In the News

Storm Runoff is More Damaging in Disturbed Watersheds
date posted: December 22 2005

Lake Tahoe Report #92
Nov. 15, 2004
By John Cobourn and Heather Segale

One of the basic principles of watershed hydrology is that when heavy runoff occurs, whether from a rainstorm or rapid snowmelt, it produces more adverse impacts in a developed or disturbed watershed than it produces in an undisturbed watershed.

In a developed or disturbed watershed, contaminants are flushed from paved or unvegetated areas into streams. Streams also increase in depth and rate of flow, causing bank erosion. In addition, streams and storm-drain systems become overloaded, causing flooding in low-lying areas.

Much of the water quality degradation in Lake Tahoe is due to the effects of thousands of small disturbances -- roads, streets, parking lots, rooftops, soil compaction and loss of vegetation – that accompany urban and residential development.

Water that is conveyed to a lake by an undisturbed watershed is usually quite pure, because the watershed’s soils, plants and organisms act as a natural water-purification system. In many undisturbed and well-vegetated watersheds, more than 95 percent of rain and snowmelt percolates into the ground, where it is filtered on its way to the nearest stream. Natural drainage systems deliver precipitation to lakes slowly and safely. Water lingers in wetlands, and plant nutrients are used up by abundant water-loving vegetation, which in turn provides some of the Tahoe Basin’s most valuable habitat for birds and wildlife.

In subwatersheds that have been disturbed by urbanization, the pavement, rooftops and other impervious surfaces that we have created shed over 90 percent of all precipitation. Instead of being filtered by the soil, the water rapidly runs off the surface. Surface runoff collects sediment whenever it runs across bare soil. Runoff water typically concentrates and builds up speed in ditches and gullies, causing further soil erosion. Drainage ditches and storm sewers further increase the drainage efficiency of the area. When higher-than-natural flows reach streams, increased streambank erosion occurs, carrying a heavy sediment load into the lake.

Because of the harmful effects of runoff caused by impervious surfaces, urban planners and natural resource managers across the country have begun to impose limits on the location and amounts of such surfaces in their watersheds. At Lake Tahoe, impervious coverage limitations have been in effect for nearly 30 years. In addition, much of the ecosystem restoration work of the Lake Tahoe Environmental Improvement Program (EIP) creates infiltration systems to capture urban runoff and put it back into the ground, where it would have gone if we had not created urban developments.

Scientific research on the effects of soil coverage is also the basis for EIP Project # 16, Best Management Practices (BMP) Retrofit. This is the only EIP project that private property owners of the Tahoe Basin need to install. BMPs reduce runoff and soil erosion. TRPA ordinances require that property owners infiltrate into the ground all the runoff that could be generated on their property by a 20-year, one-hour storm. That is the amount of runoff that would flow off roofs and pavement if it rained an inch in one hour. To obtain free assistance in determining whether you need to install BMPs, you can call the Nevada Tahoe Conservation District, (775) 586-1610, ext. 28; or the Tahoe Resource Conservation District, (530) 543-1501, ext. 6.

In cases of extreme disturbance, such as in steep watersheds that have been stripped of their vegetation by wildfire, the adverse impacts of a heavy rainstorm can include not only severe water pollution, but also threats to human lives and property. Without vegetation to absorb the precipitation and hold the soil, slope surfaces can begin to liquefy and slide. At the bottom of such watersheds, where a canyon reaches an alluvial fan, a flash flood of mud, gravel, boulders and tree stumps can emerge with strong momentum. Such debris flows could occur at the foot of burned watersheds in Southern California or, closer to home, at Ash Canyon, Vicee Canyon, and the North Fork of Kings Canyon in Carson City. Fortunately, none of the 63 subwatersheds at Lake Tahoe have been completely burned in recent decades, and many organizations are working hard to keep it that way.