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Where do Valley rivers start – and end? Examining our ‘tremendously engineered’ system

Terminus Dam impounds Lake Kaweah in Tulare County. The zig-zagged fusegates in the foreground mark the top of the dam’s emergency spillway, which was activated when the reservoir reached capacity in early March.
Chris Gray
U.S. Army Corps of Engineers
Terminus Dam impounds Lake Kaweah along the Kaweah River in Tulare County. The zig-zagged fusegates in the bottom right mark the top of the dam’s emergency spillway, which was activated when the reservoir reached capacity in early March.

This article highlights the rivers that run through the San Joaquin Valley and form Tulare Lake, and explores water storage and flood prevention infrastructure like dams and levees. Click here for more coverage of the 2023 floods in the San Joaquin Valley.

FRESNO, Calif. – California has one of the most complex water systems in the world. And so, the factors giving rise to our region’s floods are more complicated than the simple cascading of rain and snowmelt downhill during a rainier-than-average wet season.

We are well into one of the wettest winters on record in the San Joaquin Valley. Historic precipitation levels have buried the high Sierra Nevada under more than 50 total feet of snow. And parts of the Valley, stricken for years by severe drought, are underwater.

The water in our region is delivered by rivers. Most of the major rivers in our region flow through reservoirs and dams, which, during normal years, help prevent flooding, but now are being forced to discharge huge volumes of water.

Meanwhile, many of those rivers don’t actually flow all the way to the ocean – and all the water that drains here is overtaking the infrastructure that typically keeps it contained.

What questions do you have about the Valley’s current flood crisis? Email or send a voice memo with your questions to reporter Kerry Klein at kerry@kvpr.org.

These water delivery systems are “tremendously engineered,” said Roger Bales, professor of civil and environmental engineering at UC Merced. “Every place that has irrigated agriculture at industrial scale has this sort of engineered system.”

Here is a basic introduction to our region’s water storage system - where our water comes from, where it ends up, and some of the infrastructure it passes through along its path.

How do dams and reservoirs work?

California contains more than 3,000 bodies of water, of which roughly 1,300 are human-made reservoirs. While many are known for recreation like camping and hiking, one of their primary purposes is to store water. That’s particularly because of the wet winters and dry summers of our Mediterranean climate, said Bales.

“Warm dry summers are the growing season but that’s not when we get precipitation,” said Bales. “We need to irrigate our food and fiber that we’re growing…so we need to store that water somewhere, and these dams are one factor in that seasonal storage.”

Water spews out of outlet works at Friant Dam on Millerton Reservoir on March 22, 2023, to make room for incoming snowmelt from the Sierra Nevada.
U.S. Bureau of Reclamation
Water spews out of outlet works at Friant Dam on Millerton Reservoir on March 22, 2023, to make room for incoming snowmelt from the Sierra Nevada.

California’s reservoirs can collectively hold up to 45 million acre-feet water. An acre foot is equivalent to about 326,000 gallons, or the amount of water that will cover an acre of land one foot deep.

Reservoirs also help prevent flooding, and most provide some amount of hydroelectric power by harnessing the flow of water out of the dams that impound them.

Dams are typically constructed of either concrete or natural materials like compacted dirt and rocks. They can take the form of wide embankments, like the earthen dams at San Luis Reservoir, Lake Isabella and Lake Kaweah, while others, like Friant Dam on Millerton Lake, are gravity dams that have been angled in such a way that gravity pulls them in the opposite direction than the water is pushing.

A reservoir’s capacity is the volume of water it can safely hold without running the risk of overflowing or overtopping. But in order to safeguard against overtopping, which can erode dams and compromise their structural integrity, they’re typically equipped with multiple ways of letting water out in what’s known as flood releases.

One of those release mechanisms is outlet works, in which valves or pipes pump out water downstream of the dam. Another is spillways, which commonly look like ramps or ladders for water to flow down. Most dams contain at least one emergency spillway near their crest to release excess water when the reservoir reaches capacity.

Can this infrastructure prevent flooding?

Dam authorities release water via the spillway, outlet works, or both, and that water flows into rivers downstream of dams. Flood releases serve to make space in the reservoir for more water to flow in, but releases in other situations can also deliver water to communities or irrigation districts and provide water for downstream ecosystems.

This all means that water authorities wield relatively precise control over the amount of water flowing in our region’s rivers, which in turn allows them to prevent most floods. But in unusually wet seasons, dam authorities must release high volumes of water in order to ensure that reservoirs don’t overflow.

“There is an expectation that still, this spring, with the snowpack and rain more water can come into those reservoirs than probably we can hold,” said Tyler Stalker, spokesperson with the U.S. Army Corps of Engineers, which manages dozens of dams across California. “We are now having to, more so than earlier in the year, balance holding water in the reservoir, releasing water downstream, and coordinating that with the timing of different rain events.”

In a March 2023 press conference, Fresno County Sheriff John Zanoni warned that flood releases from Pine Flat Reservoir into the Kings River will continue for months and will leave rivers too dangerous for recreation.

“With the anticipated snowmelt, we will let water out and effectively have filled Pine Flat over three times,” he said, a sum that totals three million acre-feet. “That is the level of water that we're talking about coming down through the watersheds into the lakes and down to the rivers.”

Other structures used for local flood control and water storage include levees, weirs, floodwalls, ponding basins and canals.

