In order to calculate the discharge of a river, you need a cross-sectional area of part of the river and you need the velocity of the river. To find the cross-section area of the river, you can simplify the equation by assuming the cross-section is a rectangle. This helps give a general estimate since finding the true area is complicated because the water is flowing and you cannot get an accurate description of the shape. Use the formula q=va to calculate the discharge of a river, where "q" is the discharge, "v" is the velocity and "a" is the cross-sectional area of the river.

Measure the depth and the width of the river. For example, assume a river is 4 metres deep and 5 metres wide in one area.

Multiply the depth by the width to find the cross-sectional area. In the example, 4 metres x 5 metres equals 20 metres^2, or 20 metres squared.

Find the velocity of the river. In the example, assume the velocity 3 metres per second.

Multiply the cross-sectional area by the velocity to find the river discharge. In the example (20 metres^2) x (3 metres/second) equals 60 metres^3/second. The calculated discharge of the river is 60 cubic metres per second.