The annual Guernsey silt run, a process of intentionally lowering water levels of the Guernsey Reservoir, bringing silt laden water to irrigators annually since 1959, continues to be a crucial process of providing stability to irrigation districts.
After completion of the Guernsey dam in 1927, the Guernsey Reservoir began to provide irrigation water to the three downstream irrigation districts: Goshen, Gering-Ft Laramie and Pathfinder, consequently involving the Fort Laramie and interstate canals.
Within 30 years of the dam’s completion, 29,000 acre-feet (one acre foot, equal to 326,000 gallons of water) of accumulated sediment occurred in the reservoir, reducing storage capacity from 73,180 acre-feet to 44,800 acre-feet, according to a report analysis prepared by Water Resources and Environmental Consultants, Lidstone & Anderson, for the Wyoming Water Development Commission in 1993.
The release of silt laden water from the reservoir to the canals of irrigation districts, allowed for an increased amount of storage and decreased erosion within the Fort Laramie and interstate canals.
“What it does is, the silt that is in the water which comes out of Guernsey actually lines the channels and helps prevent water loss in the channels, its pretty effective means of water efficiency in the canals themselves,” Mike Follum, branch chief of the Water and Civil Works sector of the Wyoming agency said.
Stability provided through the annual silt run, significantly reduces seepage amounts throughout canals, which Pathfinder Irrigation District Manager Dennis Strauch also said is crucial to their irrigation system.
“By laying down a layer of silt along the canals, the silt run protects us against large water losses,” Strauch said, “If we didn’t have the silt run, we would see a lot more water loss through seepage.”
Paid for and contracted by the Pathfinder, Gering-Ft Laramie and Goshen Irrigation districts, Strauch said, money spent allowing the Guernsey silt run to occur annually is well worth its price, simply due to the amount of water saved through the reduction of seepage in canal systems.
According to Lindstone & Anderson’s environmental analysis report, decreased canal seepage and increased bank stability were observed during and following the silt run investigations conducted by the Bureau of Reclamation in 1959, 1960, 1961 and 1962 (BOR, 1963).
“These investigations estimated the silt run conserved an average of 30 acre-feet of water per mile per year. Annually, this results in a total of 10,000 acre-feet (equal to 3.3 trillion gallons) of water conserved within the Fort Laramie and Interstate Canals,” according to Lindstone & Anderson’s 1993 report.
Assuming the silt run has a 30-day effect, the Bureau of Reclamation further states the silt run provides the equivalent of “1 acre-foot per day per mile or 0.5 cfs per mile in additional capacity,” according to the 1993 report.
The construction of the Glendo Dam located 25 miles above the Guernsey Dam, completed in 1958, trapped large amounts of river sediment, leading to the process put to use annually, which includes decreasing the water flow from the Glendo Reservoir, allowing the water level of Guernsey to rapidly decrease, resulting in sediment flowing downstream lining irrigation channels.
The 2020 silt run will begin on the morning of July 7, beginning with the release of water from Glendo Reservoir, then refilling weeks later.
“On the morning of July 7, the release of water from the Glendo Reservoir will be decreased from approximately 4,500 cubic feet per second (cfs) to a flow of approximately 1,500 cfs. The decreased flow will cause a rapid decline of the Guernsey Reservoir level of approximately 25 feet starting the morning of July 8 and continuing through July 11. By Thursday, July 11, the boat ramps at Guernsey Reservoir will no longer be usable due to the low reservoir level,” according to Bureau of Reclamation news release, “The silt run will begin on July 12 and is anticipated to continue through July 25. Beginning on the evening of July 25, the release of water from Glendo Reservoir will be rapidly increased to refill Guernsey Reservoir. The level of Guernsey Reservoir will continue to rise by approximately 6 feet per day and is expected to reach the normal reservoir operation level by the evening of Tuesday, July 30.”