Meandering Streamsitc.gsw.edu/faculty/bcarter/physgeol/river/Meander.pdf · 2016-04-07 · Notice that even meandering streams do no meander everywhere – there can be long straight

Meandering Streams

Unless otherwise noted the artwork and photographs in this slide show are original and © by Burt Carter. Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin. Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities. Other copyrighted material is used under the fair use clause of the copyright law of the United States.

Streams and stream segments dominated by deposition (L>E) are fairly simple. Sediment clods the channels and forces them to branch. If this happens at an abrupt drop in gradient or at the mouth of a stream, the branched channels do not rejoin, but radiate as distributaries from the head of the resulting fan or delta. If it happens within a valley then the branches do rejoin, or anastomose, and a braided stream results. Erosional streams or segments, if downcutting or headward erosion dominates, are even simpler – a single more or less straight channel sits at the foot of steep valley walls in a canyon. Meandering streams are much more interesting and complex, though the complexity results from just a couple of things – mostly lateral erosion. Lateral erosion dominates because these streams form far from the head (no headward erosion) and close to base level (no downcutting).

Alapaha River near Howell, Echols Co, GA, (ca 1990). Image from USGS/Microsoft TerraServer satellite.

Alapaha River near Willacoochee, Atkinson Co., GA. Image from Google Maps.

FLOOD PLAIN

FLOOD PLAIN

INSIDE OF MEANDER

OUTSIDE OF MEANDER

The parts of a meandering stream obvious in map view are shown here: the channel and the floodplain, of course, but also the insides vs. outsides of the bends. For each bend identify the inside and outside.


Now notice that the insides of the meanders have wide sand bars and the outsides lack them. We’ll come back to this.

You can see the same thing here, a little downstream of the first picture. The red arrow shows the direction of the view on the next page.


This was taken at quite low flow from the pointbar (the gently sloped sand surface) toward the cutbank (the steep opposite bank on the outside of the bend). Notice that erosion has undercut the bank and some trees are in danger of

falling.


Suwannee River, Echols Co., GA

Cutbank Pointbar

Here’s another view.

Notice that even meandering streams do no meander everywhere – there can be long straight stretches of the

channel as well as the arrows indicate.

Alapaha River near Howell, Echols Co, GA, (ca 1990). Image from USGS/Microsoft TerraServer satellite.

A profile across a river at a straight stretch, or a the midpoint between two adjacent meanders (shown on photograph at left) would have the shape shown in the diagram and photograph at right.

Flood Plain Flood Plain

Natural Levee

Natural Levee

Channel

Thalweg

A A’

A A

A’

A’

The channel at such places is virtually symmetric, so it doesn’t really matter which side of the profile is A and which is A’. The thalweg (deepest, fastest water) is midstream and the two banks slope at similar angles up toward a natural levee – a ridge of sand that follows both banks almost continuously and separates the channel from the floodplain, which is called that because it is inundated when the river floods.


Suwannee River at the Highway 6 bridge, Hamilton/Columbia Co., FL

The channel at such places is asymmetric, so it does matter which side of the profile is A and which is A’. The thalweg is on the outside of the bend, where it erodes the cutbank and the two banks slope at different angles up toward the natural levee. If the cutbank has recently slumped the levee might not be present. Across the river, on the inside of the bend, deposition of sand builds the pointbar.

A profile across a river at a meander bend has a much different form. Note that A is on the inside and A’ on the outside of the bend.

Flood Plain Flood Plain

Natural Levee

Natural Levee??

Channel

Thalweg

A A’ A

A’


Pointbar Cutbank

Lateral Erosion Deposition

As the stream erodes its cutbank, sediment accumulates on the pointbar to maintain about the same overall shape and size for the channel. This happens mostly during high water events. As a consequence, the river “migrates” in the direction of the cutbank (or the outside of the bend) over time.

Channel Migration

(Note: If the channel migrates far enough to bring the cutbank into contact with the valley wall then this end of the diagram is not accurate. There would be neither floodplain nor natural levee, but rather a bluff where undercutting and mass wasting of the valley wall into the channel would be widening the valley.


