Earth: Portrait of a Planet 3rd edition - Sheffield

Chapter 17

Streams and Floods:

The Geology of Running Water

LECTURE OUTLINE

Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak

earth

Portrait of a Planet

Third Edition

©2008 W. W. Norton & Company, Inc.

Chapter 17: Streams and Floods: The Geology of Running Water

Streams and Floods:

The Geology of Running Water

Prepared by

Ronald Parker

Earlham College Department of Geosciences

Richmond, Indiana

Streamflow

 Streams – Ribbons of water that flow down channels.

 Runoff – Water in motion over the land surface.

 Stream runoff is crucial for humans.

 Drinking water.

 Transportation.

 Waste disposal.

 Recreation.

 Commerce.

 Irrigation.

 Energy.

Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak Chapter 17: Streams and Floods: The Geology of Running Water

Streamflow

 Stream runoff also causes many problems.

 Flooding destroys lives and property.


Streamflow

 Stream runoff is an important geologic agent.



Flowing water…

 Erodes, transports, and deposits sediments.

 Sculpts landscapes.

 Transfers mass from continents to ocean basins.

 Earth: only planet in the solar system with flowing water.

 Without flowing water, Earth might resemble Mars.


The Hydrologic Cycle

 Stream runoff is a component of the hydrologic cycle.

 Hydrologic cycle processes.

 Evaporation.

 Transpiration.

 Precipitation.

 Infiltration.

 Runoff.


Forming Streams

 Streamflow begins as water is added to the surface.


Forming Streams

 Streamflow begins as moving sheetwash.

 Thin surface layer of water.

 Moves down the steepest slope.

 Erodes substrate.

 Sheetwash erosion creates tiny rill channels.

 Rills coalesce, deepen, and downcut into channels.


Forming Streams

 Intense scouring marks entry into the channel.

 Rapid erosion lengthens the channel upslope.

 This process is called headward erosion.


Forming Streams

 Over time, nearby channels merge.

 Smaller tributaries join a larger trunk stream.

 The array of linked channels is a drainage network.

 Drainage networks change over time.


Drainage Networks

 Drainage networks often form geometric patterns.

 These patterns reflect underlying geology.

 Common drainage patterns.

 Dendritic – Branching, “treelike” due to uniform material.




 Radial

– From a point uplift (mesa, volcano, etc.).




 Rectangular

– Controlled by jointed rocks.




 Trellis

– Alternating resistant and weak rocks.


Drainage Basins

 Land areas that drain into a specific trunk stream.

 Also known as catchments or watersheds.

 Divides are uplands that separate drainage basins.


Drainage Divides

 Watersheds exist in a variety of scales.

 Tiny tributaries.

 Continental rivers.



Large watersheds…

 Feed large rivers.

 Section continents.

 Continental divides separate flow to different oceans.


Permanent vs. Ephemeral

 Permanent streams

 Water flows all year.

 At or below the water table.

 Humid or temperate.

 Sufficient rainfall.

 Lower evaporation.

 Discharge varies seasonally.

 Ephemeral streams

 Do not flow all year.

 Above the water table.

 Dry climates.

 Low rainfall.

 High evaporation.

 Flow mostly during rare flash floods.


Discharge

 The amount water flowing in a channel.

 Volume of water passing a point per unit time.

 Cubic feet per second (ft 3 /s).

 Cubic meters per second (m 3 /s).

 Given by cross-sectional area x flow velocity.

 Varies seasonally due to precipitation and runoff.


Discharge

 Velocity is not uniform in all areas of a channel.



Friction slows water along channel edges. Friction is…

 Greater in wider, shallower streams.

 Lesser in narrower, deeper streams.

 In straight channels, highest velocity is in the center.

 Few natural channels are straight.


Discharge


 In curved channels, max. velocity traces the outside curve.

 The outside curve is preferentially scoured and deepened.

 The deepest part of the channel is called the thalweg.

 Flow around curves follows a spiral path.


Discharge


 Stream flow is characteristically turbulent.

 Chaotic and erratic.

 Abundant mixing.

 Swirling eddies.

 High velocity.



Turbulence caused by…

 Flow obstructions.

 Shear in water.

 Turbulent eddies scour the channel bed.


Erosional Processes

 Erosional processes – Streamflow does work.

 The energy imparted to streamflow is derived from gravity.

 Streams do work by converting potential to kinetic energy.

 Erosion is maximized during floods.

 Large water volumes.

 High water velocities.

 Abundant sediment.



