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...The river became a steady, <br />clear-flowing stream that no <br />longer annually overflowed its <br />banks to create lagoons and <br />silt flats. The building of <br />this and other dams produced <br />large lakes of clear, open <br />water that drowned much excel- <br />lent bird habitat. Most of <br />the surviving river-bottom <br />habitat has been cleared, <br />leveled, and converted to <br />farmlands.... Perhaps nowhere <br />else in Arizona have the chan- <br />ges been more dramatic. <br />The second major event took place <br />sometime around 1920, when an exotic <br />species of tree, saltcedar, spread <br />into the lower Colorado River Valley <br />from the Gila River. Saltcedar found <br />optimal ecological conditions for its <br />spread and eventual dominance. In <br />1894, Mearns (1907) estimated that <br />there were about 160,000 to 180,000 ha <br />(400,000 to 450,000 acres) of alluvial <br />bottomland between Fort Mohave and <br />Fort Yuma covered by riparian vegeta- <br />tion. As of 1986, total riparian <br />vegetation was about 40,000 ha <br />(100,000 acres) (Anderson and Ohmart <br />1984c; Younker and Andersen 1986), <br />approximately one-quarter of the <br />available bottomland estimated by <br />Mearns. Roughly 40% of the remaining <br />area in 1986 was covered by pure salt- <br />cedar stands, an additional 43% con- <br />sisted of native plants mixed with <br />saltcedar, and only 0.7% (307 ha [768 <br />acres]) could be considered mature <br />cottonwood or willow habitats. <br />The successful spread of saltcedar <br />is an example of an introduced species <br />optimally exploiting an environment <br />disturbed by humans, to the detriment <br />of native vegetation. Initially, <br />saltcedar became established in areas <br />where native vegetation had been <br />cleared and the land left fallow <br />(Ohmart et al. 1977). Saitcedar has a <br />high rate of seed production with as <br />many as 600,000 seeds per plant pro- <br />duced from April through October <br />(Robinson 1965). The long period of <br />seed production allows saltcedar to <br />germinate well into fall, when most <br />native trees are no longer producing <br />viable seeds. Saltcedar has become <br />dominant along the lower Colorado <br />River by being salt-, fire-, and <br />flood-adapted. <br />Where channel I zation and river- <br />flow management have resulted in very <br />little native plant regeneration, <br />senescent stands of mesquite or willow <br />are replaced by saltcedar. In addi- <br />tion, soil and water salinity levels <br />have risen dramatically in association <br />with irrigation practices and evapora- <br />tion from reservoirs. Native plants, <br />with the exception of salt bush and <br />quail bush, exhibit a low tolerance to <br />saline soils. In contrast, saltcedar <br />thrives under highly saline condi- <br />tions. <br />Saltcedar is typically deciduous <br />and, without floods, large amounts of <br />leaf litter accumulate. Therefore, <br />the possibility of a stand igniting <br />increases, especially during the dry <br />summer months. After such fires, <br />saltcedar and arrowweed quickly regen- <br />erate, whereas cottonwood and quail <br />bush usually fail to return (Figure <br />20A). Thus, in stands of mixed vege- <br />tation saltcedar will be the first to <br />regenerate, and through successive <br />fires eventually displaces most native <br />species (Figure 20B). Currently, <br />saltcedar is the numerically dominant <br />tree along the entire length of the <br />lower Colorado River. <br />Riparian areas, especially on <br />Indian lands, are still being cleared <br />for agricultural and residential <br />developments. The last of the large <br />continuous mesquite bosques remaining <br />on the lower Colorado River were <br />beginning to be cleared in 1984. <br />About 800 ha (2,000 acres) of the <br />24