Rivers & Streams
Every river on Earth is doing two things at once: carrying water from high ground to the ocean (completing the water cycle), and carving, carrying, and depositing sediment (shaping the landscape). A river is a conveyor belt of rock, running 24 hours a day.
Key idea: a river erodes where water moves fast (steep gradient, outside of bends) and deposits where water slows down (gentle gradient, inside of bends, where it meets a lake or ocean).
Drainage Basins & Watersheds
A drainage basin (watershed) is the entire area of land that drains into a single river system. Every point on land is in a watershed.Divides: ridgelines or high ground that separate one watershed from the next. Rain falling on one side flows into one river system; rain on the other side flows into a different one.Tributaries: smaller streams that feed into a larger river. The point where they meet is a confluence.The Mississippi River watershed covers about 40% of the lower 48 US states, water from as far west as Montana and as far east as New York contributes to the system, all flowing to the Gulf of Mexico.Stream Gradient & Velocity
Gradient is the steepness of the channel, the elevation drop per unit of horizontal distance, measured in meters per kilometer (m/km). A mountain stream might have a gradient of 30–50 m/km; a lowland river near its mouth might be less than 0.5 m/km.
Higher gradient → faster water, more energy, more erosive power.Lower gradient → slower water, more deposition.Discharge: the volume of water flowing past a point per unit time (m³/s). Discharge increases downstream as tributaries add water.Velocity varies within the channel: fastest in the center and near the surface (less friction), slowest along the bed and banks (more friction). On a bend, fastest on the outside, slowest on the inside.Erosion & Deposition
Erosion is the wearing away and transport of rock and sediment by flowing water. Rivers erode by:
Hydraulic action: the force of moving waterAbrasion: sediment scraping the bed and banksSolution: dissolving soluble rockDeposition happens when water slows down and loses energy, dropping its sediment load. Largest particles (gravel, cobbles) settle first; smallest particles (silt, clay) are carried farthest.
Meanders: S-shaped curves that develop as a river erodes the outside of bends (faster water, more energy) and deposits on the inside of bends (slower water, less energy). Over time, meanders migrate across the floodplain.Floodplain: the flat area beside a river covered with water during floods. Built from layers of fine sediment deposited by repeated flooding, some of the most fertile agricultural land on Earth.Delta: a fan-shaped deposit of sediment where a river enters a lake or ocean and velocity drops sharply. The Nile, Mississippi, and Ganges deltas are major examples.Rivers in the Water Cycle
Rivers are the primary surface pathway for returning precipitation to the ocean, completing the water cycle. They also recharge wetlands, lakes, and some groundwater systems. Residence time of water in rivers averages about 2 weeks, much shorter than groundwater (years to millennia) or oceans (~3,200 years).
Human modifications: dams alter flow, trap sediment, and change downstream erosion and deposition patterns, affecting ecosystems. Levees prevent natural flooding of floodplains, reducing fertility but protecting development.
Key Vocabulary
Drainage basin (watershed): All land that drains into one river systemTributary: A smaller stream feeding a larger riverGradient: Steepness of a river channel (elevation drop per distance)Discharge: Volume of water flowing past a point per unit timeMeander: S-shaped bend formed by erosion and depositionFloodplain: Flat, fertile area beside a river built from flood sedimentDelta: Fan-shaped sediment deposit where a river meets standing water