CARSON CITY, Nev. (AP) — Lake Tahoe's famed clarity improved more than 4 feet last year from 2010, when the average annual clarity level was the second-worst on record, scientists reported Wednesday.

The annual report released by University of California-Davis' Tahoe Environmental Research Center and the Tahoe Regional Planning Agency said the average annual clarity of the lake was 68.9 feet. That represents a 4.5-foot improvement over 2010.

"The factors that contribute to lake clarity are complex, and not necessarily linked to factors occurring in the current year," said Geoffrey Schladow, director of the research center.

"For example, the 2011 clarity improvement followed a winter that was one of the wettest in recent years, something that is usually associated with clarity declines," he said. "Understanding what controls the long-term trends is at the heart of what we are attempting to do."

Overall, the lake's clarity has remained relatively stable since 2000, the report said.

Clarity is measured using a 10-inch white plate, called a Secchi disk. Researchers lower the disk in the water and record the depth at which it remains visible.

Measurements have been taken since 1968, when the disk could be seen to a depth of 102.4 feet. In 1997-1998, it reached an all-time average low of 65.1 feet.

Readings taken during the winter months of December through March last year showed an average clarity of 84.9 feet in 2011 — a 12-foot improvement over the previous year.

But summer clarity levels continued to decline. At 51.5 feet, last summer's clarity level was the second worst on record. Scientists point to last year's epic snowpack and late spring runoff as a possible cause, because snowmelt deposits nutrients and sediment into the lake straddling the Nevada-California line in the Sierra.

Last year, Nevada, California and federal officials pledged to restore Tahoe's clarity to 97 feet by 2076 — an ambitious goal that will require healing a rift between the two states over how a bistate compact set up more than 40 years ago by Congress is administered.

A bill passed by the 2011 Nevada Legislature and signed by Republican Gov. Brian Sandoval threatens Nevada's withdrawal from the compact unless changes are made to the voting structure to approve development projects in the Tahoe Basin.

During legislative hearings, Nevada lawmakers complained regulations and development projects were stymied by a pro-environmental bias from California representatives on the 14-member TRPA governing board.

In mid-February, California Sen. Darrell Steinberg, D-Sacramento, responded with a letter to Nevada lawmakers, calling the Nevada law "unnecessarily inflammatory and deeply counter-productive."

Both states have named delegations to discuss their differences, and an initial meeting is tentatively scheduled sometime in March.