# Hydraulic Geometry in Water Flow

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• 0:02 What is Hydraulic Geometry?
• 0:34 The Cross Section of Rivers
• 1:52 Lateral Motion of Rivers

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Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this video, you will be able to explain what hydraulic geometry is. You'll also be able to describe the geometric features of a river at different points and how they are formed. A short quiz will follow.

## What Is Hydraulic Geometry?

The hydraulic geometry of rivers is the study of how the features of a river affect its geometry - its width, depth, shape, and flow patterns. A river might be small and fast, wide and large, steep downhill or relatively flat. The bedrock and soil might be made out of different materials, and a lot of that material might be carried by a river at a certain point, or a little. How does the flow of the river affect its shape and size? The answer to that question is what hydraulic geometry is all about.

## The Cross Section of Rivers

A river's cross-section can vary hugely depending on the features of the river at a particular point. Rivers begin at the source, usually in mountain ranges or uphill areas. There, the river will quickly flow downhill. These streams are small and fast flowing. Because of their high speed, the water is able to produce a lot of erosion, and that erosion is mostly focused on the bottom of the stream. This is because the water in a stream moves faster near the center and slower at the outside edges. This produces a V-shaped cross-section for these steep, mountainous parts of rivers.

As rivers leave the mountains, the land starts to level off. Since the land is more level, the water flows more slowly, but water is also added to the river over time from other joining streams. Because of the extra water (the larger discharge) and greater debris, it can still erode a reasonable amount of material despite its slow speed. However, over time, it becomes less and less able to erode downwards, and the erosion is focused more on the riverbanks. This is why this part of the river is usually U-shaped - wide and relatively shallow. At the very end of its life, a river may stop eroding completely, depositing more material than it erodes.

## Lateral Motion of Rivers

Lateral motion is left or right motion. So, the lateral motion of rivers is the way it moves left or right. Young, fast, downhill rivers tend to be super straight. All the erosion is happening on the bed of the river. But as the river gets less steep and slows down, erosion on the banks becomes more of a big deal. And this causes the rivers to move left and right a lot more. Rivers start to meander. Meandering is where a river bends back and forth across a landscape in characteristic curves.

Rivers start high up in the mountains. But after a while, random rocks and obstacles, or a difference in the material on each side of a river, will produce slight curves, especially in lower-lying areas where the water flows slower.

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