Tributaries Similar changes have occurred to tributary streams. Historically, surface flow often became intermittent or totally dried up in the stream‘s lower reaches. Today, that condition has not only intensified but has become permanent. Upstream water storage on the Bill Williams <strong>and</strong> Gila <strong>River</strong>s has dried up the lower portions of these streams. On the Salt <strong>and</strong> Gila <strong>River</strong>s, Roosevelt, Horse Mesa, Mormon Flat, Stewart Mountain, Bartlett, Horseshoe, <strong>and</strong> Coolidge Dams capture water that historically flowed to the Colorado <strong>River</strong> <strong>and</strong> delta. The river channel through the —Valley of the Sun“ is normally dry except for a few deep ponds left behind from s<strong>and</strong> <strong>and</strong> gravel mining <strong>and</strong> an incongruous, 2-mile-long artificial lake. The river corridor is leveed <strong>and</strong> highly disturbed, forming little more than a scar across the valley <strong>and</strong> state (Figs. 41 <strong>and</strong> 42). Recent maps seldom show sections of these streams, testifying to their conspicuous absence. Fish Fish found in the lower Colorado <strong>River</strong> were unique, having the highest proportion (75%) of endemism in the Nation. They occurred nowhere else in the world. Only nine species were native to the lower mainstem river <strong>and</strong> they evolved over millions of years in one of the harshest river environments known. They had learned how to survive floods, prolonged droughts, extreme temperatures, <strong>and</strong> salinities that few other fish could tolerate. The majority of nonnative fish found in the river today could not have survived long in the pristine system, nor could they invade the basin on their own. Nonnative fish only gained access to the Colorado <strong>River</strong> when they were stocked by Europeans. Common carp were imported to southern Nevada <strong>and</strong> first noticed in the wild in 1881. Others soon followed. Bullhead <strong>and</strong> channel catfish were stocked in 1894 <strong>and</strong> by 1910, all had spread through the lower river <strong>and</strong> were replacing natives at a rapid pace. By 1930, the majority of native fish had already been replaced by these three species. There is little doubt that water control projects sealed the fate of natives. Dams blocked upstream spawning migrations, irrigation reduced flows, fish were str<strong>and</strong>ed in canals <strong>and</strong> fields, while critical habitat was claimed by agriculture. The wide, shallow, braided river was squeezed by levees <strong>and</strong> further deepened by dredging <strong>and</strong> the natural forces of streambed scour. Storage reservoirs captured floods, reduced sediment, supplied consistent summer flows to water users, <strong>and</strong> provided optimal conditions for recreational fisheries. Deep, cool water habitats benefitted species like channel catfish, common carp, largemouth bass, trout, <strong>and</strong> sunfish that were less tolerant of desert MUELLER AND MARSH 37 conditions. State <strong>and</strong> federal game agencies recognized the recreational potential of this resource <strong>and</strong> started to manage these waters to meet an ever-growing angler dem<strong>and</strong>. Resource agencies sold fishing licenses to fund their programs <strong>and</strong> sport fishing generated revenue for local merchants. What physically <strong>and</strong> biologically remains of the river more closely resembles conditions <strong>and</strong> fish communities found in the Upper Mississippi <strong>and</strong> Missouri <strong>River</strong> drainage. Today, the Lower Colorado <strong>River</strong> has the dubious distinction of being among the few major rivers of the world with an entirely introduced fish fauna. The following section describes the historical fish community, the decline of the native fishery, <strong>and</strong> the fishery that exists today. The <strong>Native</strong> Fish Community (Pre-1880) The river‘s estuary contained more than 75 marine species, but prior to 1880, only nine freshwater species were found further upstream in the main river channel (Table 5). These represented four families (cyprinids, catostomids, cyprinodontids, poeciliids) of freshwater fishes <strong>and</strong> two (mugilids, elopids) marine species that frequently invaded the lower river. Evermann <strong>and</strong> Rutter (1895) reported that —the lower Colorado is one of the muddiest rivers in America <strong>and</strong> is unfit for any but mud-loving species.“ This was especially true during floods. Fish in the channel were blasted by suspended mud <strong>and</strong> s<strong>and</strong>. Abrasion to their bodies <strong>and</strong> gills must have been tremendous. Early reports suggest that fish were not plentiful in the channel <strong>and</strong> most were found in off-channel habitats where conditions were more hospitable. Floods were unquestionably important for maintaining nursery <strong>and</strong> spawning habitat, but the size of the fish community not only depended on successful spawning during wet periods, but also the severity of droughts. It did not make any difference how many young fish were produced, if there was not enough water for them to survive. The influence that drought played in the native fish community is poorly understood <strong>and</strong> unfortunately largely ignored. Researchers <strong>and</strong> hydrologists have been fascinated with flood mechanics <strong>and</strong> stream morphology but little study has gone into underst<strong>and</strong>ing the long-term biological implications of droughts <strong>and</strong> how it must have shaped the fish community. The first drought-related fishkill was reported by Father Garces in 1774œ1776 (Coues, 1900). He reported observing the remains of thous<strong>and</strong>s of dead fish while traveling through the delta. At the beginning of the twentieth century, MacDougal (1917) reported —a windrow of remains of fish which appeared to be carp,“ that —extended
38 INFORMATION AND TECHNOLOGY REPORT--2002-0010 Fig. 41. Aerial view of the Salt <strong>River</strong> floodplain near Phoenix, Arizona. Photograph by Michael Collier.
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