Dredging will provide access to the last 5 feet of irrigation water in the parched McPhee Reservoir – The Durango Herald
Historic drought and low water show sediment blockage
The drought in southwest Colorado triggered a dredging operation on the McPhee Reservoir to remove sediment that is blocking the flow of water to the irrigation channels during the reservoir’s lowest level.
The United States Bureau of Reclamation and the Dolores Water Conservancy District are working to remove approximately 13,000 cubic meters (14,300 tonnes) of sediment from the Great Cut Canal that connects to the Dove Creek Canal and the Narraguinnep Reservoir.
The amount of sediment to be removed is equivalent to 1,300 dump truck loads, or a volume of soil 50 feet long, 50 feet wide, and up to 230 feet high.
Loading sediment into the canal blocked access to the last 5 feet of the active irrigation supply at McPhee, said Robert Stump, Cortez director for the Bureau of Reclamation.
âWe wouldn’t be able to provide this water without doing this sediment removal,â he said. “The goal is to use all the water allocated for irrigation.”
This year, the McPhee Reservoir, which has a capacity of 380,000 acre-feet, is expected to drop to its idle supply basin of 151,000 acre-feet, the lowest level since it was first filled in the mid-1980s.
The accumulation of sediment in the channel becomes an urgent problem when the lake level is very low. It obstructs the last part of the active irrigation feed to a water intake drain at the Great Cut Dike which feeds the irrigation canals.
A bathometric survey revealed that two sediment peaks of 5 to 10 feet formed where the drainages from the dry lake bed reached the channel. The heavy rains accelerated the loading of the sediment, said Rob Walker, maintenance manager for DWCD.
The dredging operation is focused on removing the two high points while cleaning up the 5 feet of sediment that has accumulated along the 2,400 foot channel.
The floating dredge uses an adjustable auger that sucks a sediment slurry through a pipe at a rate of 1600 gallons per minute. The 300 foot hose delivers it to a retention basin on the lake bed. The sediment settles in the pond and the clean water returns to the channel through a filtration system.
Dredging operations started last week. As of Thursday, 4,000 cubic yards (4,400 tons) had been removed. Operations are expected to continue for another one to two weeks.
âWe are restoring the canal to its original elevation from which it is designed to operate,â said Ben Harclerode, chief operations and maintenance engineer for DWCD. “Sediment becomes a factor in the canal during very low water years like now.”
Project costs are expected to be approximately $ 225,000 and are shared between the Bureau of Reclamation and the DWCD. The dredge itself costs $ 40,000 per month to rent.
The accumulation of sediment in the channel has been gradual for more than 30 years. As a mitigation measure, the office plans to install rip-rap along the Great Cut Canal to slow sediment loading during dry years when the lake bed is exposed to erosion.
âUsually we don’t hit the bottom of active capacity until the tank starts to fill again. That’s why this year it’s such a big deal, âStump said.
An archaeological study was also carried out on the work area before the project.
The municipal and industrial water supplies of Cortez, Towaoc and Montezuma County are not threatened as the water is drawn from the Dolores Tunnel, a diversion point much lower below the surface of the lake.
Sediment buildup is not a problem at the Dolores Tunnel or the McPhee Dam, Stump said.