Airy’s hydrological law states that the mass of objects drifted by the stream is directly proportional to the sixth power of the water velocity. Airy’s law is easy to derive with the use of secondary school physics. In our article, the law is derived from the law of conservation of energy. The result explains the destructiveness of brooks and rivers in flood. At the same time, it presents an interesting example of a power function with an exponent greater than two which rarely occurs in secondary school physics.