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1 1 1 ~~ Southerly International Boundary. Major lower basin tributaries include the Virgin, Moapa, Paria, Little Colorado, Bill Williams, and Gila rivers. Inflows from the Rio Hardy and Rio Sonoita in the states of Baja California Norte and Sonora, respectively, are also part of the drainage. The waters in this basin are more heavily used than any other river drainage in the USA and perhaps the world (Fradkin 1989). For purposes of this document, the lower basin has been divided into six subunits: Grand Canyon, Lake Mead, Lake Mohave, Lake Havasu, Parker-Imperial, and Gila River basin. Historically, the Colorado River in the lower basin had both canyon-bound and alluvial valley reaches. Canyon reaches were narrow with rapids, fast flowing runs, and shoreline eddies. Where the river meandered through the alluvial valleys it formed braided channels, backwaters, oxbow lakes, and marshes. Riparian forests of cottonwood (Populus fremont~~ and willow (Sa/ix spp.) formed on the lower terraces with stands of mesquite (Prosopis spp.) on upper terraces. River flows were highly variable. Before impoundments, the hydrologic conditions to which these fishes were adapted included relatively short periods of torrential flooding or severe drought alternating with far longer periods of variable but less extreme flow conditions. Its annual flow reflected snowmelt from the Rocky Mountains, high and cold in the spring and early summer, then dropped abruptly and remained relatively low in summer through winter. The River drained to the Sea of Cortez forming a large estuarine delta at its mouth. Extensive alluvial deposits resulted from an annual sediment load in excess of 100 million metric tons (USBR data), and water temperatures changed with ambient temperatures throughout much of the year. Beginning in the mid-1800s, settlers along the Colorado River began constructing temporary diversions for agriculture and by 1905 had constructed Laguna Dam, the first permanent dam in the lower basin. By the 1960s, Hoover, Davis, and Parker dams were completed and controlled river flow. Smaller diversion dams such as Headgate Rock, Palo Verde, and Imperial, controlled and removed water released from the large dams, decreasing the volume of water in the river as it flowed south. After water deliveries were made to Mexico at Morelos Dam at the Southerly International Boundary, the river channel was dry in all but exceptional runoff years. Floodplains were. developed for agriculture or flooded by reservoirs. Remaining river sections were channelized and controlled by bank stabilization or levees. Backwaters and marshes cut off by channelization were lost and those remaining were maintained by human efforts. 2