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deeper parts of the habitat. When ready, a female enters the territory of a male to spawn on the substrate. The males inadvertently, although effectively, protect the eggs with their aggressive territorial behavior. Young are generally ignored, so they move and feed throughout the male's territory with impunity. Growth is rapid, and young hatched in early spring can mature and reproduce in mid-summer. Most fishes deposit their eggs outside the female's body. Topminnows, however, practice internal fertilization and development of young. Sperm packets (spermatophores) are delivered to the female's genital opening by use of a highly modified anal fin of the male. A female can store sperm, so a single sex act can serve to fertilize eggs throughout her life. Topminnows in the lowermost Rio Yaqui include one kind that differs even more markedly from most other fishes in its mode of reproduction. Most topminnows are bisexual: males and females breed normally. Some, however, have developed systems where males do not exist. Females breed with another kind of topminnow male, producing all female offspring of hybrid origin. The way this system operates remains poorly understood. In any case, young develop within the female's ovaries (Fig. 54) to a large size (up to 10 mm total length), and can swim, feed, and avoid predators a few seconds after birth. As with pupfish, sexual maturation is rapid, sometimes less than six weeks after birth in warm water In summer. Low Desert Streams Most low desert streams are now diverted by dams in their headwaters, or have their subterranean supplies interdicted by well fields that pump them dry. Pumpage for irrigation and other uses has lowered water tables more than 50 m in most valleys and more than 200 m in some, and beheading of streams by impoundment in montane parts of the watersheds prevents aquifer recharge. Only streams with reliable spring inflows remain below 600 m, and many of these no longer flow except through manmade delivery channels. Where they do, such as in and below the Grand Canyon, humans are simply using the natural channels for water deliveries from upstream reservoirs. The present section is therefore largely a description of the past, a reconstruction of an original state that may never be re-created. Some native fishes persist in the mainstreams, but only in special places. All large rivers of the region are (or historically were) characterized by high runoff from spring and Figure 54. Near-term embryos in a female Gila topminnowlcharaJito de Gila (40 mm in total length) from Monkey Spring, Arizona. early summer snowmelt in distant mountains, and a second period of lesser flooding during summer rains. Prior to man's modifications, the Colorado River near Yuma fluctuated in volume from less than 0.35 to more than 7000 cubic meters per second. All these streams flow across geologic structure, at least in part, and thus alternately pass through broad alluviated valleys and narrow gorges (Fig. 4). Valleys represent downthrow blocks, and canyons are cut through uplifted blocks that comprise the northeast- southwest-oriented mountain ranges of the region. Water tables in valleys, even in the hottest parts of the Sonoran Desert, were within a few feet of the surface prior to human alteration. Oxbow lakes, marshes behind natural levees (Fig. 55), and underflow that maintained permanent pools during drought were fed by these water tables. Perhaps more importantly, bedrock causing formation of Figure 55. Topock Marsh along the lower Colorado River, Arizona, is artifically maintained, but must resemble conditions that existed in oxbow lakes, marshes behind natural levees, and backwaters prior to human modifications of the system. Photograph provided by the U.S. Bureau of Reclamation. 27