Water Lines: Gunnison flows lower than average this year
Free Press Weekly Columnist
Who cares how a river flows? Farmers who divert the water to their fields, of course, and boaters, anglers and the fish themselves, as well as the plants, people and other creatures living along the banks. All have different flow preferences at different times of year; and all were represented, in some form or another, at an April 23 meeting on the operation of the Aspinall Unit reservoirs.
Flows in the Gunnison River, like most rivers in the western United States, are heavily regulated by dams, reservoirs and diversions on the river’s main stem and tributaries. Primary among these are the three Aspinall Unit reservoirs: Blue Mesa, Morrow Point, and Crystal.
Also of note are the Taylor Reservoir, on the Taylor River just below the Continental Divide; the Gunnison Tunnel, which takes water from the head of the Black Canyon to the Uncompahgre Valley near Montrose; Ridgway Reservoir on the Uncompahgre River; Paonia Reservoir on the North Fork of the Gunnison; and the Redlands Water and Power diversion, which takes water for irrigation and power generation just upstream from the Gunnison’s confluence with the Colorado River in Grand Junction.
This doesn’t mean that flows in the Gunnison River are totally disconnected from the natural processes of rain and snow, but the connection is mediated by physical structures and management decisions. The U.S. Bureau of Reclamation attempts to optimize releases from the Aspinall Unit reservoirs to meet senior water rights in the Uncompahgre and Grand valleys; a water right for the Black Canyon National Park intended to support ecosystem health; and the needs of endangered fish in the lower section of the river. In high water years, the Bureau of Reclamation also has to consider the risk of flooding low-lying neighborhoods in the town of Delta.
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Natural hydrology plays a role in several ways. Peak, “shoulder” and base flow targets for the Black Canyon and endangered fish are adjusted up or down depending on the year’s snowpack. To hit peak flow targets for the endangered fish in the lower river, managers have to take into account the timing and amount of flow coming down the North Fork. This joins the main stem of the Gunnison upstream from Delta.
This year is considered “moderately dry” in the Gunnison Basin as a whole, with a May 1 forecast for April through July inflows to Blue Mesa of 440,000 acre feet, which is 65 percent of the 1981-2010 average. The snowpack in the portion of the Grand Mesa and West Elks that drain into the North Fork has been exceptionally dismal, however, so releases from the Aspinall Unit were expected to be higher than they otherwise would be to meet flow targets on the lower river.
As of April 23, April through July inflows into Paonia Reservoir were forecasted to be 32,000 acre feet, which is just over 30 percent of average, while total forecast runoff into the North Fork near Somerset over the same period is 165,000 acre feet, or about 55 percent of average.
This year’s base flow target for the Gunnison River at Whitewater is 890 cubic feet per second (cfs) for April and May. According to a notice issued by the Bureau on May 5, the target peak flow at Whitewater was 4,991 cfs for one day, which was expected to occur May 9 or 10. In actual fact, unexpectedly heavy rainstorms brought flows up to 5,800 cfs on May 7, and they were still near 5,000 cfs on May 10 — so even in this highly regulated system, Mother Nature can still bring surprises.
For comparison, in a “wet” year, with more than 1,123,000 acre feet flowing into Blue Mesa Reservoir, the peak flow target at Whitewater would rise to 14,350 cfs for a duration of 15 days.
Flexibility and judgement calls within their many management constraints sometimes allow dam managers to adjust releases to accommodate river sampling work by the Colorado Division of Parks and Wildlife or other activities. Rates of change in release volumes also take into account the impacts on trout in the Black Canyon and Gunnison Gorge.
Managing flows in this way is a complex juggling act, but it seems to work reasonably well — as long as there is water behind the big dams to release. On May 4, Blue Mesa Reservoir was 72 percent full.
In 2002 and 2012, reservoir levels fell alarmingly low before relief came in the form of more generous snow years. More prolonged drought in the headwaters of the Gunnison or demands for bigger releases to meet downstream needs could make it much harder to keep all the balls in the air.
To learn more about Aspinall Unit operations, go to 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. You can also find the Water Center on Facebook at Facebook.com/WaterCenter.CMU or Twitter at Twitter.com/WaterCenterCMU.
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