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Trouts are of northern derivation, and thus do well in such places. Headwater streams above perhaps 2100 m at lower latitudes in Mexico are also high and cool enough to support trouts, but they are not too high or cold to exclude temperate species. Thus, a far more diversified fauna is present on Mexican mountaintops. Most high-altitude, perennial southwestern streams depend on ground storage of melted snow for sustained flow during drought. Severe drying can induce intermittency, with pools maintained by underground seepage. Alternatively, scouring floods may occur at snowmelt, especially when warm rains fall on an accumulated snowpack or after violent convectional storms in summer. In their natural state, however, banks of mountain creeks are protected by dense riparian forests. Their channels are choked with downed logs and other organic debris and lined with armouring of cobbles and boulders, so the erosive power of floods is minimized. In fact, most mountain streams may be so heavily shaded by trees that few aquatic plants can grow. Low light intensity impedes photosynthesis, and colonization by all but specially adapted algae is precluded. Reaches exposed to sunlight in meadows support flourishing aquatic and semiaquatic plant communities, which may fill the channel to form complex marshes. Considerable organic material may be produced in such sections. Most mountain streams are nonetheless heterotrophic, with organic input coming from the watershed rather than produced within the aquatic system. Pine needles, leaves, and woody debris are attacked by bacteria and fungi to form detritus, which serves as the food base. Invertebrates capable of shredding leaves, others that gather and eat finely divided organic debris, and some that feed on decomposing wood are common. These animals are eaten, in turn, by predatory invertebrates and fishes. High Mountain Trouts Native trout of the Southwest can only be relicts of the past that achieved their present distributions either by entering the rivers from a much cooler sea or by the far more difficult process of dispersal over drainage divides. Two endemic species are isolated in highlands of Arizona and New Mexico, one or more undescribed kinds (collectively called trucha de Yaqui; Figs. 10, 11) live in the rugged Sierra Madre of Sonora and Chihuahua, the Mexican golden troutltrucha dorada mexicana lives in Sinaloa, and a native subspecies of rainbow trout (trucha de San Pedro Martir) lives on top of Sierra San Pedro Martir in Baja California Norte. The last two kinds are out of the area of present coverage. In the United States, the Apache trout (Fig. 12) is native to the White Mountains of Arizona, in headwaters of the Little Colorado, Black, and White rivers. Gila trout (Fig. 13) once lived in tributaries of the Verde River, Arizona, from which natural populations have long been extirpated. It persists naturally only in tributaries of the upper Gila River, New Mexico. Both formerly ranged downslope into larger streams. Both the Apache and Gila trouts became increasingly restricted in range and numbers as non- native species were introduced to enhance sport fishing. Each progressively disappeared as alien fishes became established and increased in abundance. Rainbow (and likely cutthroat) trout hybridized with them, brown trout ate them, and brook trout Figure 10. Yaqui troutltrucha de Yaqui, 13.5 cm in total length, a yet-to-be- described species from Arroyo Ahumado, Chihuahua. 7