© 2011 pearson education, inc. landscapes fashioned by water chapter 3

© 2011 Pearson Education, Inc.

Landscapes Fashioned by WaterChapter 3


Weathering, mass wasting, and erosion are all called external processes

• Occur at or near the Earth’s surface• Powered by energy from the Sun

Weathering- the physical breakdown and chemical alteration of rock Mass wasting- the transfer of rock and soil downslope by gravityErosion- the physical removal of material by water, waves, wind, or ice

Earth’s external processes


Mountain building and volcanic activity are internal processes

• Derive their energy from Earth’s interior

Erosion, mass wasting, and weathering breakdown rocks, while mountain building and volcanic activity add rocks

RECYCLING!

Earth’s internal processes


Mass wasting is the downslope movement of rock and soil due to gravity

• Causes loss of life and property (USGS- 25 to 50 people killed annually in the U.S.)• Natural disasters•Different from erosional processes - does not require a transporting medium (H2O, wind, glaciers, etc.)• Follows after weathering in the evolution of landforms


First the rocks weather….

Then they get mass wasted…..

Then carried out by the river


Types of mass-wasting

Debris Flow


Most rapid and spectacular mass-wasting events occur in areas of rugged, geologically young mountains

As mountain building subsides, erosion and mass wasting take over to subdue the terrain

Slope change through time


Gravity is the controlling force of mass wasting and does not require a trigger to start, but sometimes it doesExamples of triggers

• Water• Oversteepening of slope• Removal of vegetation• Earthquakes


Water• Heavy rains or snow melt• Pores in sediment become filled, cohesion is lost• Added weight


Oversteepened Slopes• A stream under cutting a valley wall, or waves pounding against the base of a cliff • Angle of repose (25 to 40 degrees), steepest angle at which unconsolidated sediments will remain stable


Removal of vegetation• Plant root systems bind soil and regolith (weathered rock debris)• If slopes are steep, have rainfall, and remove vegetation….get mass wasting


Earthquakes• Most dramatic mass wasting trigger• Dislodge enormous volumes of rocks and unconsolidated material• Liquefication- unconsolidated, saturated sediments are turned into a “liquid”


Salmon Beach landslide area, near Tacoma, Washington, after the 2001 Nisqually earthquake. Puget Sound is in the fore-ground


• The Earth’s hydrosphere– Water content is about 1.36 billion km3 (326

million miles3)– 97% in oceans, 2% ice and glaciers, and 1% in

lakes, streams, groundwater, and atmosphere

Water Cycle


• The hydrologic cycle is a summary of the circulation of Earth’s water supply– Worldwide system– Powered by the Sun– Atmosphere is the link between oceans and

continents

• Processes in the water cycle• Precipitation

• Infiltration- H2O soaks into the ground• Runoff- H2O oversaturation does not allow infiltration

• Evaporation• Transpiration


• The water cycle is balanced– Total amount of H2O vapor in the atmosphere remains

the same– Average annual precipitation worldwide must be equal to

the quantity of water evaporated– Evaporation over the continents is exceeded by

precipitation– Precipitation over the oceans is exceeded by

evaporation


Running waterSurface Water

• If water is not infiltrated, it becomes runoff. The amount of runoff depends on• Intensity and duration of rainfall• Amount of water already in the soil• Nature of the soil or material• Slope of the land• Extent of vegetation

• Is runoff high or low in urban areas?


• Streamflow• The ability of a stream to erode and

transport materials is determined by velocity

• Factors that determine velocity• Gradient, or slope• Channel characteristics including shape,

size, and roughness• Discharge—The volume of water moving

past a given point in a certain amount of time


• Changes along a stream• Cross-sectional view of a stream is

called the profile• Viewed from the head (headwaters or

source) to the mouth of a stream

• Changes from upstream to downstream• Profile

• Profile is a smooth curve• Gradient decreases downstream

• Factors that increase downstream• Velocity• Discharge• Channel size


Longitudinal profile of California’s Kings River


• Base level and stream erosion• Base level is the lowest point to which a

stream can erode• Two general types of base level

• Ultimate (sea level)• Local or temporary

• Changing conditions causes readjustment of stream activities

• Raising base level causes deposition• Lowering base level causes erosion

Base Level


Adjustment of base level to changing conditions


• Stream erosion• Lifting loosely consolidated particles by

• Abrasion• Dissolution

• Stronger currents lift particles more effectively

The work of streams


• Transport of sediment by streams• Transported material is called the

stream’s load• Types of load

• Dissolved load• Suspended load• Bed load

• Capacity—the maximum load a stream can transport

• Competence

• Indicates the maximum particle size a stream can transport

• Determined by the stream’s velocity


• Deposition of sediment by a stream• Caused by a decrease in velocity

• Competence is reduced• Sediment begins to drop out

• Stream sediments• Generally well sorted• Stream sediments are known as alluvium


