California Tahoe Conservancy: Progress Report

The Conservancy has undertaken a comprehensive program to reduce the sources of soil erosion and the amount of sediment and algae-encouraging nutrients that reach the lake.

This is essential to achieving water quality objectives, for runoff from eroded land is carrying an increased load of sediment and nutrients into the lake, a major factor in the dramatic growth of algae that is robbing Lake Tahoe of its world-famed clarity.

Increased Soil Erosion
With the urbanization of the past 40 years, various kinds of development have eliminated much of the natural vegetative cover, creating large areas of bare soil vulnerable to erosion. Exposure of the basin's highly erodible soil through the grading of roads and building sites on steep slopes can increase the rate of erosion by as much as 2,000 times. Such areas can erode for years, because it is difficult to re-establish vegetation with Tahoe's steep slopes, short growing season, cold winter climate, and relatively infertile soils. The basin's hundreds of miles of unpaved roads, and all the unpaved lots worn bare by parking, also contribute significantly to erosion.

Drainage and Runoff Patterns Altered
Development has also altered natural drainage and runoff patterns. Roofs, streets, and parking areas keep water from percolating into the ground, adding to the volume and speed of the runoff. In addition, graded areas -- where vegetation, pine needles and leaf litter are removed -- become nearly impervious to rain and snowmelt.

The increased flow into earthern drainage ditches serving most of the nearly 500 miles of roads on the California side of the basin strains the ditches' capacity, and erodes the ditches themselves. When this water reaches natural streams, the streams erode more quickly because they are forced to carry substantially more than their natural flows. Once this type of streambank erosion begins, it normally continues for many years, until the streams are widened and deepened to several times their original size. And most of this eroded soil ends up in the lake.

Some roadside ditches cross natural watershed boundaries and overload streams in other watersheds. Others bypass marshes and meadows and other filter systems and pour untreated water directly into the lake. All this has added significantly to the amount of sediment and nutrients in Tahoe's waters.

Loss of Marshes and Meadows
The conversion of marshes and meadows into subdivisions and commercial areas has decreased the area available for absorbing and filtering out the sediments and nutrients before they reach the lake. More than 75% of the marshes and 50% of the meadows have already been filled in for development in the Tahoe Basin.

The cumulative impact of these changes is to greatly increase the amount of nutrient-rich sediment entering the lake--now as much as 67,000 tons a year. This is about 19 times the natural deposition rate. As a result, the growth of algae in Lake Tahoe has doubled since 1950.

To deal with this massive problem, the Conservancy is implementing a comprehensive program to reduce the sources of soil erosion, stabilize the transport of runoff, and create special basins to filter out sediment and nutrients, restoring natural marshes and meadows for this purpose, also.

Reducing Sources of Erosion
Reducing the sources of erosion requires such measures as planting trees, shrubs and grasses on the bare soil of road banks and shoulders; stabilizing steep, eroded slopes with retaining walls or rock when revegetation is not practical; and paving heavily-used dirt roads. Paving protects the road surface, but ways must then be found to store, treat, or infiltrate the concentrated runoff from the paving or much of the benefit is lost.

Revegetation is preferred since trees, shrubs and grasses absorb nutrients, cushion the impact of raindrops, and hold soil in place with their roots. They also help increase water infiltration into the soil and slow the velocity of runoff. In the long run, vegetation increases soil stability with age, and is self-healing, self maintaining, and attractive.

Stabilizing Transport of Runoff
Stabilization of the systems that collect and carry away runoff involves a variety of activities, such as restoring streams; installing concrete curbs and gutters along roads to replace eroding earth ditches; widening drainage ditches and installing larger culverts to handle higher flows; and slowing the velocity of the runoff by vegetating or rock-lining ditches.

Encouraging Natural Treatment
The flows are then routed into specially created marshes and meadows where the sediment can settle out and the nutrients can be taken up by the wetland plants and soil bacteria. Improving techniques have increased the effectiveness of these efforts, with traps at the inlets to remove coarse sediments, and conveyance systems to carry the runoff into the settlement basins after the coarse material has been removed.

Other improvements, such as perforated pipe or buried plastic domes, have been developed to increase sub-surface infiltration. Wherever possible, degraded marshes, meadows, and wetlands are restored to provide natural treatment, for these provide increased wildlife habitat and open space as well as other benefits.

The Conservancy is currently working with a number of local governments -- El Dorado County, Placer County, the City of South Lake Tahoe, the South Tahoe Public Utility District, the Tahoe City Public Utility District, and the North Tahoe Public Utility District -- to implement a comprehensive erosion control grants program for the basin. Its purpose is to repair portions of watersheds that are contributing large amounts of sediment to the lake, and to mitigate the unavoidable effects of current and future development in the basin. Local governments play a key role in this effort since they control many of the areas involved and are in a position to design, construct, and manage the projects.

Program Status
To date, the Conservancy has awarded grants totalling more than $35 million for 71 projects. These will result in the revegetation of about 120 acres of disturbed land and the construction of 82 miles of roadside drainage facilities, 13 miles of rock-lined and vegetated channel protection, and 2.6 miles of retaining walls. Some 360 sediment traps and treatment basins will also be constructed. More than 50 acres of wetland and meadow are being restored as well.

Two projects involving innovative two stage runoff treatment systems are of special note. The Conservancy is providing funds to the City of South Lake Tahoe and Placer County for projects in which runoff will be collected in large basins designed both to store water and drop out sediment. The stored urban runoff will then be discharged slowly through newly created wetlands where nutrients and other pollutants will be removed by plants and soil bacteria. This approach holds particular promise for treatment of runoff in highly urbanized areas where runoff volumes are great and space is limited.