Lecture 23- more rivers and glaciers

River Landscapes

• As rivers meander, they transform landscape

• Floodplain: flat, low-lying area adjacent to stream channel

• When a river floods, its velocity and capacity slows

• Depostion of alluvium occurs as river spreads out to floodplain

• Leaves a natural levee alongside river

River Deltas

• When rivers meet the sea, velocity slows rapidly

• Decreases stream capacity; deposition occurs

• Delta: deposited material at mouth of river

• Rivers deposit huge amounts of sediment into deltas

  • Coarser near mouth, finer near end

  • Some sediment may be underwater

• Areas of active land growth

• Deltas play important ecological role

• Can protect inland areas by absorbing storm impact

Delta landscapes

• Low-lying marshes from deposited sediment

• The current Mississippi Delta has been forming for 500 years at this location, gradually pushing into the sea

• Deltas depend on sediment from the river

• If sediment decreases, the land sinks towards the sea

• Dams built since 1950s have trapped more than 70% of the Mississipi’s sediment

• Much of the delta marshland has receded

• Increases vulnerability to hurricanes and storms

Deltas and River Evolution

• Deposition at deltas gradually decreases river gradient.

• During floods, river will attempt to take a more direct route to the ocean

• More direct route usually through a nearby river

• The river will begin to divert some of its flow to the new, more direct, river system (delta switching; avulsion.)

• Old delta stops receiving sediments, slowly sinks

• New delta forms at the river’s new endpoint

Mississippi River

• Has had many deltas in the recent past

• It has followed its current path for the past 500 years

• Delta extends so far into the Gulf that gradient is shallow

• If the Mississippi switched paths it would devastate New Orleans and southern Louisiana

• Army Corps of Engineers has dams and lock between the Mississippi and Atchafalaya to prevent this

• Old River Control Project: cost $2.3 billion in today’s dollars when built in 1963

• In 2011, the Army Corps of Engineers opened the Morganza Spillway to allow Mississippi water into the Atchafalaya (and prevent flooding in New Orleans.

Atchafalaya River

• has a much more direct route to the Gulf than the Mississippi (140 vs. 330 miles)

• Mississippi has tried to divert its flow to the Atchafalaya.

Rivers and Flooding

• Flood occur when river volume is too large for channel

• Extra water spreads out over the floodplain, depositing river sediment

• In New England, often during snowmelt and spring rains

• Ice jams can exacerbate the problem

• Floods are natural - if rivers are unaltered by humans, floods of varying stregth will occur yearly

• Frequent flooding and flat topography makes for good cropland

Floods and Humans

• Levees and dikes protect people and property, make floods worse

• Floodplains are a ‘release valve’ for excess water.

• If water is kept from flooding by levee, volume will continue to grow

• Eventual flooding will be worse

Flood risk

• Flood risk probablities can be calculated like other natureal hazards

• Limit of 100-year flood (1% probability) bigger square in image

• Liit of 2-year flood (50% probability)

Glaciers

• Large mass of ice on land that shows evidence of being in motio, or oof once having moved, under the force of gravity.

• More than 75% of Earth’s freshwater is frozen

• Earth’s frozen ice and glaciers make up the cryosphere - the portions of hydrosphere and lithosphere that are permanently frozen

Type of glaciers

• Glaciers from where temperatures are low and precipitation falls primarily as snow

• Found at high elevations and at high latitudes

  • Alpine glaciers

  • Continental glaciers

Alpine glaciers

Forms in mountain ranges where snow accumulates, then moves downslope

Continental glaciers

Thick, slow-moving sheet of ice that covers a large part of a continent or other large landmass

Recipe for glaciers

Freezing temperatures + snow accumulation = glacier

Glacier Accumulation and flow

• Form when snowfall exceeds summer snowmelt -→ snow compacts into ice

• As ice grows thicker, the pressure on the ice increases

• Eventually, the ice becomes plastic (loses its rigidity) and flows downhill

• Glaciers move slowly due to their own weight and gravity

Glacial movement - Plastic Flow vs Basal Slip

Glacial Shrinkage

• Ablation: the total amount of ice that a glacier loses each year

  • Melting

  • Iceberg calving

  • Sublimation

  • Wind erosion

• These days most glaciers are in retreat

Glacial Landscape

Glacial sediment

• As glaciers flow they erode, transport and deposit material (like a river)

• Till: unsorted, unstratified debris deposited on the ground from glacial melt

Moraines

• A depositional landform

• A glacial landform produced by deposition of till

• The lower part of the glacier melts, thin

• Glacier ends where the rate of glacier movement can no longer keep up with the rate of melt

Alpine (valley) glaciers

• Form in mountain ranges, in locations where snowfall exceeds snowmelt

• Ice thickens, loses its rigidity, flows downhill

• Glaciers erode the landscape where ice meets the ground-along the sides and the bottom of the ice

• This creates distinctive landforms at higher elevations

Alpine Glacier Erosional Landforms

• Cirque

• Aretes

• Glacial trough (U-shape)

• V-shaped valley

• Hanging valley

Cirque form

Amphitheater-like hollow carved at the head of a glacial valley, where snow accumulates.

Example: Tuckerman Ravine, Mt. Washington, former glacial cirque, collects > 50 feet of windblown snow each winter.

Aretes

Form where two cirque walls meet when 3+ cirques meet, a sharp peak (horn) is formed

• Examples: Mt. Katahdin, ME (tallest peak in state) “knife edge” route is an arete from past glaciation

• Matterhorn, glacial horn on the switzerland / Italy borded

Glaciated valleys

• Glaciated valley are U-shaped.

• Example: Glacial trough, Austria

V-shaped valley

• Formed by fluvial (river) processes, not glacial processes

Hanging valleys:

• Where a tributary glacier met the main glaciere, often have waterfalls into the trough

Important facts

• Alpine glaciers are relatively small features

• Continental glaciers are the largest in the world

• Currently restricted to a few areas:

  • Antartica (90% covered)

  • Greenland (81% covered)

• Smaller ice caps, ice fields found in Iceland, Patgonia region of South America

• Continental glaciers have covered much larger areas in the past

Ice shelves

Glaciers usually form on land, but they can flow into the ocean, forming ice shelves. Calving forms icebergs. (For example Iceberg calving off Greenland travel down the east coast.)

Continental Glacier Depositional Landforms

Terminal (end) moraine:

Cape Cod is a giant terminal moraine from the last Ice Age

Till plain

Outwash plain

Esker

Drumlin

Drumlin shape shows:

Central MA has several drumlin fields

Several near Pine Hill Reservoir

Boston Harbor islands are drumlins

Kettle Lake/Pond