The drought in Southwest Colorado has triggered a dredging operation on McPhee Reservoir to remove sediment that blocks delivery of water to irrigation canals during the reservoir’s lowest level.
The U.S. Bureau of Reclamation and Dolores Water Conservancy District are working to remove an estimated 13,000 cubic yards (14,300 tons) of sediment from the Great Cut channel that connects to the Dove Creek Canal and Narraguinnep Reservoir.
The amount of sediment to be removed is equivalent to 1,300 dump truck loads, or a volume of dirt measuring 50 feet long, 50 feet wide and reaching 230 feet high.
Sediment loading in the channel blocked access to the last 5 feet of the active irrigation supply in McPhee, said Robert Stump, Cortez manager for the Bureau of Reclamation.
“We would not be able to deliver that water without doing this sediment removal,” he said. “The goal is to get all the allocated water to irrigation use.”
This year, McPhee Reservoir, which has a capacity of 380,000 acre-feet, is expected to drop to its inactive supply pool of 151,000 acre-feet, the lowest level since it first filled in the mid-1980s.
Sediment buildup in the channel becomes a pressing issue when the lake is at very low levels. It obstructs the last portion of the active irrigation supply to an intake drain at the Great Cut Dike that feeds the irrigation canals.
A bathometric survey revealed two 5- to 10-foot peaks of sediment have formed where drainages from the dry lake bed reach the channel. Heavy rains have accelerated the sediment loading, said Rob Walker, maintenance manager for DWCD.
The dredging operation is focused on removing the two high points while also cleaning out the 5 feet of sediment that has accumulated along the 2,400-foot channel.
The floating dredge uses an adjustable auger that sucks up a sediment slurry into a hose at a rate of 1,600 gallons per minute. The 300-foot hose delivers it to a retention pond on the lake bed. The sediment settles in the pond, and clean water returns to the channel through a filter system.
Dredging operations began last week. As of Thursday, 4,000 cubic yards (4,400 tons) had been removed. Operations are expected to continue for one to two more weeks.
“We are restoring the channel to its original elevation that it is designed to work off of,” said Ben Harclerode, DWCD chief engineer of operations and maintenance. “The sediment becomes a factor in the channel during really low water years like now.”
Project costs are expected to be about $225,000, and are being shared between the Bureau of Reclamation and DWCD. The dredge itself cost $40,000 per month to rent.
Sediment buildup in the channel has been occurring gradually for more than 30 years. As mitigation, the bureau plans to install riprap along the Great Cut channel to slow sediment loading during dry years when the lake bed is exposed to erosion.
“Usually, we don’t reach the bottom of active capacity before the reservoir starts filling again. That is why this year it is a big issue,” Stump said.
An archaeological survey was also conducted of the work area before the project.
Municipal and industrial water supply for Cortez, Towaoc and Montezuma County is not at risk because the water is drawn from the Dolores Tunnel, a diversion point that is much lower under the lake surface.
Sediment buildup is not an issue at the Dolores Tunnel or at McPhee Dam, Stump said.