Sacramento River

The Sacramento River (Spanish: Río Sacramento) is the principal river of Northern California in the United States and is the largest river in California. Rising in the Klamath Mountains, the river flows south for 400 miles (640 km) before reaching the Sacramento–San Joaquin River Delta and San Francisco Bay. The river drains about 26,500 square miles (69,000 km²) in 19 California counties, mostly within the fertile agricultural region bounded by the Coast Ranges and Sierra Nevada known as the Sacramento Valley, but also extending as far as the volcanic plateaus of Northeastern California. Historically, its watershed has reached as far north as south-central Oregon where the now, primarily, endorheic (closed) Goose Lake rarely experiences southerly outflow into the Pit River, the most northerly tributary of the Sacramento.

The Sacramento and its wide natural floodplain were once abundant in fish and other aquatic creatures, notably one of the southernmost large runs of chinook salmon in North America. For about 12,000 years, humans have depended on the vast natural resources of the watershed, which had one of the densest Native American populations in California. The river has provided a route for trade and travel since ancient times. Hundreds of tribes sharing regional customs and traditions have inhabited the Sacramento Valley, first coming into contact with European explorers in the late 1700s. The Spanish explorer Gabriel Moraga named the river Rio de los Sacramentos in 1808, later shortened and anglicized into Sacramento.

In the 19th century, gold was discovered on a tributary of the Sacramento River, starting the California Gold Rush and an enormous population influx to the state. Overland trails such as the California Trail and Siskiyou Trail guided hundreds of thousands of people to the gold fields. By the late part of the century mining had ceased to be a major part of the economy, and many immigrants turned to farming and ranching. Many populous communities were established along the Sacramento River, including the state capital of Sacramento. Intensive agriculture and mining contributed to pollution in the Sacramento River, and significant changes to the river's hydrology and environment.

Since the 1950s the watershed has been intensely developed for water supply and the generation of hydroelectric power. Today, large dams impound the river and almost all of its major tributaries. The Sacramento River is used heavily for irrigation and serves much of Central and Southern California through the canals of giant state and federal water projects. While it is now providing water to over half of California's population and supporting the most productive agricultural area in the nation, these changes have left the Sacramento greatly modified from its natural state and have caused the decline of its once-abundant fisheries.

The Sacramento River originates in the mountains and plateaus of far northern California as three major waterways that flow into Shasta Lake: the Upper Sacramento River, McCloud River and Pit River. The Upper Sacramento begins near Mount Shasta, at the confluence of North, Middle and South Forks in the Trinity Mountains of Siskiyou County. It flows east into a small reservoir, Lake Siskiyou, before turning south. The river flows through a canyon for about 60 miles (97 km), past Dunsmuir and Castella, before emptying into Shasta Lake near Lakehead in Shasta County. The McCloud River rises on the east slope of Mount Shasta and flows south for 77 miles (124 km) through the southern Cascade Range, roughly parallel to the Upper Sacramento, eventually to reach the McCloud Arm of Shasta Lake.

The Pit River, by far the largest of the three, begins in Modoc County in the northeastern corner of California. Draining a vast and remote volcanic highlands area, it flows southwest for nearly 300 miles (480 km) before emptying into Shasta Lake near Montgomery Creek. Goose Lake, straddling the Oregon–California border, occasionally overflows into the Pit River during wet years, although this has not happened since 1881. The Goose Lake watershed is the only part of the Sacramento River basin extending into another state. Unlike most California rivers, the Pit and the McCloud Rivers are predominantly spring-fed, ensuring a large and consistent flow in even the driest of summers. At the lower end of Shasta Lake is Shasta Dam, which impounds the Sacramento River for flood control, irrigation and hydropower generation. Before the construction of Shasta Dam, the McCloud River emptied into the Pit River, which joined the Sacramento near the former mining town of Kennett, submerged when Shasta Lake was filled. The Pit River Bridge, which carries Interstate 5 and the Union Pacific Railroad over the reservoir, is structurally the highest double-decked bridge in the United States (although most of the bridge piers are submerged under Shasta Lake when the reservoir is full). The Upper Sacramento River canyon also provides the route for I-5 and the railroad between Lakehead and Mount Shasta.

Below Shasta Dam the Sacramento River enters the foothills region of the northern Sacramento Valley. It flows through Keswick Dam, where it receives about 1,200,000 acre⋅ft (1.5×10⁻⁶ million km³) of water per year diverted from the Trinity River. It then swings east through Redding, the largest city of the Shasta Cascade region, and turns southeast, entering Tehama County. East of Cottonwood it receives Cottonwood Creek – the largest undammed tributary – from the west, then Battle Creek a short distance downstream. Below Battle Creek it carves its last gorge, Iron Canyon, emerging from the hills at Red Bluff, where a pumping station (which replaced the Red Bluff Diversion Dam) removes water for irrigation. Beyond Red Bluff the river reaches the low floodplain of the Sacramento Valley, receiving Mill Creek from the east and Thomes Creek from the west near Los Molinos, then Deer Creek from the east near Vina.

Southeast of Corning, the Sacramento forms the boundary of Tehama County to the west and Butte County to the east. A few miles downstream it forms the border of Butte County and Glenn County to the west. Stony Creek joins from the west in Glenn County, near Hamilton City and about 15 miles (24 km) west of Chico. The river then forms the Glenn–Colusa County line for a short distance before crossing entirely into Colusa County. It passes by the Sutter Buttes, a group of volcanic hills that rise abruptly from the middle of the Sacramento Valley, where it receives Butte Creek from the east at Colusa. Below Colusa the river flows south-southeast, forming the border of Colusa County and Sutter County to the east.