Even in the most hostile climatic zones, the fertile alluvial soils of river valleys have offered unique refuges for a great number of human and animal populations. The Euphrates and Tigris, the Nile, the Indus etc. have hosted some of the world's most ancient and glorious civilizations and been home to valuable cultural heritage. When confronted with waves of immigrants superior in access to information and resources to the valley, riverine communities may be at a disadvantage. Movements to other areas and efforts to adapt to a new environment are only the visible part of the problem. They may be accompanied by serious social trauma and conflicts not often accounted for.
Large dams are also instruments of change in the livelihoods of riverine communities.
Gains and losses accompany these changes. In many cases, large dam projects, specifically irrigation dams, restructure food production, consumption and distribution patterns, for example, by bringing a shift from recession or rainfed to irrigated agriculture, introducing new crops, increasing or reducing the labor demand and altering agricultural cycles. New production technologies and an increased influence of commodity market prices may accompany such transformations. These changes could mean greater food insecurity or greater vulnerability for certain social groups, while bringing prosperity for other groups. It is essential to understand what the gains and losses are and how they are distributed.