Lecture 19 Flashcards
Geomorphology
Study of landforms, including processes that shape them.
Tectonic processes build up landscape (uplift).
Erosional processes wear it down (denudation).
Depositional processes also build up landscape.
Powell
Best known for leading the first group down the Grand Canyon. Led US Geological Survey.
Was contributor for geography, geology, and anthropology.
Base level
Idea introduced by Powell.
Streams can erode to a certain level; sea level is ultimate base level.
Local base level; resistant rock. Like dams.
Gilbert
Worked with Powell in USGS. Many contributions to geomorphology.
Thought in terms of systems; landforms tend to equilibrium.
Theory of Graded River.
Graded River Theory
Stream adjusts its slope to be able to transport available sediment, with no extra energy.
Energy of stream based on discharge of slope. Sediment delivered by tributaries and erosion of slopes.
Discharge and sediment supplies are constant.
Davis
Dominated geomorphology in the first half of 1900s.
Studied landscape change over millions of years. Evolution of landscapes; cycle of erosion.
Cycle of erosion
Simple humid version - flat surface at base level (peneplain) undergoes rapid uplift - now above base level.
SEE CHART.
Cycle takes millions of years. Uplift can occur anytime.
Kinetic energy
Based on mass and speed.
Potential energy
Not used now, but can be used in the future.
Anything above base level has potential energy.
Q=AV
Discharge defined as cross sectional area times average velocity.
If you change A or V, the other has to adjust, as long as Q stays the same.
Laminar flow
Slow water may move in a straight line.
Turbulent flow
More chaotic flow, typical in a stream.
Factors affecting velocity
Channel slope; steeper = faster (positive).
Discharge; more mass, more away from bed (positive).
Depth of flow; less friction if deeper (positive).
Roughness of channel; more friction (negative).
Positively related variables
Variables that change the same direction.
Negatively related variables
Variables that change in opposite directions.
Shear stress
AKA tractive force.
Friction transfers some energy from water to bed. High erosion shear stress makes sediment move.
SEE CHART.
Stream load
Sediment carried by stream.
Bed load; large particles roll along bed or bounce (saltation).
Suspended load; small particles stay up above bed.
Dissolved load; products of chemical weathering, in solution.
Competence
Size of largest particle stream can carry at a given discharge.
Capacity
Amount of sediment stream can carry at a given discharge.
Types of erosion
Hydraulic action; flowing water picks up loose material.
Bank caving; sides of channel can fall into flow.
Abrasion; colliding particles break down (size decreases downstream, become more rounded).
Corrosion; chemical weathering in stream, rocks dissolve.
Factors affecting deposition
Dissolved load stays in water, but if velocity decreases, solid particles fall out.
Decreases velocity by; reduce slope, reduce discharge, decrease depth, enter standing water. OR exceed competence of capacity.
Erosional landforms
Created by removing material and the landform is what’s left.
Depositional landforms
Created by depositing sediment and building up the landform.
Waterfalls
Created by erosion. Resistant rock layer over a weak rock layer. Water flows over strong rock and erodes and the edge of it. Migrates upstream.
Deltas
Deposition as stream enters ocean or lake. Stream often changes course, due to deposition (distributary channels).
Alluvial fans
Found along mount fronts, typically in deserts. Stream leaves canyon, spreads out and slows down - deposition. Distributary channels like deltas.
Alluvium
Stream deposited sediment. Streams near base level erode wide valley that often get a layer of alluvium on either side of the stream.
Floodplains
The flat surface next to the channel that occasionally gets flooded, each flood deposits more sediment.
Flooding about every 1.5-2 years.
Meandering channels
Faster moving water tends to spiral down the channel.
Reasoning not well understood.
Some waters moves faster and slower. The faster water is close to side of the stream you where you will get erosion, and slower water is deposition.
Cut bank
Occurs where erosion is happening and there is a steep drop off into the water.
Point bar
Occurs where deposition of sediment is and leads to a gentle drop off into the water.
Braided channels
Found where discharge varies and high sediment loads -graded glacial streams and deserts.
Can’t carry all the sediment, so it deposits some in bars (islands).
Bars decrease cross sectional area, so velocity increases (Q=AV). Another way streams move toward graded state.
Drainage network
Series of connected streams (main stream and its tributaries).
Drainage basin
Area (valley) drained by a drainage network.
Divide
Boundary of drainage basin; high point between networks.
Exterior drainage basin
Flow exits basin at one point, most common.
Interior drainage basin
No outlet, flow accumulates in low spot (playa).
Longitudinal profiles
Slope of channels tends to change in downstream direction - steeper in headwaters, less steep at mouth.
