Near the source, a river flows over steep slopes with uneven surfaces. It often flows over a series of waterfalls and rapids. As a river flows down steep slopes, the water performs vertical erosion.
This form of erosion cuts down towards the river bed and carves out steep-sided V-shaped valleys. As the river flows towards the mouth, the gradient of the slope becomes less steep. Consider a continuous stream of water flowing down from a tap. Since the water flow is continuous, by equation of continuity, the cross sectional area of the stream decreases. But what makes the water flow sideways, ie, which force is responsible for decreasing the area of cross-section?
How does a river change course downstream? A river changes shape as it flows from its source to its mouth. A section of the course of a river drawn from source to mouth is known as a long profile.
That is why it is common to see cross sections snap at different angles outside the main channel we call this doglegging. The trick is to keep them from intersecting, while also staying perpendicular to flow lines. In the figure below, the dark blue line represents the main channel. The brown lines represent the edge of the flood plain. Begin typing your search term above and press enter to search. Press ESC to cancel.
Hence rivers flow progressively faster on their journey downstream. Gradient can have an impact on velocity but rivers tend to very shallow and narrow in their upper steep courses, which increases the friction acting on the water and slows it down despite the steep gradient. Velocity is also highly variable from location to location on a river, and velocity is highly influenced by channel shape or form. Wider shallow channels have larger wetted perimeters so more friction and hence flow slower than narrower deeper channels.
Velocity profiles also differ between symmetric and asymmetric channels, as shown in the diagrams. The discharge of a river is just the amount of water passing a certain point every second, and is calculated by multiplying the cross sectional area of the river by the velocity.
The cross section is measured in metres 2 and the velocity is measured in metres per second, so the discharge is measured in metres 3 per second. The discharge should therefore increase with increasing distance down Stream as tributaries swell the amount of water and velocity increases as channels become smoother and less rough.
Try these exercises to find out what happens to various river characteristics;. Calculate Spearman's rank figures and draw scatter graphs for this river data. Calculate cross sectional area and Discharge in this exercise. Analyse this bedload data for its spread or dispersion. Bags of river and stone data from to analyse. Width and depth are simple variables and generally increase downstream as more water is added from tributaries. Width can be derived either from the water surface width occupied channel width or from measuring from bank to bank, giving the bank full width.
We can calculate the stream order from source to mouth too, and this gives us an indication of the size and shape of river channels. The stream order is calculated as follows;.
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