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Control of water, salt, and sand is correctly ensured in the old irrigation areas of the « traditional system », with secular knowledges and technics. With the great new irrigation projects, environmental impacts are much more important ; water management problems are very different and more difficult to solve. Valuation and correction overstay traditional knowledges and technics, and lead to depreciate « the self-sufficient system » for the benefit of the « technical system » which is controlled by other socio-spatial systems.

The development scheme of Gabcikovo-Nagymaros on the Hungarian and Czechoslovakian section of the Danube River consists of a reservoir (Hrusov), a by-passed section equipped for navigation and energy supply (Gabcikovo), and a downstream compensation dam (Nagymaros). This present development has been preceded by an embankment phase (15th-19th century) and a Girardon-type fluvial rectification (19th- 20th century), which stabilized the Danube Channel (on an alluvial fan built on a slope break at the exit of the alpine section) and improved navigational conditions. However, this former activity did not permit the movement of barges such that this section constituted a gap in a system that could be extended to the Rhine River system and possibly into Poland. In face of these international considerations, the Hungarian opinion remains sensitive to boundary questions and to the quality of the fluvial environment upstream of Budapest. Indeed, the fluvial system will experience an appreciable degradation induced by changes in the flow conditions, by the alteration of a very valuable alluvial forest, by the reduction of aquatic biomass (particularly fish) and a degradation of water quality.

Whether as a dam to limit the spread of the desert or as a dam to promote development, Diama has stirred up and still arouses heated controversy about its expediency. The carrying out of the O.M.V.S. programme — for the integrated development of the Senegal river drainage basin — does indeed raise the problem of the appropriateness of this techno-economic management model for a region weakened by economic and climatic crisis as are the countries of the Sahel associated with this project. Doubt must remain whetther subjecting the natural and human landscape to so brutal will not induce a dysfunction in the ecosystem.

The drought which has affected the Senegal valley since 1977 has led to the development of small irrigated fields fed by pumps and use for the cultivation of rice. The success of this system has in turn led to the setting up of large-scale irrigation schemes by both public and private sectors, to the extent that 35 000 ha are now under irrigation. This development lies behind the building of the Diama and Manantali dams, but competition between the various methods of organisation will no doubt work to the detriment of the small schemes.

After the reconstitution of the 18th century fluvial pattern, when Man experienced a limited direct control upon the river dynamics, this study shows the successive alterations of the Arve-Giffre confluence : the plan and long profile succession is surveyed and shows an acceleration of the stream degradation processes, mainly since 1950. A cartographic statement of the present fluvial dynamics is carried out. The human interventions on the tributary and the Arve River (embankment, dragging...) appears to be the main factor which explains the strong physionomical and functional alteration of the confluence. Some watershed perturbations and their incidence upon the fluvial dynamics are proposed.