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

Terms Definitions
Stream flow
Rivers of water flowing down channels
Runoff
Water in motion over the land surface
Crucial for Humans
Drinking, Transport, Waste disposal, recreation, irrigation
Problems
Stream runoff also causes problems
-flooding destroys lives and properties
Important Geological Agent
Flowing Water
-Eroding, transports, deposits sediments
-sculpts landscapes
-moves mess from continents to ocean basins
Hydrologic Cycle
Stream runoff is a component of this
Processes
Evaporation
Transportation
Precipitation
Infiltration
RUNOFF
Sources of Earth's Water
Oceans - 97.2 %
Other- 2.8 %
-Glaciers - 2.15%
-Ground Water- .62%
-Freshwater Lakes- .009%
-Saline Lake/Inland Seas- .008%
-Stream Channels- .0001%
-Atmosphere-.001%
Forming Streams
Stream flow begins as water is added to the surface
-Melted snow adds water
-snow and rain add water
-swamps and puddles collect water on flat land
Sheetwash
Stream flow begins as a moving sheetwash
-thin surface layer of water
-moves down steep slopes
-erodes the substrate
Rill Channels
Sheetwash erosion creates these
The rills coalsece, deepen and downcut into CHANNELS
Headward Erosion
Intense scoring marks entry into the channels
-rapid erosion lengthening the channel further upslope
Overtime
Overtime, the nearby channels merge into a large single TRUNK STEAM.
Drainage Network
Array of linked channels
-the networks change overtime
Drainage Networks
Often form geometric patterns
-Reflect underlying geology
Several common patterns
Dendritic
Branching "treelike" pattern due to a uniform material. Formed on flat lying land
Radial
From a point uplift (Mesa, Volcano, etc)
Rectangular
Controlled by jointed rocks (limestone, fractures)
Example: San Andreas
Trellis
Alternating resistant and weak rocks
Drainage Basins
Land areas that drain into a specific trunk stream
-Also known as CATCHMENTS or WATERSHEDS
-
Divides
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 (Streams)
-Flows all year
-At or below the water table
-Humid or temperate
-sufficient rainfall
-lower evaporation
-Discharge varies seasonally
Ephemeral (Streams)
-Not flow all year
-Above the water table
-Dry climates
-low rainfall
-high evaporation
-flow mostly during rare flash floods
Discharge
Amount of water flowing in a channel
-volume passing a point per unit of time
-cubic feet per second
-cubic meters per second
-Given by a cross-sectional area times flowing velocity
-varies seasonally due to precipitation and runoff
Example: Niagara Falls (200,000 ft3/sec)
Discharge = Q x A
Velocity
Not uniform in all areas of channels
Friction shows water along the edges creates greater friction
Water Along the Edges
Greater in wider, shallower streams
Lesser in narrower, deeper streams
Straight Channels
Highest velocity in the center
-very few natural straight channels
Curved Channels
Maximum velocity travels the outside curve
-outside curve is preferentially scoured and deepened
-Deepest part of the channels is the THALWEG
-flow around curve follows a spiral path
Thalweg
Deepest part of the channel
Turbulence
Stream flow is characteristically turbulent
-chaotic and erratic
-abundant mixing
-swirling oddities
-high velocity
Causes of Turbulence
Flow obstructions
Shear in water
-Turbulent eddies scour channel beds
Erosional Processes
Stream flow doeswork
-energy imparted to streamflow is derived from gravity
-streams do work by converting potential to kinetic energy
Erosion Maximization
Maximized during floods
-Large water volume
-high water velocities
-Abundant sediment
Streams SCOUR, BREAK, ABRADE, and DISSOLVE
Scours
running water picks up sediment and moves it
Breaking and Lifting
Force of water can...
