You don’t go out fishing on the Sacramento River above Red Bluff without “a cushion for your tush,” according to the locals. The water floating your raft or rowboat is too darn cold, especially when the salmon are spawning. This mid-summer chill isn’t natural in a river you could once walk all the way across in warm shallows, or swim through without turning blue. But then, not much is natural about the way water flows out of the mountains down into California’s Central Valley anymore.

Ever since workers poured 6.5 million cubic yards of concrete into a canyon above the town of Redding, backing up the waters of the Sacramento, Pit, and McCloud Rivers for 35 miles behind Shasta Dam, Californians have been less thirsty and freer of floods. It’s dams like this that Buford Holt, a biologist with the U.S. Bureau of Reclamation, says have “made possible a bounty of food production and kept us functioning as a state, because obviously we don’t have any rain for six months out of the year.” His agency runs the world’s largest water development and management system: the Central Valley Project, with 20 dams, 11 power plants, and 500 miles of canals. Shasta is one of California’s five large foothill dams around the Central Valley that help control floods and store snowmelt for water customers up and down the state (the others are Oroville, Folsom, New Melones, and Friant); hundreds of smaller, private dams criss-cross rivers up in the mountains, built long ago by miners, private landowners, PG&E, and various public entities.

Standing on the top, looking down the sheer, streaked face of the 602-foot-high dam, you cannot help but feel a wave of vertigo. Everything around the dam seems small and far away--snow-topped Mount Shasta in the distance, the other end of the green-blue lake created by the dam, the specks of ducks bobbing in the light chop, the pin-sized pines along the river at the bottom of this massive edifice.

Inside the dam lie some hollow galleries, but it’s mostly solid. Touring these inner hallways, visitors will see swastikas imprinted on some pipes, evidence that those ordering plumbing supplies during the dam’s construction (1938 to 1945) got some from Germany before World War II broke out. Newer hardware includes a device that enables operators to withdraw and release water from different lake depths--selecting the coldest bottom water, rather than the warmer upper layers, so that the eggs of spawning salmon stuck below the dam won’t die in the river. That’s why you need a cushion to boat on the river.

Before the dam got in their way, salmon spawned in the 187 miles of snow-chilled streams of the upper watershed. The dam brought with it a constellation of new facilities, including a hydroelectric power plant, a connection to the Coast Range’s Trinity River via a tunnel and Whiskeytown Reservoir, and a smaller dam, Keswick, nine miles downstream. Spawning salmon that make it as far upriver as Keswick are trapped and trucked to a fish hatchery at the mouth of nearby Battle Creek. Keswick also serves as what is known in water engineering lingo as an “afterbay,” a place where the powerful flows released from Shasta for maximum power and revenue generation can be stored temporarily, then meted out slowly to the river. This way, the water level downstream doesn’t change too dramatically.