Why is the northern Caspian Sea much shallower than the southern Caspian Sea?

The northern Caspian Sea is significantly shallower than the southern portion due to fundamental differences in their geologic structure and history.

The northern Caspian is a section of the northern Caspian tectonic depression, which is a vast downwarp in the ancient Russian Platform. This area features a continental-type crustal structure that has remained relatively stable and shallow, with an average depth of only 13 to 26 feet (4 to 8 metres). Additionally, the northern region receives the vast majority of the sea's river inflow—including from the Volga, Ural, and Terek rivers—which results in a significant accumulation of alluvial material that contributes to its shallow nature.123

In contrast, the southern Caspian rests on a very ancient suboceanic-type basalt crustal structure. This depression is much deeper, containing the sea's greatest depths, and is characterized by a flat plain that has been shaped by complex tectonic processes, including the Alpine mountain-building movements that formed the nearby Caucasus ranges.12