It joins the Mississippi just southeast of Chester, near Kaskaskia.

Because of its proximity to Kaskaskia — the capital of Illinois Territory and the first capital of the State of Illinois — Marys River was the site of early settlements leading into the interior of Illinois.

Marys River is approximately 41 miles (66 km) in length.

[1] The following cities and towns are in the Marys River basin: Parts of the following counties are in the Mary's River basin:

This article related to a river in Illinois is a stub.