Foraminiferal assemblages in the stratigraphically lower part of the Bouse Formation in the Blythe Basin indicate marine conditions whereas assemblages in the upper part of the Bouse Formation indicate lacustrine conditions and suggest the presence of a saline lake. Benthic foraminiferal assemblages in the lower part of the Bouse Formation are similar to lagoonal and inner neritic biofacies of the modern Gulf of California. Evidence suggesting a change from marine to lacustrine conditions includes the highest occurrence of planktic foraminifers at an elevation of 123 m asl, the change from low diversity to monospecific foraminiferal assemblages composed only of Ammonia beccarii (between 110 to126 m asl), an increase in abundance of A. beccarii specimens (above ~110 m asl), increased number of deformed tests (above ~123 m asl), first appearance of Chara (at ~85 m asl), lowest occurrence of reworked Cretaceous coccoliths (at ~110 m), a decrease in strontium isotopic values (between 70-120 m), and δ¹⁸O and δ¹³C values similar to sea water (between 70-100 m asl). Planktic foraminifers indicate a late Miocene age between 8.10 and 5.3 Ma for the oldest part of the Bouse Formation in the southern part of the Blythe Basin. Benthic and planktic foraminifers correlate with other late Miocene sections and suggest that the basal Bouse Formation in the Blythe Basin was deposited at the northern end of the proto-Gulf of California. After the marine connection was restricted or eliminated, the Colorado River flowed into the Blythe Basin forming a saline lake. This lake supported a monospecific foraminiferal assemblage of A. beccarii until the lake spilled into the Salton Trough and the Colorado River became a through-flowing river.