RENO, Nev. — The clarity of Lake Tahoe's cobalt blue water improved last year from its worst level in a half-century after weather and runoff returned to more normal conditions at the alpine lake straddling the California-Nevada line.

A dinner plate-sized disc used to measure clarity was visible at an average depth of 70.9 feet (21.6 meters) in 2018, scientists at the University of California, Davis, Tahoe Environmental Research Center said Thursday.

That's an improvement of 10.5 feet (3.2 meters) from 2017 when a winter with unusually heavy snow followed years of drought, sending sediment into the scenic lake known for its pine tree-lined beaches and ski resorts. The research center typically releases its annual clarity findings in the spring after analyzing the data it gathers each month in the previous year.

Scientists hope efforts to combat threats to clarity posed by development and climate change will eventually return Lake Tahoe to its historical clear depth of 100 feet (30.5 meters).

Dozens of public and private partners have been working to reduce stormwater pollution from roads and urban areas and restore streams and floodplains to reduce the amount of fine particles and nutrients that can cloud the lake, research center director Geoffrey Schladow said.

Clarity can swing widely day to day and year to year due to weather conditions and the flow from streams that can accelerate or slow algae growth and erosion that sends sediment particles into the water, he said.

"In 2018, Lake Tahoe's clarity regained the expected seasonal patterns that were disrupted by the extreme conditions of the previous year," Schladow said.

Scientists took 26 individual depth readings in 2018, including one in March that exceeded 100 feet (30.5 meters) of clarity. The worst readings typically are in the summer, and the best are in fall and winter.

A reading of 59.7 feet (18 meters) in 2017 was the worst in the 51 years Tahoe's clarity has officially been recorded. The second worst was 64.1 feet (20 meters) in 1997.

"We are thrilled to see Lake Tahoe's clarity improving from the all-time low of just 60 feet in 2017," said Darcie Goodman Collins, CEO of the League to Save Lake Tahoe. "These results encourage us to continue restoring critical habitat and improving our urban areas to keep pollution from entering our lake."

The new clarity measurement is in line with the five-year average of 70.3 feet (21.4 meters) — an improvement of nearly a foot (30 centimeters) from the previous five-year average.

Clarity was best when it first was recorded in 1968, with an average depth of 102.4 feet (31.2 meters). The disc used to measure it typically was visible at depths of 85 feet (30 meters) or deeper through the 1970s and hovered near the worst levels during a severe drought in the late 1990s.

Since then, efforts have been underway to restore natural wetlands and meadows displaced by past development. Those areas play an important role in filtering water before it enters the lake, Schladow said.

"Seasonal weather extremes will most likely drive greater swings in clarity from year to year in the future, so it's imperative we continue to invest in the lake's restoration to combat new and emerging threats," said Joanne Marchetta, executive director of the Tahoe Regional Planning Agency.