The diversity of running water environments is enormous. When one considers torrential moun- tain brooks, large rivers of lowlands, and great rivers whose basins occupy subcontinents, it is apparent how location-specific environmental factors contribute to the sense of uniqueness and diversity of running waters. At the same time, however, our improved understanding of ecological, biogeochemical, hydrological, and geomorphological processes provides insight into the structural and functional characteristics of river systems that brings a unifying framework to this field of study. Inputs and transformations of energy and materials are important in all river systems, regional species richness and local spe- cies interactions influence the structure of all riverine communities, and the interaction of phy- sical and biological forces is important to virtual- ly every question that has been asked. It seems that the processes acting in running waters are general, but the settings are often unique.

We believe that it helps the reader, when some pattern or result is described, to have some image of what kind of stream or river is under investigation, and also where it is located. Stream ecology, like all ecology, depends greatly on context: place, environmental conditions, season, and species. This text includes frequent use of descriptors like ‘‘small woodland stream,’’ ‘‘open pastureland stream,’’ or ‘‘large lowland river,’’ and we believe that readers will find these useful clues to the patterns and processes that are reported. For most studies within the United States we have included further regional description, but have done so less frequently for studies from elsewhere around the globe. We apologize to our international readers for this pragmatic choice, and we have made every effort to include examples and literature from outside of North America

J. David Allan & MarI¤ a M. Castillo