-break chunks of rock off the channel
-lift rocks off the channel bottom
Abrasion
-Sediment grains in flow "sandblast" rocks
-bedrocks exposed in channels often polished and smooth
-Gravel surrounding turbulent eddies drills holes in the bedrock
-bowl shaped depresions - POTHOLES
-Unusual and intricate sculptes
Dissolution
mineral matter dissolves in water
Sediment Load
matter moved by streams
Three Load Types
Dissolved Load-
Suspended Load-
Bed load-
Dissolved Load
Ions from mineral weathering
Suspended Load
Fined particles (silt and clay) in flow
Bed Load
Larger particles roll, slide, and bounce along
Competence
Maximum size transported
Capacity
maximum load transported
Capacity and Competence
Change with discharge
High Discharge
Large cobble & boulders may move
Low Discharge
large clasts are stranded
When velocity of flow decreases
-Competence is reduced and sediment drops out
-Sediment grain sizes sorted by water
-sands removed from gravel, mud from both
-Gravel settles in channels
-Sands drop out near channel environments
-silts and clays drape flood plains away from
channel
Streams Guide Sediment with Transport
Biggest particles typify steep gradients in head waters
Fine particles typify gentler gradients near mouth
Alluvium
Fluvial sediments are called this
-Channels are decorated with mud-channel bars
-sands build up to t he point bars inside meander beds
-muds deposit from channel during floods
-streams builds a sediment delta upon entering fan
Longitudinal Changes
-Changes character with distance
-In profile, gradient describes a concave-up curve
Near the HEAD WATER source of the stream
Gradient is steep
discharge is low
sediments coarse
channels are straight and rocky
Toward the Mouth...
Gradient Flattens
Higher Discharges
Smaller grains typical
Channels describe broad meander belts
Base Level
Lowest point a stream can erode
-ultimate base is sea level
-can't erode below
-A lake serves as a local (or temporary) base level
Base level changes cause stream...
-Raising base level results in an increase in deposition
-Lowering base level accelerates erosion
-A ledge of resistant rock may define local base level
-Erosive forces act to slowly remove the resistant layers
-Act to restore longitudinal profile
Valleys and Cannons
Stratsgraphic variations often yield a stair step profile
-Strong rocks yield vertical cliffs
-Weak rock produce sloped walls
-Geologic processes stack strong and wear rocks
-Valleys stowe sediment when base level is reached
-Renewed incision creates stream terraces
-terraces mark former flood plains
V-Shaped Valley-
Rapids
Are turbulent water with rough surface
-Reflect geologic control
-Flow over bed rock steps
-Over large clasts
-prompt narrowing of a channel
-sudden increase in gradient
Waterfalls
Streams cascade or free fall
Energy causes a PLUNGE POOL at base
Erosion initiates collapse of overlying rocks
-temporary base levels
Niagara Falls
Lake Erie drops 55 m flowing toward Lake Ontario
-Dolostone rock is resistant, underlying shale erodes
-Blocks unsupported dolostone collapse and fall
-Falls continuously erode south to lake erie
-Erosion shows deglaciation has formed Niagara Gorge
-Diversion of falls reveals huge blocks of rock
-Rate of waterfall retreat- present .5 m/yr
-Erie will drain when falls reach it
Alluvial Fans
Sediments rapidly dropped near stream source
-coarsest material found near stream source
-sediments fine and thing away from source
Sediments create conical, fan shaped stretches
Forms at base of a mountain front
Braided Stream
Channels choked by sediments
Flow formed around sediment obstructions
-DIVERGING- converging flow creates sand and gravel BARS
-Bars unstable, sand and gravel cannot stick together so it eventually collapses (no cut in the channel from stream)
Meandering Streams
Channels can form intricately looping curves
-snake-like curves
-travel over broad flat-plains with underlying soft substrate
-Region where river has a gentle gradient
-Meanders increase volume of river by making longer
-Become sinuous with time
Cut Bank
Water on the outside of the channel flows fastest and erosion eats away at the channel walls creating a cut bank.
