What was the purpose for the land rush? encourage western settlement take away tribal lands pay for military posts build a railroad

The purpose for the land rush was to encourage western settlement.

Unfortunately, majority of the land that white people settled on were Indian territory. These lands were initially allotted for Native Americans to reside in when they were driven away from their original territories to give way for white men settlements. 

A lot of white men coveted said lands after they saw how prosperous and bountiful the land was under the management of the Indian people. They did not think of the hardships and hard work the Indian people did in order to make the land fruitful. All they saw was the result of the hard labor and they wanted it for themselves, to the detriment of the Indian people. 

These islands/chinampas then provided rich soil for crops with easy access to water. i hope this is correct, im so sorry if its not !