The hostilities between two racial subgroups of a riverfront community were dramatically reduced when the threat of the river flooding its banks required that they work together to save their town. this best illustrates the impact

Superordinate goals.

Explanation:

A superordinate goal is a goal so big that is enough to overlook personal or group differences in order to achieve it. It is, also, a goal rather big for anyone of this individuals to reach it alone, therefore, having the need to overlook said differences.

answer; primary source information;

the answer to your question is,

old stone age.

-mabel < 3