The Spanish word for “feather” coincidences with the Latin word; the Spanish plural is plumas. Hence, el rio de las plumas means “the river of feathers,” or “feather river” for short. With a little imagination it can be translated as a “river with plenty of birds.”
This imagination was reality in the first half of the 19th century, when early European explorers and gold-rush pioneers discovered and exploited the Feather River in northern California. During that time it got its beautiful name :
The Feather River originally was named “el Rio de las Plumas” in 1821 by Spanish explorer Luis Antonio Argüelo for the multitude of waterfowl seen upon its waters. In about 1850, the name was anglicized to “Feather River.”
Several branches of the Feather River run through Plumas County . It can be considered as Feather River County, but continues to stick with the Spanish name. Plumas County's Frazier Creek, for example, is a tributary of the Feather River: it flows from Gold Lake via the scenic Frazier Falls into the Feather River's Middle Fork by the resort town of Graeagle.
The Latin/Spanish word pluma also lives on in open-space and park names such as Plumas National Forest and Plumas-Eureka State Park, associated with the Feather River headwaters.
Keywords: geography, languages, etymology, place naming, anglicisation (anglicization), American history.
References and more to explore
 Plumas County, California GenWeb Project: A Brief History of Plumas County [www.cagenweb.com/plumas/hist.htm].
 About Feather River Country [featherrivercountry.com/about/].