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Landscapes Fashioned by Water

Chapter 3 Lecture

Natalie Bursztyn

Utah State University

Foundations of Earth Science

Eighth Edition

Three important external processes.

Describe where they fit into the rock cycle.

Focus Questions 3.1

External processes

Occur at or near Earth’s surface

Internal processes

Earth’s External Processes

External processes include:

Weathering

Disintegration and decomposition of rock

Mass wasting

Transfer of rock and soil downslope under influence of gravity

Erosion

Physical removal of material by a mobile agent (e.g., flowing water, waves, wind, ice)

Earth’s External Processes

Explain the role of mass wasting in the development of valleys.

Discuss the factors that trigger and influence mass-wasting processes.

Focus Questions 3.2

Earth’s surface is covered by slopes

Slopes are unstable

Gravity causes material to move downslope

This movement is called mass wasting

May be slow and imperceptible, or catastrophic

Does not require a transporting medium

Mass Wasting: The Work of Gravity

Mass Wasting and Landform Development

Landform evolution:

Weathering breaks rocks apart

Mass wasting transfers materials downslope

Erosion (transportation) carries the materials away

Most sediment is eventually transported to the sea

Mass wasting shapes stream valleys

Most common landform

Generally much wider than they are deep

Mass wasting increases width

Eventually transforms steep, rugged landscapes into gentle, subdued terrain

Mass Wasting and Landform Development

Gravity is the controlling force

Other factors overcome inertia to create downslope motion

Slope material is gradually weakened

Slope gets closer and closer to being unstable until a trigger initiates downslope movement

Saturation with water

Oversteepening

Removal of vegetation

Earthquakes

Controls and Triggers of Mass Wasting

Saturation

Water in pore space reduces cohesion and allows particles to slide

Water adds weight to sediment

Oversteepening

Unconsolidated sediment forms a stable slope at a certain angle of repose depending on the size and shape of the particles

Stream undercutting a valley

Waves undercutting a cliff

Human activity

Controls and Triggers of Mass Wasting

Removal of vegetation root systems that bind sediment

Forest fires, deforestation, development, farming

Earthquakes can dislodge rock and unconsolidated material

Many mass wasting events occur without an identifiable trigger

Controls and Triggers of Mass Wasting

List the hydrosphere’s major reservoirs.

Describe the different paths that water takes through the hydrologic cycle.

Focus Questions 3.3

Water moves between the ocean, atmosphere, and land via the hydrologic cycle

Hydrosphere is all of the reservoirs where water is held

Oceans

Glaciers

Rivers

Lakes

Air

Rock

Soil

Living tissues

The Hydrologic Cycle

96.5% of hydrosphere is the global ocean

1.76% is ice sheets and glaciers

~2% is lakes, streams, groundwater, and atmosphere

The Hydrologic Cycle

Hydrologic cycle is powered by the Sun

Water enters atmosphere from the oceans via evaporation

Winds transport water through the atmosphere

Precipitation either falls to the ocean or continents

Precipitation to the oceans completes the hydrologic cycle

Precipitation to the continents must return to the ocean

The Hydrologic Cycle

Some water soaks into the ground (infiltration)

Surplus water flows over the surface (runoff)

Water absorbed by plants is eventually released via transpiration

Evapotranspiration is the combined effects of evaporation and transpiration

Precipitation in cold regions becomes part of glaciers

Significant reservoirs: melting all glaciers would cause sea level rise of dozens of meters

The Hydrologic Cycle

Hydrologic cycle is balanced

Average annual precipitation equals amount of water entering the atmosphere from evapotranspiration

Precipitation exceeds evaporation over land

Evaporation exceeds precipitation over oceans

The Hydrologic Cycle

Describe the nature of drainage basins and river systems.

