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A July 31 afternoon thunderstorm dumped nearly three inches of rain in Cheesman Canyon in less than an hour. The deluge sent decomposed granite into the South Fork of the South Platte River below Chessman Reservoir, creating as many as 20 sediment piles that nearly blocked the river. (Pat Dorsey, Special to The Colorado Sun)

This story first appeared in The Outsider, the premium outdoor newsletter by Jason Blevins.

In it, he covers the industry from the inside out, plus the fun side of being outdoors in our beautiful state.

Pat Dorsey has been fishing the granite-walled Cheesman Canyon below Cheesman Reservoir for 31 years. The renowned angler has seen some changes in the legendary fishery that rewards hiking anglers with deep pools filled with brown, rainbow, cutthroat and even some cut-bow trout.

The Hayman fire in 2002 scorched 138,000 acres around the reservoir and canyon, devastating the fishery. A summer thunderstorm five years ago funneled tons of decomposed granite into the South Fork of the South Platte above and below Cheesman Reservoir, ending the fishing season. 

But nothing compares to the storm Dorsey weathered on July 31. He measured almost 3 inches of rain in an hour as he huddled with angling clients in a cave that Monday afternoon. 

“It was like someone was just pouring buckets on us,” Dorsey said. “You would think the burn scar from Hayman would have healed by now but clearly it has not.”

Dorsey fled Cheesman Canyon during a lull in the storm and returned earlier the month. He’s never seen such damage in the 2.5-mile stretch of South Platte below the reservoir. His pictures show at least 20 sediment fans, with some of them reaching two-thirds across the channel and stretching as wide as 70 yards. 

“That habitat has been severely impacted. There are holes that were 8- to 10-feet deep that you can walk across now,” Dorsey said. “Those pools start to fill in and the fish start to get skinny and we end up losing a lot of fish. It’s pretty sad there.”

Downstream of Cheesman Canyon, through the Deckers stretch of the South Platte, Denver Water’s critical Strontia Springs Reservoir funnels 80% of the utility’s water to 1.5 million customers. The state’s largest water utility has been struggling with rain-triggered erosion of reservoir-choking granite in Strontia Springs for decades. 

Decades ago, a typical year saw as much as 50,000 cubic yards of decomposed granite fill the reservoir. Then wildfires in Denver’s foothills above the South Platte increased that erosion. Today, more than 13% of the reservoir has been filled with sediment, taking up capacity that could hold enough water to serve 4,000 homes for a year. 

Denver Water estimates about 20,000 cubic yards of sediment was swept into the South Fork of the South Platte River by recent rainstorms. A veteran angler counted 20 sediment piles like this in Cheesman Canyon below Cheesman Reservoir. (Pat Dorsey, Special to The Colorado Sun)

In 2010, Denver Water spent $18.5 million to dredge 228,000 cubic yards of sediment from Strontia Springs Reservoir that had washed into the South Platte upstream. Denver Water, the U.S. Forest Service, the Colorado State Forest Service and the Natural Resources Conservation Service have spent $96 million since 2010 to reforest 120,000 acres in the South Platte Basin in an effort to rebuild forests to help them absorb rain. The collaboration has planted 1.4 million new trees in burn zones. 

Denver Water is planning another project to remove sediment from Strontia Springs Reservoir “in the coming years,” utility spokesman Todd Hartman said. 

Hartman said the utility estimates 20,000 cubic yards of debris has entered the South Fork of the South Platte above Strontia Springs Reservoir from a pair of big storms in the past two weeks. Not all that sediment will make it to the reservoir, he said. 

“Any impacts to water quality in the reservoir were short-term and included brief spikes in turbidity. Those effects are readily managed through our treatment plants,” Hartman said. 

Hartman said the challenges with sediment in the South Platte highlights the importance of expanding Gross Reservoir. The work underway there now will add capacity to the north end of Denver Water’s system and “reduce our heavy dependence on the South Platte system that anchors our south side.”

Dorsey, a lifelong angler, is more worried about his beloved trout in one of the most treasured stretches of river in the state. 

“People say this is normal and natural, but it’s still a tragedy that is out of the norm,” Dorsey said. “Yes, it’s Mother Nature doing what she does, but it’s gut-wrenching, man. I guide in that canyon every day in the summer and this has got me inside out.”

Jason Blevins lives in Eagle with his wife, two teenage girls and a dog named Gravy. He writes The Outsider, a weekly newsletter covering the outdoors industry from the inside out. Topic expertise: Western Slope, public lands, outdoors,...