• Deposition of sediment by a stream• Delta—Body of sediment where a stream

enters a lake or the ocean• Results from a sudden decrease in velocity

• Natural levees—Form parallel to the stream channel by successive floods over many years

• Floodplain deposits• Back swamps• Yazoo tributaries


Structure of a simple delta


Natural levees


Stream valleys

• The most common landforms on Earth’s surface

• Two general types of stream valleys• Narrow valleys

• V-shaped • Downcutting toward base level• Features often include rapids and waterfalls


• Two general types of stream valleys• Wide valleys

• Stream is near base level• Downward erosion is less dominant• Stream energy is directed from side to side

forming a floodplain


• Features of wide valleys often include• Floodplains

• Depositional floodplains• Meanders• Cut banks and point bars• Cutoffs and oxbow lakes


Drainage basins and patterns

• Drainage networks• Land area that contributes water to the

stream is the drainage basin• Imaginary line separating one basin

from another is called a divide


• Drainage pattern• Pattern of the interconnected network of

streams in an area• Common drainage patterns

• Dendritic• Radial• Rectangular• Trellis


Floods and flood control

• Floods and flood control• Floods are the most common and most

destructive geologic hazard• Causes of flooding

• Result from naturally occurring and human-induced factors

• Causes include heavy rains, rapid snow melt, dam failure, topography, and surface conditions


Water beneath the surfaceGroundwater

• Largest freshwater reservoir for humans

• Geological roles• As an erosional agent, dissolving by

groundwater produces• Sinkholes• Caverns

• An equalizer of stream flow


• Distribution and movement of groundwater• Distribution of groundwater

• Belt of soil moisture• Zone of aeration

• Unsaturated zone• Pore spaces in the material are filled

mainly with air• Zone of saturation

• All pore spaces in the material are filled with water

• Water within the pores is groundwater• Water table — The upper limit of the zone of

saturation


Features associated with subsurface water


• Movement of groundwater• Porosity

• Percentage of pore spaces• Determines how much groundwater can be

stored

• Permeability• Ability to transmit water through connected

pores• Aquitard — An impermeable layer of

material• Aquifer — A permeable layer of material


• Springs• Hot springs

• Water is 6 – 9 °C warmer than the mean air temperature of the locality

• Heated by cooling of igneous rock

• Geysers• Intermittent hot springs• Water turns to steam and erupts


Video 1Video 2


• Wells• Pumping can cause a drawdown (lowering)

of the water table• Pumping can form a cone of depression in

the water table

• Artesian wells• Water in the well rises higher than the

initial groundwater level • Artesian wells act as “natural pipelines”

moving water from remote areas of recharge great distances to the points of discharge


Formation of a cone of depression


An artesian well resulting from an inclined aquifer


• Environmental problems associated with groundwater• Treating it as a nonrenewable resource • Land subsidence caused by its withdrawal

• Sinkholes

• Contamination


• Geologic work of groundwater• Groundwater is often mildly acidic

• Contains weak carbonic acid• Dissolves calcite in limestone

• Caverns• Formed by dissolving rock beneath Earth’s

surface• Formed in the zone of saturation

• Features found with caverns• Form in the zone of aeration• Composed of dripstone• Common features include stalactites

(hanging from the ceiling) and stalagmites (growing upward from the floor)


Carlsbad Caverns National Park


• Karst topography• Formed by dissolving rock at, or near,

Earth’s surface• Common features

• Sinkholes – surface depressions• Sinkholes form by dissolving bedrock and

caver collapse• Caves and caverns

• Area lacks good surface drainage


Features of karst topography

© 2011 pearson education, inc. landscapes fashioned by water chapter 3

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