The cutbank is not, of course, just a point on the outside of the bend, it is the entire outside of the bend, so erosion of the cutbank moves the meander in such a way that it expands – becomes larger and more rounded with time, as the arrows on this photograph suggest.


Of course every meander is doing the same thing. You can presumably predict what will eventually happen somewhere in the vicinity of the asterisk.

*

Alapaha River near Howell, Echols Co., GA. Image from Google Earth.

This image was taken in January, 1993. The arrow indicates a place where my brothers and I used to portage our canoes to save a little time. (If we saved any time it was certainly a very little.) Note that both sides of the portage are on cutbanks and predict which way the two meanders will expand.


This image of the same place was taken in November, 2005. Notice that the two cutbanks have eroded entirely through the neck between them and the channel now bypasses an abandoned meander. The new channel is called a meander cutoff and the old channel is an oxbow lake (when the water level is higher and there’s actually water in it.)


This image of the same place was taken in October, 2015. At this point the old meander is barely noticeable.


Notice on the original 1993 photograph that there is a similar feature across the river from the new oxbow, as indicated by the line. This is also an oxbow lake that is called the “Dead River”. Apparently even my uneducated great great grandfather understood what happened here. If you look around you can probably spot other old abandoned meanders on the photographs.

A

An appreciation for how the water moves in a meander helps explain why the outside of the bend is erosional.

At point A the water emerges from one bend and heads for the next. Here it moves in a straight line, as its momentum prefers that it do. The channel profile is symmetric just here. At point B that momentum (the arrow shows its vector) comes into conflict with the barrier created by the outside of the next bend. The water “hugs” the outer bank and it’s depth and velocity are greatest here. All around the bend (points C-G) the same interaction between the water’s momentum and the bank’s interference is in play, subjecting the bank to the high energy of the fast, deep water. At point H the water is between two bends and again flows as its momentum dictates, in a symmetric channel. At point I the water is constrained again by the cutbank of the next meander and the same forces come into play, now on the opposite river bank.

B

D

C

F

H

E

G

I

Given that the fastest, deepest water (the thalweg) is on the outside of the bend, flow on the inside of the bend must be slower – slow enough, in fact, to allow deposition of sand eroding off of upstream cutbanks (and maybe the cross-stream one too).

The red line shows the hypothetical path of a sand grain eroded off the cutbank and transported to/deposited on the next pointbar. Remember that as the cutbank is eroded the pointbar across from it is aggraded so that the channel’s general shape and width are maintained.

Eastern Mississippi Valley in western MS. Image from Google Earth.

Meander scrolls and oxbow lakes in the floodplain of the Mississippi River in northwestern Mississippi reveal that the river (now visible on the western edge of the photo) used to occupy different parts of the valley. Meandering streams can migrate across their valleys by lateral erosion, or they can “jump” into a different part of the valley by laterally eroding into a previous channel system. The small Tallahatchie and Yazoo Rivers in the eastern part of the floodplain cannot have made the meanders they occupy because their discharge is inadequate. They flow through former Mississippi River meanders. It is around 50 miles from the valley wall at Oxberry (east of Philipp) to the Mississippi.

95’ 90’ Valley Wall

Valley Wall

1 mile

95’ 90’ Valley Wall

Valley Wall

1 mile

The meanders of a stream allow it to manipulate its E/L balance in part by the temporary erosion and transport of sediment, but this is not a complete fix because the sediment cannot be transferred very far. In other words, increasing load is only a limited option. However, meandering also allows a stream to reduce its gradient (and therefore its energy) as shown here.

Stream elevation drops 5’ in one mile of channel length for a gradient of 5 ft/mi.

Stream elevation drops 5’ in two miles of channel length for a gradient of 2.5 ft/mi.

Documents

Meandering Streamsitc.gsw.edu/faculty/bcarter/physgeol/river/Meander.pdf · 2016-04-07 · Notice that even meandering streams do no meander everywhere – there can be long straight