Rapid economic development and population growth exert great pressure on hydrological, erosion/sedimentation and biogeochemical cycles. The cycles are closely related, with the hydrological cycle having the leading role. In the first instance only water aspects of the impact of human activity are discussed: namely, the roles of water withdrawals, reservoirs and transfers as compared with the natural components of the water balance; impacts of irrigation and agriculture in general; the problem of closed lakes; water management problems stemming from the current water resources situation. This is followed by a discussion on impacts of human activity on water and erosion/sedimentation with particular emphasis on consequences of agriculture. Then there is a discussion on impacts of economic activity on water and hydrochemistry: changes of chemical transport by rivers; perturbations of the nitrogen and phosphorus cycles. Mankind faces the need for an active control of water resources and hydrologic regime on a large scale. The control inevitably requires an interdisciplinary approach based on understanding the wide scope of hydrological processes. For hydrology there is no other way but to accommodate itself to this interdisciplinary setting.