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:

When there is a bigger problem or issue faced by two opposite sides of a group or individual have personal differences and this bigger problems was compelling enough to make the two group set their differences aside to achieve something significant, something that none of the group or individual can privately claim, and its closer to nature, then this illustrates superordinate goals. From the example, the flood would wash their town and probably cause damage to life and properties without segregating one group and the other. But the threats of the hostilities reduced because they are faced with a greater evil, which has to do with nature, and hence illustrates the impact of superordinate goals.

i honestly speaking this is too long