July Watershed Snapshot: High Temperatures and Fish Stress

Upper Colorado River Watershed Snapshot: July 17, 2020

Flow levels and water temperatures taken from USGS flow gauges around the Upper Colorado River watershed.


Water temperatures were high across most of the Upper Colorado River Watershed in July,

with low flows in key parts of the watershed from trans-basin diversions. Over 500 cubic feet per second (CFS) were diverted to the Front Range through the Moffat and Adams tunnels, marked by the purple lines on the right side of the map.

Flows at Windy Gap represent water sent further downstream, with only 176 cfs flowing further down the Colorado in mid-July. The red dot to the left of Granby marks Windy Gap Reservoir, where the two major confluences of the Upper Colorado River – the North Fork and the Fraser – meet.

Green plus signs mark areas in the watershed with lower water temperatures, where fish won’t already be stressed by water conditions, while the yellow and red crosses on the map mark areas in the watershed that should only be fished in the morning, before the heat of the day, or not at all.

Knowing water temperatures is key when it comes to fly fishing. Trout are cold water fish, which means they experience stress at higher water temperatures. Catch-and-release fishing on a stream or waterway with elevated temperatures can kill trout already stressed from the warm waters.

Extreme to abnormally dry conditions were measured across the state of Colorado on July 16, 2020. Source: U.S. Drought Monitor

The Fraser River in 2 sites near Tabernash and at the popular St Lois Creek fishing destination were all above 68F/20C on July 18th, which is in the danger zone for trout. Fishing should definitely NOT be happening in the afternoon hours in these places, and preferably not happening at all.

Where the temperatures hit the yellow level (64.4 – 68F/18 – 20C), fishing is NOT recommended during the hotter part of the day in the afternoon, as fish are already stressed by the high temperatures.

Low flows were measured at a number of locations throughout the watershed, though Grand County isn’t as dry as watersheds in the south of the state. With almost half of Colorado experiencing extreme drought conditions, the Upper Colorado River watershed is only considered abnormally dry, as opposed to exceptionally or extremely dry.


We’re still in the yellow on the map at the right from the U.S. Drought Monitor, while large swaths of Colorado in the south are deep red. 

The western U.S. has been in a decades-long megadrought linked to anthropogenic climate change since 2000. This year marks the launch of the Colorado River Drought Contingency Plan, as water managers grapple with how to adapt to changing conditions. 

To learn more about how high water temperatures affect trout, check out our tips on fly fishing in warmer waters.

Leave a Reply