Update on 1/27/2020:
The habitat was a wooded riparian strip along a levee adjacent to agricultural fields at the edge of the Sacramento Bypass Wildlife Area.
Three photos have been added
in the absence of a trunk or stone to lean on, a capybara may be an option;
I have observed these two individuals do this twice;
see also
https://www.inaturalist.org/journal/nelson_wisnik/21258-the-friendly-capybara