Vast river diversion projects could be under construction by the end of the century in the USSR and ultimately reach several hundred cubic kilometers annually. The most seriously considered would take water from major Arctic draining rivers (Ob, Yenisey, Pechora, and Northern Dvina) and transfer it to western and southern regions of need (Figures 1 and 2). These grandiose undertakings would result in significant hydrologic, climatic, cryogenic, biotic, pedologic, and geomorphic changes. Most of these would be of local or regional scale and confined to the Soviet Union [Micklin, 1979]. However, some macroscale alterations with international implications are possible. Of these, modifications in the sea ice cover of the Arctic Ocean that are induced by diminution of freshwater discharge are the most serious. Sea ice plays a key role in the Arctic mass and energy budgets, diminishing water vapor, heat, and momentum exchange between the ocean and atmosphere and affecting pressure and circulation patterns over the entire Northern Hemisphere [Budyko, 1974; Lamb, 1978; Flohn, 1979]. Significant alterations in its extent, thickness, concentration, duration, and distribution would have important consequences not only for Arctic but Northern Hemisphere climate.