Hydrology, geomorphology and management: Implications for sustainability of native southwestern fishes

Native southwestern fishes have declined markedly in range and numbers. The factors responsible for their decline are many and varied. However the primary stressors to native fish assemblages in southwestern rivers and streams are habitat alteration and introduction of non-native species. We present data that compare the fish assemblages in two desert rivers-the Gila and Verde (Arizona-New Mexico)-over a period of 7-12 years, respectively. We also present data on hydrographs, broadscale and local geomorphology, and past fisheries, water and land management activities. Peak flow mean volume of flow, variability of flow, canyon-bound and broad alluvial reaches, dams, and introduced fishes are all either directly or indirectly related to fish assemblages in southwestern rivers and streams. We suggest that three primary influencing factors-two natural and one human induced (hydrograph, geomorphology, management)-are critical features in delimiting native fish assemblages. Conserving and sustaining native fish assemblages in these and other southwestern rivers and streams will require land managers to address all aspects of these three major influencing factors with administrative and legal mandates.