By A.D. 800, as the Mayans declined in Mexico and Europe muddled through the Dark Ages, Cahokia was ripe for the emergence of a great civilization based on agriculture. The physical and cultural environments there offered the fruits of thousands of years of development to the emerging Mississippian people.

In A.D. 800, the climate was similar to that of today. Over the next 400 years, it gradually warmed and was quite moist. Those conditions produced longer growing seasons than in previous centuries.

Cahokia's geographic setting at the confluence of three major rivers and four ecozones was ideal. The meeting of the Mississippi, Missouri, and Illinois Rivers created an exceptionally fertile and expansive flood plain called the American Bottom. It stretched 70 miles along the Mississippi from present day Alton, Illinois, to Chester, Illinois, and was up to 12 miles wide from the river east to its bluffs. In the spring, when rains swelled the bottom land's streams and their myriad tributaries, water carrying rich silt from the riverbeds renewed the nutrients essential for consistent and wide-scale farming. This extensive network of waterways also gave the Mississippians access to distant areas where they hunted, traded, and learned through contact with other cultures.

The Mississippians found a wealth of natural resources in the ecozones that immediately surrounded Cahokia. The forested Ozark Mountains to the southwest offered important rocks and minerals like granite, sandstone, limestone, and especially chert for making tools. The Ozarks also abounded in white-tailed deer, the Mississippian's primary source of meat and skin for rawhide and clothing. The Prairie to the north and west was a seemingly endless expanse of tall grasses that were useful for building and furnishing homes and other structures. The Woodlands to the east of the American Bottom were rich in nuts and berries, in animal life--white-tailed deer, raccoon, turkey, squirrel, wolf, gray fox, black bear, possum, and bobcat- and in oakes, hickories, and other deciduous trees that provided excellent hardwoods for canoes, fires, tool handles, bows and arrows, and building construction. The Eastern Woodlands also were dotted with salt licks which yielded essential dietary salt for people and animals alike. And the Mississippi Valley itself gave these farmers not only its rich soil, but also its fish and other aquatic life; ducks, geese, and all the migrating waterfowl that traveled the Mississippi Flyway; game animals including deer, beaver, racoon, and otter; mulberries, persimmons, nuts, and other edible plants; and a wide variety of trees.