• In the case of canals there is no difficulty in measuring the cross-sectional area.
  • Canals are always of some regular shape whose area can be found out by any geometrical formula.
  • But in case of natural streams and rivers, it is not very easy to determine the area of cross-section as their sections are not regular.
  • In this case area of cross-section of the river can be determined as follows.
  • A river is flowing from left to right.
  • Select AB, CD and EF sections of the river and pull cables at all these sections across the river.
  • The distance between sections AB, CD, and CD, EF may be anything, say 100 m.
  • Wooden battens are attached to all the cables and sections of river at the cable points are

Fig. 10.1. River length.


  • divided into equal parts.
  • In our case we have divided each cable into seven equal parts as shown in Fig.10.2.
  • Thus the river width gets divided into seven equal parts longitudinally.
  • Now take a staff or any other graduated rod and measure the depth of the river at the centre points of the each of the seven parts.
  • Compute the areas of all seven compartments by multiplying the width of the water of each compartment by the depth of the water
  • The centre of each corresponding compartment. The sum of all these areas is the cross-sectional area (A) of the flowing water of the river.

Fig 10.2. Cross-section of river.

  • If it is not possible to measure the depth of water with staff or graduated rod, it can be measured by using measures like Kelvin’s tube, Echo sounder etc.