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77 <br />sympatric in these areas of the Colorado River system with <br />C. P. clarki, a close relative of the bluehead sucker. <br />C. P. clarki was isolated from the bluehead in late <br />Miocene to Middle Pliocene times by the Kaibab upwarp <br />which may have allowed a period of upper and lower Colorado <br />River basin isolation (Bunt 1956). The £lannelmouth sticker, <br />never abundant in headwaters regions, may have aquired <br />adaptations to the torrential flows and unique chemical <br />conditions of the lower Colorado River. Relief of the <br />Kaibab upwarp and re-establishment of the upper and lower <br />basin connection may have brought the flannelmouth and blue- <br />head suckers into sympatry in the reaches above the Grand <br />Canyon in late Pliocene times. This would allow sympatric <br />existence of the flannelmouth and bluehead suckers for a <br />minimum of 8 million years. Most probably the species <br />were not completely isolated by the upwarp and have evolved <br />together for an even longer period. <br />Flannelmouth sucker distribution presently extends to <br />the more tributary segments of the Yampa and White Rivers <br />which is probably a result of limitations in downstream <br />habitat (Miller 1961). Populations of both bluehead and <br />flannelmouth suckers have declined near or in impounded <br />segments of the Colorado River. The result is a clouding <br />of the actual niche separations of native catostomids which <br />certainly existed prior to construction of the impoundments.