Focus Question 3.4

Precipitation that forms runoff depends on:

Intensity and duration of rainfall

Amount of water already in the soil

Nature of the surface material

Slope of the land

Extent and type of vegetation

Running Water

Runoff starts as unconfined thin sheets across hillslopes

Flow develops threads of current in tiny channels called rills

Rills converge to form gullies

Gullies converge to form streams and rivers that carry water from broad areas

Running Water

Drainage basins (separated by divides) are the land area that contribute water to a river system

Divides vary in scale

Drainage Basins

A river system carries water from an entire drainage basin

Includes three zones:

Sediment production (erosion dominant)

Where most water and sediment is derived

Headwater regions

Sediment transport

Transportation through the channel network occurs via trunk streams

Sediment deposition

Rivers slow when they enter a body of water; sediment accumulates forming a delta

River Systems

Discuss streamflow and the factors that cause it to change.

Focus Question 3.5

Water flow in slow-moving streams can be laminar

Moves in roughly straight-line paths parallel to stream channel

Most streamflow is turbulent

Water moves erratically in a swirling motion

Lifts sediment from streambed

Increasing flow velocity increases turbidity

Streamflow Characteristics

Streamflow

Flow velocity varies along a stream and through time

Flow velocity depends on:

Channel slope or gradient

Channel size and cross-sectional shape

Channel roughness

Amount of water flowing in the channel

Factors Affecting Flow Velocity

Gradient is the vertical drop over a specified distance

Varies from stream to stream and over a single stream’s length

Steeper gradient provides more energy for flow

Shape, size, and roughness of channel affect the amount of friction between channel and water

Higher friction creates turbulence and slower flow

Discharge is the volume of water flowing past a certain point in a given unit of time (m3/s)

Intermittent streams only flow during wet periods

Ephemeral streams carry water after heavy rainfall

Factors Affecting Flow Velocity

The cross-sectional view of a stream from headwaters to mouth is called longitudinal profile

Overall shape is concave curve with local irregularities

Gradient, sediment size, and channel roughness decreases from head to mouth

Discharge and channel size increases

Flow velocity increases

Changes from Upstream to Downstream

Outline the ways in which streams erode, transport, and deposit sediment.

Focus Question 3.6

Streams are an important erosional agent

The Work of Running Water

Raindrops knock sediment particles loose

Flow of water in a stream can dislodge and lift particles from the channel

Erodes poorly consolidated material quickly

Can undercut banks

Hydraulic force can also cut bedrock

Enhanced by particles carried in water

Swirling pebbles can carve potholes in channel floors

Stream Erosion

Streams transport sediment in three ways:

Dissolved load is material in solution

Delivered by groundwater

Not effected by velocity

Suspended load is material suspended in the water

Clay and silt particles

Larger particles can be moved during floods

Largest component of load

Bed load is material moving along the channel bed

Sand, gravel, large boulders

Only in motion intermittently

Smaller particles move via saltation

Larger particles roll or slide

Transportation of Sediment by Stream

Transportation of Sediment by Streams

Capacity is the maximum load of solid particles a stream can transport per unit time

Increases with discharge

Competence is a stream’s ability to transport particles based on size

Increases with flow velocity

Transportation of Sediment by Streams

As flow decreases competence is reduced

Particles settle when flow reaches critical settling velocity for that particle size

Sorting separates particles of various sizes

Alluvium is material deposited by a stream

Deposition of Sediment by Streams

Contrast bedrock and alluvial stream channels.

Distinguish between two types of alluvial channels.

Focus Questions 3.7

Streamflow is confined to a channel

Two types of stream channels:

Bedrock channels are actively cut into solid rock

Alluvial channels are composed of unconsolidated sediment

Stream Channels

Bedrock channels are cut into rock

Common in headwaters with steep gradient

Transport coarse particles

Alternate between gentle gradients (alluvium accumulates) and steep segments (bedrock is cut)

Rapids and waterfalls common

Channel pattern is controlled by underlying geologic structure

Often winding and irregular

Bedrock Channels

Alluvial channels are composed of loosely consolidated sediment

Continually being eroded, transported, and redeposited

Shape is controlled by average sediment size, gradient, and discharge

Two common types

Meandering channels

Braided channels

Alluvial Channels

Meandering channels have sweeping bends called meanders

High suspended load

Deep, smooth channels

Banks are resistant to erosion

Most erosion occurs on the outside of the meander, or the cut bank, where velocity is highest

