Complex sentence: Its contour was defined by the headwaters that formed the watershed along the Appalachia--all land with rivers that flowed into the Atlantic was designated for the colonial entities, while all the land with rivers that flowed into the Mississippi was reserved for the native Indian population.
Split into simpler sentences: Its contour was defined by the headwaters that formed the watershed along the Appalachia. All land with rivers that flowed into the Atlantic was designated for the colonial entities, while all the land with rivers that flowed into the Mississippi was reserved for the native Indian population.