Why some reservoirs get filled faster

“Dear Tom, “Why do a few of the reservoirs seem to lag so far behind the others in terms of their water levels? In particular, why is Trinity Lake so low, when relatively nearby Shasta is now above historical average?” — Chris Kagay

Years of drought followed by the recent pounding storms to hit the Bay Area and Northern California has turned water watching into a spectator sport.

It has been a spectacular show this month. The formula is different for every watershed and every lake.

Lakes in the foothills of the Bay Area are fed 100 percent by rain. Those above 8,000 feet elevation are fed 100 percent by snowmelt. In the foothills of the Sierra, Cascades and ShastaSiskiyous, it’s a volatile mix.

The showpiece of California right now is Shasta Lake, the state’s largest reservoir. It’s 87 percent of full, 110 percent of average, and since early December, the lake level has come up 133 feet. Giant Shasta will reach peak in mid- May, and based on snowmelt projections, probably will hit 95 to 98 percent full and reclaim its status as the No. 1 recreation lake in the Western U. S.

According to the Bureau of Reclamation, Shasta is fed about 70 percent by rain from a vast network of watersheds.

Trinity Lake, on the other hand, is fed about 30 percent by rain and 70 percent from snowmelt. Trinity is only 47 percent full. The annual April 1 survey, in which surveyors measure water content in snow, will provide the best forecast of how full Trinity could get by June.

Over the weekend, the best news is that the Department of Water Resources reported that of 30 major reservoirs in Northern California, 20 were higher than 100 percent of average for the date.

Snowpack provides 30 percent of the state’s water that is stored in reservoirs. In rough estimates from sensors, the snowpack in many watersheds from the central Sierra north to Shasta- Trinity is projected to be better than 100 percent of normal.

The April 1 snow survey also will answer this year’s biggest water mystery in the southern Sierra, where snowmelt in April, May and June feeds a series of still- barren reservoirs above the San Joaquin Valley.