Which river did the Americans want access to?

Reports of the retrocession caused considerable unease in the United States. Since the late 1780s, Americans had been moving westward into the Ohio River and Tennessee River valleys, and these settlers were highly dependent on free access to the Mississippi River and the strategic port of New Orleans.

Explanation:

the answer to your question is a. the salt plains

in 1811, he led an expedition, known as the george c. sibley expedition, to improve relations with the pawnee and kansa tribes, and also to locate the rumored jefferson's salt mountain