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Redside shiner Richardsonius balteatus *Common shiner Notropis cornutus *River shiner Notropis blennius Red shiner Notropis lutrensis Sand shiner Notropis stramineus Bigmouth shiner Notropis dorsalis Spottail shiner Notropis hudsonius Golden shiner Notemigonus crysoleucas Grass carp Ctenopharyngodon idella Tench Tinca tinca Clupeldae Gizzard shad Dorosoma cepedianum Threadfin shad Dorosoma petenense River Basins in Colorado There are several distinct river basins in Colorado. Each is physically isolated from other basins. Over the course of time different species communities developed in each basin. Stream capture, whereby direction of flow is changed from one drainage to another by earthquake, uplift or some other geologic phenomenon, accounts for some movement of species between basins. Eventually each basin came to have a distinct fauna, adapted to various habitat types within the drainage. In most Col- orado river drainages the fauna of a given system is rather unique. Several species are limited to only one drainage and others are found statewide. One interest- ing aspect of the fisheries of Colorado is the rather few number of species native to each drainage basin. Presently, over 40 species may be found in a drainage basin while less than a dozen are native to the system. The Colorado River Basin drains all por- Catostomidae River carpsucker Carpiodes carpio White sucker Catostomus commersoni Longnose sucker Catostomus catostomus Flannelmouth sucker Catostomus latipinnis Bluehead sucker Catostomus discobolus Mountain sucker Catostomus platyrhynchus *Rio Grande sucker Catostomus plebeius *Razorback sucker Xyrauchen texanus Cyprinodontidae Plains killifish Fundulus zebrinus Plains topminnow Fundulus sciadicus tions of the state on the western slope of the Continental Divide. The White, Yampa, Green, Dolores, Gunnison, San Juan, and Animas rivers are all tributary to the Colorado River which drains about one-half the state's area. Despite the vast size of the Colorado River Basin, only nine species of fish were native in the drainage Ellis (1914). Behnke and Benson (1980) put this figure at 13 or 14 species. The Rio Grande drains the southern portion of the state. Not all streams in this drainage merge with the mainstem river, several streams which arise in the moun- tains in the north part of the drainage disappear into the arid sands of the San Luis Valley without ever reaching a larger river. Jordan (1891) collected only four species from the Rio Grande basin while Ellis (1914) described five species from the system. The Arkansas River drains the south- east portion of Colorado. Originating in the mountains of central Colorado, the Poeciliidae Mosquitofish Gambusia affinis Cottidae Mottled sculpin Paiute sculpin Percidae Johnny darter *Iowa darter Orangethroat darter *Arkansas darter Cottus bairdi Cottus beldingi Etheostoma nigrum Etheostoma exile Etheostoma spectabile Etheostoma cragini Arkansas then flows across the arid east- ern plains of the state. There are few permanently flowing tributaries that enter the Arkansas River on the eastern plains. The Fountain, Purgatoire, Huerfano and a few others are only a few inches deep and ten yards or so wide when they flow into the Arkansas mainstem. Seventeen species were listed as native to this drainage by Ellis (1914). The Cimmaron River flows through the very southeast tip of Colorado. There is no permanent flow in the river segment inside Colorado. A few isolated potholes in the main channel or tributaries might contain water during periods of low flow. Only species that are able to tolerate the severe conditions in these potholes could survive in this drainage. Three species have been collected in the Cimmaron system. The headwaters of the Smoky Hill River are in the eastern edge of Colorado, north of the Arkansas River Drainage. In western Colorado's Little Fish