how do you find the discharge of a river in full detail?

Answer 1

You need to know the slope the river, the cross section and the river channel charateristics. Then you have to conduct open channel calculations using mannings formula.

This is not easy to do.

See HEC-RAS for free software put out by the Fed. Govt.

Answer 2

First, you have to pick a spot where the flow is approximately laminar, with no channels where the current flows faster than at other places in the stream.

Then you have to know the cross sectional area of the stream by measuring its depth at various points across the entire width.

Then you measure the average velocity at the centroid of the stream. This is typically at a point in the center of the stream 2/3 of the distance between the surface and the bottom. There are several instruments available for measuring this.

Then, when you know the average velocity and the cross secional area, the discharge volume = cross-sectional area x average velocity.

(Note: Some people think you can throw a floating object on the surface and measure the speed with which it travels between two points a known distance apart. Close, but no cigar. The surface flow is not entirely representative of the flow cross section due to surface tension and other boundary effects.

Source: 10 years measuring stream flows

Answer 3

I think you multiply the volume by the speed of the water?