Determing flow direction of a river

[jones123]

Hello,

 

I do not really understand how one can determine the flow direction of a river by looking at a topographic map (see attachment). Does the river flow from north to south or from south to north?

 

I think it has to do something with the steepness of the meanders on either side, but I do not really understand the full process.

Can anyone help me figure it out?

Thanks already!

[EdEarl]

It is a contour map. In other words, many of the lines on the map indicate constant altitude along the line. Dotted parallel lines are probably a road or railroad. My eyes cannot make out all the detail. It is difficult to explain here, but sets of curved lines show hills or mountains and valleys, The river (blue) is in a valley and brown lines beside the river (e.g., left-bottom just above the bend) show a steep rise in altitude, because the brow lines are close together. A small loop in these lines shows the top of a hill (or a depression). The altitude is marked in numerals (e.g., 800) and there will be other altitude numbers on lines to show altitude increasing or decreasing from contour line to contour line. If you count ten lines between 800 and 700, then each line is 10 (feet/meters) difference in altitude from the others. Refer to the legend on the map.

 

As water flows down hill, you must determine which way is downhill.

 

I hope this helps. I apologize if my explanation is difficult to understand.

Edited May 21, 2013 by EdEarl

[overtone]

Rivers often flow a long way before crossing a countour line, which would be the gold standard, but one can make a reasonable guess at flow direction by examining the shapes of islands and the pattern of bank cutting - the steepest cuts (closest packing of counter lines) will tend to be (overall, generally) where the current hits the bank most directly, toward the upstream end of the outside of sharp bends or the upstream ends of islands etc.

 

Also, check the contour lines closest to the waterline at the two ends of the map - the actual levels. If one of them is lower than the other, you have determined downhill (there would be another line inside the higher one, if the river level were as low there). Usually they will be the same, unfortunately - rivers in my areas anyway do not often drop steeply enough to show like that on a small map.

 

By that kind of guesswork, it looks like maybe, probably, your river there is flowing from bottom to top. If the contour line in the low bank inside the bend toward the top is a different level than the one on the low bank inside the bend toward the bottom, you could tell for sure.

Edited May 21, 2013 by overtone

[michel123456]

Ed is very correct but I have the feeling this is not the exercise.

In a river with meanders the erosion of the ground takes place on the outer side and deposit takes place on the inner side.

Is this a homework?

[jones123]

Ed is very correct but I have the feeling this is not the exercise.

In a river with meanders the erosion of the ground takes place on the outer side and deposit takes place on the inner side.

Is this a homework?

No it isn't. We've breen through it in class today but I didn't really get it. We are supposed to see the flow direction by looking at smooth slopes on one side of the (inner/outer) curve and steep slopes on the other. But as I said, I don't understand that reasoning...

[overtone]

But as I said, I don't understand that reasoning...

See my post above: The river will tend to cut and undermine harder,often producing a steeper bank, where the water hits the bank directly, head on as it were.

 

That will often be at the upstream end of the outside bank of sharp bends - the river flow hits most squarely, and is deflected most, there. Imagine you are drifting on the river, and can't steer - where do you think you would hit the bank hardest?

 

That's a guess, but a reasonable one.

Edited May 21, 2013 by overtone

[ewmon]

 

Use the contour lines for another purpose — a non-parametric purpose (which means forget about the altitude they represent).

 

Their closeness to each other show the steepness of the banks on either side of the river. It makes more sense that a steep bank would steer the river than a low bank.

 

If the river flows north in the diagram, you can see where only steep banks steer the river. If the river flows south, you can see northeast of the number "30" where a rather shallow bank would supposedly steer the river. The same would supposedly occur in the lower left corner of the map.

 

Besides, there's an intermittent tributary (shown by ∙∙∙—∙∙∙—∙∙∙) near the map's lower right corner that travels westward and then turns north to join the river northeast of the number "30". Plus, there's another intermittent tributary between the "30" and the "L" that flows downhill/north, apparently to join with the river.

 

Thus, the river most likely flows north.

Edited May 21, 2013 by ewmon