r-In a 2.5 year (1978-1980) study of the lotic warmwater fish communities of the Platte River Basin, Colorado, 25 native and 9 non-native species were collected. Three natives (Nocomis biguttatus, Couesius plumbeus, and Notropis heterolepis) have been extirpated. Salmo clarki stomias was not collected, nor were the provisional natives, Stizostedion vitreum and Carpiodes cyprinus. Eleven native species were rare in the basin. The rarity of each was strongly correlated with their limited historic distributions, rather special habitat requirements, and intolerance of environmental stresses. Six native species were historically more common and widespread, but habitat deterioration associated with European settlement has caused their decline. The remaining native species (8) were common and widely distributed. The common carp (Cyprinus carpio) was the only common non-native fish. Lack of suitable habitat is believed responsible for the rarity of other non-native species. The Platte River arises in the Rocky Mountains of central and northern Colorado; the river system is comprised of two major basins: the North and South Platte. The North Platte River System in Colorado is limited to its headwaters area, a broad intermountain basin (3,706 km2). In its warmwater reaches, coalescing streams are small (<15 m wide and <1 m deep) but have a mixture of flow patterns. The relative isolation and high elevation (2,250 m) of the basin have restricted human activities primarily to ranching. In contrast, the South Platte River basin is 59,927 km2 and has a greater diversity of stream morphologies. Upon emerging from the mountains, the streams are in transition from montane to plains streams. Stream dimensions vary, but streams exhibit a commonality of heterogeneous flow patterns and substrate types. As the system progresses eastward and anastomoses, streams evolve from ones characterized by variable gradients and substrate types but stable, sinuous thalwegs to broad, shallow, sand- bottomed rivers. Except for the South Platte River, the eastern portion of the basin is drained only by intermittent streams. Peak discharge in the basins normally occurs from late spring to mid summer (May-July). Mean discharge of the South Platte River varies greatly, but decade means have remained fairly constant since 1900 (Williams, 1978) (Fig. 1). The South Platte River System has been significantly altered by human activities. As the streams flow eastward, they are deleteriously affected by industrial, municipal, and agricultural effluents. Irrigation, flood control, and municipal water supply reservoirs control the pulse of water through the streams. Transmountain diversions and ground-water pumping supplement natural discharge. The North Platte System in Colorado remains comparatively unmodified. Because this research was funded by the Colorado Division of Wildlife, political rather than ecosystem boundaries defined the limits of the study