World renowned for its crystal clear, azure water, Lake Tahoe straddles the Nevada-California border.

At 1,645 feet, it is the second deepest lake in the United States and the 10th deepest in the world. Lake Tahoe sits 6,225 feet above sea level, and is 22 miles long and 12 miles wide.

Approximately 40 percent of the Tahoe Basin’s rain and snow fall directly into the lake, contributing to Lake Tahoe’s legendary clarity. The remaining precipitation drains through granitic soils, which are relatively sterile and create a good filtering system.

Formed by uplifting, glaciation and erosion about 2 million to 3 million years ago, Lake Tahoe today is facing major challenges.

Lake Tahoe Challenges

Lake Tahoe’s vitality is threatened by several factors including invasive species such as trout and bass, stormwater runoff and increasing temperatures as a part of ongoing climate change. The historic five-year drought, which ended in 2017, led the lake to be the driest it has been in a century. But the historic precipitation that followed caused the lake to fill to the brim, making popular beaches smaller in size.

Lake Tahoe’s famed clarity has also declined in the last 40 years due to accumulated effects of development.

Experts believe the biggest problem affecting Lake Tahoe’s clarity is algae. Because the lake is a contained body of water with only one outlet, the Truckee River, the lake has difficulty “flushing,” despite inflow from several surrounding creeks.

Instead, urban runoff such as feritlizers feeds algae growth in the lake while other sediment flushed into the lake causes it to be less clear. Once in the water, these ultra-fine sediments are one of the primary culprits for loss of clarity as they reflect sunlight.

Meanwhile, many of the basin’s natural wetlands have been filled in to make room for development, which increases runoff into the lake  and decreases the natural filtration process provided by wetlands.

Lake Tahoe Restoration Efforts

Since the late 1990s, the Tahoe Regional Planning Agency, a joint planning effort between California, Nevada and the federal government, has worked to implement a comprehensive environmental improvement program. A summit led by President Clinton in the 1990s helped draw attention to the issues facing the lake.

Financed by nearly $1 billion in local, state and federal funds, the program aims to restore and maintain the ecosystem health of Lake Tahoe. Projects and programs undertaken by TRPA and other agencies include:

• replacing 23 acres of wetlands near the mouth of the Upper Truckee River; a $10 million restoration project
• monitoring the amount of sediment and nutrients flowing into the lake from the Tahoe basin’s tributary streams and groundwater aquifers
• constructing rock trenches and retention basins; a $7 million program by the California Department of Transportation to keep stormwater runoff and sediment out of the lake by allowing polluting sediments time to settle out and be absorbed into land

These efforts may be paying off; UC Davis Tahoe Environmental Research Center and the Tahoe Regional Planning Agency have reported recent increases in average annual clarity. In 2011, researchers reported the average annual clarity level at 68.9 feet, a 4.5-foot improvement over 2010 but still a significant decline from the first year of measurements in 1962 when clarity measured 136 feet. (Researchers use a variety of methods to determine Lake Tahoe’s clarity including a Secchi disk, a dinner plate -sized object dropped into the lake until it is no longer visible, and buoys that monitor blueness.)

In its 2017 annual report, the Tahoe Environmental Research Center found that the number of days during which Lake Tahoe exhibits summer-like conditions have increased by 26 days since 1968. Furthermore, the lake has been warming by one-half degree Fahrenheit each year, which is 14 times faster than the long-term warming rate.

Looking ahead, one of Lake Tahoe’s biggest future challenges may not be environmental but managerial. California and Nevada’s joint jurisdiction over the lake is now being tested. In 2013, Nevada lawmakers proposed legislation to end the arrangement but ultimately decided to continue the joint jurisdiction.