The National Park Service's Southern Colorado Plateau Inventory and Monitoring Network (SCPN) treats aquatic macroinvertebrates as a high-priority "vital sign" of stream health, because they respond sensitively to physical and biological disturbance. In 2006 SCPN set up two monitoring sites on the Mancos River in Mesa Verde National Park as a pilot for its monitoring methods. Park staff were concerned about the effects of past wildfires and of water taken from the river upstream.

A report covering 2008–2019 looked at trends in the river's physical habitat and its invertebrate community.

Key findings

• The Mancos River in the park has experienced chronic hydrologic and thermal stress.
• Colorado's multi-metric index shows the monitoring sites were impaired in multiple years between 2008 and 2019.
• Sensitive taxa, especially caddisflies (trichopterans), declined, a trend attributed to periods of low flow and heat.

Less water, warmer water

Reservoirs and diversion ditches along the Mancos take water out, reducing streamflow by the time the river enters the park — especially harmful in warmer, drier months.

The Mancos River watershed, with the two long-term monitoring sites, MAN01 and MAN02, and water diversions. Kristin Straka/NPS.

Continuous temperature records show several years in which daily maximum stream temperatures exceeded the water quality standard for aquatic life on this stretch — 28.6 degrees from March to November and 25.2 degrees from December to February. The exceedances lined up closely with some of the lowest flows recorded at the U.S. Geological Survey gauging station at the upper site, suggesting that extremely low discharge drove the higher temperatures.

Daily mean (orange), maximum (red) and minimum (blue) stream temperatures at MAN01, with the March–November and December–February standards as dashed lines. Stacy Stumpf/NPS.

An impaired community

Colorado's Multi-Metric Index (MMI) rates the ecological integrity of a river reach from the biology of its invertebrate community. Approximately 28% of samples from the upstream site and more than 20% from the downstream site were rated impaired over the study.

Multi-metric index scores for each sample at MAN01 (top) and MAN02 (bottom); orange circles mark impaired samples. Stacy Stumpf/NPS.

Impairment showed most clearly in three measures, which fell in impaired samples by approximately:

Why caddisflies

Caddisflies count toward all three. Most genera cling to the undersides of stones in riffles and are extremely sensitive to change, particularly heat, which can kill them and cause eggs to fail to hatch — and prolonged warming has been shown to remove them from stream communities. At the upstream site, caddisfly richness and abundance declined from 2008 to 2019, and the years of temperature exceedances matched years of lower caddisfly numbers.

A caddisfly larva in a protective case of silk and debris. Matt Reinbold, CC BY-SA.

What it means for managers

• Continued monitoring of flow and temperature will be key to understanding how a warming climate affects the river's aquatic life.
• Restoration that shades the stream corridor and slows water through the system would help buffer stream temperatures against warming air.
• The caddisfly decline most likely reflects their sensitivity to chronic heat and low-flow stress.

The Mancos River in Mesa Verde National Park. NPS.

Sources

Rewritten from the National Park Service brief "Long-term Trend in the Aquatic Macroinvertebrate Community of the Mancos River at Mesa Verde National Park," prepared by Christopher Calvo (November 2024) from Stumpf, S., and L. Ploughe, 2024, Science Report NPS/SR—2024/209 (public domain), in hubnx's own words. The caddisfly photograph is by Matt Reinbold under CC BY-SA.