Erosion by rivers is affected by water velocity which is influenced by
Gradient (slope) of the land, discharge (volume of water over time), shape of the river channel, amount of debris in the channel
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Gradient (slope) of the land
Velocity tends to be higher on steeper slopes.
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Discharge (volume of water over time)
Increasing discharge increases velocity.
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Shape of the river channel
Semicircular-shaped channels have less friction with water than narrow or shallow channels.
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Amount of debris in the channel
Water flows faster in the absence of any type of impediment - fallen trees, rocks, etc.
This includes rapids.
Water will move faster between the rocks in rapids, but the overall velocity of the river decreases.
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Velocity of a straight channel
in the middle, about ¼ of the way to the bottom.
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Velocity of a curving channel
deflected to the outside of the bends
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Symmetric stream channel
Erosion force potential greatest in the middle of the stream
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Asymmetric stream channel
Erosional force potential greatest on the right side of the stream.
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Longitudinal Profile
As a river flows from its source to its mouth: Gradient decreases, discharge increases, the number of rapids decrease, the river becomes wider and shallower
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Base Level
A river cannot erode its channel much deeper than the elevation of the body of water that it flows into.
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Ultimate Base Level
Sea level for most major rivers, but there are exceptions.
The Jordan River’s base level is the Dead Sea.
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Local Base Level
Lakes, waterfalls, man-made reservoirs behind dams, other rivers, etc.
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When a river reaches base level
velocity falls close to zero
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If base level falls
The river will resume downcutting and erode a deeper channel, possibly forming a canyon.
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If base level rises
The gradient is reduced, the river slows and begins to deposit sediment. The channel will build higher to reach the new base level.
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Base level decreases due to
Lower sea level and Rising land elevations
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Base level increases due to
Higher sea level Lower land elevations
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Dissolved Load (smallest sizes)
chemically weathered materials
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Suspended Load
Fine sand, silt, and mud that are held in the water
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Bed Load (largest sizes)
Larger material that moves by rolling or sliding
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Competence
A measure of the largest particle size that is in motion. Increases during floods.
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Capacity
The total amount of sediment that a river is able to transport
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Deltas
tend to form during periods of rising sea levels.
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Alluvial Fans
the largest tend to form in deserts. (delta on land)
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Downcutting
deepens valleys
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Lateral Erosion
widens valleys
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Headward Erosion
lengthens valleys
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Mass Wasting
widens valleys
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High Gradient
Often far above base level (mountains) Downcutting is dominant The channel is often narrow, straight, and may contain rapids and waterfalls. V-shaped canyons are common in these areas.
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Moderate Gradient
Downcutting is no longer dominant. Lateral and headward erosion and mass wasting are all significant. The channel is wider and shallower. Meanders, floodplains, and levees form
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Low Gradient
Elevations are often very close to base level. Downcutting is not significant, lateral erosion is dominant. The channel is often very wide and shallow, with many meanders, oxbow lakes, large floodplains, and broad levees.
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Incised meanders
meandering channels with steep-sided canyons and no floodplain
Spherical or rounded particles generally have the greatest degree of empty spaces between them. Flat particles often stack more closely together.
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Particle Size
if all else is equal, size alone won’t affect porosity. However, larger grains often lead to lower porosity due to sorting, packing, and cementation
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Sorting
well sorted material has much higher porosity than poorly sorted material.
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Packing
A greater degree of packing will lower porosity. Seen mainly in sediment.
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Cementation
lowers porosity substantially. Graywacke has less porosity than quartz sandstone.
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Permeability
The ability to transmit a fluid (water)
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permeability and porosity in rocks
Pumice has porosity, but little permeability. Clay and shale have porosity, but the pores are very small and poorly connected. Most igneous and metamorphic rocks have little permeability.
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aquifers
Materials with both high porosity and high permeability (most sediment and sedimentary rocks)
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aquitards
Materials with low permeability (shale and most igneous/metamorphic rocks).
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Effluent stream
found in humid climates
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Influent stream
found in dry climates
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unconfined aquifer
have a direct connection to the surface
recharged directly by rainfall and/or snowmelt
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confined aquifer
Aquifers located below an aquitard layer
They recharge with water slowly, but groundwater is often cleaner due to filtration through the aquitard (often clay or shale).
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Springs
places where groundwater returns to the surface
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Geysers
exist where groundwater is superheated by magma. When pressure builds, the groundwater erupts.
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Hot Springs
heated to a smaller degree and don’t erupt.
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Groundwater Erosion
Based not on velocity, but on the effectiveness of chemical weathering
affect rocks such as limestone, dolostone, and rock gypsum much more than sandstone, shale, etc.
