Engineering works and agricultural activity which change the relationship between rainfall and river flow lead to modifications of river channels with attendant erosion and deposition problems. In the Swiss Jura Lakes area, the natural flooding of the River Aare became such an acute problem by the mid-nineteenth century that extensive engineering works were carried out to alleviate flooding. The land thus reclaimed became a valuable agricultural asset, but the fall of the water table following removal of the annual flood risk, led to a fall in the level of the land as peat was changed into humus. Renewed flooding occurred. The natural readjustment following the first series of flood alleviation works reproduced the original problem and a second series of engineering works has had to be undertaken to remedy the situation. On the Belgian coast, harbour construction and the spread of buildings over the sand dunes have resulted in severe beach erosion in the eastern seaside resorts. Extensive engineering works have had to be undertaken to restore the beach. These examples illustrate how man's challenges to nature are often recurrent phenomena, and how the alteration of one aspect of the physical environment may lead to a succession of readjustments. Each phase of engineering activity may be considered a break in natural equilibrium, and each period of natural erosion or deposition a trend towards a new equilibrium.