Excerpt
Do you know where your tap water comes from? Two thirds of Oregonians get their water from surface sources and most Oregon tap water originates on federal lands, primarily national forests. Rain and snowmelt from forests are collected, treated (sometimes only minimally because the initial quality is excellent) and delivered to businesses, schools and household taps.
Intact forests are natural reservoirs that absorb, store, filter and gradually release water to forest streams. Logging and road building in forest watersheds degrades their natural hydrology. Water that once percolated slowly through stable soils runs off more quickly, carrying with it soil and other sediments. Logged watersheds have both earlier peak flows and greater storm volumes than do pristine watersheds that maintain more consistent flows through the hot summer months.
Logging reduces water quantity in other ways as well. An intact old-growth conifer forest "harvests" water from fog, as droplets condense from the moist air onto the needles, then drop to the ground. The surface area of the needles of a single old-growth Douglas-fir tree, if spread flat, would cover a football field. This "fog drip" contributes up to one third of all precipitation in Portland's Bull Run municipal watershed and, in many Northwest watersheds, may be the only source of summer precipitation.
Of course, when the trees are logged, fog drip no longer occurs. Moreover, without shade from the standing forest, the sun evaporates even more water from the soil. This decreases the amount of water "migrating" into the streams and rivers during the dry summer months when the demand for municipal and irrigation water is greatest. The combination of high summer demand (related to increased population and per capita consumption) and reduced supply (related to roading and logging) may ultimately force us to drink from dirty rivers full of agricultural chemicals, dioxin and sewage. Clackamas area water planners, for example, could be driven to tap the polluted lower Willamette River (home to toxic three-eyed fish), while timber interests continue to deforest and dewater the Clackamas River watershed.