RIST CANYON — Mulch dropped by helicopters and timbers laid across charred slopes mostly held through the recent flood — proving the value of a $10 million gambit Greeley and Fort Collins took to protect urban water supplies after wildfires last year.

Nobody expected the erosion controls to stay in place as a foot of rain fell in two days. But an aerial inspection Wednesday showed the wood mulch and timbers withstood the deluge and likely kept soot and sediment from flowing into the Cache la Poudre River.

A two-year effort to treat 5,600 acres of critical watershed continues this month, with helicopters dropping 1 ton heaps of mulch across steep slopes.

“I wouldn’t say mulch is a silver bullet, but to reduce sedimentation and erosion on burned slopes, it works really well,” said Todd Boldt, district conservationist for the U.S. Department of Agriculture’s Natural Resources Conservation Service said.

The early evidence here is sought around the Western states as wildfires increase in frequency and severity. The initial huge costs of fire suppression to protect homes lead to even bigger long-term costs of dealing with damage to water supplies.

Last year, as the High Park and Hewlett Gulch wildfires scorched 136-square miles along the river and near Milton Seaman Reservoir, officials immediately began consulting with Denver Water about how the 2002 Hayman Fire hurt Denver’s watershed.

Denver Water still is struggling, spending more than $10 million for the still-incomplete dredging of Strontia Reservoir.

The advice was adamant: “Stabilize the burned slopes as quickly as you can,” said Eric Reckentine, Greeley’s deputy director of water resources.

So leaders of both cities and Larimer County resolved to launch watershed protection work, even if federal funding wasn’t available. Some taxpayers at public forums questioned the wisdom. Aerial mulch drops cost around $2,000 an acre.

“This is our watershed, and we really didn’t want Strontia Springs to happen to our Milton Seaman Reservoir, so we started immediately,” Greeley water director Jon Monson said.

Eventually, federal agencies engaged — Agriculture Secretary Tom Vilsack has prioritized forest health and watershed protection — and Congress approved funding to reimburse 75 percent of overall costs, Reckentine said.

Wood mulch is an alternative to straw, which can spread cheat grass and other invaders into forests. The idea is to mimic dropped pine needles in healthy forests that help retain water and encourage new growth.

Trucks carry mulch from a mill in Wyoming. Ground crews cut burned trees and lay them across drainages.

“We worried rain would wash everything down the slopes,” said Colby Reid of Western States Reclamation, the contractor coordinating current air drops. The mulching “was not designed to withstand a 500-year or 1,000-year flood.”

When skies cleared, he, Boldt and others dropped into high terrain and found 98 percent of the mulch still in place.

While floods turned waterways temporarily black, chopper pilot Jim Hennessy said in his cockpit, water soon cleared enough that, on Wednesday, stones were visible in creek beds carrying water toward cities.

“We’re very pleased,” Fort Collins water manager Kevin Gertig said. “What we learned from the flood is that the timber-felling and mulch has been very successful.”

Bruce Finley: 303.954.1700 twitter.com/finleybruce or bfinley@denverpost.com

Editor’s note: This story has been edited to correct the title of Eric Reckentine, who is Greeley’s deputy director of water resources.