Identified from a photo by David H. Kavanaugh, Senior Curator of Entomology, California Academy of Sciences. Found on sandy, silty, and gravelly banks of medium to large streams all over western North America. More photos and a short video clip at
Opisthius richardsoni.
Species page from University of Alberta says this species is similar to the genus
Elaphrus (see below). Carabidae; Carabinae; Opisthiini
(Originally misidentified by comparison to a photo labelled
Elaphrus riparius in Peter Haggard & Judy Haggard's Insects of the Pacific Northwest.)