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, . "late I. The Colorado River system: (a) the Colorado River at a regulated headwater location )elow Granby Reservoir (photo JVW), (b) Lower Black Canyon of the Gunnison River, showing )rccarnbrian basement near the river and Mesozoic sediments on the uplands (JAS), (c) Grand :anyon of the Colorado River from the North Rim (JAS), (d) Colorado River in the Grand CanY(lll JVW), (e) turbid flood waters of the Little Colorado River at Grand Falls (Sykes 1937), and (f) the 'olorado Delta in 1932 (Sykes 1937). In 1907 the salinity of the Salton Sea was c, 3600 mg I-I; it has since increased )y an order o[magnitude due to evaporation and irrigation runotI. The bottom vaters develop seasonal anoxia, and surface temperatures may exceed 35 C 3lue-green algae (e.g. Phormidium) dominate the phytoplankton, and brackish- vater zooplankters (e,g, Brachionus plicatilis) are food for various introduced ish (Walker 1961), Massive fish kills occur each year from algal toxins and :hemicals in agricultural and urban runotI (Kim 1973). 162 .~ River limnology Temperature Before regulation, desert reaches of the Colorado in winter rarely dropped below 5-lOoC, and summer temperatures did not exceed c. 300C despite air temperatures sometimes above 400C. Backwaters and marshes, however, often reached 350C (Deacon & Minckley 1974). Seasonal temperatures in the head- waters range from OOC to 15-200C (cf. Deacon & Minckley 1974; Stanford & Ward 1983). Thermal patterns have been changed by flow regulation, and the mean mainstream temperature has decreased by 10-15Co (e.g. below Lake Powell; Fig. 5). Regulated reaches are characterised by summer-cool, winter-warm c<;mditions (Ward & Stanford 1979). On the Gunnison River water from the 9-ypolimnion of a headwater impoundment (3500 m) prevents the downstream river from freezing, even though air temperatures may be below - 300C for extended periods (Stanford & Ward 1983). Chemistry Dissolved solids loads reflect the geology of the various sub-basins, In the Upper Basin, the Mancos Formation (Fig. 3b) is extensively drained. Mancos shales are predominantly gypsum, as are Mesozoic shales elsewhere in the drainage (Laronne & Schumm ] 982). Limestones are locally important, particularly in the Grand Canyon (Fig. 3b). Thus Ca2+, SO; and HCO) dominate the com- · position of Colorado River water. Sulphate consitutes over half of the Total Dissolved Solids (TDS) in the river at the delta. TDS loads increase 2-4 fold as streams flow into the sedimentary formations. In the Gunnison River, average SO; concentrations increase from less than 10 mg] - t in the headwaters (3000 m) to more than 250 mg I-I at the Colorado River (1400 m), primarily due to drainage from the Mancos Formation (Stanford & Ward 1983). Some reaches drain NaC] formations. The Paradox Valley of the Dolores River, where salt encrusts the river bank, is a textbook example of "salt dome" formation (cf. Clark & Stearn 1960). Even so, Ca2+ and SO; dominate at the Dolores-Colorado confluence. The Little Colorado River is the only major tributary contributing mainly Na + and CI - , due mostly to high-discharge springs near the confluence. Salt loads also originate from similar springs throughout the Upper Basin (United States Dept Interior ] 98]). Various springbrooks in the Grand Canyon issue from the RedwalI Forma- tion (Fig. 3b), an impure dolomitic limestone, The water is saturated with calcium and magnesium carbonates (Kub]y &. Cole ]979), and intricate travertine formations have developed in turbulent areas. 363