The Cosumnes River is a river in
northern California
Northern California (commonly shortened to NorCal) is a geocultural region that comprises the northern portion of the U.S. state of California, spanning the northernmost 48 of the state's List of counties in California, 58 counties. Northern Ca ...
in the United States. It rises on the western slope of the
Sierra Nevada
The Sierra Nevada ( ) is a mountain range in the Western United States, between the Central Valley of California and the Great Basin. The vast majority of the range lies in the state of California, although the Carson Range spur lies primari ...
and flows approximately
[ into the Central Valley, emptying into the ]Mokelumne River
The Mokelumne River ( or ; ''Mokelumne'', Miwok for "People of the Fish Net") is a -long river in northern California in the United States. The river flows west from a rugged portion of the central Sierra Nevada (U.S.), Sierra Nevada into the C ...
in the Sacramento-San Joaquin Delta.
The Cosumnes is one of very few rivers in the western Sierra without major dams. The Nature Conservancy's Cosumnes River Preserve is located just upstream from the Delta. Towns and cities along the Cosumnes River include Plymouth
Plymouth ( ) is a port city status in the United Kingdom, city and unitary authority in Devon, South West England. It is located on Devon's south coast between the rivers River Plym, Plym and River Tamar, Tamar, about southwest of Exeter and ...
, Rancho Murieta, Sloughhouse, Wilton, Elk Grove, and Galt.
The California Office of Environmental Health Hazard Assessment has issued a
advisory
regarding fish caught from the body of water.
Name
The Cosumnes River is thought to have been named as the Mokelumne and Tuolumne rivers were, using the "-umne" suffix meaning "people of". The prefix is derived from the Miwok
The Miwok (also spelled Miwuk, Mi-Wuk, or Me-Wuk) are members of four linguistically related Native Americans in the United States, Native American groups indigenous to what is now Northern California, who traditionally spoke one of the Miwok lan ...
word "kosum" meaning "salmon". Chinook Salmon
Salmon (; : salmon) are any of several list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the genera ''Salmo'' and ''Oncorhynchus'' of the family (biology), family Salmonidae, native ...
runs are rarely, if ever, seen above Rancho Murieta as a result of diversions in the area.
Many locals pronounce the river's name , inserting an "n" where there is none in the first syllable. Given the etymological similarity between "Cosumnes," on the one hand, and "Tuolumne" and "Mokelumne" on the other (see next paragraph), a case can be made that the most accurate historical pronunciation is "kasumme," since the "n" is essentially silent in the pronunciation of both "Tuolumne" and Mokelumne," and there is no "s" or "z" sound at the end of those names. An older pronunciation common among Central Valley locals is ), which includes the epenthetic in the first syllable, omits the initial of the final syllable (as in "Mokelumne" and "Tuolumne"), has the expected value for the letter in an open syllable (created by the omission of the last , and uses a final rather than a
Cosumnes River College, a two-year community college located in the southern part of Sacramento, California
Sacramento ( or ; ; ) is the List of capitals in the United States, capital city of the U.S. state of California and the county seat, seat of Sacramento County, California, Sacramento County. Located at the confluence of the Sacramento Rive ...
is named after the river being located only a few miles off campus. CRC's mascot is the Hawk
Hawks are birds of prey of the family Accipitridae. They are very widely distributed and are found on all continents, except Antarctica.
The subfamily Accipitrinae includes goshawks, sparrowhawks, sharp-shinned hawks, and others. This ...
due to the number of hawks that inhabit the river area.
Geography
Flowing from the western slope of the central Sierra Nevada mountains, the Cosumnes starts as North, Middle and South forks cutting canyons through the El Dorado
El Dorado () is a mythical city of gold supposedly located somewhere in South America. The king of this city was said to be so rich that he would cover himself from head to foot in gold dust – either daily or on certain ceremonial occasions � ...
and Amador County Gold Country before converging just east of Highway 49. The North Fork, about long, begins at the Singleton Springs in El Dorado County, at the head of Leek Spring Valley, above sea level. It flows west, past the unincorporated community of Meiss, receiving Van Horn Creek and Butte Creek from the right. At Somerset
Somerset ( , ), Archaism, archaically Somersetshire ( , , ) is a Ceremonial counties of England, ceremonial county in South West England. It is bordered by the Bristol Channel, Gloucestershire, and Bristol to the north, Wiltshire to the east ...
, it is joined by its largest tributary, Camp Creek, from the right. Below Camp Creek it flows through the granite narrows of the Cosumnes River Gorge, a popular rock-climbing area. The river then turns abruptly south and passes Nashville
Nashville, often known as Music City, is the capital and List of municipalities in Tennessee, most populous city in the U.S. state of Tennessee. It is the county seat, seat of Davidson County, Tennessee, Davidson County in Middle Tennessee, locat ...
before joining the Middle Fork near Enterprise
Enterprise (or the archaic spelling Enterprize) may refer to:
Business and economics
Brands and enterprises
* Enterprise GP Holdings, an energy holding company
* Enterprise plc, a UK civil engineering and maintenance company
* Enterpris ...
.
The Middle Fork, long,[ also starts in El Dorado County, just two ridges south of the headwaters of the North Fork. It flows westward for its entire course, roughly parallel to the North Fork. The upper half of the river flows down a steep canyon, past Croft, receiving Dogtown Creek from the right at Omo Ranch. Below Omo Ranch it flows through a gentler valley, past Outingdale, then enters another short canyon where it receives the South Fork Cosumnes River from the left. The smaller South Fork, about long, flows generally west then northwest along the El Dorado/Amador County line, to join the Middle Fork. Shortly below this confluence the combined rivers join with the North Fork to form the Cosumnes River.
