While Trinity County has experienced a week or more without rain and the short-term forecast calls for more clear weather, it leaves one wondering if Trinity Lake might still refill this year from its current low status.
While any amount of rain or snow certainly helps, local officials say it’s likely impossible to predict, especially this early in the water year, which started in October.
When asked what it would take to fill the lake, Todd H. Buxton, Ph.D. hydrologist/geomorphologist with Trinity River Restoration Program said it would be impossible to answer.
As of Wednesday, Jan. 11, the lake was at 774,000 acre-feet of water. When full, the lake holds 2,448,000 acre-feet, creating a current deficit of 1,674,000 acre-feet.
Buxton said the Bureau gets its data from the California Nevada River Forecast Center and tracks the numbers for planning purposes. However, any predictable trend created by the week of moisture and snow was made less and less valid by the high-pressure system that brought sunny skies to the area this past week, he said.
Using snow levels to predict how much water will reach the lake is tricky. Regional weather is hard to predict for more than about 10 days, and other factors go into how much water will reach the lake. Buxton said high mountain snow can evaporate back into the sky, making it difficult to predict its yield. What doesn’t melt and run off or evaporate will soak into the ground and be drawn by gravity through the soil to creeks and rivers. Since soil has a certain amount of retention, it’s also hard to predict how much will get through.
Buxton said that a simple equation for turning snowpack into predicted inches of lake water simply doesn’t exist.
Buxton noted that the Bureau of Reclamation diverts water to Whiskeytown reservoir and the schedule for doing so is unknown at this point. The Bureau also allows water to continue into the Trinity River, further complicating lake level predictions. However, if the numbers from Jan. 11 were used as an indicator, it’s not likely Trinity Lake would completely refill this water year, even if no water was diverted to Whiskeytown or the Trinity River.
Trinity Public Utilities District Manager Paul Hauser said he has had several discussions with Bureau of Reclamation staff this year, and the consensus is that the lake stands little chance of refilling this year.
“Trinity is appropriately sized,” he explained. “The reservoir is 2.448 million acre-feet and the Trinity watershed, above the dam, is about 2 million acre-feet.” He said that means when the reservoir is extremely low, it’s also very difficult to refill in a single year.
Hauser echoed Buxton’s statement that it would be even harder to fill since water has to come out of the reservoir. The Record of Decision (ROD) mandates that during an “extremely wet” water year, the dam must release 810,000 acre-feet of water.
“Even if we get all kinds of precipitation and snow, it would have to practically be a biblical event to overcome not just an extremely low reservoir, but the 810,000 acre-feet they’ll release.”
By comparison, Shasta Lake holds 4.5 million acre-feet and is fed by a 6.2 million acre-feet watershed, he said, noting that the dam there was originally set to be taller and the reservoir larger.
“Statistically, it’s easier to fill Shasta,” he said.
There have been periods where the lake filled a significant amount in a short period of time.
The Trinity Journal reported the lake level rose by almost 36.6 feet between Dec. 15, 1964, and Jan. 5, 1965. In that three-week period, the lake went from being 61 percent full, holding 1,485,603 acre-feet of water to 80 percent full (1,965,383 acre-feet) and by early June of that year it was 99.9 percent full, holding 2,444,700 acre-feet of water.
As of Dec. 30, the lake was measured at 29 percent of its 2,447,650 acre-feet capacity, according to the California Data Exchange Center.