According to Gudde, in "California Gold Camps", Bath was "one mile above Foresthill. It drains into Volcano Canyon and was (1850) originally called Volcano, according to the county history, 1882 (p.375). The name was changed to Sarahsville in 1851 and to Bath in 1858, when the Post Office was established. Doolittle's map, 1868, shows 'Bath P.O.' with Sarahsville as an alternate or former name." Gudde has more detail, but it's mostly about mining history.