 Streams scour, break, abrade, and dissolve material.

 Scouring

– Running water picks up sediment and moves it.

 Breaking and lifting – The force of moving water can…

 Break chunks of rock off of the channel.

 Lift rocks off of the channel bottom.



 Abrasion – Sediment grains in flow “sandblast” rocks.

 Bedrock exposed in channels is often polished smooth.

 Gravel swirled by turbulent eddies drills holes in bedrock.



These bowl-shaped depressions are called potholes.



Potholes are often unusual and intricately sculpted.

 Dissolution

– Mineral matter dissolves in water.


Sediment Transport

 The material moved by streams is the sediment load.

 There are 3 types of load.

 Dissolved load – Ions from mineral weathering.

 Suspended load – Fine particles (silt and clay) in the flow.

 Bed load – Larger particles roll, slide, and bounce along.


Sediment Transport

 Competence – The maximum size transported.

 Capacity – The maximum load transported.

 Capacity and competence change with discharge.

 High discharge – Large cobbles and boulders may move.

 Low discharge – Large clasts are stranded.


Sediment Deposition



When flow velocity decreases…

 Competence is reduced and sediment drops out.

 Sediment grain sizes are sorted by water.

 Sands are removed from gravels; muds from both.

 Gravels settle in channels.

 Sands drop out in near channel environments.

 Silts and clays drape floodplains away from channels.



 Streams grade sediment with transport.

 Coarsest particles typify steep gradients in headwaters.

 Fine particles typify gentler gradients near the mouth.



 Fluvial sediments are called alluvium

 Channels may be decorated with mid-channel bars.

 Sands build up the point bars inside meander bends.

 Muds are deposited away from the channel during floods.

 A stream builds a sediment delta upon entering a lake.


Longitudinal Changes

 The character of a stream changes with flow distance.

 In profile, the gradient describes a concave-up curve.






Near the headwater source of the stream…

 Gradient is steep.

 Discharge is low.

 Sediments are coarse.

 Channels are straight and rocky.






Toward the mouth…

 Gradient flattens.

 Higher discharges.

 Smaller grain sizes typical.

 Channels describe broad meander belts.


Base Level

 The lowest point to which a stream can erode.

 Ultimate base level is defined by the position of sea level.

 Streams cannot erode below sea level.

 A lake serves as a local (or temporary) base level.

 Base level changes cause stream readjustments.

 Raising base level results in an increase in deposition.

 Lowering base level accelerates erosion.


Base Level

 The lowest point to which a stream can erode.

 A ledge of resistant rock may define the local base level.

 Erosive forces act to slowly remove the resistant layer.

 This acts to restore the longitudinal profile.


Valleys and Canyons

 Land far above base level is subject to downcutting.

 Rapid downcutting creates an eroded trough.

 Valley – Gently sloping trough sidewalls define a V-shape.

 Canyon – Steep trough sidewalls form cliffs.

 Determined by rate of erosion vs. strength of rocks.



 Stratigraphic variation often yields a stairstep profile.

 Strong rocks yield vertical cliffs.

 Weak rocks produce sloped walls.

 Geologic processes stack strong and weak rocks.



 Active downcutting flushes sediment out of channels.

 Valleys store sediment when base level is raised.

 Renewed incision creates stream terraces.

 Terraces mark former floodplains.


Rapids

 Rapids are turbulent water with a rough surface.

 Rapids reflect geologic control.

 Flow over bedrock steps.

 Flow over large clasts.

 Abrupt narrowing of a channel.

 Sudden increase in gradient.


Waterfalls

 Streams that cascade or free-fall.

 Waterfall energy scours a plunge pool at the base.

 Basal erosion initiates collapse of overlying rocks.

 Waterfalls are temporary base levels.


Waterfalls

 Niagara Falls.

 Lake Erie drops 55 m flowing toward Lake Ontario.

 Dolostone caprock is resistant; underlying shale erodes.

 Blocks of unsupported dolostone collapse and fall.

 Niagara Falls continuously erodes south toward Lake Erie.

 Erosion since deglaciation has formed Niagara Gorge.


Waterfalls

 Niagara Falls.

 Diversion of American Falls revealed huge blocks of rock.

 The rate of waterfall retreat is presently 0.5 m/yr.

 Lake Erie will drain when the falls reach it.


Alluvial Fans

 Alluvial fans build at the base of a mountain front.

 Sediments rapidly dropped near the stream source.

 Coarsest material found near the stream source.

 Sediments fine and thin away from canyon stream.

