Study probes shoreline erosion's effect on Tahoe's clarity

RENO - Shoreline erosion may contribute more to Lake Tahoe's declining clarity than previously thought, a new study by the Reno-based Desert Research Institute shows.

The $100,000 study commissioned by the Tahoe Regional Planning Agency found that 3 percent of the phosphorus entering the lake every year comes from shoreline erosion.

Phosphorus feeds the algae that is stealing the lake's famed clarity at a rate of a foot or more per year, and is thought to be the controlling factor in efforts to reverse this trend.

But given its small contribution to the problem, controlling shoreline erosion may not be a key component of the effort.

According to TRPA public affairs coordinator Pam Drum, compared with other sources, shoreline erosion contributes the least phosphorus to the lake. She said stream loading and direct runoff combine to contribute 61 percent of the problem.

Nevertheless, its contribution was somewhat of a surprise. "The numbers I came up with were larger than what was thought," DRI researcher Ken Adams said.

The study found that 228,000 square yards of Lake Tahoe's shoreline were lost to erosion during a 60-year period ending in 1998.

Nearly 472,000 tons of sediment eroded into the lake over that time, contributing more than 128 tons of phosphorus to Tahoe's waters at the rate of more than two tons per year.

While that exceeds previous estimates by the University of California's Tahoe Research Group, shoreline erosion still contributes less phosphorus than other sources such as atmospheric deposition, streams and runoff.

The greatest source is runoff, which contributes an estimated 17 tons of phosphorus to the lake each year, or 33 percent of the total.

"Shorezone erosion is still at the bottom of the pile," Adams said, but cautioned that it shouldn't be ignored. "It's certainly a concern to property owners and it is a source of sediment and nutrients to the lake."

The study was launched in March to determine how much of a role shoreline erosion plays in Tahoe's declining clarity.

Many scientists think the problem increased significantly after a string of recent wet winters.

Most scientists have thought that prevailing winds and resulting wave action would make Tahoe's north shore more vulnerable to erosion.

"Really that doesn't appear to be the case," Adams said.

Instead, Tahoe's south shore - mostly made up of lake and stream sediments as opposed to the north's rocky shoreline - experienced the most erosion over the years.

"Where the erosion is occurring is a function of what type of materials are at the shorezone," Adams said.

A second phase of the DRI study will try to determine how piers, seawalls and other similar structures affect erosion.

TRPA will meet early next year to finalize its near-term blueprints for healing Lake Tahoe. These plans will include specific projects to be funded through the just-enacted Lake Tahoe Restoration Act.

Rochelle Nason, executive director of the League to Save Lake Tahoe, said she expected proposals for controlling shoreline erosion to be based on property preservation, and not on reducing the flow of algae nutrients such as phosphorus into the lake.

Incline Village resident and Crystal Bay Club owner Chuck Bluth questions the value of such studies when their outcomes are, in his opinion, self-evident.

"Why are we spending $100,000 on studies to tell us that shoreline erosion affects lake clarity. Common sense tells us that," he said. "A simple fix for this problem would be to keep lake levels a bit lower."


Use the comment form below to begin a discussion about this content.

Sign in to comment