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85 historically as a function of high water velocities during spring runoff. It exists below a natural migration impedi- ment (Cross Mountain Canyon) and is the richest Yampa station in indigenous fish species. Only one major tri- butary (the Little Snake River) enters the area, and it has no permanent fish communities. No stream channel altera- tions have occurred, and flows are not influenced by irri- gation diversions. The river is able to reproduce itself within historical extremes every year, and this stability, with a lack of exotic catostomid influence, has probably aided the bluehead sucker population at Station Y-4 in maintenance of its morphologic integrity. Upstream populations are subject to less stability about any of the above-mentioned conditions, and maintenance of genetic integrity within their catostomid populations is not assured. The nature of and factors influencin tior_ in the Yampa River ntnctnmid hvbridiza- Hybridization of the catostomid fishes of the Yampa River is proven beyond most questions in this report. A precise evaluation of the nature of the pheno- menon is not possible with the present lack of life history and specific cytological evidence which might clarify pre- and post-spawning isolating mechanisms. Preliminary con- clusions made possible by the present data, however, may be of value to future researchers.