The level of Lake Tahoe fell precipitously in the last 48 hours, reaching the official natural rim at 9:00 am PDT on October 12, 2021, six days sooner than anticipated. Due to the natural sloshing (or “seiching”) of the lake surface, it had risen again by a quarter inch at 11:00 am, but by 5:00 pm, it was again at the natural rim and has continued to fall. Lake Tahoe is now a terminal lake!
Why did the final drop occur so suddenly, despite the snow showers of the last couple of days? The storm that blew in added very little water to the lake, and certainly not enough to cause an increase in the stream flows into the lake. On the contrary, the high winds that accompanied the storm greatly increased the rate of evaporation from the lake. In one 36-hour period, the lake level fell by 1.2 inches due to evaporation. This is almost four times the annual evaporation rate of about 72 inches per year.
What will the future bring? Without significant precipitation we can expect the lake level to continue falling at about 1-1.5 inches/week. Flow from the lake to the Truckee River will be at zero, although the Truckee River will continue to get inflow from groundwater and from streams downstream of Lake Tahoe.
How much does 1-1.5 inches/week of evaporation from Lake Tahoe amount to? It is the same as 4–6 billion gallons of water per week, or enough water to meet the daily needs of 48-72 million people. That’s not a drop in the bucket.
The official lake level measurements are reported hourly by a U.S. Geological Survey gage station located at the US Coast Guard Pier near Tahoe City, CA. See the moment it crossed the natural rim below and continue to track the rate of change yourself from home.