Question

Roughly what line of longitude was the westernmost boundary of the United States in 1790?

Answer 1

Answer:

It is not an easy one to evaluate since the Mississippi river runs across several longitude lines. How about 93° W ?

Explanation:

The history of the Western frontier of the United States is a complex one. For one thing, until Magellan's voyage, no one new that there was the physical limit of an Ocean on the other side. 
Cavelier de La Salle in 1692 claimed all of the Western territory draining into the Mississippi for the King of France; such a vast territory was obviously impossible to exploit and protect. 
France lost most of it with the Paris treaty of 1763 (that ended the 7 Year War) when it ceded all her American possessions (with some small exceptions) to England and Spain.

The small exceptions were in fact significant since the boundaries were again modified in 1783 so that, in 1790, the United States had their western frontier again at the Mississippi, yet not with France but with Spain. 
The story goes on for more than 50 years of negotiations and all out wars, notably not with Spain anymore but with Mexico.

Answer 2

Answer:

As the western frontier of the United States in 1790 was located at the Mississippi River, the line of longitude that was the westernmost boundary of the nation was 93º W meridian.

The Mississippi River springs 450 feet above sea level in Lake Itasca in northern Minnesota. Mississippi flows into the Gulf of Mexico approximatly 160 km below New Orleans, Louisiana. The Mississippi River is the second longest river in the United States; the longest is the Missouri River, which flows into the Mississippi River. The total length of the two rivers is approx. 6,270 km, and together they form the largest river system in the United States.