A bridge over the North Fork Cosumnes River
|- left||Middle Fork Cosumnes River, Ocmulgee River|
|- right||North Fork Cosumnes River|
|Cities||Plymouth, Rancho Murieta, Sloughhouse, Wilton, Elk Grove.|
|- location||Confluence of North and Middle Forks, El Dorado County|
|- elevation||787 ft (240 m)|
|- location||Near Galt, Sacramento County|
|- elevation||13 ft (4 m)|
|Length||52.5 mi (84 km) |
|Basin||724 sq mi (1,875 km2) |
|Discharge||for Michigan Bar|
|- average||492 cu ft/s (14 m3/s) |
|- max||93,000 cu ft/s (2,633 m3/s)|
|- min||0 cu ft/s (0 m3/s)|
The Cosumnes River (pronounced ka-sum-neez) is a river in northern California in the United States. It rises on the western slope of the Sierra Nevada and flows approximately 52.5 miles (84.5 km) into the Central Valley, emptying into the Mokelumne River in the Sacramento-San Joaquin Delta.
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 word "kosum" meaning "salmon". Chinook Salmon runs are rarely, if ever, seen above Rancho Murieta as a result of diversions in the area. The Cosumnes is often mis-pronounced as "ka-SOOM-ness", "kun-SOOM-ness", or "ko-SUE-meez", which is incongruent with pronunciation of other regional rivers with the same root (e.g., Tuolumne, Mokelumne) and the original linguistic origins of its name.
Cosumnes River College, a two year community college located in the southern part of Sacramento, CA is named after the river being located only a few miles off campus. CRC's mascot is the Hawk due to the number of hawks that inhabit the river area.
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 and Amador County Gold Country before converging just east of Highway 49. The river proper begins at the confluence of the North and Middle Forks near Enterprise. It then winds through the scenic Sierra foothills before passing into southern Sacramento County in the Sacramento Valley, joining with the Mokelumne River which in turn empties into the San Joaquin River.
The Cosumnes is the only river in the western Sierra without major dams, although two small diversion dams do cross the river just upstream from Rancho Murieta near Van Vleck Park. The Nature Conservancy's Cosumnes River Preserve is located just upstream from the Delta. Towns and cities along the Cosumnes River include Plymouth, Rancho Murieta, Sloughhouse, Wilton, and Elk Grove.
River modifications 
As the last undammed river flowing from the west slope of the Sierra, the Cosumnes poses a major flooding hazard to southern Sacramento County and may dry up in some years due to the lack of river regulation. Dams have been proposed to be built on the river to control flows, but these plans have largely stalled because of the river's unique free-flowing status. Sacramento County officials continue to lobby for the construction of a flood-control dry dam above Rancho Murieta, which would open much of the lower Cosumnes River valley to development. Such proposals are still the subject of ongoing debate.
See also 
- "Cosumnes River". Geographic Names Information System, U.S. Geological Survey. 1981-01-19. Retrieved 2011-10-06.
- U.S. Geological Survey. National Hydrography Dataset high-resolution flowline data. The National Map, accessed March 11, 2011
- "USGS Gage #11336000 on the Cosumnes River at McConnell, CA (Annual Statistics)". National Water Information System. U.S. Geological Survey. 1942-1980. Retrieved 2011-10-06.
- "USGS Gage #11335000 on the Cosumnes River at Michigan Bar, CA: Water-Data Report 2010". National Water Information System. U.S. Geological Survey. 1908-present. Retrieved 2011-10-06.
- California Place Names of Indian Origins. 1916. University of California Publications in American Archaeology and Ethnology, 12(2): 31-69, p.41
- ACME Mapper. USGS Topo Maps for United States (Map). Cartography by United States Geological Survey. http://mapper.acme.com/. Retrieved 2011-10-06.
- "About Cosumnes River Preserve". Cosumnes River Preserve. Retrieved 2011-10-06.
- "Beach Stone Lakes and Point Pleasant Flood Control Study – Evaluation of Cosumnes River Dry Dam Alternative". Sacramento County Municipal Services Agency. 2006-07-27. Retrieved 2011-10-06.
|Wikimedia Commons has media related to: Cosumnes River|
- 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
- U.S. Geological Survey Geographic Names Information System: Cosumnes River (Geographic Names Information System)