The river that makes up the Westerly Board of Illinois

"Mississippi river" makes up the Westerly Board of Illinois.

Explanation:

The second-lengthiest river and Major River of the North American continent is "the Mississippi River" and placing as second position in immense drainage system to "Hudson Bay". The mid-western state bordering from east the Indiana while from the west Mississippi River is known as "Illinois State" or "the Prairie State".

The Illinois connects the Mississippi near Grafton nearly 40 km northwest of downtown St. Louis and about 32 km upstream to the River Missouri and the Mississippi confluence. It have great importance of wetlands, farmland, forests and rolling hills.