Instructor’s Notes: Chapter 10 Streams and Flooding
The Distribution of Water on Earth
The Hydrologic Cycle- movement of water as it evaporates from the sea and returns to the
sea
15-20% of rainfall becomes surface runoff:
Stream- a body of running water confined to a channel moving downhill due to gravity
Streams Have Longitudinal Profiles
Headwaters- upper part of stream near its source
Mouth- where the steams meet the sea, lake, or larger stream
Base level-the elevation at which the stream can no longer erode -often found close to the
mouth
Sheet flow (unconfined flow of water) controlled by:
Infiltration capacity- how fast the ground can absorb the water
Rainfall intensity- precipitation rates
How wet is the soil to start with- if the ground is saturated all rainfall becomes runoff
Soil composition and texture- clay rich soil will compact and become impermeable more
quickly in comparison to a soil with abundant quartz sand
Slope- angle and thickness of sediment
Vegetation cover - amount and type of vegetation
Streams are constantly trying to reach equilibrium
A stream adjusts to channel and profile to changing:
Discharge- the amount of water flowing in the stream
Erosion- current causes cutting down (vertical) or lateral erosion in a stream channel
Base level- elevation to which the stream is flowing
Sheet wash- at flood stage, thin layers of un-channeled water flows over a broad area as
the stream water exceeds the volume of the channel – this is associated with sheet erosion
Stream cross-section:
Stream channel-long narrow depression
Stream bed- bottom of the channel
Stream bank-sides of the channel
Drainage Basin- total area drained by a stream and its tributaries
1/3 of US is included in the Mississippi River Drainage basin which includes the Mississippi River and its
tributaries the Ohio and Missouri rivers
Tributaries-smaller streams that flow into larger streams
The Mississippi River is classified as a long stream system where material is eroded at the
headwaters in the northern U.S., transported, and ultimately deposited in the Gulf of
Mexico. The transported sediments are considered "mature" because during the long
period of transportation, most of the minerals weather to clay. The resulting sediment
reaching the Gulf of Mexico is resistant quartz grains and abundant clay minerals.
In contrast, California Coastal streams are described as short stream systems where the
headwaters are not very far from the coastline. The material that is eroded at the
headwaters reaches the coastline before it can thoroughly be weathered. The sediment
found on these beaches is considered "immature" sediment with mineralogy that can vary
greatly (arkosic for example).
Royal Gorge in the American River in California drops 375 feet in ¼ mile
Drainage Patterns
Drainage Patterns (another model)
Pedernales Falls, TX
Trellis
Rectangular
Stream Erosion and Discharge
Erosion and discharge are controlled by:
Water Velocity and Stream Gradient
Water Velocity
Moderately fast; is 3 miles per hour-flood stage as high as 15 miles/hour
Water velocity is directly related to a stream ability to erode and deposit sediment
As a stream bends around a curve, the water in the outside of the bend is moving faster and
therefore, it erodes. The water on the inside of the bend is slowed down by drag and
therefore, it deposits.
Steam Gradient
Downhill slope of a stream bed- important in controlling steam velocity
Stream Gradient = elevation x - elevation y / distance between x and y
Example:
Royal Gorge 375 ft in ¼ mile =1500 ft /mile gradient
Gradient is measured in feet/mile or meters/kilometer
Stream gradient is typically greater at the headwaters than at the mouth
Rapids indicate local increases in stream gradient
Calculation Stream Gradient
Stream Discharge: the volume of water that flows past a given point in a unit of time
Stream discharge = (stream width x depth) x velocity
Stream discharge= x-sectional area of channel x velocity
X-section is square meters or square feet
Velocity is measured in cubic meters/second or cubic feet/second
Climate Effect on Stream Discharge:
Humid climates- streams increase discharge downstream because water is added from
ground water and tributaries
Increase in width and depth of stream downstream
Flood discharge can be 50-100 times normal flow
Arid Climates- discharge can decrease down river due to evaporation and irrigation
Decrease on discharge causes deposition
Stream Erosion
Hydraulic action- the ability of water to pick up and transport sediment
Pothole- holes eroded by abrasion
Stream Channel- eroded by abrasion
Abrasion- erosion due to friction and impact of sediment within the stream channel- the
higher the sediment load of a stream the higher the erosion rate
Sediment Transport:
Bed load- large particles that travel on the stream bed
Traction- rolling sliding or dragging
Saltation- bouncing off the bottom
Suspended load- light enough to be suspended in the water- generally clay but in flood
conditions could be cows, houses and cars!
