The red dotted lines represent the present course of the Mississippi River. The small darker yellow block represents the village of Kaskaskia. The Kaskaskia River is a tributary to the Mississippi.
Kaskaskia, the first capitol of the State of Illinois, was destroyed by Mother Nature in the form of the Mississippi River:
Forsaken by the legislature, Kaskaskia was not forsaken by the rivers between which its founders had located it. Gradually these encroached upon the site. Finally floods threatened to make Kaskaskia an island, the Mississippi reaching across the neck above the town to the Kaskaskia (or Okaw, as the river came to be called, because of the French way of saying that they were going aux Kan, to Kaskaskia). [Source]
Here's a New York Times article about present day Kaskaskia, Illinois.