The southwesterly course of the probably pre–early Miocene and possibly Oligocene Crooked Ridge River can be traced continuously for 48 km and discontinuously for 91 km in northern Arizona (United States). The course is visible today in inverted relief. Pebbles in the river gravel came from at least as far northeast as the San Juan Mountains (Colorado). The river valley was carved out of easily eroded Jurassic and Cretaceous rocks whose debris overloaded the river with abundant detritus, probably steepening the gradient. After the river became inactive, the regional drainage network was rearranged three times, and the nearby Four Corners region was lowered 1–2 km by erosion. The river provides constraints on the early evolution of the Colorado River and Grand Canyon. Continuation of this river into lakes in Arizona or Utah is unlikely, as is integration through Grand Canyon by lake spillover. The downstream course of the river probably was across the Kaibab arch in a valley roughly coincident with the present eastern Grand Canyon. Beyond this point, the course may have continued to the drainage basin of the Sacramento River, or to the proto–Snake River drainage. Crooked Ridge River was beheaded by the developing San Juan River, which pirated its waters and probably was tributary to a proto–Colorado River, flowing roughly along its present course west of the Monument upwarp.