Rivers: Fluvial Processes

Yukon and Charley Rivers

Topics

1. Introduction to fluvial processes

2. Types of channels

3. Processes associated with large meandering rivers

1. Introduction to fluvial processes

Streams “work” by eroding, carrying and depositing sediment

Solid load: suspended load & bedload (alluvium)

Suspended LoadLittle Colorado River

Large bedload, carried during monsoon rains in Nepal

Constant suspended load of glacial flour (ground up rock)

Also dissolved load

Suspended Load

Suspended Load

Dissolved Load

Khartoum

Manaus

Suspended Load

Dissolve

Load

Organic

Acids!

Manaus

Erode? Carry? Deposit?

200 cm/s ~ 4.5 mi/hr

Discharge & Velocity:key

Elements Of Channel Formation

Erosion in headwaters Transport in

middle

Deposition closer to ocean

High gradient (mountain) streams – erode and carry large bedload (high competence)

High competence load creates potholes & other evidence of abrasion

Middle – mixture of erode, transport, deposit, and for much of the year – like this

Depends on flood or not

Lower end - deposit

Mississippi

River delta

has very high

capacity

(lots of sediment,

Just small competence)

These are generalizations that work for larger system and not smaller

pieces

Flow Velocity Relationships

Note: always concave upward profile – characteristic of streams

Even these small washes

Base Level & Grade

The ability of a stream to

erode is based on velocity

of water. Velocity is

proportional to slope. At a sufficiently low slope, streams will run without

eroding: this is called the base level. The ultimate base

level is sea level, although local base levels exist, since

flow paths are seldom one consistent slope.

Grade – concave upward

profile

Base Level

Base level is a short-term

concept. Change in sea

level, for instance, can

either increase or

decrease slope of a

stream profile.

Dams create artificial

local base levels, and in

so doing force steeper

slopes (and hence

erosion) downstream.

Profile –

concave

upward

Effects Of Dams

From FEMA

When base level drops, every tributary incises because streams are steeper and have more energy

When Grand Canyon formed, it “entrenched” meanders

Base level dropped ~ 5 million years ago, freezing in place the old stream patterns

2. Types channels

3 basic types of channel patterns:• Straight Channels • Meandering channels • Braided Channels

Straight – in nature, requires a weakness (fault,

jointing) for stream

Straight – when you see, usually from human activity (channelized, Nasca, Peru)

Meandering rivers typically contain one channel that winds its way across the floodplain. As it flows, it deposits sediment on banks that lie on the insides of curves (point bar deposits), and erode the banks on the outside of curves.

Meandering

Point Bar Cut Bank

Asymmetrical

Point Bar Cut Bank

Braided Channel

Braided Rivers exhibit numerous channels that split off and rejoin each other to give a braided appearance. They typically carry coarse-grained sediment down a steep gradient

Braided – carries mostly bedload, so

stream spreads out to bring high velocity

close to bed

Meandering – carries much suspended load, so stream

narrows to carry load more efficiently

Can get very complicated

3. Large Meandering Rivers

Meandering stream, Phnom Penh, Cambodia

Oxbow lake

Point barCut bank

Zooming into a Reach

Pool

Riffle

Bank Erosion at Cut Bank

Bank ErosionNewaukum River (WA)

Point Bar Deposit

Point bars

Floodplains

These are satellite images before and during Summer, 1993 floods of the Mississippi river north of St. Louis.

A Floodplain

FloodsIn natural streams,

floods occur

periodically when

seasonal discharges

grow extremely large

and waters exceed

channel banks.

The first thing that

happens after water

goes over bank is the

velocity drops, and

sediment is deposited.

This makes natural

levees along channels.

Yazoo Stream

Societal Problem

Living below the river!

Cities grow & obligation to protect

Certainty: levee will always break. Only question: when

Tendency to increase meander amplitude over time

Tendency to increase meander amplitude over time

Tendency towards avulsion (sudden change in channel)

Sacramento R.

Oxbow lakes