One of the last undammed perennial rivers in the desert Southwest, the San Pedro River in southeastern Arizona illustrates important processes common to many desert riparian ecosystems. Although historic land uses and climatic extremes have led to aquifer depletion, river entrenchment, and other changes, the river still sustains a rich and varied selection of life. Resilient to many factors, portions of the San Pedro have become increasingly threatened by groundwater pumping and other impacts of population growth. This book provides an extensive knowledge base on all aspects of the San Pedro, from flora and fauna to hydrology and human use to preservation. It describes the ecological patterns and processes of this aridland river and explores both the ongoing science-driven efforts by nonprofit groups and government agencies to sustain and restore its riparian ecosystems and the science that supports these management decisions. An interdisciplinary team of fifty-seven contributorsabiologists, ecologists, geomorphologists, historians, hydrologists, lawyers, political scientistsaweave together threads from their diverse perspectives to reveal the processes that shape the past, present, and future of the San Pedroas riparian and aquatic ecosystems. They review the biological communities of the San Pedro and the stream hydrology and geomorphology that affect its riparian biota. They then look at conservation and management challenges along three sections of the San Pedro, from its headwaters in Mexico to its confluence with the Gila River, describing legal and policy issues and their interface with science; activities related to mitigation, conservation, and restoration; and aprognosis of the potential for sustaining the basinas riparian system. These chapters demonstrate the complexity of the San Pedroas ecological and hydrological conditions, showing that there are no easy answers to the problemsaand that existing laws are inadequate to fully address them. Collectively, they offer students, professionals, and environmental advocates a better grasp of the San Pedroas status as well as important lessons for restoring physical processes and biotic communities to rivers in arid and semiarid regions.