ABSTRACT
The Colorado River presents a varied habitat to support the fish fauna located within its basin. The river begins as a series of cold, clear streams in the Rocky Mountains. It drains an area of about a quarter of a million square miles as it travels across portions of seven states and a piece of Mexico to reach the Gulf of California. As the river flows across this arid to semiarid land with its hot climate and highly erodible soils, the water temperature rises and the silt load increases. Historically, annual and seasonal flow regimes appear to have been quite variable. This river, with its highly varied and somewhat harsh habitat conditions, existed in a virtually isolated condition until the early 1900s. Under these conditions, the Colorado River eventually supported the largest assemblage of endemic fishes of any known river system in North America (Miller, 1959; Holden and Stalnaker, 1975). Dr. Molles very appropriately treats the Colorado River system as an island. I am in basic agreement with him. This insular concept reasonably explains the highly endemic nature of the fish fauna of the Colorado River.
