As the largest wildfire in Colorado history spread beyond 200,000 acres, Mark Kempton began to worry it would incinerate so much of the Fort Collins watershed that the city would be unable to guarantee water to its residents.

When the spring rains come next year, ash and debris will pour down the slopes of the Rocky Mountains and clog the city’s water intake on the Cache la Poudre River, said Kempton, interim director of Fort Collins Utilities, which oversees the water supply for Colorado’s fourth-largest city.