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1 002715 Opportunities for Ecological Improvement Along the Lower Colorado River Mark Briggs and Steve Cornelius 7/24/97 Historic accounts appear to indicate that the lower Colorado River's riparian ecosystems changed little from the time of early Spanish exploration in the 17th century to the 1930s when construction of Hoover Dam was completed (Ohmart et al. 1977). ' The completion of Hoover Dam in 1935 sparked a wave of major construction and agricultural projects along the river. Today, as the Colorado River flows from Hoover Dam to the delta, it passes through 28 dams, irrigates over 1 million hectares (2.5 million acres) of agricultural land, and serves or supplements water supplies for over 20 million people in the U.S. and Mexico (Colorado River Basin Salinity Control Forum 1990). The Mainstem The accumulative weight of river impoundment, river diversions, groundwater pumping, spread of non-native species, agricultural activities, and other human activities has had a devastating effect on the river's riparian ecosystems. In particular, the buffering of annual overflows and altering of natural channel dynamics by river impoundment has compromised habitat for native fishes and limited the creation of sandbars and channel islands, which are critical features for the propagation of many native riparian plants (Ohm art et al. 1977) The completion of Hoover Dam, and then Glen Canyon Dam in 1963, had dramatic effects on river streamflow. Prior to the dams, the Colorado River was a warm, muddy flow with tremendous seasonal fluctuations. After dam construction, the river became a much clearer flow of cold water that fluctuated relatively little. Such hydrologic changes have adversely effected the river's native population of warm-water fish, such as the razorback, bluehead sucker, flannelmouth, Colorado squawfish, humpback chub, and bony tail chub, while benefiting such non-native fish as the rainbow and cutthroat trout, which are better adapted to the river's artificially created clear, cold waters (Minckley 1991). With the exception of the bluehead sucker and speckled dace, all of the river's native fish are either endangered or under consideration for listing. The distribution and extent of native wetland plant communities, such as the cottonwood/willow (Populus fremontii/Salix gooddingii) riparian forests and the cattail/rush (Typha spp./Juncus spp.) marshlands, along some reaches of the lower Colorado River appear to have changed significantly. Ohmart et al. (1977) observed, for example, that cottonwood communities along the mainstem have declined from over 2,000 hectaress (5,000 acres) in the 1600s to less than 200 hectaress (500 acres). Saltcedar (Tamarix spp.), which was introduced to the western U.S. during the mid- 1800' s as a soil stabilizer and ornamental plant, now forms homogeneous stands along significant reaches of the lower Colorado River (Ohmart et al. 1977). Indeed, reaches of the lower Colorado River that are not significantly effected by saUcedar are few and far between, even in clreas where significant native wetland plant communities persist Page 3