Wedge-Shaped Bank
Inside edge of channel has slow moving water such that the competence decreaes and sediment accumulates creating a wedge also known as POINT BAR
Oxbow Lake
The meander that has been cut off from via a cutoff in a straight reach
Floodplain
Meandering stream channels cover a small portion
Typically has escarpment or bluff along sides and channels over flow to flood plains in entire region
Deltas
When stream enters standing water
-current slows and loses competence, dropping out sediment
-Forms a number of distributaries
Distributaries
Fan of smaller streams
Subsidence
Vertical sinking of the Earth's surface in a region, relative to a reference plane
Drainage Evolution
Landscapes evolve over time
Stream flow causes most changes
EX: -Uplift sets new base level
-Stream cuts former surface
-Valleys widen, hills erode
-Landscape divided to base level
Stream Rejuvenation
When streams cut down into landscape that was originally near the stream's base level
Stream piracy
Stream Capture
-A situation in which headward erosion causes one stream to intersect the coarse of another stream
Drainage Reversal
Tectonic Uplift can alter a course of a major river
Early Mesozoic, South American drained westward
Western uplift raised Andes
- now flows east
Superposed Streams
Streams carve channels into the flat strata; when they eventually erode down through the unconformity and start to downcut into the folded strate, they maintain their earlier course, ignoring the structure of the folded strata
-pre-existing geometry has been laid down on the rock structure
Antecedent Streams
As a mountain uplifts, the stream downcuts just as fast maintaining its original course, cutting across the range.
Raging Waters
Devestate people and property
During a flood
-flow exceeds volume of the channel
-Velocity (capacity and competence) increase
-Water leaves channel and immerses adjacent land
-Moving water and debris scour flood plains
-Water slows away from the river
Monsoons
Tropical rains on the Indian Subcontinent
-Large rains and severe flooding
-Many live in floodplain and delta plains
-1990, Bangladesh...100,000 dead
Flash Floods
Rapid water rise with little warning
From unusual intense rainfall or dam failure
-Typified by rapid moving wall of debris laden water
-1889, Johnstown
Case History: Mississipii and Missouri Rivers
1993- jet stream high altitude winder current drifted southward
-Trapped moist, humid air from Gulf.
-Air rose and cooled releasing torrents of rain (years worth)
-Entered two rivers, flooded
-50 dead, 250,000 no drinking water, houses covered silt, 55,000 homes destroyed
Ice Age Megafloods
11 thousand years ago, ice dams failed
-Glacial Lake Missoula
-Scarred eastern Washington
-Created Channeled Scablands of Washington
-barren craggy, stripped rock
Living with Floods
Flood control is expensive and sometimes trivial
-Dams on tributaries hold back water from trunk stream
-Fake levees and flood walls increase channel volume
New Orleans
...
Living with Floods
People living in floodplains have danger
-Flood risks for insurance, home ownders, lenders, gov agencies
-
Floodrisk Analysis
Hydrologic data is used to measure this
There are maps regulating areas to manage risks
Building in floodplains strictly regulated
Floodways
regions likely to be flooded
-prevent from building
-abandon buildings in the areas
Annual Probability
likelihood that a flood of a given size or larger will happen at a specified locality during any given year
Recurrence Interval
Average number of years between successive floods
Calculated Probability
Floodrisk analysis
-Discharge plotted vs. Recurrence Levels
-Straight line
Graph Inspection
determines the chance of occurence
Urbanization
Cities cover large areas with impermeable concrete
-Stream water runoff from cities destructive
-shorter lag time between rainfall and flood flow
-large discharges for shorter duratiosn
-Water does not go in ground but floods
Pollution
Turning some rivers into deadly cesspools
Raw sewage and drainage from urban areas
-Fertilizers, industrial sites, animal waste
Dam Construction
Resevoirs make irrigation and hydroelectric power
Recreation areas
Alter ecosystems, barriers to fish, decreased nutrients downstream, removing delta sediment source
Overuse of Water
Thirst increases worldwide but water does not
Need so much more today
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