What central asian body of water devolved into a major environmental disaster in the late 20th century?

ARAL SEA.

Explanation:

Aral sea is a central Asian fresh water lake. I is the world's fourth largest lake and it form a centripetal drainage system as the two rivers Amu Dariya and Syr Dariya falls into it. But as per the images of NASA's Terra Satellite the lake is shrinking very fast as its volume has reduced to one tenth of it's actual volume. Some of the reasons behind the shrinking are:-

1. Construction of dams over the river which drain into the Aral sea.

2. Less rain.

3. Snow in the Pamir mountains.