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occupied by nine minnows and suckers at the same time and place, all of which are characteristic of "warmwater" habitat. These co-occurrences may be partially explained by a latitudinal effect. High- altitude streams in Mexico are warm enough to support minnows and suckers, but are not (quite) too warm in summer to exclude trout. They also may be an artifact of sampling. Remote streams in Mexico remain inaccessible or unknown to foreign workers unfamiliar with the region, and some of them not yet sampled may be high enough to exclude temperate fishes. It is also notable, however, that non-native trouts have yet to be widely stocked in Mexico, so natives may still occupy natural ranges that include lower-elevation waters. There is good evidence that Apache and Gila trouts descended to lower elevations in the past, where they also encountered "warmwater" species. Streams at Intermediate Elevations An Historic Perspective Creeks and rivers in lower coniferous forests through woodlands and into desert grasslands (from about 1800 to 900 m elevation) support most of the remaining native fish populations in the region. These channels carry surface runoff as well as intercepting groundwaters from the highlands, which enter as springs or seeps along bedrock fracture zones or bedding planes or is lifted from deep alluvial fills when channels cross bedrock dikes (Fig. 19). Many streams are highly erosive in deep, Figure 19. Bedrock dikes exposed by channel incision in Cienega Creek, Arizona, force subterranean flow to the surface, creating refuges for fishes and other aquatic organisms during drought. Figure 20. Canyons of the Colorado River in northern Arizona were cut in the Colorado Plateau surface through millennia. Forces resulting in these spectacular landforms are now under partial control of mainstream dams. Photograph by D. A. Hendrickson. precipitous canyons for parts of their courses (Fig. 20). From an historic perspective, such places have changed little from the way they appeared hundreds of years ago. Other reaches, flowing through broad valleys (Fig. 21), are dramatically different from a century ago due to human activities resulting in changes in the equilibrium of erosion and deposition. A stream at equilibrium transports precisely as much material as produced within its watershed. Thus, as a ton of material weathers and erodes frorr a headwater mountain, a ton is carried out. In practice, what one sees in a given stream results from the average condition over an undefined periol before the observation. In bedrock, canyon-bound segments, transport through the channel may have little to do with how the system appears; apparent equilibrium is dictated by the non-erodibility of bedrock. In alluvial channels on unconsolidated sediments deposited in times past, the stream bed at equilibrium may remain at a constant level, with 11