Oct 06 2014Share

Wildfires Raise Cost Of Treating Valley Drinking Water

By Kathleen Ferris

Wildfires that tear through Arizona's north country destroy our playgrounds, ravage ecosystems, kill and displace wildlife, and pollute the Valley's water supply. If the wildfires are not contained, the Salt River Project fears more of these hotter and faster-moving fires also could threaten the life of its reservoirs. To slow and cool the fires, SRP is leading an effort to thin the state’s ponderosa pine forests to reduce the threat. That means cutting down trees, which is expensive and controversial.

The Problem

Before 1850, ponderosa pine forests in the state’s unsettled north country averaged 40 large, rugged trees to an acre. During monsoon season, lightening caused fires smoldered through the forests’ low underbrush of grasses, seedlings and pine needles. The mature, tough-barked ponderosa pines were nearly untouched.

Animals brought by settlers fed on the forest grasses and left the soil bare. That encouraged the growth of seedlings. Slowly, the forest became thick with smaller, weaker trees.

After decades of suppressing fires, the forests changed.

Today’s ponderosa pine forests can have up to 1,000 trees and more per acre. The underbrush includes young trees in a thick layer of pine needles. Fire fighters call this underbrush “ladder fuel” because it carries fire up to the crowns of the largest pines in the forest where the flames grow hotter, move faster and are more destructive. A typical wildfire can progress at three miles a day. During the 2011 Wallow Fire firefighters reported flames moving up to 15 miles a day as winds whipped the flames.

Ponderosa pines near Fort Valley is indicative of the dense conditions that exacerbate fire risks.  Photo: Ecological Restoration Institute, NAU

Ponderosa pines near Fort Valley is indicative of the dense conditions that exacerbate fire risks. Photo: Ecological Restoration Institute, NAU

These hot crown fires leave behind devastated landscapes covered in a thick layer of sterile soil that cannot absorb water. Storm runoff brings tons of soil into the rivers, lakes and streams that feed SRP’s reservoirs. This erosion can continue for up to five years after a fire.

SRP provides half of the Valley’s drinking water. The sediment makes the water delivered by SRP difficult and expensive for cities to treat and turn into drinking water. After large fires, such as the Wallow and Rodeo-Chediski in 2002, that soil also contained ash and carbon making it even more of a problem to treat.

The sediment from these large and unnatural wildfires begins to grow in the reservoirs. The more sediment, the less water the reservoirs can hold. SRP fears that if these newer and hotter wildfires are not controlled, sediment could fill the reservoirs until they are useless. This has already happened to smaller reservoirs in Colorado, similar to Arizona’s C.C. Cragin Reservoir near Strawberry.

The Solution

Research shows that returning the state’s ponderosa pine forests to an average of 100 to 150 trees per acre would significantly reduce the likelihood of crown fires. That would allow fewer trees to grow larger and tough enough to survive wildfires. Wildfires would burn lower and cooler and move slowly through the underbrush without small trees to feed on.

Conservation groups have mixed reactions to forest thinning, but the biggest hurdle is expense. Thinning all of Arizona’s ponderosa pine forests would cost nearly $2 billion. Private industries just aren’t that enthusiastic about cutting small trees because the wood products, such as chips and mulch and home heating pellets, are not as lucrative as planks from larger trees.

Where we are now.

SRP in partnership with the National Forest Foundation has started the Northern Arizona Forest Fund. The fund encourages businesses, individuals and organizations to donate time and money to specific projects that protect the Valley’s watersheds. For example, this year the fund will pay to thin about 1,000 acres around Upper Beaver Creek, 30 miles south of Flagstaff. Chainsaw crews will clear 150 acres of the thickest forest, taking out trees 9 inches or less in diameter. Then the project will use prescribed fire to clear underbrush from the 1,000 acres.

Now consider this: Arizona has over 3 million acres of ponderosa pine forests.

Find out more about SRP’s forest restoration efforts and the statewide Healthy Forests, Vibrant Economy conference Oct. 16 and 17.

For 45 years, Arizona Municipal Water Users Association has worked to protect our member cities’ ability to provide assured, safe and sustainable water supplies to their communities. For more water information visit

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