The Soviets did a lot of damage to the hydrology of Central Asia, almost drying up the Aral sea, damming of the Kara Bogaz Gol from the Caspian, nearly drying it out as well, diverting rivers, etc. etc. Besides that, the Caspian Sea is 90 ft below sea-level.
What if the large-scale engineering was conducted to improve the hydrological cycle for the region instead of (unintentionally) damaging it. The Don-Volga Canal in this case was for more than just shipping traffic - the flow of the Don was diverted into the lower Volga and thence the Caspian in the '60's. The increased flow raises the level of the entire Caspian Sea to general sea-level, and the D/V canal now becomes a Dardenelles-like waterway connecting the Sea of Azov with the Caspian - and Volgograd the Constantinople-like city overlooking the vital connection.
Note the Aral Sea is also restored to its full level by its rivers
not being diverted.
The effect is a moderated climate in the Central Asia (large bodies of water make winters warmer and summers cooler for surrounding regions) with more rainfall on the Kirgiz Steppe (more Sea-surface evaopration leading to rainfall downwind to the East) leading to less-arid conditions. Lake Balkash also rises with increased rainfall in the area. This makes the area more arable (the reason the Aral was almost killed, but by other means.
What're the geo-poltical effects of this regional environemt transformation? The area might become the CCCP's bread-basket, it's connected to world trade through the Capian-Azov link, much more important to the Russians. More Russian immigration/ethnic displacement?
