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canyon reaches also acted as subterranean dams. Underground flow of water was forced to the surface creating reliable reaches of stream, even during drought. These large desert rivers aggraded their valleys except in time of channel-straightening flood. Substrates of sand and fine gravel shifted continuously with the current. In reaches flowing through valleys, pools were formed by degradation on the outside of meanders or by undercutting of riparian trees, or were associated with large logs or other debris in the channels. Permanent features such as boulders or irregularities in stony walls of canyons caused scour, and deep places persisted despite continuous bedload transport. Water quality varied radically. Snowmelt produced water low in total dissolved solids, while evapotranspiration in summer and inflows of mineralized springs promoted high salinities. As an example of the last, residents near Yuma reportedly impounded a low flow of the Gila River near the turn of the century to prevent contamination of downstream irrigation supplies by "alkali water". A small flood in the Gila breached the dike, and toxic water flowed into the Colorado River mainstream, supposedly killing fishes for a distance of more than 160 km, all the way from Yuma to the Gulf of California. Temperatures also must have fluctuated radically, yet extremely high water temperatures likely did not occur because of evaporation into the dry desert air. Dangerously high water temperatures can develop, however, when dark substrates back radiate heat into clear, overlying water, or, as noted before, when unusually high humidities suppress evaporation from water surfaces. Canyon segments remained cooler since stony walls shaded the stream and evaporation resulted in even more rapid cooling when direct sunlight was excluded. The lowermost portions of regional rivers originally consisted of large, sand-bottomed channels meandering over broad deltas (Fig. 56). Water on undisturbed deltas must have been seasonally warm, silt laden, and rich in organic debris transported from their vast watersheds. Some were lined by dense riparian woodlands (Fig. 57), forests of cottonwoods and willows and understories of arrowweed, seepwillow, and other woody plants. Deltaic channels were so broad, however, that only a small percentage could have been shaded from intense summer sunlight. Rocks and boulders of headwaters had long before been ground to finer particles, and aggradation of silt and sand was the rule except at the highest flow. Organic materials arriving on deltas also had been pulverized; even Figure 56. View of the Colorado River Delta southward from near the International Boundary in 1975. This vast and complex region has now been largely desiccated as a result of water diversions and depletions upstream. Photograph provided by the U.S. Bureau of Reclamation. logs washing from mountain forests were mostly reduced to microscopic particles or actually dissolved before nearing the sea. Dissolved and particulate organic material provided fertilizing nutrients not only for deltaic communities, but also for the Gulf of California. Impacts of entrapment of these nutrients by upstream reservoirs on the ecology of the gulf has never been assessed. Remarkable variation in discharge was the rule. Water spread widely over the broad, nearly level floodplain, and each flood resulted in channel changes. Oxbow lakes and sloughs were formed, only to be filled with organic and inorganic debris during periods of low discharge, then cut again by later flooding events. This was especially true where soft deltaic sediments were marked by complex series of spreading channels or distributaries (Fig. 56). Channels were created and disappeared in days or hours. Aggradation formed natural levees and lakes, Figure 57. Riparian woodland on the Colorado River Delta, ca. 1890. Photograph provided by R. D. Ohmart. 28