Sediment is deposited along the inside of the meander where turbulence and velocity are low, forming point bars

Meanders migrate laterally and downstream

May form a cutoff and oxbow lake through narrow neck of land

Alluvial Channels

Braided Channels are a complex network of converging and diverging channels

Form where most of stream load is coarse (sand and gravel) and discharge is variable

Wide and shallow (bank material erodes easily)

Common at the end of glaciers

Alluvial Channels

Contrast narrow V-shaped valleys, broad valleys with floodplains, and valleys that display incised meanders.

Focus Question 3.8

A stream valley is the channel and surrounding terrain that contributes water to the stream

Includes valley bottom and sloping walls

Top is generally broader than channel width because of mass wasting

Divided into two general types:

Narrow, V-shaped valleys

Wide valleys with flat floors

Shaping Stream Valleys

Base level is the lower limit to how deep a stream can erode

Usually occurs where a stream enters another body of water

Velocity and ability to erode are greatly reduced

Sea level is the ultimate base level

Temporary or local base level includes lakes, resistant rock layers, main streams, etc.

Change in base level causes readjustment of stream

Base Level and Stream Erosion

Downcutting is dominant when gradient is steep and channel is above base level

Abrasion and hydraulic power

Produces V-shaped valley with steep sides

Rapids and waterfalls common

Valley Deepening

Downward erosion becomes less dominant as channel reaches base level

Channel becomes meandering

Lateral erosion creates a broad, flat valley floor called a floodplain

Valley Widening

Incised meanders flow in steep, narrow valleys

Meanders develop when stream is near base level, but base level falls and stream starts downcutting again

Sea level fall

Uplift

Stream terraces are the remnants of former floodplains

Form after river adjusts to relative drop in base level then floods again

Floodplain is produced at a level below the old one

Incised Meanders and Stream Terraces

Discuss the formation of deltas and natural levees.

Focus Question 3.9

Streams transport sediment and deposit it downstream

Bars are deposits of sand and gravel

Temporary: material will eventually be carried to the ocean

Longer life span depositional features:

Deltas

Natural levees

Depositional Landforms

Deltas form where streams enter still bodies of water

Flow decreases and sediment falls

Delta grows outward and gradient lessens

Channel chokes with sediment, divides, and moves to higher-gradient areas

Distributaries carry water and sediment away from main channel

Deltas

Natural levees are built by successive floods on rivers in broad floodplains

Flow decreases when streams overflow

Coarse sediment deposited in thin strips parallel to channels

Fine sediment distributed across floodplain

Back swamps form because drainage is poor behind levees

Yazoo tributaries parallel the river until they can breach the levee

Natural Levees

Discuss the causes of floods and some common flood control measures.

Focus Question 3.10

Floods occur when stream discharge exceeds channel capacity

Among most common and most destructive natural hazards

Floods and Flood Control

Most floods occur because of weather

Snowmelt, heavy rains over large regions

Flash floods

Limited geographic extent

Influenced by rainfall intensity, surface conditions, and topography

Common in urban areas (rapid runoff)

Failure of dams or artificial levees

Causes of Floods

Floods can be controlled by:

Artificial levees

Earthen mounds increase volume of water the channel can hold

Flood control dams

Store flood water and let it out slowly

Channelization

Artificial cutoffs shorten the stream and increase gradient and velocity

Nonstructural approaches may be more efficient

Flood Control

Discuss the importance of groundwater.

Describe its distribution and movement.

