The (Temporary) Rebirth of California’s Once-Huge Tulare Lake?

The southern half of California’s vast San Joaquin Valley is almost never depicted as a desert nor is it officially classified as one. But it clearly is a desert by climatological criteria. Most of the San Joaquin Valley gets less than 10 inches of precipitation a year, with much of the southern valley receiving less than seven, and it has an extremely high rate of evaporation from late spring through early autumn. But with abundant water flowing from the adjacent Sierra Nevada range, the southern San Joaquin Valley is a verdant, intensely cultivated land. Before the late 1800s, it was the site of the third largest freshwater lake entirely within the United States (as measured by surface area). But when the rivers that formerly flowed into Tulare Lake were diverted into canals to irrigate crops, the huge lake disappeared. Today, the former lakebed is highly productive farmland with only a few small seasonal wetlands providing natural habitat.

As the paired maps posted below indicates, the extent of Tulare Lake varies greatly in different cartographic depictions. This is because the lake itself varied significantly in size on both a seasonal and multi-year basis. As Tulare Lake did not drain in most years, it would expand in winter and spring and then contract through summer and early fall. It would also grow to an especially large size in wet years and shrink dramatically in dry ones. In particularly wet years, the lake would rise high enough to drain to the sea through the San Joaquin River, thus flushing out any accumulated salt and ensuring that its water remained fresh.

A shallow and nutrient-rich lake, Tulare was extremely productive. The Yokuts people who lived around its shores were reputed to have had one of the highest levels of population density of any indigenous American ethnic group. For several decades after the gold rush, Tulare’s aquatic resources from were shipped in huge quantities to San Francisco. As the Wikipedia article on the lake notes:

Even well after California became a state, Tulare Lake and its extensive marshes supported an important fishery: In 1888, in one three-month period, 73,500 pounds of fish were shipped through Hanford to San Francisco. It was also the source of a regional favorite, western pod turtles, which were relished as terrapin soup in San Francisco and elsewhere.

Turtles in Tulare Lake were so abundant that they were even fed to hogs. Today the western pond turtle is classified as a vulnerable species, suffering from competition with invasive exotic turtle species and undermined by the loss of habitat.

Environmentalists occasionally dream about bringing back Tulare Lake, emphasizing the vital habitat that it once provided and contending that its revival would be a relatively easy way for California to store excess runoff. Such a scenario, however, is extremely unlikely. Not only is the former lakebed highly productive farmland, but it also contains the city of Corcoran, home to some 22,000 residents.

But regardless of human plans and desires, Tulare Lake will probably reappear this spring, if only for a short period, owing to the extremely heavy precipitation that has been experienced this winter in the southern Sierra. Tulare County has already seen levee-breaks and the flooding of several towns, and water is now beginning to accumulate in the old lakebed. Local flooding could easily persist as snowmelt begins in April or May. Noting such factors, a recent article by Dan Walters claims that “It’s almost certain that Tulare Lake will once again spring to life.” Walters concludes by arguing that, “the probability is generating some hopeful, if unrealistic, speculation that state and or federal governments could buy up the lakebeds fields and bring back to Tulare lake permanently.”

This season’s reborn Tulare Lake will probably evaporate over the course of the summer, which will almost certainly be hot and bone dry – as is always is in the San Joaquin Valley. But if California enters a multiyear wet cycle, which is possible although not probable, winter and spring drainage could become a big problem for the farms and towns of the Tulare Basin. The city of Corcoran well known for its continual subsidence, dropping in elevation by about two feet a year due to the overuse of groundwater. Subsidence has already created major headaches for Corcoran. As noted in The Science Times,

The town levee had to be reconstructed for $10 million after the casings of drinking-water wells were crushed, flood areas changed, and the town levee had to be rebuilt. The situation has increased homeowners’ property tax bills by around $200 a year for three years.

Another powerful storm is slated to slam into California on Tuesday, March 21. Like most of this year’s major storms, it will be most pronounced in central and southern California, largely missing the normally much-wetter northern third of the state. More than 48 inches of additional snow is expected in the southern Sierra, which drains into the Tulare Basin. Thus far this winter, the southern Sierra has received an astounding 268 percent of average annual snowfall.

As can be seen on the map posted above, the northern and central parts of the Sierra have also received much higher-than-average amounts of snow this winter, but not to the same extent as the south. This pattern is highly unusual and was not expected. Until recently, the eastern Pacific was under La Niña conditions, which usually means a drier than average wet season, especially in Southern California. By winter 2024, El Niño conditions may assert themselves, which usually means a wetter than average winter for southern and central California. If so, Tulare Lake might fill up yet again.

Bay Area Rainfall Forecasts—and Results

A recent GeoNote praised the U.S. National Weather Service for its accurate rainfall prediction maps, noting as well the specific forecast that had just been made for northern California’s impending storm. As that storm has passed, it now seems reasonable to ask how well the Weather Service did. The answer: not very, but in a very interesting way. The foreseen rainfall did appear, but it remained focused in a rather narrow band, drenching the Santa Cruz Mountains and Marin County. The particularities of the weather system greatly enhanced the effects of orographic lift, concentrating of rainfall the windward side of the mountains. As a result, the precipitation forecasts were much too low for wet areas and much too high for dry areas.

The Bay Area is characterized by extreme geographical variability in rainfall. While much of northern Sonoma County receives more than 70 inches of rain a year on average (1,778 mm), parts of eastern Alameda County get less than 10 inches (254 mm). The gradient is particularly steep between the Santa Cruz Mountains and the Santa Clara Valley, ranging from over 50 (1,270) inches a year to less than 12 (305 mm) in the span of about 20 miles (32 km; indicated by the black line on the rainfall map). The most recent storm was marked the greatest differential I can recall. In a little more than two days, some weather stations in the Santa Cruz Mountains received more than 14 inches (356 mm), whereas much of the Santa Clara Valley received barely a trace. Currently, the San Lorenzo River in Santa Cruz is at flood stage while San Jose is still gripped by an extreme drought, having received only about 25 percent of average precipitation during the current water year (which runs from the beginning of July to the end of June).

The differences in precipitation between the Santa Cruz Mountains and the Santa Clara Valley stems from the rain-shadow effect: air rises as it passes over the mountains, as it rises its pressure decreases, cooling the airmass and hence increasing its relative humidity to the point of condensation, resulting in rainfall. On the lee-side of the mountains, the air descends, increasing pressure, warming the airmass, decreasing relatively humidity and hence reducing or even eliminating precipitation. In most storms, other mechanisms also induce rainfall, such as frontal passages, steep lapse-rates (warm surface, cool upper air), vorticity, and so on. The Bay Area’s recent storm generally lacked such atmospheric dynamics. It did, however, have a steady stream of warm, moist air moving from the southwest to northeast that remained trained on the zone between Santa Cruz and Marin counties, drenching their mountainous areas.

An upper-level low pressure system is forecast to drop south over California today, promising more pronounced atmospheric dynamics, more evenly distributed rainfall, and lower temperatures.