river in southwest Colorado, U.S., rising in the La Plata Mountains and flowing southwest through deep canyons, past Dolores, then northwest through Paradox Valley, at the north end of which it is met by its chief tributary, the San Miguel River. It continues on past Gateway and across the Utah border to join the Colorado River near Moab after a course of about 250 miles (400 km). Melting snow produces high water in spring and early summer, but in midsummer the flow is a trickle. The Dolores and its tributaries are used for irrigation; McPhee Dam, near the town of Dolores, impounds a portion of the upper river for this purpose. The river probably was named Río de Nuestra Señora de los Dolores (“River of Our Lady of Sorrows”) by the Spanish trader Juan Maria Rivera c. 1765.