Focus Questions 3.11

Groundwater exists in tiny pore spaces between grains of soil and sediment plus narrow joints and fractures in bedrock

Groundwater: Water Beneath the Surface

Groundwater is the largest reservoir of freshwater readily available to humans

Source of 40% of water

Drinking water for ~44% of population

40% of irrigation water

25% of water used in industry

Overuse can cause streamflow depletion, land subsidence, and increased pumping cost

The Importance of Groundwater

Important erosional agent

Forms sinkholes and caves

Stabilizes streamflow

Groundwater’s Geologic Roles

Comes from infiltration of rainfall into the ground

Amount is influenced by slope, surface material, intensity of rainfall, vegetation

Belt of soil moisture

Film of water on soil particles near the surface

Zone of saturation

All pore space is filled with water: groundwater

Upper limit is water table

Area above the water table is called the unsaturated zone

Distribution of Groundwater

Water table is irregular

Subdued replica of the surface

Highest below hills

Contributing factors:

Groundwater moves slowly

Water “piles up” between stream valleys

Variations in rainfall

Changes in permeability of sediment

Water table falls during droughts

Distribution of Groundwater

Porosity

Percentage of total volume of rock or sediment that consists of open pore space

Spaces between particles, joints, faults, dissolution cavities, vesicles

Depends on size and shape, packing, and sorting of grains

10–50% in sediment

Quantity of groundwater depends on porosity

Factors Influencing the Storage and Movement of Groundwater

Permeability

A material’s ability to transmit fluid

If spaces are too small, water can’t move through

Aquitards

Impermeable clay layers that prevent water movement

Aquifers

Rock or sediment that water moves through easily

Factors Influencing the Storage and Movement of Groundwater

Groundwater moves slowly from pore to pore

Typical rate is a few cm/day

Moves from high water table to low water table because of gravity

Usually towards a stream channel, lake, or spring

Pressure increases with depth in zone of saturation

Groundwater Movement

Compare and contrast springs, wells, and artesian systems.

Focus Question 3.12

A spring is a natural outflow of groundwater

Occurs where the water table intersects Earth’s surface

Aquitard prevents downward movement of water

A perched water table is a localized zone of saturation above an aquitard

Springs

A well is a hole drilled into the zone of saturation to remove groundwater

Drawdown is the lowering of a water table when water is withdrawn

Decreases with increasing distance from the well

Creates a cone of depression

Wells

An artesian system

Free flowing groundwater from an outlet far above the water table

A confined water table

The aquifer is inclined

Aquitards border above and below an aquifer

Increased pressure in a confined water table causes water to rise and create an artesian system

Artesian Systems

Springs, Wells, and Artesian Systems

List and discuss three important environmental problems associated with groundwater.

Focus Question 3.13

Overuse threatens groundwater supply

Excessive groundwater withdrawal causes land surface to sink

Contamination

Environmental Problems of Groundwater

Groundwater system is at equilibrium

Imbalance raises or lowers water table

Long-term drop can occur with a prolonged drought of an increase in discharge or withdrawal

Depletion of groundwater can be sever in regions of intense irrigation

Treating Groundwater as a Nonrenewable Resource

Ground subsidence occurs when water is removed faster than it is replenished

Pronounced in areas underlain by thick layers of loose sediments

Land Subsidence Caused by Groundwater Withdrawal

Common sources of contamination include septic tanks, sewer systems, and farm wastes

Purification by natural processes can occur with correct aquifer composition

Sand or permeable sandstone

Once pollution is identified water supply can be abandoned or treated

Groundwater Contamination

Explain the formation of caverns and the development of karst topography.

Focus Question 3.14

107

Most groundwater contains carbonic acid

CO2 dissolved from air and decaying plants

Dissolves limestone

Forms caverns, sinkholes, and karst landscapes

The Geologic Work of Groundwater

Caverns form due to the erosional work of groundwater

Created in the zone of saturation

Dissolved load is discharged into streams

Decorated by calcium carbonate deposits

Form when cavern is above water table

Stalactites hang from the ceiling

Stalagmites develop upward from the floor

Caverns

Karst topography results from groundwater dissolution

Common in Kentucky, Tennessee, Alabama, Indiana, and Florida

Not enough groundwater in arid or semiarid regions

Karst Topography

The Geologic Work of Groundwater

Sinkholes, or sinks, are depressions where limestone has been dissolved

Tower karst landscapes have isolated, steep-sided hills

The Geologic Work of Groundwater