Members of this species are relatively pale, and among the largest in subgenus Liocosmius. The head, pronotum, and abdomen are relatively pale brown or reddish-brown (in contrast to the dark brown or black colors of all other Liocosmius except B. "Cooper's"). The elytral margins are pale. The hind elytral spots are distinctly comma-shaped. The antennae are quite pale, either yellowish brown or pale reddish. The male genitalia have a very large, sinuate flagellum, and in this are distinct from all other Liocosmius, except B. "Cooper's".
There are no obvious differences between B. festivum as a whole and B. "Cooper's" in external appearance except for the slightly more obtuse hand angles of the pronotum in B. festivum. However, the individual subspecies of B. festivum can be told from B. "Cooper's" more easily: specimens pale prothoraces, large sinuate flagella, and with more than two discal setae on each elytron or with elytral microsculpture are B. festivum. If it were not for the notable differences in DNA sequences, these might be considered to belong to the same species.
Body length 3.5-4.5 mm, with most specimens 3.8-4.2 mm.
Found from southern California north to Corvallis, Oregon.
Found on steep, sand or silt banks of creeks and rivers, frequently in the shade of trees.
Bembidion festivum festivum
This subspecies differs from B. festivum hilare in having no visible microsculpture on the elytral disk (which is therefore very shiny), and in having three or more discal setae on each elytron. A very few specimens have been observed with only two setae on one elytron, but if so, the other elytron has three or more setae. For comparisons with other members of the subgenus Liocosmius, see the description of B. festivum. Specimens of B. festivum festivum tend to be larger than B. festivum hilare, and in the southern end of the range, paler.
Body length 3.5 to 4.5 mm, with most specimens longer than 3.9 mm.
Found in southern California, from just south of the Bay Area to the area around San Diego.
Found on steep sand or silt banks of creeks and rivers, often where shaded.
Bembidion festivum hilare
This subspecies differs from B. festivum festivum in having distinct microsculpture on the dorsal surface of the elytra; the microsculpture consists of transverse sculpticells, formed from finely engraved lines. It is only visible at higher magnification on a good microscope. In addition, almost all specimens of this subspecies have only two discal setae on each elytron (as opposed to three or more in B. festivum festivum). For comparisons with other members of the subgenus Liocosmius, see the description of B. festivum.