What natural feature form the boundary between british and spanish territory in 1763?

At the end of the Seven Years War, Great Britain and France were having many problems to achieve peace together, much of this, because Spain did not want to recognize the conquest of Spanish lands by the British Empire. France, (in a memorable act, which allowed the Paris Treaty to work) resigned all of its North American lands, giving the territories east of the Mississippi River to England, and the French territory in the west of the Mississippi, to the Spanish Crown. Then it was forbidden for the settlers to inhabit the regions west of the Mississippi River, although in practice this was not very obeyed.

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