Answer:
1. Domestication of maize, and the subsequent widespread cultivation of it - this is perhaps the most important factor that led to the development of the Mississippian cultures. Practically all civilizations have followed this pattern: first they domesticate a few vegetable species, then, they settle along a major river, and finally, they build cities.
2. The establishment of trade networks - A reason why civilizations settle along major rivers is to facilitate trade. Rivers serve as trade networks. The Mississippian cultures traded with people in the Rocky Mountains, in Mexico, and the Eastern United States.
3. The construction of mounds - this came to be the most important architecture achievement of the Mississippian civilization. Atop of the mounds, houses and palaces were built, they most famous of these mound cities being Cahokia.