Fishes, Macroinvertebrates, and Aquatic Habitats of the Purgatoire River in Pinon Canyon, Colorado

Abstract

We describe physical habitat and aquatic biota of a relatively undisturbed canyon reach of the Purgatoire River and its tributaries in southeastern Colorado. Flow regimes are highly variable due to unpredictable, brief, intense summer floods. River habitat consists of long, deep, silty pools with few large boulders separated by short cobble riffles, whereas tributaries contain isolated pools maintained by groundwater. Water chemistry and temperatures were within ranges tolerable by plains stream fishes at all sites. Benthic macroinvertebrate assemblages included representatives of four regional faunas. At lotic sites, a few taxa of two detritus-collecting functional groups predominated. The fish fauna is depauperate, consisting of only 11 native species which we divided into river, perennial stream, and generalist faunal associations. However, most fishes have generalized habitat, trophic, and reproductive requirements, which seem to adapt them to survive the harsh environmental conditions in this plains stream. Although the four species that made up 95% of individuals generally persisted at river sites over the 5to 7-year period sampled, the abundance of red shiners declined markedly from 1983 to 1987 despite favorable flow regimes. Little is known of the ecology and physical characteristics of streams of the western Great Plains (Matthews, 1988). However, fish and macroinvertebrate assemblages in such systems can be expected to differ from those in more mesic regions due to extremes in physical characteristics such as flow, turbidity, and temperature. Relatively pristine habitats in streams of the western plains are scarce due to a profusion of human activities, such as channelization, damming, dewatering, mining, overgrazing, use of agricultural chemicals, release of municipal and industrial wastes, and the introduction of nonnative species (Cross and Moss, 1987). Although the Purgatoire River is not immune to these perturbations, the reach we discuss here is one of few comparatively undisturbed remnants in the region and, therefore, offers an opportunity to study habitat and aquatic biota in a relatively natural stream of the western plains. The purpose of this paper is to describe the flow regime, physical habitat, and water chemistry of the Purgatoire River and its tributaries in Pifion Canyon and to document the species composition, distribution, and ecological characteristics of assemblages of benthic macroinvertebrates and fishes of these waters. Few reports of aquatic biota exist for this general area (Cope and Yarrow, 1875; Jordan and Evermann, 1891; D. Vana-Miller, in litt.), and none for the specific reach. We show that this reach harbors depauperate assemblages of entirely native aquatic biota which have generalized ecological requirements that adapt them for persistence in plains