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Wildfires leave impact on California water source.
By Norman Pillsbury

People used to take air for granted. We breathe it in and out and it sustains our very existence, but people seldom thought about it. This year's fires have changed all of that. Similarly, many Californians today take water for granted. Turn the tap and out it comes. But our growing population has had an affect on water quality and availability. California is putting unprecedented demands on its water supply.

Most Californians live far removed from the front lines of water quality. The vast majority of our water originates in forested watersheds, out of sight, out of mind. Being disconnected from our water sources does not mean Californians can afford to be in the dark about the forest-water connection. The trend of restricting forest management, often in the name of protecting water quality for salmon and other aquatic species, is having serious, negative consequences on California's water quality.

Well-managed forests play a critical role in assuring that Californians have abundant, clean water. Sustainable forest management can reduce the cost of providing clean drinking water, provide spawning gravels and cool temperatures fish need, and mitigate the affects of storm flooding and mudslides. Forest where "hands-off" management prevails, conversely are more prone to overcrowding, wildfire and mudslides that can degrade water quality for years.

Healthy forests act like a filter and a sponge, helping to remove impurities and control runoff. In well-managed forest, the canopy, or tree branches and leaves, intercept rainfall, absorbing their erosive energy. Roots bind soils to resist erosion and stabilize slopes.

Despite the commonly held misperception, forest management or harvesting trees rarely leads to unacceptable increases in erosion or sediment reaching streams. In fact, studies have shown may cases where harvesting has led to no increase in sediment delivery to watercourses.

Forested watersheds left to nature, however, can wreak havoc on water quality for aquatic species and human consumption. Unmanaged forest can become overgrown, and create overly dense stands of trees stressed by the competition for nutrients.

Nature will ultimately thin its forest. But how?

Over many decades, insufficient soil moisture will lead to increased tree mortality. Disease and insect infestation will set in, creating conditions ripe for wildfire. Fire becomes the thinning agent, with more devastating effects on water quality than harvesting could produce.

Fire burns away the vegetation and duff that protects soils, exposing them to rain and erosion. Catastrophic fire D increasingly common in California where a century of fire suppression has led to unnatural fuel accumulations in many forest D does the worst damage. High-intensity fires do more than scorch the surface; they create a crust-like hydrophobic layer below the surface, an oil-based film that greatly slows the penetration of water. When rain follows catastrophic fire, water quickly saturates the exposed topsoil and hits the hydrophobic layer about two inches underground. Since the water cannot seep into the ground any further, the topsoil, ash and debris gets washed away. Mud fills nearby watercourses, devastating wildlife habitat and polluting drinking water.

This cycle of fire and erosion plays itself out all too frequently in California, where this year's fire becomes next year's flood. Southern California's 2003 and 2007 wildfire sieges had terrible consequences on water quality, as did the McNally, Fred’s, French Gulf, Power, Star, and Gap fires in Northern California. This year, more than 1.1 million acres have burned in California. Every significant wildfire leaves its costly mark on the state's water quality.

Managing forested watersheds to reduce the threat of catastrophic wildfire and enhance aquatic species habitat can help. The danger lies not only in the next inevitable fire, but in Californians sitting on the sidelines unaware of the connection between forest management and water quality. Our forests and water are at stake, right now and for the years to come.

Norman Pillsbury, Ph.D., is a professor emeritus of forest hydrology and watershed management at California Polytechnic State University, San Luis Obispo who has conducted research into watershed systems for more than 30 years.