Hello. Today, we just did some revision and we were asked to read up on the water formations along the river such as waterfalls, meanders and oxbow lakes. So, I shall now try to revise for you what meanders are and teach you what oxbow lakes are.
Meanders
A meander is a bend in a river, normally at its middle or lower course where the channel is deeper and wider, which is caused by two things:
a) the difference in velocity between the inside and outside bend of the river
b) helicoidal flow
A. There is a difference in the velocity of water between the inside and outside bend of the river. Water flows faster on the outside bend of the river (because water is flung out of the bend as it curves through the outer bend, causing greater erosion, resulting in a deeper channel and thus, less friction). This is why the bank of the outer bend is steep (refer to picture).
It is opposite in the inner bend where the water is slow moving due to a shallower channel and more friction, causing deposition rather than erosion. This material deposited builds up and creates a slip-off slope (refer to picture).
Hence, because there is higher velocity at the outer bend, erosion happens and vice versa for the inner bend, creating this wondrous formation - MEANDERS!
B. Besides that, helicoidal flow, which is water flowing in a spiral, means that water does not flow straight, so there will be meanders.
Oxbow lakes
Following up from meanders, it is (so) obvious that as a meander gets more and more meandered, the difference in velocity will become even larger because the inner bend will have collected so much deposition that it is much shallower than the extremely eroded outer bend, so as a river is more and more meandered, it will cause it to become even MORE AND MORE MEANDERED! It is a kind of positive feedback thing….
So what happens when it gets so meandered that the river actually joins together again? You will get an oxbow lake! Look at the picture and you can see that as it gets super meandered, the neck of the straight paths is only separated by a little land. So, the water can just break through, leaving the meander out of the river. Soon, this oxbow lake will dry out because it won’t receive any of the river’s water and it will become a meander scar.
So, there. Now you know what oxbow lakes are. Thanks for reading this. You have just wasted 5 minutes of your life trying to understand me because all you had to do was watch this animation (http://www.cleo.net.uk/resources/displayframe.php?src=309/consultants_resources%2F_files%2Fmeander4.swf) which is everything I just said. :D
The end.
Done by WENG KIN.
Geography rocks! (get the pun? :D)
PS: You can also read http://geobytesgcse.blogspot.com/2006/11/middle-course-of-river-meanders-ox-bow.html
posted by 08ip01 at
8:16 PM
6 Comments:
In other words, meander is caused by uneven distribution of energy within the river. At the outer bank, the flow has greater energy because its velocity is greater that that of the inner bank.
By the way, i get the pun but it doesnt make any sense !! :D
(Vu Ngoc Quang)
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That's right, as a river flows, any curve in its course is intensified by the current. On the outside of the curve the current is the greatest, and therefore the erosion. Here the river cuts into the outside bank, producing a river cliff. On the inside of the curve the current is slow and so it deposits any transported material, building up a gentle slip-off slope, and hence meanders.
And look at how cool a real oxbow lake can be! :D http://www.travelblog.org/Photos/3132515.html
And guess what, oxbow lakes are called billabongs in Australia! O:
Well I did some more reading up on the velocity of the oxbow lakes and it turns out that the velocity is also affected by centripetal force(the inward central force when it occurs in conjunction with rotation such as to cause a body to follow a curved path). This formula is given by F=mv^2/r. So if we manipulate it, it will be [v = (Fr/m)^1/2]. So if the radius is greater, the velocity will be greater, which explains why the velocity is greater in the outer bends of a meander.
Well, its really a wonder how bodies such as meanders can be formed just from the mere movement of water. In my opinion, I feel that water behaves just like human beings. because most of the time, we shape our surroundings because of our actions.. like how meanders are formed by the erosion and deposition, caused by the movement of water.
Just a little elaboration on helicodial flow: it is the corkscrew (kind of a vertical spiral/ circle) movement of water. The top current is responsible for the erosion and undercutting of the outer bank while the lower current carries the erroded sediment and deposits it on the next inner bank of the river, where the velocity is lesser, a little way downstream.
Also, there has, I think, been no universal agreement on how exactly meanders came about. What caused there to be an outer bend and an inner bend in the first place? In a perfectly straight river, one would be just the same as the other. Perhaps the cause is pools and riffles? But then again, how did these come about in the first place? We only know how a meander developes, but not how it starts. This may prove to be a mystery we will be forced to leave unsolved forever, although there have been some theories...
1.Stochastic theory: due to obstacles disrupting the flow of the river (eg. big rocks...?)
2.Equilibrum theory: To use up excess energy and mantain an equilibruim.
3.Geomorphic Theory: obstacles to river flow, of the tectonic kind (eg. river might flow over a fault line)
*the theories are taken from wikipedia, and summarised to the best of my understanding, so they might not be entirely accurate.
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