Where do the rivers in our area start, where do they end, and have some been altered by reservoirs and dams?

Every river in the San Joaquin Valley region begins in the Sierra Nevada, and almost all of them have been engineered to contain at least one reservoir. And so even though all their flows originate from rain or Sierra snowmelt, much of the water that reaches the Valley floor has likely passed through a dam somewhere upstream.

The headwaters of one of the largest forks of the San Joaquin River near Thousand Island Lake in the High Sierra.
Justin Gaerlan
Wikimedia Commons
The headwaters of one of the largest forks of the San Joaquin River near Thousand Island Lake in the High Sierra.

The San Joaquin River is one of California’s longest. It originates in the Eastern Sierra, flows downhill through Fresno and Madera Counties then turns north and empties into the ocean by way of the Sacramento-San Joaquin Delta. Its main fork and many tributaries, including the Fresno and Tuolumne Rivers, pass through more than a dozen reservoirs, including Millerton Lake, Mammoth Pool Reservoir and Bass Lake.

The Merced River, which meanders through Yosemite Valley, is one of the larger tributaries of the San Joaquin and eventually flows into the bigger river near the city of Newman. Its main reservoir is Lake McClure.

Unlike the San Joaquin River, the Kern, Kings, Kaweah, Tule and White Rivers all drain inland in the basin of the historical Tulare Lake (more on that below). All of them except the White River contain at least one reservoir, including Lake Isabella on the Kern River, Pine Flat Reservoir on the Kings River, Lake Kaweah on the Kaweah River, and Lake Success on the Tule River.

In early March, this infrastructure was no match for a barrage of storms that delivered a deep snowpack and then warm rain that rapidly melted the lower elevation snow. For instance, much of the flooding of communities and agricultural land in Tulare County has occurred after a one-two punch: heavy rain, then voluminous flood releases from dams along the Kaweah and Tule Rivers.

This produced an unfortunate irony in a region that typically reports hundreds to thousands of drinking water wells running dry each year.

“We've been one of the most drought stricken regions of California and we're begging and begging and begging for water,” said Tricia Stever Blattler, Executive Director of the Tulare County Farm Bureau. “But sadly when the good Lord delivered it, he brought it all in way too quickly.”

Where is Tulare Lake, and why have I never heard of it before?

If you’ve never heard of Tulare Lake, you’re not the only one. Most of us have never seen it. Fresno writer Mark Arax wasn’t familiar with it until he received a phone call in early 1997 – the last time floods filled the lakebed.

“‘Tulare Lake has come back to life,’ my colleague from the Sacramento bureau of The Los Angeles Times shouted over the phone. What the hell was Tulare Lake? I pulled out my AAA map and there in a corner of Kings County, next to the town of Corcoran, the cotton capital of the West, was a square of blue that designated the lake,” Arax recently wrote in the New York Times.

A barn rises out of thousands of acres of water that has filled the usually-dry Tulare Lake basin near the city of Corcoran.
Joshua Yeager
A barn near the city of Corcoran rises out of thousands of acres of water that has filled the usually-dry Tulare Lake basin.

Until the 20th century, Tulare Lake was an enormous body of water whose size would fluctuate between the wet and dry seasons. Situated largely in Kings and Kern Counties, It was considered the country’s largest freshwater lake west of the Mississippi River, covering an area that was by some estimates more than four times the size of modern-day Lake Tahoe.

Originally the home of tens of thousands of Yokuts Native Americans, the lake supported thriving salmon, trout and sturgeon fisheries. Over time, however, settlers sought to permanently cultivate the lakebed. To prevent flooding and store the water for irrigation, they dammed rivers upstream and diverted flows within the Valley using an extensive system of canals and levees.

A map of Tulare Lake from 1873.
Wikimedia Commons
Wikimedia Commons
A map of Tulare Lake from 1873.

By the early 1900s, the lake had been essentially engineered out of existence. Today, any water that flows to the area is used to irrigate thousands of acres of cotton, safflower, pistachios and tomatoes that are grown year-round in the lakebed.

The lake has only flooded and reappeared a handful of times since it disappeared, most recently in 1997. In those instances, the water has not only covered agricultural fields, but has at times breached levees and bested other flood prevention infrastructure to flood nearby communities including Corcoran, Allensworth and Alpaugh.

Today, now that the lakebed has again flooded, water experts estimate it’ll be months before those fields are dry again.

“Tulare lake is a closed basin…it has no outlet. So the water will either evaporate or sink into the groundwater,” said UC Merced’s Bales.

Kings County Sheriff David Robinson said seeing a full Lake Tulare is a once-in-a-lifetime experience.

“Some have described it as a biblical moment in time,” he said during a March 2023 press conference. “This will impact the world…we’re going to have a million acre-feet of water covering up an area that feeds the world.”

Kerry Klein is an award-winning reporter whose coverage of public health, air pollution, drinking water access and wildfires in the San Joaquin Valley has been featured on NPR, KQED, Science Friday and Kaiser Health News. Her work has earned numerous regional Edward R. Murrow and Golden Mike Awards and has been recognized by the Association of Health Care Journalists and Society of Environmental Journalists. Her podcast Escape From Mammoth Pool was named a podcast “listeners couldn’t get enough of in 2021” by the radio aggregator NPR One.