The spring runoff from the exceptional 2019 snowpack has receded, and Pine River Irrigation District is moving forward while declaring the carefully balanced water management season “conquered.”
The snowpack around Vallecito Dam, northeast of Durango, reached 160% of normal levels this winter, according to the Colorado Basin River Forecast Center. For the PRID hydrology team, that meant a complex dance of balancing the gushing spring runoff, keeping Vallecito Reservoir full and controlling water levels rushing downstream. In early July, the snowmelt officially began to recede. The PRID team fought one of the largest spring runoff periods and won.
“We have 77 years of record, and this is going to fall probably around the third or fourth highest,” said Greg Smith, a senior hydrologist at the Colorado Basin River Forecast Center. “It was definitely an impressive year.”
As of Monday, the reservoir was at full capacity. Inflows, or incoming water, were at 1,000 cubic feet per second. PRID managers lowered the reservoir a tenth of an inch because of incoming summer storms.
Managing the spring runoff was difficult, said Ken Beck, superintendent of the irrigation district. Not only did the district have large amounts of water coming down from the mountains, it also dealt with late-season cold temperatures that affected water management calculations.
The district’s goal is to keep the reservoir close to its maximum capacity to meet system-wide irrigation needs throughout the season, to make sure that the dam is operationally sound at full capacity and to keep downstream water flows at safe levels as much as possible, Beck said.
While snowmelt increases streamflow as temperatures warm, the tail end of the runoff comes with a hard-to-predict, sudden drop in streamflow. The water level in the reservoir needs to be at a certain elevation to maintain full capacity for as long as possible during the summer season.
“If you miss that calculation, when it falls, you’re toast,” Beck said.
By late June, the reservoir was almost full, with an elevation of 7,665 feet. The Colorado Basin River Forecast Center projected that there were about 30,000 acre-feet of remaining flows for July at the beginning of the month. The hydrology team wanted to keep water releases below 2,500 cfs, the maximum safe release capacity, according to an update letter Beck sent July 6.
Meanwhile at the Rocky Creek Ranch in Bayfield, a non-vehicle bridge was inundated with water flowing at 2,460 cfs. Near Pine River Hops and LePlatt’s Pond, a berm eroded and rechanneled a portion of the river, Beck wrote.
The U.S. Bureau of Reclamation constructed the dam, which was completed in 1941, to prevent the flooding of crops, farmland and structures along the river during spring runoff by storing the floodwater for controlled releases to benefit irrigation, according to the bureau’s website.
“Without the dam, flows of roughly 4,000 cfs would have shock-waved down the Pine River on Sunday, June 9, 2019,” Beck wrote. Water flows greater than the maximum safe capacity would have been commonplace instead of a three- or four-day period with flows around 2,460 cfs.
The hydrology team watched “vigilantly” for the water to recede or storms to approach, keeping water flowing below the safe release capacity and increasing the reservoir levels a tenth or hundredth of an inch at a time. Finally, by the morning of July 4, they saw forecasts showing that the recession had arrived.
“We had conquered the winter of 2018-2019 and its accumulated snowpack!” Beck wrote.
smullane@durangoherald.com
