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Importance to Geology
River (stream) - flowing water at Earth's surface usually confined to channel; water derived from rain or melted snow that runs over Earth's surface or through ground into river. Rivers flow downhill due to gravity, eroding + transporting sediment, eventually to ocean. Sediment carried by river = river's load - grains (clay through boulders) that are carried up in water (suspended load) or near bottom (bed load) by grain hopping or rolling. With > velocity, river can carry greater load + larger grains. Load also includes dissolved salts (dissolved load).
I) Streamflow Characteristics
Velocity (v) = water speed, 5 km/hr = fast river. Maximum velocity is in river center, just below surface. Why?
Discharge (Q) = volume of water passing point over period of time (m3/sec or commonly ft3/sec).
A = cross-sectional area of river (m2), width = horizontal distance from bank to bank, depth = vertical distance from water surface to river bottom.
Discharge normally > after rainstorm (> v + A) + downstream
where tributaries add to main river. Discharge is used to characterize
flood size. Velocity also > slightly downstream. Why?
Gradient (downhill slope, rise/run in m/km or ft/mile)
II) River Channel Characteristics - Rivers tend to have characteristic downhill shape called longitudinal profile = river elevation vs. distance along flow direction. Rivers usually begin in steep, actively eroding areas, with fast-moving water that carries all but boulders (source). Eventually landscape flattens out + river slows down (on inner parts of bends), depositing sand + gravel. When river reaches base level (lowest elevation), it deposits remaining sediment (silt + mud) at river mouth.
Dam construction traps all sediment in reservoir (temporary base level), resulting in upstream deposition + downstream erosion (water has no sediment load), including coastal erosion.
III) Different kinds of rivers
A) Mountain rivers - in mountainous areas (near source). Valley has characteristic v-shape + channel occupies nearly all of valley bottom. River tends to follow relatively straight pathways (map view) due to steep topography (water flows in most direct pathway). Typical gradient = 10 - 40 m/km.
B) Meandering rivers - rivers that wander (meander = sinuous or snake-like channel course) back + forth across broad valley + flat plain called floodplain (broad, flat area consisting of sediment that is deposited during flood, when river overflows its channels). Typical of rivers in plains (near mouth) where typical gradient = 0.1 m/km. Meanders = path of least resistance for river. Natural levees (low, wedge-shaped lenses of sediment along river channel) commonly form due to deposition during flooding.
Meandering rivers change their course with time due to constant erosion + deposition that occurs along meanders. Outer part of meander has higher velocity, therefore erosion occurs. Inner part of meander has lower velocity, therefore deposition occurs. Differences in velocity are due to inertia (mass), water wants to move in straight line. Area of maximum water velocity is shifted to outer part of meander. Eventually, meander swing can get so large that river cuts through to form straighter channel, leaving oxbow lake (abandoned river meander). Rivers are dynamic systems that are constantly changing their course.
Area drained by river + its tributaries (small rivers that drain into larger ones) = drainage basin. Usually high ground separates one drainage basin from another (e.g., continental divide). Dendritic drainage = pattern of river + tributaries resemble branches in tree (map view).
When river enters body of standing water (lake or ocean), flow stops + remaining sediment load is deposited in delta. Shape (map view) is often like triangle.