The earliest alignment of US Route 101 from Hopland of Mendocino County south to Cloverdale of Sonoma County followed a much more difficult grade than the modern highway west of the Russian River. This older alignment is presently occupied by Mountain House Road (Mendocino County Route 111) and California State Route 128 between Hopland south to Mendocino. The original alignment of US Route 101 had been in use during the California Gold Rush prior to the construction of the San Francisco & North Pacific Railroad as a stage road. This early highway was adopted into Legislative Route Number 1 and the early Redwood Highway following the passage of the 1909 First State Highway Bond Act. US Route 101 would finally be realigned to a much more even grade along the Russian River between Hopland and Cloverdale during the summer of 1934. What had been the original alignment of US Route 101 from Mountain House Road east to Cloverdale Boulevard was recycled as California State Route 28 in 1934 which in turn became California State Route 128 in 1953.
https://www.gribblenation.org/2021/03/former-us-route-101-between-hopland-and.html
That goes on my bucket list, for next time I'm in No Cal. Looks like a really nice ride in Street View.