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ture of the flooded gravel pit in the 15-mile reach from which two running ripe razorbacks were captured in 1986 was 22.5 C, while the main channel temperature was 13-13.5 C. We believe the razorback sucker evolved to exploit the ephemeral but predictable flooded bottomland habitat type as part of its reproductive strategy. Today, the magnitude of spring flows has been reduced to little more than half of that which occurred historically resulting in much loss of lowland flooding. Those areas in the Grand Valley that might still routinely flood, and thus provide potential spawning habitat for razorback suckers, have been diked by land owners. Razorback suckers must now spawn in suboptimum habitats such as gravel-pit ponds, where carp (Cyprinus carpio) and introduced predators are abundant, or in the main channel where temperatures are low and the opportunity for hybridization with flannelmouth suckers is increased. As mentioned above, the frequency of years in which peak flows of 30,000- 40,000 cfs are attained at the State line (or 19,500-25,000 cfs in the 15- mile reach) has not changed significantly from historic times, but the frequency of years with peak flows higher than this has been greatly reduced (Fig. 7 and 8). Spawning requirements of razorback sucker that are uniquely tied to very high flows in spring would greatly help to explain why the population of razorback sucker has collapsed while a small popula- tion of Colorado squawfish has managed to persist. Historically, peak flows greater than 40,000 cfs at the State line occurred in 56% of the years; in recent times, they have occurred in only 8% of the years. To recover razorback sucker in the upper Colorado River, the process of river channelization must be reversed so that suitable spawning habitat is 41