initial focus was on determining the minimum flow necessary to preserve an individual species, such as trout, in a river. Environmental flows evolved from this concept of "minimum flows" and, later, "instream flows", which emphasized the need to keep water within waterways.

By the 1990s, scientists came to realize that the biological and social systems supported by rivers are too complicated to be summarized by a single minimum flow requirement. Since the 1990s, restoring and maintaining more comprehensive