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Karst
Areas where the land surface has been greatly modified by the collapse of caves
sinkholes, disappearing streams, and springs are common features.
usually appears in limestone regions where cave development is already in an advanced state.
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Glaciers
form when snow accumulates at a greater rate than it melts, converts to the granular forms of névé and firn, then compacts due to pressure into glacial ice.
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Types of Glaciers
Alpine (Valley) and Continental (Ice Sheet)
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Alpine (Valley)
Form in, and are restricted to, high gradient river valleys in mountainous areas. Most common type in existence today.
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Continental (Ice Sheet)
Form in high latitudes and are not constrained by topography. Much thicker than valley type. Most common during Ice Ages.
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Glacial Ice Movement
basal sliding and plastic flow
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glacial ice movement by basal sliding
The glacier, as a single mass, slides over the underlying rock on a layer of meltwater.
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glacial ice movement by plastic flow
The individual ice grains slide over each other. They also deform and recrystallize.
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Rates of Movement
Movement varies over the course of the year, but averages between 0.1 and 10 meters per day.
Plastic flow is fastest near the center.
Glaciers may temporarily move much faster, over 100 meters per day, by surging. This is often caused by a build-up of meltwater beneath the ice which may float the glacier.
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Ablation
the loss of glacial ice by melting sublimation or calving (pieces falling off the end).
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terminus
a glacier will advance if accumulation of new snow and ice exceeds ablation.
will retreat if ablation exceeds accumulation - but the glacial ice will still flow forward.
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Plucking
erosion combined with mechanical weathering. Ice fractures the underlying bedrock, then freezes it to the glacier.
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Abrasion
moving ice and imbedded rock grind down the bedrock into rock flour (fine silt and clay-sized material).
Amphitheater-shaped recesses carved into a mountain at the head of a glacial valley. They usually have very steep slopes on three sides.
often later occupied by tarns
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tarns
bedrock-floored lakes
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Horns
Eroded peaks formed when three or more cirques cut back into a mountain.
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Arêtes
Eroded ridges of bedrock between glacial valleys.
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Fjords
flooded glacial valleys that form from post-glacial sea level rise or sub-base level erosion.
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Striations and Grooves
scratches in bedrock from mm to meters in width and depth. Used to interpret the direction of former ice movement. Kelleys Island has well-preserved grooves.
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Roches Moutonnées
Eroded hills of bedrock that were resistant to glacial erosion. They are usually elongated and streamlined. The gentle side of the hill faces into the direction from which the ice came, the steeper side faces in the direction of ice movement.
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Depositional Landforms
till and outwash
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Till
Material deposited directly by glacial ice. It is unsorted and unstratified mud, sand, gravel, and boulders.
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Outwash
Material laid down or reworked by meltwater. Sorted and stratified sand and gravel.
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Landforms Made of Till
Glacial Erratics, Moraines, Drumlins
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Glacial Erratics
boulders derived from non-local bedrock
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Moraines - End Moraine
a ridge piled up along the front end of a glacier.
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Terminal moraine
the last moraine, marks the greatest extent of the glacier.
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Recessional moraine
forms as a glacier retreats then temporarlly stops.
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Drumlins
streamlined hill sculpted beneath glacial ice. The steep side points toward the glacier.
Hill made of till
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Other Moraines
Lateral moraine, Medial moraine, Ground moraine
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Lateral moraine
forms along the sides of many valley glaciers.
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Medial moraine
a dark streak marking the union of two lateral moraines
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Ground moraine
relatively thin blanket of till left behind as a glacier retreats.
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Landforms Made of Outwash
Outwash Plain, Eskers, Kames Kettles - Shallow depressions marking the location of old, buried ice blocks that melted. These are often later filled with water and become kettle lakes. Kettle lakes are floored by sediment. (tarns are floored by bedrock) As kettle lakes begin to fill with sediment, they become kettle bogs (wetlands).
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Outwash Plain
Thick blanket of sand and gravel.
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Eskers
Long, sinuous ridges that form in subglacial stream channels.
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Kames
irregular, mound-like hills
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Kettles
Shallow depressions marking the location of old, buried ice blocks that melted.
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kettle lakes
Kettles that are later filled with water
floored by sediment
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kettle bogs (wetlands)
As kettle lakes begin to fill with sediment
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Deserts
areas that receive very little rainfall (
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What Creates Deserts?
The Rainshadow Effect (topographic deserts), Mid-continental Effect, Effect of cold ocean water (coastal deserts), Geographic Effect (trade wind/polar deserts)
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The Rainshadow Effect (topographic deserts)
a consistently high elevation mountain range may block significant rainfall from reaching downwind locations.
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The Rainshadow Effect examples
The Patagonian and Atacama Deserts (Andes Mts.), and the Gobi desert (Himalaya Mts.)