The Gila River Indian Reservation lies in the Sonoran Desert of south-central Arizona, with an elevation range from 941 to 4512 feet (287 to 1375 m). Three major desert streams (the Salt, Gila, and Santa Cruz Rivers) have their confluences on the reservation. The human occupancy of the Gila floodplain is believed to have been continuous for the past two millenia. In the past century loss of stream flow and deterioration of riparian and marsh habitats have resulted from drastic changes in hydrologic regimes of the major streams. Documentation of original habitat conditions is based on Hispanic accounts (1694-1821) and subsequent Anglo accounts, together with oral history from the Pima Indians. Riparian timber and emergent vegetation were gone by 1950 due to destructive floods and lowered water table. Irrigation run-off and Phoenix sewage effluent have reestablished locally riparian communities and marshes. Eleven major habitats occur today on the reservation. Their predominant vegetation is described. Field work on the reservation was conducted from 1963 to 1976. Modern distributional data are compared with evidence from archaeological, ethnographic, and historical sources. The total avifauna (all time horizons) consists of 232 species, of which 207 are supported by specimen evidence. At least 101 species are breeding or have bred in the past; five other species are probably breeding. On geographic grounds an additional seven species are suspected of having bred aboriginally. The taxonomy and migration are discussed in accounts of 46 species with two or more subspecies occurring on the study area. The Piman ethno-taxonomy of birds distinguishes 67 taxa, most of which correspond to Linnaean (biological) species. In the past 100 years 28 species (21 as breeding species) have been extirpated from the reservation. Of these 24 are directly related to loss of riparian woodlands or open water and marshes. Since 1958 at least 13 species have recolonized as a result of the redevelopment of riparian communities with willow, cottonwood, and cattail. The present reservation habitat and avifauna are contrasted with two modern analogs with perennial surface water and intact riparian communities. These are at similar elevations, less than 30 miles (48 km) from the reservation. Ten Neotropical species have colonized Arizona within the 20th century and an additional eight have extended their breeding ranges northward. Four Neotropical species have occupied the reservation during this period. Formerly allopatric subspecies of two species (Great-tailed Grackle, Quiscalus mexicanus, and Horned Lark, Eremophila alpestris) are now interbreeding on the reservation in areas of secondary intergradation. The northward movements are attributed to post- Pleistocene recolonizations, in part facilitated by human modifications of habitats. At least 15 wintering species are departing earlier in spring than they did at the turn of the century. This is attributed to habitat deterioration. The habitats with the greatest avifaunal diversity, both summer and winter, are the traditional Pima farms (rancherlas) and the recently redeveloped riparian communities along the Salt River. The least diversified and most disturbed habitats are the Gila River channel and the large-scale mechanized farms on lands leased to non-Indians. The future of both the natural habitats and the avifauna of the reservation is in the hands of tribal leaders.

This item was digitized as part of the Million Books Project led by Carnegie Mellon University and supported by grants from the National Science Foundation (NSF). Cornell University coordinated the participation of land-grant and agricultural libraries in providing historical agricultural information for the digitization project; the University of Arizona Libraries, the College of Agriculture and Life Sciences, and the Office of Arid Lands Studies collaborated in the selection and provision of material for the digitization project.

The question posed in this study is why alluvial soil on a floodplain is finer than the bed material of the river that supplied the sediments deposited to form the floodplain. A schematic, simplified river/floodplain system is used in an approximate analysis to find the size distribution of the floodplain soil. It is assumed that the stable limiting condition is a suspended load in the floodplain flow of the same concentration and composition as the sediment load in that portion of the river channel flow above the level of the floodplain. It was found that floodplain soil should be finer than the channel bed material; how much finer depending on the bank height, and to a lesser degree the width of the floodplain.

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