In the News

New Research Points to Real Causes of Clarity Loss in Lake Tahoe
date posted: December 27 2005

Lake Tahoe Report #109
March 18, 2005
By John Cobourn and Heather Segale

Scientists have known for decades that Lake Tahoe’s water is steadily losing transparency. Research on the causes of this clarity loss is being used by many agencies in the Tahoe Basin to develop better ways to reduce water pollution.

In a new report, still in draft form, the California Tahoe Conservancy has asked some key questions about how to make storm-water treatment basins more effective. Alan Heyvaert, of the UC Davis Tahoe Research Group, will present the key findings of this report at the Lake Tahoe Interagency Monitoring Program meeting at 9 a.m., April 6.

In the new report, Heyvaert references the research of Ted Swift, also of UC Davis, which finds that very fine particles of inorganic matter (tiny bits of rock) are the most important factor in Tahoe’s clarity loss. Measured in thousandths of millimeters or “microns,” these silt and clay-sized particles are only 0.5 to 10 microns in diameter. (As a reference, there are 25,400 microns in an inch.) This is an important finding because these fine sediments are also the most difficult to remove from runoff heading for the lake.

There are several reasons why these very fine sediment particles are so damaging to the lake’s water clarity. Such particles are very common, originating in runoff from roads, construction sites, unpaved parking areas, unpaved streets and driveways, mobile home parks, roadside ditches and other bare soil areas. Additionally, particles of this size remain suspended in the lake’s water for months and even years before settling to the bottom. Finally, fine sediment particles scatter light very efficiently, clouding the water.

Theoretical calculations show that a 10-micron soil particle of typical shape and density will take over 100 days to settle to the average depth of the lake floor (313 meters or 1,027 feet) under ideal conditions, while a 4-micron particle will take almost 1,000 days to settle out. However, since windy storms usually mix the lake water to a depth of 300 meters or more each winter, many of these particles are brought back up to the surface before they can reach the bottom.

Particles that are 0.5 to 10 microns are extremely effective at scattering light, which is a major cause of clarity loss. Heyvaert interprets the research of Ted Swift to say that about 50 percent of the light scattering that causes clarity loss in Lake Tahoe is caused by fine inorganic sediment particles, while only about 30 percent of the clarity loss is caused by algae particles. The other 20 percent of clarity loss is caused by light absorption by pure water and dissolved organic matter.

When one peers down close to 20 meters at a Secchi disk (an object lowered in the lake to help determine its clarity), it is obscured from view more because the fine sediments in the water scatter the sunlight than because they absorb the light or block the view of the disk. Much of the light reflected off the Secchi disk is scattered off to the side by the surfaces of the suspended particles, preventing the light from reaching the observer’s eyes on the research vessel. In fact, about four times more light is scattered than is absorbed by contaminants in the water.

If resource and property managers at Lake Tahoe want to improve the lake’s clarity, they must retain fine sediments up in the watershed, in the soil-plant community. Erosion must be controlled by implementing best management practices (BMPs). Property owners must infiltrate runoff as near to its source as possible, before it can erode soil. One effective method is to plant native and adapted plants, such as those on the TRPA Recommended Plant List, which are effective at infiltrating water and holding soil in place.

To remove sediment that is suspended in runoff, public jurisdictions often construct storm-water detention basins that cause the sediment in runoff to settle to the bottom of the basin. Unfortunately, storm-water detention basins are not usually effective at removing particles smaller than about 20 microns in diameter. Data is now being collected to determine if wetland retention basins, designed as biological systems that treat storm water, might be more effective at removing fine sediments suspended in runoff.

If wetland retention basins are more effective, then more of them will be designed and constructed. The Tahoe Basin is employing adaptive management. Scientists are doing the research on the lake, and the jurisdictions are applying the knowledge to make practices more effective at preventing lake pollution.