Why Boulder is investing in forest thinning to protect the city’s water sources ahead of the next fire

One of the biggest risks of wildfire — outside of flames welcoming themselves, uninvited, into our neighborhoods — is its potential effect on water quality. Ash and sediment from burn sites happily ride rainwater downhill to clog stream channels and lower water quality for all those downstream.

Last week, Boulder Reporting Lab spoke with Kate Dunlap, Boulder’s water quality project manager, about the city’s strategy for protecting local water sources after a wildfire. But just as fire mitigation is almost always more cost-effective than fighting a fire, so too is protecting water sources before the forest around them burns. Boulder is adhering to this economic lesson and addressing fire risk around the city’s water supply before fires begin of their own accord. 

“We’ve been scaling up investment in forest health projects in the upper watersheds to conduct forest thinning projects,” Dunlap said. “We’re restoring meadow area by removing a significant portion of lodgepole pine which have encroached upon ponderosa pine.”

The projects, which for now are focused on the Barker Reservoir watershed, also aim to remove trees infested with Bark Beetle or Mistletoe (a parasitic plant), and improve first responder access should a fire break out in that area.

Driving up Boulder Canyon Drive, or Highway 119, Barker Reservoir opens to your left with the Town of Nederland close behind. In terms of storage capacity, it holds less water than flood control reservoirs like the Cherry Creek and Chatfield reservoirs. When Barker is full, the excess water flows over the spillway and downstream into Boulder Creek.    

Located on Middle Boulder Creek and supplying roughly one third of Boulder’s water, much of Barker’s watershed (the area feeding the reservoir) is forested. Thinning that landscape back to its historical density, in addition to the previously cited benefits, also reduces the probable intensity of future fires.

“What you’re trying to avoid is high intensity fires, the ones where you’re going to get high [vegetation] mortality and impacts to the soil,” said Chris Wanner, Boulder’s vegetation stewardship senior manager who works with Dunlap on the forest health projects.

The soil impacts Wanner mentioned are called hydrophobicity: a condition caused by the waxy substance in plants turning into a gas while burning, infiltrating the soil, and then resolidifying around soil particles, making them repel water. “If you get a really hot burning fire, it can have impacts on runoff,” Wanner said.

Wanner said the Barker Reservoir watershed thinning projects will not just benefit our water systems, but the ecosystem as a whole.

“There’s the ecological benefit of restoring more natural conditions,” Wanner said. “You have the value to wildlife, the value to native plants — all of it sort of goes hand in hand as far as the [water] protection piece and the ecological benefits that come along with it.”

The City of Boulder is not the only one invested in these projects. The Town of Nederland, the U.S. Forest Service, Boulder County, Colorado State Forest Service and the Boulder Watershed Collective are all involved.

“We’ve been working with lots of stakeholders,” Kate Dunlap, the city’s water quality project manager, said. “It’s been a really good way to involve a lot of people because we’re not the only ones concerned about fire in that area.”

Yet even with many involved, more will be needed, as climate change continues to dry out Colorado’s already dry climate and heat both our summers and winters. Even if you don’t mind it hot, nobody likes being thirsty.

“In Colorado, we’ve been seeing this increase in wildfires across the state, really since the 2002 drought kicked off,” Dunlap said. “There’s been a substantial increase in the number of fires per year and the number of acres burned each year. We continue to see records broken in terms of the size of these fires and their severity.

“And when we get these high burn severity fires, they have a strong potential to impact water quality.”