The upper Miocene to lower Pliocene Bouse Formation in the lower Colorado River trough of the American Southwest was deposited in three basins—from north to south, the Mohave, Havasu, and Blythe Basins—that were formed by extensional faulting in the early to middle Miocene. Fossils of marine, brackish, and freshwater organisms in the Bouse Formation have been interpreted to indicate an estuarine environment associated with early opening of the nearby Gulf of California. Regional uplift since 5 Ma is required to position the estuarine Bouse Formation at present elevations as high as 555 m, where greater uplift is required in the...