This method states that it is possible to define an unique function between discharge and volume if the channel is represented by a cascade of n linear reservoirs with the length L (characteristic length). The unsteady flow in those sections can be approximated by a quasi-stationary flow.

Figure b expresses that the discharges of the cross section *r* are assigned to the water levels in the cross section *m*, so that the discharges of the stationary discharge curve at the cross section *m* can be assigned to the corresponding unstationary ones at the cross section *r*.

A straight line is assumed for QA_{i} associated with a volume S_{i}, which corresponds to profile *r*. For unstationary flow the line is AB and for stationary flow CD. And the volumes over the areas AOC and ODB are represented, respectively.

In addition, *h* represents the water depth, *J*_{w} the water level downward gradient, J_{s} the river bed gradient and J_{steady state} the gradient of the straight lines at stationary conditions.