Change in Blog Format by Minh.

Hi everybody!!!
I've just changed the blog format so that hopefully it would be easier for all of you to edit your own post and make comment.
Best regards,
Minh.

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

Braided Channels and Deltas

Ok, this is my first blog entry and may be my last post also (unless we have 22 more lessons :D).
Today lesson, we've come up with two concepts: braiding and deltas. Now I'm going to do some recap for you guys.

1) Braiding:
- A braided channel is a river within which the stream is divided into smaller and interlacing channels by small islands called "bars". (the picture looks like a bunch of epidemal cells, really)
- These bars are sediments deposited by the river and are very firm (in Bangladesh, people make use of these islands to build house on) because when old sediments are removed, new ones will arrive and be deposited at the bars
- Causes of this braiding features:
+ The river banks are easily eroded (materials from the banks will form the bars)
+ The more abundant the sediments are, the bigger and more of bars will be formed
+ If the river's discharge changes, formation of bars also changes. If less discharge, the river's velocity as well as its ability to transport sediments decrease and vice versa.
+ It also depends on the gradient of the river, if it's steep, velocity of water is greater and hence less deposition occurs. If the water flows more slowly, materials are easily dropped along the way.

2) Deltas:
- When a very large amount of sediments is deposited at the river mouth, materials will be piled up and new lands are formed here. These lands are called "delta".
- The river then has to divide to smaller channels called "distributaries" to overcome these blocks
- There are two types of Delta: arcuate and bird's foot (why not bird's feet???) but since (Miss) Peixin didn't say anything about them so i'm not going to mention here.

Extra stuffs about Delta:

Introduce you guys to the largest Delta in Vietnam!!! We call it "Đồng bằng sông Cửu Long", as translating to English, it'd mean "Nine Dragons river delta":

This Delta is formed at the mouth of the Mekong River. As you can see in the picture, the river itself has divided into 9 (nine) (:D) distributaries, that's why it's called "Nine-dragons...". In this area, people rely heavily on agricuture due to the rich of alluvium brought by the river.

OK that should be it :) Before i end my entry and go mug Chem stuffs, i want to remind you of the contents tested in assessment paper:
+ Atmosphere: - Climates, seasons
- Climograph
- Heat balance (no need to remember all the numbers but if you want to challenge your brain, then go ahead :D)
- Rain formation (adiabetic coolings, movement of air, types of rains)
* No global warming stuffs
+ Hydrophere: - Hyrpgraph
- Drainage basin (not sure about this)
- River channels processes
- Formation of waterfall
- River's features.

OK done! Have fun! :)

Vu Ngoc Quang (aka CT rep :D) 5/5/2009