Big winter challenges water officials throughout Tahoe-Truckee region
As temperatures warm and the Sierra Nevada snowpack melts, flooding along the Truckee River and in other areas becomes more of a possibility.
Whether there’s flooding on the trails between Tahoe City and Squaw Valley, as in 2017, or in other locations is dependent on several factors and is unique to each season.
“Each year is individual and has its own challenges,” said Federal Water Master Chad Blanchard. “It’s all based on multiple things: (including) what the temperature does and how fast the snow melts between now and the seasonal peak at the lake.”
Lake Tahoe’s seasonal peak, according to Blanchard, happens when the amount of water entering the lake and amount lost due to evaporation are equal. At that time, most of the gates on the Lake Tahoe Dam will be closed and little water will be spilled into the Truckee unless there is demand downstream.
“The ideal thing is to top the lake off with last melting drop right before the peak happens,” said Blanchard. “The problem is you don’t know when that happens.”
The seasonal peak at Tahoe, said Blanchard, can happen anytime between April and August, depending on conditions.
“We’ve melted very little snow so far, which is going to push back the peak,” said Blanchard. “We’re not going to hit that until a lot later in the year … we’ll go into the fall higher than we normally would. In case we go back into a drought, we’ll be better situated.”
Challenges going into summer
After a second winter in three years with well-above-average snowfall, Blanchard is faced with a unique set of challenges as summer nears.
In order to release water from Lake Tahoe during spring and summer, the Truckee River’s flow rate at Farad has to dip below 500 cubic feet per second. If the flow rate isn’t met, and the lake isn’t in danger of reaching it’s upper limit, significant water won’t be spilled into the Truckee.
“Once we have demand, then we have to release from storage, but we cannot release more water unless that specific criteria is met,” said Blanchard. “You can’t start way ahead because you don’t know what we’re going to get.”
Blanchard pointed to 2013 when the season started off very wet, before going dry, comparing it to last year when the season started drier than normal before the region was hit with record snowfall in March.
During 2017, Tahoe had its largest inflow for a season in recorded history. The lake rose by 6 ½ feet, according to Blanchard, and the equivalent of roughly another 3 feet was spilled from the dam.
With Lake Tahoe so full this past year, Blanchard said he’s often asked why water isn’t spilled from the lake earlier in the season.
“Unfortunately you need to start early on Tahoe, but on the other hand you can’t start until that criteria is met and you know you have too much water,” said Blanchard, noting significant water was spilled from Tahoe beginning in late February this year.
Tahoe’s maximum legal elevation is 6,229.1 feet. The lake was measured at 6,227.91 feet, according to the water master’s daily worksheet, on April 16. Water is being released at a rate of 1,132.3 cubic feet per second into the Truckee River, which is currently a few feet from rising over certain sections of the paved trails between Tahoe City and Squaw Valley.
Parts of trails flooded in 2017, but little damage was done, according to the Tahoe City Public Utility District. The trails were also near the end of their lifetime and were rehabilitated last fall.
“We’re in good shape since it just got completely rehabilitated, so we don’t expect any damage from future flooding that we are expecting with the snowmelt this year,” said Tahoe City Public Utility District Community Engagement Analyst Stacie Lyans.
The amount of water being released from the dam is roughly 60 cubic feet per second less than at this time in 2017. When the trails flooded in late April of 2017 the flow rate was between 1,600 and 2,000 cubic feet per second.
“Right now we’re not aware of any flooding that’s going on,” said Lyans. “As soon as the parks team knows about it, they notify me and we put it out there via social media.”
Boca Dam improvements
Another challenge facing Blanchard this year is work scheduled to be done at Boca Dam.
The Bureau of Reclamation will modify the dam beginning in May to better resist the impacts of potential earthquakes.
“We have to keep Boca down because of construction,” said Blanchard.
With just a fraction of the water of Tahoe, Blanchard said roughly half the reservoir could be released, if necessary, in 24 hours, compared to a 0.5% of Tahoe’s storage that could be released during the same timeframe.
The project is expected to last until October 2020, and during that time, according to a release from the bureau, the road over Boca Dam will be closed.
“Safety is a top priority for reclamation,” Terri Edwards, Lahontan Basin Area office manager, said in a news release. “Through our Safety of Dams program, we are correcting seismic risks at Boca Dam. The project will construct an earthen buttress on the downstream face to stabilize the dam and also modify the spillway crest structure.”
The reservoir will operate at around half its capacity, according to the bureau, which is about 28 feet lower than maximum water levels, during the summer and fall of 2019 to assist construction.
Whether the Truckee rises enough to cause flooding will depend on a number of factors, including temperatures in the coming months, any additional precipitation the area receives, and the amount of runoff entering the Lake Tahoe and the Truckee River from its tributaries.
“The issue is what the temperatures are going to be between now and that peak,” said Blanchard. “Unknowns are temperature, precipitation, and the actual runoff compared to the forecast.
“We’ve been doing this for a little over 90 years, and I’ve been doing this at the office for 26 years and it’s always a challenge. Every year has different patterns of runoff.”
Justin Scacco is a reporter for the Sierra Sun. Contact him at firstname.lastname@example.org.