If a river's bedload gets rounder, does this mean that the velocity is increasing or decreasing?

Question

we recorded that the stones at the source of the river were much more angular and the ones at the mouth were well rounded, i thought that this would suggest the velocity is increasing downstream but we also measured this and the velocity is in fact decreasing, why is this?

thanks alot!

Answer 1

Hi there.

A stone will be carried by the river as long as the force of the flow is enough to move the stone. Once the weight of the stone is too great for the diminished force of the river, the stones are deposited, which is why you get shoals at point of slow flow - strong bends, the mouth of the river. So rounder stones, which have greater surface area for the water to "push", get a lot further downstream much quicker. Unless they are of very soft rock, it is unlikely that erosion alone makes them rounder.

Cheers, Steve.

Answer 2

from waterfall to miles traveled down river causes the erosion

Answer 3

It is length of the journey, the velocity of the water, the amount of material in the bedload, the type of material over which the bedload is moving, and type of stone that determines the roundness of the bedload.

A rock caught in an eddy might not travel far at all and all it needs is a velocity great enough to move it around in the eddy.  It becomes like a rock in a tumbler.

Also if there is a lot of smaller, more abrasive material also in the bedload, it will wear down the stones faster.

If the bottom of the stream is over a hard (e.g., granitic) surface instead of a softer (e.g., shale) surface, the rock will round faster.

If the rock is sandstone it will round quicker than a similar size piece of granite.

If there is a lot of material in the bedload, the constant bumping will round the stones quicker.

If the stone has been transported a long distance it will be rounded more.

As you can see, there are a lot of factors which influence the "roundness" of the bedload.

Answer 4

Stones at the mouth have been in the river longer and travelled further.

Answer 5

The roundness of the rocks in a river bed is a function of the distance the stones have traveled.  As stones are moved downstream as part of the sediment load, they knock against each other, The result of this knocking is that edges and points get broken off.  Eventually, the stones become more rounded.

As a river enters a larger body of water, lake or ocean, the velocity of the river water decreases, and some of the sediment load is deposited.  Heavier objects, we call them stones fall out of the water stream sooner than smaller particles, like sand and even finer particles, silt.

Answer 6

it's the " journey " of the stone , from angular to round due to erosion downstream and effect of tides at the mouth bringing more erosion , thus producing a rounded stone  .

Answer 7

Increasing