San Juan (săn wän) [key], river, c.400 mi (640 km) long, rising in the San Juan Mts., SW Colo., and flowing generally W through N.Mex. and Utah to Lake Powell on the Colorado River. Navajo Dam, part of the upper Colorado River storage project, is on the river, which is unnavigable. Its chief tributaries are the Animas, Los Pinos, La Plata, Piedra, and Mancos rivers. The San Juan is used for irrigation; vegetables, fruits, and grains are grown in the river valley in northwestern N.Mex.