# How much water is flowing in the river?

One way or another, you can always determine the speed with which the water jets of the river are flowing. The second part of the preparatory work required to calculate how much water is flowing in the river is more difficult - determining the cross-sectional area of the water. To find the size of this area - what is commonly called the "living section" of the river, it is necessary to make a plan of this section. This work is done as follows.

At the point where you measured the width of the river, you place a pole on both banks. Then get into the boat with a friend and swim from one milestone to another, trying to keep a straight line connecting the milestones all the time. An inexperienced rower cannot cope with such a task, especially in a river with a fast current. Your companion must be a skilled rower; in addition, he must be assisted by a third participant in the work, who, standing on the shore, makes sure that the boat does not stray from the proper direction, and, if necessary, gives the rower with signals instructions in which direction he needs to turn. On the first crossing of the river, you only have to count how many strokes with the oars it required, and from here find out how many strokes the boat moves 5 or 10 meters. Then you make a second crossing, armed this time with a fairly long pole with marked divisions, and every 5-10 meters (measured by the number of strokes) you submerge the pole to the bottom, recording the depth of the river in this place.

In this way, you can measure the free area of only a small river; for a wide, abounding river, other, more complex techniques are needed; this work is carried out by specialists. The amateur has to choose a task for himself that meets his modest measuring tools.

When all measurements are completed, you first of all sketch on paper a drawing of the cross-section of the river. You will end up with a shape like the solid line in figure 1. It is very easy to determine the area of this figure, since it is divided into a series of trapezoids, in which you know both bases and height, and into two edge triangles also in height.

Now you already have all the data for calculating the amount of flowing water. Obviously, every second a volume of water flows through the living section of the river, equal to the volume of the prism, the base of which is this section, and the height is the average second speed of the current. If, for example, the average speed of water flow in a river is 1,5 meters per second, and the area of the free area is, say, 126 sq. meters, then every second 126*1,5 = about 190 cubic meters sweeps through this section. meters of water, or the same number of tons. This amounts to 190*3600 = 684000 cubic meters per hour. meters, and per day 684000*24 = 16396000 cubic meters. meters, over 16 million cubic meters. meters. In a year, about 6 cubic kilometers of water rushes past you in such a river - the contents of a gigantic tank a kilometer wide, a kilometer deep and 6 kilometers long. But a river with a living section of 126 sq. meters - not so great: it can have, say, 6 meters deep and 21 meters wide.

Let us add that the amount of water flowing every second through the cross-section of a river is called the "discharge" of water in this river.