Description

Like most river valleys, that of the Santa Cruz River contains a complex history of erosion, deposition, and stability spanning over a million years of geologic time. Unraveling this history is a laborious and often difficult process of mapping, analyzing, and correlating exposures of the sediments deposited by the river. Today this effort is aided by the fact that the river is a typical arroyo or entrenched drainage with vertical banks that expose the layers of sediment deposited in the past. Like pages in a book, these strata reveal the history of sequential deposition, but unlike a new book many of the pages are either missing (cut out by erosion) or mangled (altered by various processes). The Santa Cruz through Tucson has not always been a steep-walled, normally dry, ephemeral river. Less than a century ago, near "A" Mountain, it flowed intermittently in a relatively narrow and shallow channel through a broad, flat, grassy valley with numerous mesquite thickets and occasional cottonwood groves (Bryan, 1922). In some reaches the channel became indistinct and gave way to wet meadows or cienegas that were commonly exploited for water by constructing earthen dams. The stored water was then used for either irrigation or mill power. Another technique of acquiring irrigation water was to excavate a trench at headcuts to intersect the shallow water table and direct the outflow to fields downstream by ditch or pipe (Betancourt, 1978; Betancourt and Turner, in press). ( 44 pages)