Plaque at Fort Massac explaining Clark's bold move to capture the Illinois settlements from the British.
A narrative from General Clark regarding his preparations:
As I (Clark) intended to leave the Ohio at Fort Massac, three leagues below the Tennessee, I landed on a small island in the mouth of that river in order to prepare for the march.
They [a party of hunters] had been but lately from Kaskaskia and were able to give us all the intelligence we wished. ...that the militia was kept in good order--and spies on the Mississippi and that all hunters, both Indians and others, were ordered to keep a good lookout for the Rebels-- [Source]