Hostilities between a riverfront community's two racial subgroups were dramatically reduced when the threat that the river would flood its banks required them to work together to save their town. This BEST illustrates the impact of goals.

super-ordinate goals

Explanation:

Super-ordinate goals are those goals, which are accomplished using the idea and interests of two or more social groups, which cooperate each other. The super-ordinate goals are likely to promote positive interdependence in which two groups may work together and attain an identity of larger group with more significant features of having the collaboration of features of all the groups.

According to the given situation the two racial groups came together to save their town, even on the cost of loosing their racial identity and merging with the other group to seek the common goal. Thus this is an example of seeking an super-ordinate goals.