The possibility of applying one-dimensional hydrodynamic equations of a nonstationary flow averaged over the cross-section of the channel during mathematical modeling of long waves-abruptly changing the movement of the water flow in channels is substantiated in the article. The characteristic dimensions of the length are much larger than the depth of the flow-the theory of shallow water. With this averaging, it is necessary to apply some hypotheses, the most important of which is the hypothesis of the distribution of pressure over the depth of the flow