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80 fishes with fine scales ( <100 in the lateral line), posi- tive identification is difficult in the field. The fish mentioned by Wiltzius were probably bluehead X longnose sucker hybrids. The abundance of the white sucker hybrids with both bluehead and flannelmouth suckers at the upper stations on the Yampa River is probably attributable to 1) success of the introduced white sucker, 2) river channel alterations, and 3) flow reductions due to agricultural and municipal demands. White suckers were most abundant at Station Y-2, as were their hybrids with the indigenous suckers. River chan- nel alterations near Craig, Colorado, were numerous. Also, pump and gravity irrigation diversions during spring and summer typically caused downstream segments to be temporarily dewatered and resupplied with return-flow water from an up- stream application. Such waters are often warmer and more saline than waters which have not been diverted. The stream alteration effort at Station Y-2 left little quiet pool area during high-water periods. The unprotected, open stream channel was probably not a suitable spawning site. Spawning fish were found crowded in an artificial diversion loop in early June, 1976. This concentrated as- sociation of white and bluehead suckers, both in spawning condition, probably resulted in many of the lentic conditions described by Ilubbs et al (1943) which favor extensive hybridization.