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32 The relationship between flow and reproduction of other rare fish has not been established primarily because insufficient young have been found to indicate success. Available information indicates that the bony tail chub and razorback sucker require large rivers for their existance. This statement is supported in that relatively few of these species are found in tributaries, even the Yampa River, as compared to the numbers found in the mainstem Green and Colorado rivers. This in- formation suggests that these large river forms either have a direct requirement for particular flows and/or that the habitat needed for these fishes is maintained by large flows. No young-of-the-year Colo- rado squawfish have been found in the Yampa River above its mouth, even though they reproduced in the Green River when it contained almost 100% Yampa River water in 1963 (Figure 11). It appears therefore that large river habitat is very important to the maintenance of these rare fish populations. Since the Green River is now regulated by Flaming Gorge Dam, the Yampa represents most of the Green's flow below Echo Park for most of the spring and summer. i Therefore, it is the Yampa, and other \ downstream tributaries such as the Duchesne and White rivers, that are maintaining the Green as a "1arge river." and thereby maintaining one of the last potential habitats for these large river fishes. Data at present suggests the bony tail chub may be very near ex- tinction in the Green River. Vanicek (1967) collected over one hundred adults in the three years of his study in Dinosaur National Monument. These fish were used in a taxonomic study by Holden and Stalnaker (1970). Holden (1973) collected only 36 adults in the four year of his study in t' $, t .~, ~-