a flowing water channel moving from source (headwaters) to mouth (delta/estuary/ocean)
organized into drainage basins (watersheds) separated by ridges
agents of erosion, sediment transport, and deposition shaping landscapes
three stages: youthful (steep, V-shaped), mature (meandering), old (broad floodplain)
civilization catalyst: Nile, Tigris-Euphrates, Indus, Yellow River cradles of human settlement
floodplains deposit fertile soil supporting agriculture
freshwater habitat for fish, amphibians, invertebrates, and riparian ecosystems
transport corridor for nutrients, connecting the water cycle, carbon cycle, and nitrogen cycle
dams alter flow regimes, sediment delivery, and ecology
river deltas are among the most productive and most vulnerable biomes
carries dissolved minerals from weathering, linking atmosphere chemistry to ocean composition