The significance of rivers to the deserts they cross Low and variable precipitation and high evaporation are not conducive to generating perennial rivers in deserts. Rather, the source of perennial rivers in deserts is from upland, nondesert areas. The headwaters of the Nile which crosses the eastern Sahara Desert, the Gariep river of the Kalahari, the Tigris and Euphrates in the Syrian Desert, the Indus of the Thar Desert, and other desert rivers, are far from the desert edge, up in humid highlands (Figure 3.3). At a certain point of their course these rivers cross a desert to eventually discharge into the sea (like the Colorado and the Tigris rivers), or into a desert lake (like the Amu Darya and Syr Darya rivers). While flowing through the desert, the riparian banks function as elongated, winding oases, a non-desert island within the desert. Cross-desert rivers provide fish, plants, and animals that comprise the base of livelihoods for people concentrating along their courses (for example, 97 per cent of the Egyptian population lives along the Nile). River water is often diverted to irrigate extensive agriculture and to support pasture, and it carries sediment that fertilizes or generates soil. Desert rivers have cultural and spiritual significance for desert people, nurturing ancient civilizations.

Climate away from deserts modulates the flow of desert rivers

The flow of perennial rivers in deserts totally depends upon the upland headwaters, nondesert wetlands and lakes, and on their pre-desert course. For example, the Egyptian population, most of which lives under climatically hyperarid conditions with far less than 100 mm of annual rainfall, totally depends on a rainfall regime of more than 1 600 mm that precipitates some 3 000-4 000 km away. The Mesopotamian rivers cross a desert of less than 200 mm of annual rainfall, but depend on precipitation of more than 1 000 mm, much of which is snowfall maintaining seasonal peaks of desert flow, depending on the timing and rate of snowmelt 900 km away. Desert flows of rivers such as the Nile and the Colorado are extremely sensitive to variations in rainfall interception within their small headwater catchments outside the desert (Degens and others 1990).

Water diversion away from deserts reduces cross-desert flow

The amount and quality of river water reaching deserts often depend more on people than on nature. The river flows in the Egyptian and Iraqi deserts depend on the management of their headwaters in Ethiopia and Turkey, respectively. In countries like Pakistan, with both desert and nondesert areas, desert people depend on the way water is managed in the non-desert sections of the Indus river, which has a denser population and a greater impact on the river-flow. Damming that diverts water for irrigation and generation of electric power in the upland non-desert courses of rivers reduces the amount of water reaching the desert. Conflicts between highland and lowland water users are becoming common globally, but they are more apparent in desert-crossing rivers, because most deserts are in the lower river reaches, where water is more precious and population growth is often high; most dams in the Tigris-Euphrates basin are in the Turkish, non-desert course of the two rivers, while few lie in the desert Syrian and Iraqi sections, where flow has been much reduced. Yet, such conflicts have not escalated into armed confrontations; rather they often motivate cooperation among the riparian countries.

Water quality of cross-desert rivers depends on humans away from deserts

Deforestation and overgrazing at the source enriches river water with minerals leached or transported with eroded soils (hanspeter and others 1998). Residual pesticides and fertilizers, irrigation-generated salinity, and industrial and organic wastes are also drained into river flows. Much of the organic pollution dumped in the nondesert section oxidizes before it reaches the desert; but other pollutants do reach the desert flow, which lacks incoming tributaries to dilute pollutants or spring floods to wash them away. Since the damming of most desert rivers, chiefly off-desert, reduces sediment load in the river flow and hence nutrients, fisheries and wildlife have been impacted. For example, the demise of the Mesopotamian Marshlands is in part due to such reduced flows (Richardson and others, 2005).

Effects of global climate change in the nondesert source areas

Global climate change will affect the remote sources of desert rivers more than deserts themselves. Those rivers with headwaters in snow-capped mountains that depend almost exclusively on snowmelt from glaciers - such as on both sides of the Andean range (Atacama and Monte) - will be the most affected, because accelerated melting of most glaciers is predicted with high confidence (IPCC 2001). The himalayan glaciers, surrounded by relatively dry areas and sustained due to the high elevations where water is stored as ice, are highly vulnerable; melt will first generate increased flow and the eventual loss of the glaciers would reduce desert flow dramatically. With most of Pakistan's inhabitants dependent upon an irrigation network now fed by the Indus river, the effects of climate change in its basin could be devastating (Nianthi and husain 2004). The Nile catchment is located at the boundary between climatic zones, hence minor shifts of this boundary could have dramatic consequences on its desert flow, as already happened when the last glacial age terminated (Conway 2005). however, rainfall behaviour in the Nile catchment under future climate change scenarios is still unclear.