 Sediments create a conical, fan-shaped structure.


Braided Streams

 Form where channels are choked by sediment.

 Flow is forced around sediment obstructions.

 Diverging - converging flow creates sand and gravel bars.

 Bars are unstable, rapidly forming, and being eroded away.

 Flow occupies multiple channels across a valley.


Meandering Streams

 Channels can form intricately looping curves.

 Along the lower portion of the profile with a low gradient.

 Where streams travel over a broad floodplain.

 When substrates are soft and easily eroded.

 Meanders increase the volume of water in the stream.

 Meanders evolve.



 Meanders change due to natural variation in...

 Thalweg (maximum depth) position.

 Friction.

 Maximum velocity swings back and forth across flow.

 Fast water erodes one stream bank.

 The opposite bank collects sediment.



 Erosion accentuates the cut bank.

 High-velocity flow scours the outside of the meander bend.

 Collapsed cut bank material is transported away.

 Deposition builds the point bar.

 Sediment accumulates inside the meander bend.

 Continued addition expands the point bar laterally.



 Meanders become more sinuous with time.

 The cut bank erodes; the point bar accretes.



Meanders’ curves become more pronounced.

 Meanders elongate.



 Sinuosity may increase until a meander is cut off.

 Cut banks converge and a meander neck thins.

 During flooding, high-velocity flow saws through the neck.

 The meander cut-off forms an oxbow lake.

 The oxbow fills with sediment, leaving an arcuate scar.



 Occupy only a small part of the floodplain.

 Floodplains are typically bounded by eroded bluffs.

 During floods, the floodplain may be immersed.

 Natural levees form ridges parallel to the channel.

 Made of sand dropped as floodwaters jump from channel.


Deltas

 Deltas form when a stream enters standing water.

 Current slows and loses competence; sediments drop out.

 Stream divides into a fan of small distributaries.

 Shape due to the interplay of flow, waves, and tides.


Deltas

 The Mississippi is a riverdominated bird’s foot delta.

 Distinct lobes indicate past depositional centers.

 The river periodically switches course via avulsion.

 River breaks through a levee upstream.

 Establishes a shorter, steeper path to the Gulf of Mexico.


Deltas

 Abandoned delta lobes are sediment-starved.

 Sediments deposited before avulsion slowly subside.

 Compaction and dewatering.

 Decay of organic matter.

 Lack of sediment nourishment.

 Eventually, abandoned delta lobes are submerged.


Deltas

 Subsidence is a problem for cities built on deltas.

 Lack of regular flooding leads to sediment starvation.

 Subsidence below sea level magnifies flood risks.

 New Orleans is an example.

 Other cities face this threat.


Drainage Evolution

 Landscapes evolve over time.

 Streamflow is the cause of most landscape changes.

 Example:

 Uplift sets a new base level.

 Stream cuts into former surface.

 Valleys widen; hills erode.

 Landscape denuded to base level.



 Stream piracy.

 One stream captures flow from another.

 Results from headward erosion.

 A stream with more vigorous erosion (steeper gradient), intercepts another stream.

 The captured stream flows into the new stream.

 Below the point of capture, the old stream dries up.



 Drainage reversal.

 Tectonic uplift can alter the course of a major river.

 In the early Mesozoic, South America drained westward.

 Western uplift raised the Andes, diverting flow to east.



 Superposed streams – Cross deformed terrain ignoring structure.

 Streams initially develop in younger, flat strata.

 The stream then chainsaws into underlying rocks.

 Stream maintains its initial geometry.



 Antecedent drainages.

 Tectonic uplift may raise ground beneath established streams.

 If erosion keeps pace with uplift, the stream will cut through the uplift.

 Called antecedent drainage.

 If the rate of uplift exceeds erosion, the stream is diverted by the range.



 Some antecedent streams have incised meanders.

 Meanders initially develop on a gentle gradient.

 Uplift raises the landscape (dropping the base level).

 The meanders chainsaw down into the uplifted landscape.


Raging Waters

 Floodwaters are devastating to people and property.



During a flood…

 Flow exceeds the storage volume of a stream channel.

 Velocity (thus, competence and capacity) increase.

 Water leaves the channel and immerses adjacent land.

 Moving water and debris scour floodplains.

 Water slows away from the thalweg, dropping sediment.


Raging Waters

 Floods result from a number of circumstances.

 When torrential rains dump large volumes of water quickly.

 After soil pores have been filled by prior rainfalls.

 When abrupt warm weather rapidly melts winter snow.

