Although the river and the hillside…do not resemble each other at first sight…one may fairly extend the river all over its basin and up to its very divides.
W. M. Davis (1899)
This definition emphasises for us the huge problems of information, knowledge and influence inherent in our ambition to coordinate the human exploitation of the river basin ecosystem and to make that exploitation sustainable.
The United Nations Conference on Environment and Development (UNCED or ‘Earth Summit’), held in 1992, made explicit to all nation states the need to make development sustainable; far from advocating a single operable definition of sustainability there was, within another ‘S’-word (subsidiarity), the inherent challenge to define sustainable processes appropriate to particular environments at particular scales. Subsidiarity implies decision making at the appropriate level in a hierarchy of bodies from the international community, through nation states to local government bodies, popular groups and the individual. The word ‘sustainable’ is hard to define; however, many agree that working towards a practical definition is an essential technical and political venture in all fields of environmental management at the close of the twentieth century. It is, however, an appropriate adjective for the type of management which Chapter 7 implicitly prescribes for the river basin system. Pezzey (1989) describes sixty-one versions of a definition and the political, philosophical and scientific doubts which attend the broad notion of sustainable development, first widely publicised by the World Conservation Strategy (International Union for the Conservation of Nature, 1980) but mainly boosted by the Brundtland Report (World Commission on Environment and Development, 1987) which defines it as: ‘development that meets the needs of the present without compromising the ability of future generations to meet their own needs’ (p. 43).
Pearce (1993) suggests that there are three main forms of sustainability: ultra weak, weak and strong, and that one may judge the stage to which