The Cosumnes River proper continues west, receiving Big Indian Creek from the south and passing under SR 49 (Golden Chain Highway). It winds through the scenic Sierra foothills through a series of short canyons separated by small ranching valleys. It receives Big Canyon Creek from the right, then Little Indian Creek from the left, before flowing through its last gorge and emerging at the former mining site of Michigan Bar. The entire stretch of the Cosumnes River between the confluence of the forks and a point one mile (1.6 km) above Michigan Bar define the El Dorado/Amador County line; below this point, the river flows into southern ]Sacramento County
Sacramento County () is a county located in the U.S. state of California. As of the 2020 United States census, the population was 1,585,055. Its county seat is Sacramento, which has been the state capital of California since 1854.
Sacrament ...
.
The Cosumnes flows west through Rancho Murieta, an affluent outer suburb of the Sacramento metropolitan area
The Greater Sacramento area is a metropolitan region in Northern California comprising either the U.S. Census Bureau defined Sacramento–Roseville–Arden-Arcade metropolitan statistical area or the larger Sacramento–Roseville combined sta ...
. Two small diversion dams cross the river near Van Vleck Park just upstream from the town. Below Rancho Murieta the Cosumnes flows through an agricultural valley, turning southwest near Sloughhouse. A few miles downstream the Folsom South Canal crosses under the Cosumnes River via an inverted siphon. The Cosumnes continues southwest, passing Sheldon and Wilton, receives Deer Creek from the right, then is crossed by Highway 99 near Elk Grove. Below Highway 99 it enters the nearly Cosumnes River Preserve, which include some of the largest remaining wetlands and riparian areas in the Central Valley. It enters the Sacramento–San Joaquin River Delta
The Sacramento–San Joaquin River Delta, or California Delta, is an expansive inland river delta and estuary in Central California and Northern California
Northern California (commonly shortened to NorCal) is a geocultural region that ...
near Mokelumne City, turning west once more to join the Mokelumne on the border of Sacramento County and San Joaquin County.
River modifications
As one of the last rivers flowing from the west slope of the Sierra without a major dam, the Cosumnes is a vital example of a healthy watershed. However, its salmon and steelhead populations have been harmed by invasive fish species and pollution from illegal mining
Illegal mining is mining activity that is undertaken without state permission. Illegal mining is the extraction of precious metals/rocks without following the proper procedures to participate in legal mining activity. These procedures include pe ...
activity, as well as the two irrigation diversion structures near Rancho Murieta. There are plans to re-introduce salmon spawning into the river.
Some water is diverted from two tributaries of the North Fork – Camp Creek and Sly Park Creek – for irrigation in the Camino area. Sly Park Dam, which creates Jenkinson Lake, is the largest water storage facility in the Cosumnes River watershed. The dam and reservoir are part of the extensive Central Valley Project
The Central Valley Project (CVP) is a federal power and water management project in the U.S. state of California under the supervision of the United States Bureau of Reclamation (USBR). It was devised in 1933 in order to provide irrigation and ...
. However, the project has negligible effects on river flows, and does not block historic salmon spawning runs.
Dams have been proposed to be built on the main stem to control flows, but these plans have largely stalled because of the river's unique free-flowing status and its importance as a wildlife habitat. Sacramento County officials continue to lobby for the construction of a flood-control dry dam above Rancho Murieta at Michigan Bar. Such proposals are still the subject of ongoing debate.
Flooding History
Over the past few decades, the Cosumnes River Watershed has experienced significant, sometimes devastating, flooding. The most notable flooding occurred in 1986, 1997, 2017, and 2023. The 1997 storm was the largest recorded event on the Cosumnes River, with peak flow of 97,000 cubic feet per second at the Michigan Bar stream gauge estimated to be close to a 200-year event. Many Cosumnes levees breached during this storm.
Reclamation District 800, a California Reclamation District, is responsible for managing a series of levees along the Cosumnes River stretching from the edge of the community of Rancho Murieta to just upstream of California State Route 99
State Route 99 (SR 99) is a major north–south state highway in the U.S. state of California, stretching almost the entire length of the Central Valley. From its southern end at Interstate 5 (I-5) near Wheeler Ridge to its northe ...
.
See also
*List of rivers of California
This is a list of rivers in the U.S. state of California, grouped by region. Major lakes and reservoirs, if applicable, are indicated in italics.
North Coast (north of Humboldt Bay)
Rivers and streams between the Oregon border and Humboldt Bay t ...
References
External links
Cosumnes Research Group
Cosumnes River Task Force
Lower Cosumnes-Lower Mokelumne Watershed Profile
— ''U.S. EPA Region 9''
Cosumnes River Preserve
Cosumnes River Watershed Project datasets
— ''California Environmental Information Catalog''
* (Geographic Names Information System
The Geographic Names Information System (GNIS) is a database of name and location information about more than two million physical and cultural features, encompassing the United States and its territories; the Compact of Free Association, asso ...
)
{{Authority control
Rivers of the Sierra Nevada (United States)
Mokelumne River
Tributaries of the San Joaquin River
Sacramento–San Joaquin River Delta
Rivers of El Dorado County, California
Rivers of Amador County, California
Rivers of Sacramento County, California
Rivers of San Joaquin County, California
Geography of the Sacramento Valley
Rivers of Northern California
Rivers of the Sierra Nevada in California