Box 5.1 The Santa Cruz River, Southern Arizona

The Santa Cruz River is a typical example of many rivers and streams in the valleys of the western United States that have experienced pronounced ecological changes during the past century. It is not an example of a restoration activity, but rather an illustration of how human activities and rapid urbanization of the floodplain can bring about irreversible changes to a stream system.

The Santa Cruz River is a dry, and usually insignificant, stream throughout most of its length. It rises in oak woodlands and grasslands southeast of Tucson. The headwaters of the Santa Cruz are gathered into a shallow, perennial channel that courses southward into Mexico and briefly follows a 56-km westerly course before reentering the United States some 10 km east of the border town of Nogales, Arizona. In Sonora, Mexico, the river's perennial flow is captured by wells and infiltration galleries for agricultural and municipal consumption. Since the late 1960s, effuent discharges from the Nogales wastewater treatment plant have accounted for the permanence of flow for several kilometers north of the border, where all of it infiltrates into the sandy streambed, resulting in a normally dry stream further north. The river is entrenched most dramatically within the San Xavier Indian Reservation, with vertical banks up to 10 m high and 100 m apart, where the river meanders around the base of Martinez Hill. To the north of Martinez Hill, sections of the riverbanks have been soil cemented as a precaution against flood damage in the heavily urbanized floodplain.

Annual flow along the river is extremely variable. During the 68-year period of available records at the Congress Street gauging station, 72 percent of all annual flood peaks occured during the months of July and August, 19 percent during September and October, and 9 percent November through February. No annual peak flows have been recorded during the months of March, April, May, or June (Betancourt and Turner, 1988). In this century, the greatest geomorphological changes in the Santa Cruz River were caused by floods occurring in 1905, 1915, 1977, and 1983 (the greatest recorded event, which had a peak discharge of approximately 1,500 m(3)/s at the Congress Street gauge), and all are associated with El Nio conditions (warmer than average episodes in the tropical Pacific).

Prior to extensive pumpage for agriculture and consumptive use in the Tucson Basin, the amount of water leaving the basin (i.e., stream flow, evaporation, and transpiration) equaled the amount entering, and ground water storage was nearly constant (Betancourt and Turner, 1988).

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