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- Sacramento County, California: Sacramento is named for the Sacramento River.
- Saline County, Kansas: Saline is named for the Saline River.
- San Benito County, California: San Benito is named for San Benito Creek.
- San Juan County, Colorado: San Juan is named for the San Juan River and San Juan Mountains.
- San Juan County, New Mexico: San Juan is named for the San Juan River.
- San Miguel County, Colorado: San Miguel is named for the San Miguel River.
- Santa Cruz County, Arizona: Santa Cruz is named after the Santa Cruz River.
- Sangamon County, Illinois is named after the Sangamon River.
- Scioto County, Ohio: Scioto is named for the Scioto River.
- Sevier County, Utah: Sevier is named for the Sevier River.
- Shenandoah County, Virginia: Shenandoah is named for the Shenandoah River; Shenandoah is a Native American term meaning beautiful daughter of the stars.
- Shiawassee County, Michigan: Shiawassee is named for the Shiawassee River.
- St. Francis County, Arkansas: St. Francis is named for the St. Francis River.
- St. Johns County, Florida: St. Johns is named for the St. Johns River.
- St. Joseph County, Indiana: St. Joseph is named for the St. Joseph River.
- St. Joseph County, Michigan: St. Joseph is named for the St. Joseph River
- St. Lawrence County, New York: St. Lawrence is named for the St. Lawrence River.
- St. Louis County, Minnesota: St. Louis is named after the St. Louis River.
- Stillwater County, Montana: Stillwater is named for the Stillwater River, named for its very slow current.
- Suwannee County, Florida: Suwannee is named for the Suwannee River, the same river made famous by Stephen Foster in his song Old Folks at Home.
- Sweetwater County, Wyoming: Sweetwater is named for the Sweetwater River.
Read more about this topic: List Of U.S. Counties Named After Rivers