Fluvial Geomorphology

Lab 7

Concepts Basic concepts and definitions

Types of work performed by streams Alluvium Base level

Watersheds/Drainage Basins Drainage divides Drainage patterns Classifying stream order

Categories of rivers Straight channel, braided stream, meandering stream

Stream information Calculating discharge Flood recurrence Real-world example: Midwest Floods of 1993 Real-world example: Water Wars affecting Georgia

Basic Concepts & Definitions Fluvial = stream-related processes Geomorphology = analysis of how landforms

evolve

Fluvial Geomorphology is important in understanding how water shapes our earth

Basic Concepts & DefinitionsTypes of work performed by streams 3 types of work:

Erosion of landscape Streams carry mixture of water and solids Alluvium = clay, silt, and sand that is transported & deposited by

running water

Transportation of materials Deposition of materials

Concept of “Base Level” Lowest (elevation) point to

which a stream can flow and cut down to

Ultimate base level = sea level

Watersheds/Drainage Basins Streams organized into areas called watersheds or

drainage basins. All the water in the area flows into one stream and exits in one spot. Can be broken down into sub-basins (see map of Georgia).

Watersheds are separated from others due todrainage divides

High ground which separates streams that are flowingin adjacent basins.

Streams cannot cross the divide.

Watersheds/Drainage Basins

Watersheds/Drainage BasinsDrainage Patterns Within each watershed, streams develop into

distinct patterns due to the underlying geology.

7 common patterns: Dendritic Rectangular Trellis Radial Parallel Deranged or Chaotic Centripetal

Centripetal

Watersheds/Drainage BasinsDrainage Patterns

Watersheds/Drainage BasinsClassifying stream order Classify patterns of streams in a basin

“First-order” streams are smallest Any basin will have more first-order streams than any other category.

Think of these as headwater streams.

Where 2 similar-ordered streams come together, they increase in order. 1 & 1 = 2 2 & 2 = 3 3 & 3 = 4

2 streams of same order must be joined to increase in order.Otherwise, keep the higher number for the next stream.

1 & 2 = 2 1 & 3 = 3 2 & 4 = 4 5 & 12 = 12

Watersheds/Drainage BasinsClassifying stream order

Categories of StreamsStraight Channel

Uncommon, usually only occurs in canyons or when humans force rivers to straighten out

Examples: portions of the Columbia River (states of Washington and Oregon) and the Colorado River (southwest U.S.)

Forms when the stream cannot hold its sediment load and dumps it in the middle of the channel.

Channel bars are collected piles of sediment Channel bars force the river to flow around them, which

separates the river into “braids”

Categories of StreamsBraided Stream

Categories of StreamsBraided Stream example: Platte River

Notice the channel bars? Some of them are even covered in vegetation, and have wildlife living on them! These are considered established channel bars and do not easily reshape.

Sinuous channel Form through deposition and lateral erosion

Deposition = inside bank (slower velocity water allows alluvium to collect here)

Erosion = outside bank of the meander (higher velocity water erodes the bank)

Categories of StreamsMeandering Stream

Point Bar Cut Bank Cutoff Oxbow Lake Backswamp Meander Scar Floodplain and its features

Levee Yazoo stream

Categories of StreamsMeandering Stream: common features

Why streams meanderThe fastest water in thechannel moves from sideto side.

Check out the blue line: this is the path of the fastest water, which is why erosion takes place there.

Cut BankErosion taking place on the outside ofthe meander.

Point BarLook at the deposition on the rightside of the photo.

Categories of StreamsMeandering Stream: common features

Categories of StreamsMeandering Stream: common features (oxbow lake formation)

Categories of StreamsMeandering Stream examples: Chattahoochee, Mississippi, Tennessee

Stream InformationCalculating discharge

Velocity = how quickly water is moving through the stream Discharge = volume of water transported by a stream

The greater the discharge, the more ability it has to carry sediment In times of flooding, discharge is higher than during periods of

infrequent/low precipitation. Need to know velocity & area of the

stream’s channel.

Area (A) = Width (w) x Depth (d) If w = 5 feet, d = 4 feet, then A = 20 ft2

Discharge (Q) = Velocity (v) x Area (A) If v = 100 ft/s and A = 20 ft2,

then Q = 2000 ft3/s

Less infiltration water can’t soak into pavement…

More runoff More flooding

Specific causes: Deforestation Construction and paving Channelization (restricting the channel by building

man-made levees)

Stream InformationCalculating discharge: effects of urbanization

Floodplains form when the river leaves its channel during times of high flow.

Natural levees are produced from flooding Flood waters overflow the banks, river loses velocity as it spreads

out. It dumps its sediment load, which forms the levees. Yazoo streams may form on the other side of levees – cannot join the

main channel because of the levee.

Stream InformationFloodplains

Recurrence = average time period when an event will be equaled or exceeded

Likelihood of occurrence each year Used to designate different flood zones

(1 ÷ Frequency) x 100

“10 year flood” = (1 ÷ 10) x 100 = 10% 10% chance that a flood will occur each year

by that stream If a 10 year flood happens in 2007, it still has

a 10% chance to occur in 2008, 2009, 2010… Does not mean that another flood cannot

happen for another 10 years!

Stream InformationFloodplains: Flood recurrence

10 year

100 year

500 year

Stream InformationExample: Midwest Floods of 1993

Most devastating flood in recent US history ($20B in damages), 14 weeks of flooding.Towns relocated rather than rebuilt!Estimate that this was beyond a 100 or 500 year flood… perhaps a 1,000 year flood!Can you calculate the likelihood of that type of flood?

Stream InformationExample: Water Wars of the ACT and ACF

Tri-state water fight ACF = Apalachicola,

Chattahoochee and Flint ACT = Alabama, Coosa, and

Tallapoosa

Fights due to conflicting water needs

Georgia: needs for urban areas and farming

Alabama: fears GA is hogging water

Florida: oyster fisheries in Apalachicola Bay is suffering since water is being held back in GA and AL

Read more about the water wars:www.metroatlantachamber.com/macoc/initiatives/img/tri-statefactsheet.pdf