• We had said earlier that cables may be stretched across the river and river width divided into small compartments by fixing tags to the cable.
  • But in case of very large and wide rivers it is not possible to stretch cables across the river.
  • In that case pivot method is used.
  • It consists of a long high pole installed at one bank of the river.
  • A long wooden plank is fixed to this pole at some height.
  • The plank consists of a number of equidistant holes.
  • P is the point of observation.
  • The last hole B, on the plank is made in such a way that the observer from point P when views through hole B7 strikes point B7 at the opposite bank of the river.
  • B7 is the last point where river water at H.F.L. is touching the opposite bank. See Fig. 10.10.

Fig. 10.10. Dividing big river into small parts.

  • Now by observing from P through points B1, B2, B3……B7, corresponding points A1, A2,A3, in the river can be fixed.
  • The velocity of flow for each compartment is then found out with the help of a current meter.
  • Depths of water at the centre of each compartment are found out by going in boat.
  • The discharge of the river is finally found out by adding together the discharges flowing in all the compartments separately.