This is an unusual trillium to see in a garden.
"Trillium rivale is confined to the Klamath and Siskiyou mountains of southwestern Oregon and California. It has a strong affinity for banks along streams and adjacent to trickles in Darlingtonia bogs, mainly on serpentine soils. "
Source Flora of North America
This plant is also genetically quite different from other Trillium:
"Trillium rivale was distinct from both Paris and Trillium and should be placed in its own genus, for which the name Pseudotrillium is proposed."
by SUSAN B. FARMER and EDWARD E. SCHILLING Systematic Botany (2002), 27(4): pp. 674–692
Susan Farmer Trillium Web Page is most interesting as well
Trilliums are interesting from a cytology standpoint
... the genus <Trillum is unsurpassed as critical cytological material. Chromosomes are extremely large, ranging from 15 to 40 microns in length, are few in number (5 pairs), and fix and stain to show clearer internal structure than any other plant or animal which the writer is familiar, with the obvious exception of the salivary chromosomes of the Diptera.
Source Cytology of the Pacific Coast Trilliums
H. E. Warmke
American Journal of Botany, Vol. 24, No. 6 (Jun., 1937), pp. 376-383