(
Geography/
Geomorphology)
If you look at the way that tributaries empty into larger rivers, you will notice that most of them flow in the same direction as the rivers they empty into:
________________________
`--._
-> -> -> -> `--._
______________ ______ -> `--._
_.-'_.-' `--._ -> `--._
_______.-'_.-' `--._ -> `-
_______.-' `--._
-> -> `--._
But quite a large number flow the other way:
_______________________
_.--'
_.--' -> -> -> ->
_.--' -> ______ _____________
-> _.--' `-._`-._
-> _.--' `-._`-.______
_.--' `-.______
<- <-
In fact, you will find stretches of some rivers where every tributary empties into the river in the "wrong"
direction! Such an occurrence is known as a barbed drainage pattern.
The most common explanation for barbed drainage is as a consequence of stream piracy. In fact, barbed drainage appears most frequently in areas of high relief undergoing tectonic uplift, just the sort of area where stream systems are rapidly developing, just the place for stream piracy to occur. When a fast-eroding stream extends its valley to intersect a more slowly-eroding stream, there is a chance that the slowly-eroding stream will just happen to be flowing in the opposite direction. But the process that follows is more certain to produce barbed drainage: a tributary of the beheading stream begins to erode itself back into the downstream section of the beheaded stream. Such tributaries form easily, since
- The capturing stream has a much deeper valley (its slope has to be much steeper to erode the same material faster)
- The beheaded stream's dries up near its point of capture, for obvious reasons, as well as the fact that groundwater
levels near the capturing stream's new valley drop
- The beheaded stream's hanging valley is relatively lower than the rest of the new valley, a convenient starting spot
for erosion of new tributaries to begin.
The tributary eats its way back along the beheaded stream's former course, in the opposite direction, capturing more and more of the latter stream's
tributaries.
`-._ `-._ `-._ `-._ `-._ `-._ `-._ \ `-._ `-._
-._--.------.--`-._.--> -._--.-+ --`-._.--> -._--.-+-<-- -`-._.-->
.-' | .-' | .-'
| | |
| | |
v v v
1. Fast-eroding north 2. Capture of east-west 3. Consequent extends
-south stream, slow flowing stream east-west flowing
west-east stream tributary
`-._ \ `-._ `-._ `-._ \ `-._ `-._
-._--.-+---<---- .----> -._--.-+---<--<-.___\__
| .-' | .-'
| |
v v
4. W-E tributary captures one 5. Fully-developed
of the beheaded stream's barbed drainage
tributaries