In the News

How Does Shoreline Erosion Rank with Other Sources of Lake Pollution?
date posted: December 22 2005

Lake Tahoe Report #93
Nov. 23, 2004
By John Cobourn and Heather Segale

Over the years, scientists have struggled to determine the quantity of sediment and nutrients that enter Lake Tahoe from various sources. In the mid-1980s, rough estimates of the amount of sediment going into the lake suggested that, “it was as if 10 dump trucks emptied a full load of soil into the lake each day.” Current estimates are probably more precise, though perhaps less colorful.

The Lake Tahoe Watershed Assessment, produced in 2000 by the USDA Forest Service, contains the results of many scientific studies in its 735 pages. In this assessment, researchers assigned approximate amounts of sediment and nutrients to five major types of pollution sources. These sources are atmospheric deposition, stream loading, direct runoff from urban areas, groundwater, and shore-zone erosion. The total amount of sediment entering the lake was not estimated, but the authors, John Reuter and Wally Miller, estimated that stream loading alone contributed 11,300 metric tons (MT) of sediment per year to the lake. For nutrients, the following table gives the estimates for the five types of sources:

Yearly-sources for nitrogen and phosphorus for Lake Tahoe in metric tons (MT)
Nutrient Inputs Total Nitrogen (MT) Total Phosphorus (MT)
Atmospheric deposition 233.9 (56%) 12.4 (27%)
Stream loading 81.6 (20%) 13.3 (29%)
Direct runoff 41.8 (10%) 15.5 (34%)
Groundwater 60 (14%) 4 (9%)
Shore-zone erosion .75 (<1%) .45 (1%)
Source: Lake Tahoe Watershed Assessment (Murphy and Knopp, 2000)

The Watershed Assessment’s table shows the mass of total phosphorus and total nitrogen entering the lake by source. Note that it estimates over half of the nitrogen entering the lake is from atmospheric deposition, meaning it is falling out of the air. This is one reason why improved air quality could lead to better water quality in the lake. As for phosphorus, the largest single source is estimated to be direct runoff from urban areas in Tahoe’s 52 intervening areas that drain directly into the lake without first entering streams. This indicates that control of phosphorus is particularly important in Tahoe’s urbanized zones.

In the Watershed Assessment, the influence of shore-zone erosion is estimated to be the least important of the five sources. The 2000 report states that only .75 MT of nitrogen and .45 MT of phosphorus enter the lake each year from shoreline erosion. The effect of this source has been debated over the years, especially because the surface elevation of the lake or “lake level” is controlled by a 6.1-foot dam at the outlet to the Truckee River in Tahoe City. This dam is controlled by a water master according to a federal decree, in order to meet the needs of downstream water rights owners. While the lake level is currently below the natural rim due to the drought, during wet periods, the lake level rises, establishing a different high point along the shore. Annual shore-zone erosion rates probably increased after the 6.1-foot dam was constructed in 1913.

An article by Ken Adams and Tim Minor in the fall 2002 issue of the “Journal of Coastal Research” reported new findings on the extent of shoreline erosion in the lake between 1938 and 1998. The authors, from the Desert Research Institute, studied aerial photographs to track shoreline changes over the 60-year period. They estimated the amount or mass of sediment by first converting the area of lost shore zone to the volume of sediment removed. They estimated nutrient deposition to the lake by taking 80 samples of sediment at the water’s edge and analyzing them for nutrient content.

Adams and Minor’s findings indicate that shoreline erosion has probably played a larger role at Lake Tahoe than was estimated by the Watershed Assessment. The annual deposition of nutrients was estimated by Adams and Minor to be 1.8 MT of nitrogen and 2.0 MT of phosphorus. In contrast, the Watershed Assessment estimated that .75 MT of nitrogen and .45 MT were deposited in the lake each year (see table). So the 2002 article states that about twice as much nitrogen and four times as much phosphorus come from shoreline erosion as estimated in 2000.

With regard to the volume of sediment entering the lake from this shoreline erosion, Adams and Minor estimate an annual total of 7,150 MT per year, if averaged over the 60-year period since 1938. Though the Watershed Assessment did not make a corresponding estimate, Adams and Minor state that the average long-term sediment-loading rate from shore-zone erosion probably ranks second to stream loading, which Reuter and Miller estimated to be 11,300 MT per year in the 2000 assessment.

While the scientists work to improve our knowledge of pollution sources, we must continue working to reduce sediment and nutrient pollution with all the means at our disposal.