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CHAPTER 3. HYDROLOGIC, LIMNIOLOGICAL, AND TERRESTRIAL PHYSICOCH941CAL CHARACTERISTICS OF THE LOWER COLORADO RIVER SYSTEM 3.1 HYDROLOGY The Colorado River drains a total area of 630,000 km2 (378,000 m12). The recorded range of flow through Yuma was from 0.34 to 7,083 m3/sec (12 to 250,132 ft3/sec) (U.S. Geological Survey 1973). Sediment loads during flood stage averaged more than 108 t (11.02 X 108 tons) per year from 1925 to 1935. During normal flows and during drought periods the river ran clear (Minckley 1979). The average annual undepleted surface flow of the lower Colorado River was estimated to have been 1.8 X 1010 m3 (14.6 million acre-fit) from 1896 to 1935 at Lees Ferry, AZ (Lower Colorado River Comprehensive Framework Study 1971). About 1.9 X 1010 m3 (15.4 million acre-ft) virgin flow was estimated to flow into Mexico. With local runoff and tributaries con- sidered, an additional 3.9 X 109 m3 (3.12 million acre-ft) would be added to undepleted water flowing into Mexico. gravel, sand, clay, and silt layers in varying thicknesses; locally, it may be as much as 914 m (3,000 ft) thick. In general, the deposits grade in texture from large boulders near the mountains to fine-grained silt along the axis of the valleys. Where clay beds form a confining layer, the groundwater beneath is under artesian pressure. Groundwater in the coarse materials above the clay beds is under water-table conditions. Localized clay beds within coarse materials sometimes support widespread perched or semiperched water bodies (Lower Colorado River Comprehensive Framework Study 1971). The present drainages, cut in the older alluvium, have been filled to various depths with unconsolidated deposits of gravel, sand, and silt. In many basins this younger alluvial fill, along the floodplain of the present stream, provides large amounts of groundwater. The amount of ground- water that can be obtained from the younger fill in any particular area depends on the depth and extent of the deposits. Groundwater distribution and availability are determined largely by the geologic setting of the area. Subsequent to major faulting that formed the mountains and valleys, several stages of erosion and sedimen- tation filled the valleys with materi- als that now form the major aquifers adjacent to the lower Colorado River. This older alluvial fill consists of Not surprisingly, the major aquifers are located in the broad al- luvial fans found in the Mohave, Parker, Palo Verde, and Imperial Val- leys. Depth to water table in these valleys is <61 m (<200 ft). Recover- able groundwater in storage amounts to about 9.9 X 1011 m3 (803 million acre- ft) along and adjacent to the lower 30