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22 <br />According to Brown et al. (1977), only a few southwestern <br />drainages such as the Rio Magdalena in Sonora, Mexico, and the San <br />Pedro in Arizona, presently contain any extensive linear riparian <br />forest development. Remaining riparian plant communities are <br />continually subjected to upstream impoundments, channel cutting, <br />channelization, irrigation diversions, groundwater pumping, and, in <br />many cases, increased water salinity. In addition, overgrazing by <br />cattle has negatively affected remaining forests, especially <br />cottonwoods, which reproduce primarily from seed rather than sprouts <br />(Horton et al., 1960). <br />A recent study by Reichhardt et al. (1978), compared various <br />land-use practices along the San Pedro River from 1935-1978. Somewhat <br />surprisingly, conclusions were that riparian areas composed of cotton- <br />wood, mesquite, salt cedar, and willow had increased since 1935, as <br />had agricultural and cultural-industrial areas. These increases <br />occurred simultaneously with a decrease in areas such as marsh lands, <br />mesquite-scrub, river channel, and streambed thickets of annual and/or <br />immature riparian species. However, these authors also concluded <br />that, although mesquite and salt cedar areas,appear to have increased <br />significantly since the early 1900's, cottonwood and willow associations <br />requireiessentially perennial streamflow and consequently are declining. <br />Resulting loss of shade, cover, and bank stabilization are obvious <br />consequences. <br />In summary, surface water flows in the San Pedro basin have <br />declined dramatically from those historically present. Native fish <br />populations have been decimated because of habitat loss throughout <br />the basin. Aravaipa Creek remains the only relatively-unmodified <br />perennial stream section, and contains 7 of the 8 native fish <br />species which are still present.