- 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