Trapping Sediment
Before It Hits Cheesman

Denver Water continues to feel the aftershocks of the giant Hayman Fire two years ago, which burned 138,000 acres of forested watershed along the South Platte River, including 7,250 acres around Cheesman Reservoir (Map).

Sediment from the scorched region above the reservoir has filled 15 percent of a year-old rock sediment trap on Goose Creek, which feeds into Cheesman. Meanwhile DW is building a similar trap on Turkey Creek. The sediment traps consist of piles of rock, with stones ranging from 6" to 36".

Sediment trap under construction on Turkey Creek.

The stones allow water to flow through, leaving sediment and debris behind. The trap slows water down, and heavier sediment settles to the bottom.

The Goose Creek trap was built in March 2003 under a $870,000 contract; the new Turkey Creek trap will cost $560,000. The trap across Goose Creek is about 20 feet high and 180 feet wide, while the one spanning Turkey Creek will be 45 feet high and 180 feet wide. Even though twice as high as the Goose Creek trap, the Turkey Creek trap will hold only half as much sediment because the area upstream from the trap is narrower.

Late this spring, DW began removing sludge collected above the Goose Creek trap. This will take about two months. The work on Goose Creek will be the first opportunity for DW to determine how much sediment a trap collects and how fast.

The Buffalo Creek Fire above Strontia Springs Reservoir in 1997, followed by heavy rains, showed that sediment can have severe impact on a reservoir, if not held in check by trees and ground cover. That wildfire scorched 11,000 acres of watershed, and the rains sent much of the soil into Strontia. This reduced the reservoir's potential life span by decades. Increased sedimentation reduces of storage capacity and water quality.

Once the sediment is in the reservoir, the removal costs of dredging can be three to four times more than the cost of conventional excavation methods used above the reservoir.

In Cheesman's case, sediment from the Hayman Fire enters through the South Platte, Goose Creek, and Turkey Creek. Constructing a trap on the Platte would have been difficult and expensive. Based on average rainfall, it is estimated that half the sediment in the reservoir would come from the two creeks.