After a wildfire’s smoke dies down and its flames reduce to smoldering embers, most of Boulder’s residents turn their attention away from the singed foothills and return to the banalities of everyday life.
But for Kate Dunlap, water quality project manager for the City of Boulder, the aftermath of a fire means putting in place a multi-tiered approach to protect the city’s water sources.
“I work on implementing projects and policies specifically designed to protect the city’s water supply,” Dunlap said. “And wildfire, being one of the top risks to the water supply, is a significant portion of my job.”
In semi-arid climates like Boulder’s, fire plays a pivotal role in the decomposition process. Nitrogen, sulfur, phosphorus and carbon are released from burned flora as a gift to future generations. But that gift can’t be enjoyed if rain washes it from the landscape. Indeed, when moved from its place of origin, remnants of fire quickly become a bane to those downstream.
When fires burn hot enough, especially from a buildup of fuels on the forest floor, the soil underneath effectively cooks. This causes a condition called hydrophobicity: when soils repel water for several weeks to several years. Combine that repulsion with a lack of vegetative matter to absorb moisture (as fire tends to burn all pine needles, leaves and shrubbery) and there is a drastic reduction in the amount of water retained by the landscape.
Now enter a violent, wet Colorado thunderstorm, and ash and sediment are sent trundling away towards streams, rivers and reservoirs — some of which provide Boulder its drinking water.
For Dunlap, treating and preventing this post-burn runoff fills much of her days. One aspect of that work includes doing everything possible to keep tainted water away from the city’s supply points.
“In some areas we have diversion capabilities,” Dunlap said. “When there was the Cold Springs fire in 2016, we monitored flow conditions and weather, so when there was the first storm event up there we turned off some of our ditch diversions to prevent any potential runoff from getting to Boulder Reservoir.”
In 2016, Boulder Reservoir was still a primary water source for Boulder. Since 2020, however, the reservoir has been relegated to a backup after the completion of the Carter Lake Pipeline, which added yet another source to the arsenal serving the city. Others include North Boulder Creek and Barker Reservoir.
“Boulder’s water supply is pretty diverse,” Dunlap said. “Which is great.”A diverse water supply means resiliency should one supply point become compromised by, say, ash and sediment — a likely possibility when much of the city’s water comes from forested areas prone to wildfire. The recently released 2022 City of Boulder Drinking Water Quality Report identifies wildfire as a primary threat to the city’s water supply.
“Planning for wildfire is a key component of our water protection plan,” Dunlap said.
Even fires that pose no threat to the city’s water sources require attention. Because of the placement of the NCAR Fire, there was never concern for Boulder’s drinking water. “However,” Dunlap said, “we do still have a vested interest in protecting water quality and aquatic life in any water system in the city.
“The day after the [NCAR] fire took place, we conducted an assessment of the burn area. To protect water quality, we deployed a series of catchment devices in Bear Creek that allow water to pass through but retain sediment and ash from the burn scar.”
Catchment devices, or sediment booms, are often around street drains near construction sites and look like tubes filled with straw. They can be deployed in streams — as was done in Bear Creek after the NCAR Fire — or on the burn scar itself to keep the hillslope from washing into the nearest canyon at the first sign of rain. Burn scar boom deployment is more likely when the burned area is close to a water source, necessitating increased diligence.
“If we were to have a wildfire in the source water system, deploying these types of catchment devices [on the burn scar] would definitely be a strategy we could use,” Dunlap said. “You can also deploy wood mulch which is a very common and very effective strategy to hold the hillslopes in place.”
Dunlap referenced the East Troublesome Fire and the Cameron Peak Fire as examples of what Boulder might implement should wildfires burn near the city’s water sources.
“For both of those fires, a lot of different strategies have been used simultaneously to hold hillslopes in place, encourage regrowth of plants and vegetation, and minimize ash and sediment runoff,” Dunlap said. “That kind of layering of different kinds of strategies would be something we would use as well.”
Should any ash or sediment saunter past those catchment devices and wood mulch, however, Dunlap and her colleagues would be closely tracking the turbidity — or muddiness — of the water.
“We would most likely deploy a continuous water quality monitoring device in the system,” Dunlap said. “So we could continuously monitor water quality and help inform if we need to turn off an intake or operate a diversion to protect downstream water systems.”
And if a monitoring device alerts to levels of sediment too high to filter? Thankfully Boulder has other water sources to lean on.