I just happen to own California Place Names
Looks like John Sutter named the American River. He told the head of a US military exploration outfit that he [Sutter] named it the Rio de los Americanos+ after a spot near Sutter's fort that was known as +El Paso de los Americanos. Said "Americanos" were not United Statesians, but rather Canadian fur trappers. (The Hudson Bay Company was pretty active in northern California, pre Gold Rush.) The American River arises in the Sierra Nevada mountains and empties into the Sacramento River at Sacramento, about 90 miles/145 KM northeast of San Francisco. The first gold discoveries of the Gold Rush were in the American,
The Russians had colonies on the California coast, the main one being Fort Ross (Russ) a bit north of the mouth of the Russian River. They called the river Slavianka+ "Slav woman" (various spellings; first mentioned 1830). The first version of the modern name was in an 1843 Mexican land grant that mentioned +el boca del Rio Ruso.+ The English +Russian River showed up on an 1852 map "but was doubtless current before the publication of this map." The Russian River empties into the Pacific Ocean, about 80 miles/13 KM north of San Francisco.
