1952- 1967- 20thcentury amphibians antiquities arizona artistic bibliography biography byron dams debrisavalanches descriptionandtravel discoveryandexploration environmentalconditions environmentalpolicy excavations(archaeology) geologists geology glencanyondamregion grandcanyonregion history holocene indiansofnorthamerica indiantrails klett landscapearchaeology landscapechanges landscapephotography mark naturalhistory photography politicalaspects publicopinion rapids repeatphotography reptiles research riversurveys running sierraclub stratigraphic streamecology unitedstates water-supply waterresourcesdevelopment wolfe