Human-built structures can either increase or decrease hydrologic connectivity of freshwater systems, rates of water movement, and transport and movement of organisms, materials, and heat. Construction of dams and diversions and modification of watersheds have greatly altered the natural flow regimes in streams and rivers (see, e.g., Fig. 3). Many existing flow regimes, particularly in large rivers, reflect human demands for water rather than natural cycles (Naiman et al., 1995). Two of the most dramatic changes to rivers in the twentieth century have taken place with regard to (1) water quality decline from return flows of agricultural and municipal waste water (L'vovich and White, 1990) and (2) large-scale diversions of water from one watershed to another. By 1990, the agricultural use of water worldwide was almost double that of all other uses combined, and canals in the former Soviet Union alone were estimated to be diverting more than 60 km3 of water annually. In the United States diversions such as that from the Upper Colorado to Missouri and Rio Grande Basins and, more dramatically, from the Lower Basin to southern California have had significant impacts on the basins of origin and on streamflow into the Colorado River delta in Mexico.
At the global level, there has been a tripling of water use since 1950 (Postel, 1997). The number of large dams increased sevenfold to about 39,000, with reservoir capacity at about 9% of global annual river runoff. Until the 1930s, dam and reservoir designers were concerned primarily with single-purpose economic benefits (e.g., either transportation or irrigation). Since then, dams have been designed or altered to meet multiple-purpose criteria including flood control, hydropower, fishing, and recreation. Adverse social and environmental impacts include displacement, disease, siltation, scouring, reduced length of wild rivers, interference with migration and life cycles of aquatic species, introduction of exotic species, eutrophication and anoxia, and losses through evaporation and seepage.
Soil and vegetation act as intermediaries between precipitation and streamflow. Changes in landscapes brought about by urbanization, agriculture, forestry, industrialization, channelization, and construction of transportation corridors alter terrestrial and aquatic components of watersheds. Such alterations result in changes of flow (volume and timing) of water, sediments, nutrients and organisms in river channels, lake basins, wetlands, and groundwater. The rates, processes, and consequences of these changes are not well documented for most rivers. While the data on large-scale irrigated lands are for the most part reliable and comprehensive, no such detail exists for changes in drained wetlands and in low-lying grasslands.
In the following section, three cases (the Nile, the Colorado, and the Plata Basins) are chosen to illustrate problems, opportunities, and challenges in the sharing of transboundary streamflow under variable climate conditions and evolving management arrangements. By the late-1960s there were only 25 major rivers in the world with streamflow records extending back for at least 60 years (NAS, 1968). This discussion is limited to the reliable period of record for each basin considered.
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