River networks, and even individual river segments, are complex ecosystems that can be studied from many perspectives. Arguably the most common differentiation is between studies that focus on various aspects of rivers, such as contemporary physical processes (river engineering, hydrology, or geomorphology); physical processes over longer time spans (geomorphology); chemical processes (geology or aqueous chemistry); individual species or groups of organisms (fish biology); and biological communities (aquatic and riparian ecology). Each of these approaches to understanding rivers has an extensive technical literature. The works cited in this bibliographic entry draw from these sometimes disparate bodies of literature and focus on rivers in an environmental context rather than treating a specific river as an isolated feature or focusing solely on one component of rivers. River segments and river networks provide a wealth of information about past and contemporary environmental conditions, for rivers inherently integrate fluxes of matter and energy within a landscape through the entity of a drainage basin. The entire land surface that drains to a specified point makes up the drainage basin for that point. In addition to water, sediment, solutes, and organic matter enter the river network via atmospheric, surface, and subsurface pathways. Matter and energy move upstream, laterally, and vertically within a river network, as well as downstream. A well-studied example comes from the upstream migration of spawning salmon that then die and transfer ocean-derived nutrients to the river network and adjacent riparian zone. Because a river so effectively integrates diverse inputs and reflects conditions across the entire drainage basin, investigators have used physical, chemical, and biological characteristics of rivers as metrics for the environmental state of the river itself, and of the larger drainage basin. Three of the sections within this bibliographic entry include works that provide examples of these metrics for prehistoric, historic, and contemporary environmental conditions. Rivers also provide numerous ecosystem or environmental services, such as clean water and recreational fisheries, and another section provides examples of studies focusing on this aspect of rivers. Attempts to manage rivers and preserve desired attributes such as clean water, flood control, or fisheries constitute an important subset of environmental management, and are addressed in the final section of this entry.