River Systems and Landforms

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GEOG 140
Intro Physical Geography
Lecture Notes
River Systems and Landforms
Fluvial Processes
-erosion
-transport
-deposition (alluvium)
Base Level of Streams
-ultimate
-sea level (average between high and low tide)
-local/temporary
-river
-lake
-resistant rock layer
-man-made dam
-arid environments
-closed drainage basins
-no connection to ultimate base level
Drainage Basins
-wide variety of sizes from a few sq. kilometers to thousands of sq. kilometers
-basin defined by ridges that form drainage divides
-define a watershed (the water receiving area of a drainage basin)
-sheet flow to concentrated channel flow
-rills
-gullies
-stream channel
-trunk river joined by a series of tributaries
-stream order (video)
Drainage Density and Patterns
-drainage density
-length of stream channels per unit area
-higher the density the more erosion
-drainage patterns
-dendritric
-tree-like
-most common
-relatively flat sedimentary rocks
-trellis
-parallel folded or dipping rocks
-long main trunks
-short tributaries joining at right angles
-radial
-conical landforms
-composite volcanoes and cones
-parallel
-associated with steep slopes
-rectangular
-similar to trellis but for smaller areas controlled by joint or fault system
-annular
-domes
-centripetal
-drainage into closed basin
Streamflow Characteristics
Discharge
-streams flow rate
-Q = wdv
-as a streams discharge (Q) increases some combination of stream width,
depth and velocity increases
-sediment load also increases with discharge
Exotic Streams
-streams with origins in humid areas that flow through arid regions
-discharge decreases with distance
-Nile
-Colorado
Stream Erosion
-takes place by
-hydraulic action
-work of water itself
-abrasion
-work of boulders, rocks, pebbles, and sand
-corrosion
-dissolving of minerals
Stream Transport
-suspension (suspended load)
-carried by water without contact with bed
-solution (solution load)
-material dissolved by corrosion
-traction (bed load)
-rolling along river bed
-saltation (bed load)
-bounce along bed
-if load exceeds streams capacity sediments accumulate
-aggradation
-enough excess sediment
-braided stream channel
-reduced discharge lowers streams transporting ability
Chapter 14 – River Systems and Landforms (continued)
Flow and Channel Characteristics
-two types of flow
-laminar flow
-streamlined flow in deep channels with smooth surfaces
-water moves in parallel paths transporting clay and very fine
material
-turbulent flow
-found in shallow streams and /or where channels are rough
-complex flow (including eddies) caused by friction
-greatest stream velocity
-near surface at center channel (deepest point)
-form three types of channels
-braided
-high bedload content and/or decreasing discharge
-straight
-meandering
-gradual slope
-stream develops a sinuous pattern
-evidence of river systems struggle to operate with the least effort
-balance between equilibrium and chaos
-fastest velocity on outer curve
-most erosion
-cutbank or undercut bank
-slowest velocity on inner curve
-deposition (point bar)
-erosion (cutbank) and deposition (point bar) feature migrate
downstream
-neck
-cutoff
-oxbow lake
Stream Gradient
-rate of elevation decline from headwaters (source) to mouth (end point)
-longitudinal profile
-steeper in upper portions
-gentler in lower portions
-graded stream
-slope adjusts to available discharge and channel characteristics over time
- provides just the velocity required to transport sediment load
provided by the drainage basin
-graded stream different from lowest gradient
-balance between erosion, transportation, and deposition
-dynamic equilibrium
-can happen in high or low gradient streams
-normally only portions of a stream are graded
-gradient can be affected by tectonic uplift
-rejuvenation
-entrenched meanders
-nickpoints
-abrupt changes in gradient
-waterfalls
-rapids
-stream energy works to reamove nickpoints
Stream Deposition
-floodplains
-low lying area adjacent to channel of a stream which a subjected to
repeated flooding
-repeated deposits of alluvium
-levees
-river overflows banks, looses velocity, drops portion of sediment load
-sand followed by silt and then clay
-smaller particles deposited farther from the river
-grow in height
-perched river channel
-terraces
-rejuvination of stream leading to entrenchment (further downcutting
through deposited alluvium
-paired sets on either side of river
-multiple terraces-multiple episodes of rejuvination
-deltas
-braided (Ganges)
-arcuate (Nile)
-estuarine (Tiber)
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