In Oregon and Washington coastal streams, there are relatively diverse faunas on the Olympic Peninsula of Washington (Chehalis fauna), in the lower Columbia River, and on the south-central Oregon coast (Tyee fauna). Adjacent to the Tyee fauna are groups of streams with depauperate faunas (0–2 primary freshwater fishes). To the north, from the Nehalem River to the Alsea River, streams have 0–1 species (Oregon Coast Range Isolates), and to the south, from the Rogue River to the Klamath River, streams have 0–2 species (Klamath Isolates). Throughout the entire area there was no relationship between drainage area and freshwater fish diversity. Multiple processes are responsible for these patterns. Two endemic genera, Oregonichthys and Novumbra, originated by the Miocene (23–5.3 Mya) and current distributions are largely bordered by the continental flood-basalt flows of the Columbia River Basalt Group (CRBG). It is hypothesized that both genera were isolated by early Miocene Grande Ronde basalt flows, Oregonichthys to the south of the flows in an ancestral Umpqua River, and Novumbra to the north of the flows in an ancestral Chehalis River. The depauperate streams of the Oregon Coast Range Isolates are hypothesized to be the result of bisection of ancient coastal streams by uplift of the Oregon Coast Range and subsequent extinction of the ancestral fauna on west-slope streams either from changing habitats or stochastic events in smaller streams. The Klamath Isolate streams drain the southern Oregon Coast Range and Klamath Mountains, an ancient region otherwise known for its faunal and floral diversity. It is hypothesized that these streams, some of which are relatively large and would be expected to have relatively diverse faunas, have been isolated from diverse primary freshwater fish faunas to the north (Columbia) and south (Sacramento), and most taxa have simply never had access to these streams.