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Figure 3. Pre- and post-impoundment patterns of discharge in the Colorado River mainstream in Grand Canyon, downstream from Glen Canyon Dam. tributary to the Colorado (Fig. 1), originates in highlands of Arizona and New Mexico. The Rio Yaqui in Mexico similarly passes from the northern Sierra Madre Occidental through hot desert lowlands to the sea. These rivers were characterized by remarkable variation in discharge (Fig. 3). Floods from spring snowmelt in the north and both during regional winter rains and late summer monsoons in the south, are followed by mere trickles under the intense evaporation and minimal precipitation during early summer and autumn droughts. Down- cutting of canyons and transport of tremendous volumes of sediments are the rule. Canyons alternate with reaches flowing through broader floodplains, and diversity is high (Fig. 4). Where floodplains are wide, gallery forests and woodlands of cottonwood, willow, and other trees formerly lined desert watercourses; extensive marshes occupied their backwaters. Water tables were only a few feet beneath the surface in even the driest desert basins, and seepage maintained rivers, or at least filled pool in protected canyons during major droughts. The situation changed dramatically with intervention of technological humans. Now, floods alternating with intermittency in drought are an exception rather than the rule (Fig. 3). Rivers are dammed before they leave the mountains. Flood waters and sediments are caught in reservoirs, and water is released as needed for irrigation, power generation, and domestic use. Low flow occurs when humans deem it so; dams can be closed and rivers turned off like a kitchen tap. Far more surface water exists today than before, but it is in quiet reservoirs markedly different from the originally turbulent streams (Fig. 5). Canals instead of streams, lined with concrete and chain-linl fence rather than riparian trees, descend from- reservoirs to and through the lowlands (Fig. 6). Sediment carried downstream in flood is no longer 3