Dissolved load- soluble product of chemical weathering
Example: Mississippi River- 750 million tons of material /yr
502 million tons suspended load
195 million tons dissolved load
52 million tons bed load
Model of Stream Transport (Figure 10.13)
Stream Topographic Features:
Bars- ridges of sediment that are deposited when a stream velocity decreases
Placer deposits-gold, platinum, diamonds, gemstones, titanium and tin
Braided Stream-heavy sediment load
Meanders-sinuous curves in a river- associated with late stage river development
Point bars- deposition of sediment on inside bend in meander loop
Cutbacks- erosion of bank on outside bend in meander loop
Meander Cutoffs-short channels across the narrow neck of a meander
Oxbow lakes-abandoned meander loop
Development of a Meander (Figure 10.20)
Flood plains- broad strip of land built up by sediment on either side of a river
Natural levees- low ridge of flood deposited sediment
Back swamps-low lying area behind a levee
Stream terraces-
Oxbow Lakes (Figure 10.21)
Animations and Movies (Plummer, McGeary, and Carlson)
Abandon Channel Fill
Deltas- a body of sediment deposited at the mouth of a river when the water velocity
decreases
Distributaries- channels within a delta that carry water away from the main channel
Three Stages of Stream "Evolution":
Early, Middle, and Late
Early Stage:
Stream is well above sea level
Erosion is vertical
Straight with v shaped valleys,
Upgraded streams- waterfalls and rapids
Mostly erosion, trying to smooth out variations in gradient
Stream gradient decreases through headward erosion and stream capture
Middle Stage:
Lateral erosion increase and meandering begins
Valleys broaden and develop flood plains
Waterfalls and rapids disappear
Graded Stream- more balance between transport capability and sediment load
Late Stage:
stream is close to sea level (Coastal Plain)
Wide flood plains
Oxbow lakes
Yazoo rivers
Flood Plain and Terraces
A single drainage can have early, middle and late stage stream development along its
course.
Example: Mississippi River
Unusual Streams:
Superimposed Streams
Stream superimposed across narrow mountain ranges
Example: Appalachian Cenozoic Uplift
Incised Meanders- “goosenecks”
Example: San Juan River southern Utah
Incised meanders in the San Juan River in Utah- the Colorado Plateau has been subjected to a long period of
slow tectonic uplift throughout the Cenezoic. Because of the slow rate of tectoninc uplift the meandering
streams were able to cut down without streighten thecourse so the meanders are superimposed in steep
canyon ( very unusual). If the tectonic uplift had occurred at a faster rate the streams would have
straightened the course and shown early stage stream development.
The San Juan river is the tributary to the Colorado River shown on
the right edge of this map
The Grand Canyon
(view with RB glasses)
DTM the Grand Canyon
Types of Deltas:
Stream dominated deltas
The stream energy is higher than the sea
Bird foot deltas
Example: Mississippi delta
Wave dominated deltas
Energy of the sea is greater than the stream energy
Tide dominated deltas
Tidal energy is greater than the energy of the stream
Stream Deltas
Herringbone Cross Beds associated with tidal flats
Floods:
Stream Floods Flood occurs when a stream overflows its channel
Floods cause erosion, deposition and high water
Erosion- undercuts banks and levees
Deposition -silts and clays beneficial to agriculture
High water- devastating in urban areas - Storm sewers are designed for 100 yr flood
Flood size is measured by maximum discharge or by stage
Flood Size:
Maximum dischargeStream discharge = (stream width x depth) x velocity
Discharge m3/second
Width- meters
Depth meters
Velocity meters/second
Stage- the elevation of the water at the surface
Recurrence interval- the average interval of time between floods of a particular size
A 100 year flood is the largest flood expected to occur within a 100 year period. Each year
there is a 1 in 100 chance or 1% probability that a flood of the given magnitude will occur.
Flood Gaging Station
Flood plain zones are based on recurrence intervals, and a stream’s flood plain is mapped
using for example 25, 50, 100 year flood levels.
Insurance premiums are based on flood plain maps and are therefore based on recurrence
probability
Flash Floods:
Localized rapid rise in stream levelCommon in steep stream valleys that receive large amounts of water is short period of time
Common in deserts and coastal mountains with large amounts of rainfall in a short periods
of time
Video: Flash Flood
Video: Flash Flood
Pedernales Falls, TX Flash Flood Warning System
Controlled Floods:
Levees and Dams
Bypass- example Mississippi River bypass of flood waters to Lake Pontchartrain
Non-structural approaches (abandon and relocate)
Mississippi River Delta:
Mississippi “Birdfoot” Delta
Mississippi River Birdfoot Delta
Mississippi River and Delta Time Line:
Shift position over geologic time has allowed for the two hundred mile arc- shifting
depositional axis ever 1000-3000 years.
Locations of the shifiting Mississippia Delta over the last 3, 000 years
Since the 1950 the US Army Corp of Engineers has been battling the Mississippi trying to
prevent shifting from its current location to the Atchafalaya River which is currently 13
feet below the dammed Mississippi.
1940 Atchafalaya captures, (through headward erosion), the Red River another tributary to
the Mississippi. The increase in discharge allowed the Atchafalaya to downcut its channel.
By 1950 when a dam project was started at the headwater of the Atchafalaya, 30% of the
Mississippi discharge was flowing through the Atchafalaya River.
The Current Achafalaya and Wax River Deltas
1963 the dam was finished – during spring flood events the Atchafalaya Basin was used to
divert flood waters causing further downcutting
1973- Unusually high floods caused the dam and lock system to vibrate- -it weighs 200,000
tons. On the outflow side a pothole 100 m (300 ft) deep was eroded. On intake side both
guide walls were undercut and collapsed. The dam very nearly survived total collapse.
½ of New Orleans is now below sea level. The entire bank of the Mississippi has concrete
levees. The extraction of groundwater behind the levees has increased the sinking of New
Orleans
Lake Pontchartrain- bypass floodwaters diverted into Lake Pontchartrain by series of canels
built by the Army Corp of Engineers
2005 Katrina http://www.nasa.gov/vision/earth/lookingatearth/h2005_katrina.html