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.J.... _ - ~ . '\"0&.1 .... _-f" .__...L,: .... __.< ,... '. ~.---r:I!!J :I ~ --1- " .J g'-,'..a..-._ Figure 21. Lower Colorado River, Arizona-California, at Laguna Dam. Upper: Note continuous riparian vegetation and river being navigated by paddlewheeled vessel in 1906. Lower: Riparian vegetation has been replaced in 1975 by agriculture, and the river flows in a constrained, dredged channel. Photographs provided by U.S. Bureau of Reclama- tion and R. D. Ohmart, respectively. material forming the bottom at a given place constantly exchanged. If balance shifts to the positive side, the stream channel will build up, or aggrade. A negative shift results in local erosion, or degradation. In the 1800s, many southwestern streams had aggraded, and flowed near the tops of their banks. Extensive marshes and riparian plant communities slowed water currents, promoting deposition of organic and inorganic materials (Fig. 22). Beaver were common, and their dams contributed to over-all stabilization of the systems. Layers of organic debris interbedded with alluvium and laced together by the trunks and limbs of down trees soaked up flood water, which slowly leaked into the channel during dry times, thereby buffering the impacts of drought. Figure 22. Marshlands associated with Babocomari Creek, upper San Pedro River basin, Arizona, 1968. This is one of the few, relatively undisturbed cienega habitats remaining in the American Southwest. A major catastrophe occurred in the 1870s. Livestock, introduced to the region by ranchers, had become abundant. As the uplands desiccated, cattle concentrated near streams and rivers. But even that tactic soon failed, and 75 % of all livestock in Arizona are thought to have died of thirst or starvation by 1875. Ranges were severely damaged, so erosion prevailed when a wet cycle began in the 1880s. Deep arroyos were cut from downstream to upstream, incising valley fills so deeply that water tables were drained (Fig. 23; see also Figs. 7, 19). ,." -i ' .~ :~ . ~. ~~_. Figure 23. Cienega Creek, Arizona, 1977: organic deposits (dark bands) formed during periods of marshland development, are interbedded with alluvium deposited from the watershed (light bands) in eroded banks of many incised Southwestern streams. 12