 When a natural or artificial dam breaks, releasing water.


Raging Waters

 Seasonal floods recur on an annual basis.

 Monsoons

– Tropical rains of the Indian subcontinent.

 Generate long periods of rain and severe flooding.

 Many people live in floodplain and delta plain settings.

 In 1990, a monsoon killed

100,000 people in Bangladesh.


Raging Waters

 Flash floods – Water rises rapidly with little warning.

 Stem from unusually intense rainfall or dam failures.

 Typified by a rapidly moving wall of debris-laden water.

 May strike with little warning and may be very destructive.

 Johnstown, Pennsylvania, 1889.


Raging Waters

 Case history: Mississippi and Missouri Rivers, 1993.

 Spring 1993: the jet stream moved over midwestern U.S.

 Trapped moist, humid air from the Gulf of Mexico.

 Rain, rain, rain.

 July 1993: Flood waters invaded huge areas.

 Covered 40,000 mi 2 .

 Flooding lasted 79 days.

 50 people died.

 55,000 homes destroyed.

 $12 billion in damage.


Raging Waters

 Case history: Big Thompson Canyon, Estes Park, Co.

 July 31, 1976: Rising moist air drenched the Rockies.

 Rain, starting at 7 :00 pm, fell in great torrents (7.5” in an hour).

 Rock and soil were stripped away and added to the flow.

 Houses, bridges, and roads vanished; a 275-ton rock moved.

 144 people died.


Raging Waters

 Case history: Ice-Age megafloods.

 11 Ka, ice dams failed, releasing Glacial Lake Missoula.

 Water scoured the landscape of eastern Washington.

 Created the channeled scablands.

 Largest floods in geologic history?


Living with Floods

 Flood control is expensive and, sometimes, futile.

 Dams on tributaries hold water back from trunk streams.

 Artificial levees and flood walls increase channel volume.

 Levees transmit intensified flood problems downstream.

 Extreme conditions - Levees are overtopped or undermined.



1993, Mississippi River.



2005, New Orleans.



 Case study: New Orleans in the aftermath of Katrina.

 Numerous levees and floodwalls failed in the days after.

 Possible failure mechanisms include:

 Overtopping water eroding soil on the landward side.

 Soil instability beneath levees.

 Water pressure.

 More than 1,300 died.



 People living in floodplains face hard choices.

 Move or expect eventual catastrophic loss.

 Land use changes may mitigate flood damage.

 Establish floodways – Places designed to transmit floods.

 Remove people and structures from these places.



 Flood risks are borne by homeowners, insurance companies, lenders, and government agencies.

 Hydrologic data are used to produce flood risk maps.

 Maps allow regulatory agencies to manage risks.

 Building in flood-prone settings is tightly regulated.



 Flood risks are calculated as probabilities.

 Discharges are plotted against recurrence intervals.

 On semi-log paper, this plots as a straight line.

 The probability (% chance of occurrence) of a given discharge can be determined by graph inspection.


Urbanization

 Cities cover large areas with impermeable surfaces.

 Storm water runoff from cities is distinctive.

 Shorter lag time between rainfall and flood flow.

 Larger discharges for shorter durations.


A Vanishing Resource?

 Rivers have directed human settlement patterns.

 Drinking water, food, transport, energy, recreation, and waste disposal.

 In spite of their importance, rivers have been abused.

 Urbanization.

 Agriculture.

 Pollution.

 Dams.


This concludes the

Chapter 17

Streams and Floods:

The Geology of Running Water

LECTURE OUTLINE

Earth: Portrait of a Planet, 3 rd edition, by Stephen Marshak

earth

Portrait of a Planet

Third Edition

©2008 W. W. Norton & Company, Inc.

Chapter 17: Streams and Floods: The Geology of Running Water

Earth: Portrait of a Planet 3rd edition - Sheffield

Chapter 17

Streams and Floods:

The Geology of Running Water

LECTURE OUTLINE

earth

Portrait of a Planet

Streams and Floods:

The Geology of Running Water

This concludes the

Chapter 17

Streams and Floods:

The Geology of Running Water

LECTURE OUTLINE

earth

Portrait of a Planet

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Earth: Portrait of a Planet 3rd edition - Sheffield

Chapter 17

Streams and Floods:

The Geology of Running Water

LECTURE OUTLINE

earth

Portrait of a Planet

Streams and Floods:

The Geology of Running Water

This concludes the

Chapter 17

Streams and Floods:

The Geology of Running Water

LECTURE OUTLINE

earth

Portrait of a Planet

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