Why were River Valley Civilizations able to become more advanced than Neolithic and Paleolithic Eras?

Which place is most likely depicted in aaron douglass's painting, song of the towers?

Review the territory. all land from the bighorn mountains down to south dakota territory. what happened to this territory as a result of the treaty of fort laramie in 1868? it was confiscated by the us government, as a consequence for the battle of the little bighorn. it was given to the us government by the lakota sioux, who got reservation lands in exchange. it was gifted by the us government to the lakota sioux, who viewed it as a sacred place. it was allocated for lakota sioux to settle, but the us government maintained mining rights.

Why was the land in the middle colonies attractive to immigrants