Seem to remember reading that the cental valley was impassable until late summer most years because Tule lake occupied most of the valley floor. William Brewer wrote extensivly of it in his travels as the state surveyor 1851-1854. Ferrys were used between Yurba Buena (San Francisco) and Sacramento because land travel was by way of Bakersfield.
I think even Kit Carson and Fremont both talked about valley crossings being cirquitus.
Those correct dates are from 1860-64 and he was working for the US Geological Survey under Josiah Whitney (it is available online here). His diary (I have it in front of me) emphasizes a particularly wet set of years due to the great flood of January 1962, which would skew his observations for years thereafter. If I recall correctly, he went down to the southern Sierra the year after the flood.