Drought measures: Lake Tahoe water released to meet goals downstream
SOUTH LAKE TAHOE, Calif. — Water outflows at the Tahoe City dam remain high, not despite of, but because of drought conditions.
The Lake Tahoe Basin experienced its second dry winter in a row, so despite the lake being full in 2019, it is expected to drop below its natural rim this summer.
“Conditions this spring have remained dry and warm, with streamflow to date much below normal,” said Jeff Anderson, hydrologist for Nevada Natural Resources Conservation Service in the June forecast report. “April-May precipitation measured at SNOTEL sites has been 30-45% of average in the Sierra basins and 37-66% elsewhere in northern Nevada.”
According to Water Master Chad Blanchard this is the third driest winter on record in 111 years.
Although the basin did receive some precipitation, it was mostly in the form of snow, so the soil remained dry.
“Soil moisture across the SNOTEL network continues to be very dry,” Anderson said in the report. “These conditions have impacted runoff as dry soils soaked up snowmelt and what little precipitation we saw this spring.”
With the strong winds and sunny conditions the basin has been experiencing the last few weeks, the lake has lost a significant amount of water. The wind increases the surface area of the lake, which in turn increases evaporation. The wind has also caused much of the remaining snow melt to sublimate, meaning it turns into a vapor rather than melting into the lake.
Both the El Dorado County boat ramp in South Tahoe and the Tahoe Vista Recreation Area boat ramp on the North Shore are staying closed for the 2021 summer season because of the low lake level.
U.S. Coast Guard officer Cody Browning said they’ve seen an increase of incidents this year related to the low lake level.
“We’ve responded to a lot of boats on rocks or beaching on sand bars,” Browning said.
He added that many mooring lines have been broken by the wind.
With all that being said, many basin citizens have wondered why so much water is being released from the Tahoe City dam.
The answer is complicated. The Floriston Rate is the minimum flow rates that have to be met with water flowing from Lake Tahoe through the Truckee River to meet demands downriver.
“In the Sierra, the Lake Tahoe net inflow forecast is negative. This means evaporation will outpace inflow to Lake Tahoe this season,” Anderson said in the report. “Lake Tahoe’s water elevation is expected to fall below the lake’s natural rim later this year.”
So, in years with strong winters, snowmelt runoff can provide most of the water needed downstream but because this year was so dry, the river isn’t getting its much needed runoff.
“There’s very little natural flow, so we didn’t get much snowmelt from storms or anything,” Blanchard said. “So, the natural flow is very low. It’s not enough to meet our targets and very few days this year were enough to meet the targets, which is extremely rare, so we have to release from storage to supplement the natural flow to meet those targets.”
The reality is, Lake Tahoe is a reservoir, so while it seems counterintuitive to release water during drought years, that is its ultimate purpose.
“That’s the whole purpose for a reservoir, you store it in good years and you release it in bad years,” Blanchard said.
There is currently about 460 cubic feet of water coming out of the lake per second which is a little over two-thirds of 1/100th of a foot per day. Blanchard said it would take 600 cubic feet per second to be released for 100 days to lower the lake by one foot.
However, the majority of the water leaving the lake is leaving through evaporation and not through the dam. So, if Blanchard and his team were to release the water at a slower rate, most of the water would evaporate before it could get released and the Floristan Rate goals could not be met.
“We’re very close to rim control right now, which means we’re limited on how much water we can release. And we need that water for the Floristan Rate target,” Blanchard said. “So, you can’t get it out if you wait. So it has to come out before evaporation takes the lake down because you don’t have the release capacity if you wait.”
Part of the reason there is a current need for outflow was because Fish and Wildlife were spawning fish downriver. However, Blanchard said spawning is complete so, “the fish water that was being released to support the fish is being ramped down.”
Now, the basin could use cloud coverage and thunderstorms to prevent the lake from getting too low.
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SOUTH LAKE TAHOE, Calif. – A pair of utility companies may cut off power during this weekend due to high winds forecast for the Lake Tahoe Basin.