Natural and human factors affect flows in the Gunnison, which will be low this year
Irrigation water managers, trout fishermen, park personnel, boaters, fish scientists and energy distributors all gathered at the Grand Junction office of the Bureau of Reclamation April 25 for a single, unified purpose: To learn how much water they can expect to see in the Gunnison River this summer.
The short answer: Not much. But there are some interesting details.
Hydrologists, meteorologists and dam managers did their best to explain and predict at this quarterly Aspinall Operations meeting — referring to the “Aspinall Unit” complex of dams below Blue Mesa Reservoir on the Gunnison River.
The amount of water we will see in the river depends on the interaction of a complex array of factors. There’s the snowpack, of course — both total quantity and timing are important. This year, we’re below average for total quantity in the Gunnison River Basin (Colorado River Basin, too, although not by as much), but it’s holding on longer than usual, and a lot longer than it did last year.
Then there’s soil moisture locally: The dryness of the dirt under the snow in our mountains. Extremely dry soils left over from last year’s drought mean that as the snow melts, much of it will soak into the ground instead of running off right away into streams and reservoirs. If it soaks in deep enough, it will contribute to flows later in the season.
Soil moisture in the Four Corners region also affects flows in our Colorado rivers. The same storms that brought new snow to the mountains in April also brought substantial quantities of dust from dry, disturbed soils in that region. Dusty snow isn’t just ugly; it also melts a lot faster than the pristine, white stuff, since the dust captures the sun’s heat instead of reflecting it back into the atmosphere.
Ambient temperatures at both high and low elevations affect flows as well. Cool temperatures at high elevations slow the melt, keeping streams flowing later. Cool temperatures at lower elevations delay the planting of crops, delaying demand. So far this year, we’ve experienced both, although the forecast is for warmer than average temperatures for the rest of the spring.
Snowfall, soil moisture and temperatures: Those are some of the key components affecting flows in the Gunnison and all of our other mountain-born streams. To understand flows downstream from Blue Mesa Reservoir, you have to add in numerous human factors as well.
Decisions by water managers affect how much water is pulled out of the river at the Gunnison Tunnel to serve farmers in the Uncompahgre Valley between Gunnison and Montrose. This year, they opened the tunnel late and are delivering less than full water rations, in part to keep as much water in storage as possible.
Other influences include the water rights for the Black Canyon of the Gunnison National Park and target flows to meet the needs of endangered fish species downstream, as well as how Redlands Water and Power is operating their canal system at the bottom of the river near Grand Junction. Both the Black Canyon water right and the target flows for endangered fish can be adjusted downwards in drought years. This year, they will be, with flow targets at Whitewater limited to 900 cubic feet/second.
If it was a big water year, like 2011, reservoir releases would also take into consideration a desire to avoid flooding the city of Delta.
This year, total inflows into Blue Mesa Reservoir, as of April 15, are forecast to be 340,000 acre feet. For some perspective, that’s more than flowed in last year, but still in the bottom 10th percentile for inflow years. It’s forecast to end the water year next fall about six feet below where it finished last year, despite higher in-flows. That’s because it started out full last year, and now it’s at about 41%.
Toward the end of the meeting, participants were asked if they had any special flow requests. Even with the tight legal parameters that govern the river and the tight supplies this year, a remarkable spirit of cooperation prevails. Dam managers want to know when people will be sampling in the river so they don’t knock them over with big releases, and they will try to time the peak flow required by the Black Canyon water right so that it will meet downstream needs as well. It’s a complex juggling act, with far-reaching impacts.
To learn more about Aspinall Unit operations and future meetings, visit http://www.usbr.gov/uc/water/crsp/cs/asp.html.
This is part of a series of articles coordinated by the Water Center at Colorado Mesa University in cooperation with the Colorado and Gunnison Basin Roundtables to raise awareness about water needs, uses and policies in our region. To learn more about the basin roundtables and statewide water planning, and to let the roundtables know what you think, go to http://www.coloradomesa.edu/WaterCenter.
Hannah Holm is coordinator of the Water Center at Colorado Mesa University.
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