The Alamosa River is a river in the southern part of the U.S. state of Colorado.

It is about 64 miles (103 km) long,[2] flowing roughly east through the San Luis Valley.

Its watershed comprises about 148 square miles (380 km2).

The river's name means "shaded with cottonwoods" in Spanish.

[3] The river was affected by the Summitville mine disaster, the worst cyanide spill in United States history.