It is separated from the sea by a sandy Kuyalnytsky-Khadzhibey peresyp sandbar up to 3 km wide.

The lowest point of Ukraine is located near the Kuialnytsia estuary: 5 meters below sea level.

Once on the place of Kuialnytsia estuary was located the mouth of the river Big Kuialnyk.

Separation from the sea took place in about the fourteenth century, much later than the branch located near the Khadzhybeian estuary.

This can be judged at least by the fact that Kuialnyk's overflow is three times narrower than that of Khadzhybey.