The delta smelt is endemic to the Sacramento-San Joaquin River Delta in California, where it is distributed from the Suisun Bay upstream through the Delta in Contra Costa, Sacramento, San Joaquin, and Solano Counties.
Historically, delta smelt were distributed from San Pablo Bay upstream to Sacramento on the Sacramento River and Mossdale on the San Joaquin River, which varied seasonally and with freshwater outflow [REF].