The Northern River Reversal, proposed back in the 1930's and underwent serious consideration in the 60's through to the 80's, was a massive Soviet plan to divert the flow of the Pechora, Tobol, Ishim, Irtysh, and Ob rivers. Instead of flowing "uselessly" into the Arctic Ocean, the idea was to divert them south toward Central Asia for irrigation. About 120 institutes and agencies participated in the impact study coordinated by the Academy of Sciences in over a dozen conferences. The consensus was, if carried out, this project would produce enough food to feed 200 million people and the resulting fallout (parts of the canals were going to be dug with nukes) would be "negligible."
In addition, some expansions on the plan included bringing water south to refill and expand both the Aral and Caspian Seas. In particular, the Davydov Plan also called for the flooding of the West Siberian Plain to create a new inland sea. Naturally, all of this brought with it environmental concerns. Some worried that diverting the rivers would deprive the Arctic of fresh water, leading to it becoming more saline and lowering its freezing point. This would cause the sea ice would begin melting, possibly starting a warming trend. Others worried that the opposite might happen and stopping the flow of warmer fresh water would lead to the polar ice expanding.
For this and many other reasons, the plan was officially abandoned in 1986, although calls for its resumption have occasionally cropped up over the years.
*Credit to KitFisto1997 for
the Davydov Patch.