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70 80 <40 from its natural state. Furthermore, the evidence that this has been caused by the regulation and reduction of streamflow in the Platte is convinc- ing. There is little evidence, how- 50 that the loss of habitat has ever, had any effects on the survival prospec ts of the whooping crane. Judging from the number of sightings, the whoopers seem to be using the 30 Platte as a stopover point no less frequently than they did in earlier days. Moreover, the fact that the '7 20 cranes have made such a remarkable' 10 o 19<40 1980 1970 1980 1950 Fig. 1. Population of Whooping Cranes, 1938-1976 (Aransas- Wood Buffalo Flock). Conclusion This review suggests that some of the statements in the chain of cause and effect connecting Grayrocks Dam with the survival of the whooping cranes is better supported by evidence than others. Clearly, the width of the channe 1 of the Plat te River, and hence the area suitable for crane habitat, declined drastically has recovery in the past quarter century suggests that the availability of suitable migratory habitat has not heretofore been an important factor in limiting crane populations. Even so, it seems likely, if not inevit- able, that at some point the further loss of habitat would begin to have an effect, perhaps even a catastrophic effect if it promotes the changes of a disease outbreak. The problem is how to determine the point at which further development of water re- sources would be harmful to the cranes. Research now in progress may soon a 11 ow this question to be an- swered, at least with more precision than is currently possible. But for the present, the Fish and Wildlife Service has taken a very conservative 43