The Cosumnes River (often pronounced /kəˈsʌmniːz/) 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. Many locals pronounce the river "kunsumniss," inserting an "n" where there is none in the first syllable.