With the most recent census

The first state capital of Illinois, and old French settlement, dropped from first to second smallest municipality in the state.

Kaskaskia is also the only city in Illinois on the far side of the Mississippi. A book I recently read about the history of steam engines mentioned that after steamboats became popular on the river, all the trees along the river were cut for wood. Without them, the banks eroded and the river became shallower and wider, enabling larger floods. In some spots the effects were so large the river eventually found a new course, as with this town.