About 20,000 years ago, glacial dams plugged up Lake Tahoe and raised its surface 15 feet higher than it is today.
Water covered the land and eroded soil into the lake. But erosion and lake clarity weren't concerns then because the lake had time to heal itself.
Times have changed. With construction of a dam at Tahoe City in the 1880s, the level of Lake Tahoe has been kept unnaturally high for years, causing a substantial amount of erosion.
Today there is a greater demand for Tahoe's water and more reasons - agriculture, fisheries, recreation, consumption - to allow the level of the lake to stand 6 feet above its natural rim, in essence creating a reservoir.
Between 1938 and 1998, the elevated level caused about 2 million square feet of shorezone to erode and created 550,000 square feet of new beach, according to a study released by the Desert Research Institute.
"All of a sudden, there's water on landscape that's not been under wave attack for 15,000 years," said Ken Adams, a DRI geologist who studies lakes and rivers. "So there was rapid change."
Some areas at South Shore - the most erosive part of the basin because it is flat and comprised of softer soils - have lost almost 50 feet of land in the shorezone.
But Adams says that shorezone erosion, in the grand scheme of the basin, does not appear to have had a large impact on the health and clarity of Lake Tahoe.
Adams has estimated that shorezone erosion is responsible for less than 5 percent of the phosphorous and nitrogen loading that plays a major role in reducing the clarity of the lake.
But, Adams said, the data in his 2002 paper - "Historic Shoreline Change at Lake Tahoe from 1938 to 1998: Implications for Water Clarity" - is important because it helps to establish facts about the lake that can be fit into a larger puzzle.
"It is important to understand how much erosion has occurred, where it has occurred, and how much sediment was introduced to the lake from that source," Adams, 40, said. "Those questions we're pretty far down the road in quantifying."
Funding for the paper was provided by Tahoe Regional Planning Agency and the U.S. Bureau of Reclamation.
"It's something we pay attention to," said Larry Benoit, water quality program manager for the TRPA. "There is not necessarily a desire or anything to try and keep (the lake) at a certain level."
Would the TRPA ever decide the dam is bad for the lake and ask that it be removed? No way.
"There'd be a big lawsuit over that from the folks downstream," Benoit said.
A water master appointed by the federal government decides how much should be released from Lake Tahoe.
The water, as well as water from six other reservoirs around Truckee, flows to Truckee, Reno, Carson City, alfalfa farmers along the Truckee and to Pyramid Lake, which belongs to the Paiute tribe.
Adams agreed that removing the dam is not a reasonable option.
"It's a politically impossible scenario," Adams said. "There's no way that dam is going to be removed. The cities of Reno and Sparks depend on that, and the flow to Pyramid Lake depends on that ... to change that is just fantasy."
And removing the dam at this point might be useless. Adams said the shorezone is close to reaching an equilibrium with the level of the lake. The rate of erosion has been decreasing and will continue to decrease through time.
Shorezone erosion still occurs, Adams said, but it typically takes place during storms when the level of the lake is at least 4 feet higher than its natural rim of 6,223 feet.
Gregory Crofton can be reached at (530) 542-8045 or by e-mail at gcrofton@tahoedailytribune.com
