ebb and flow

A decline and increase, constant fluctuations. For example, He was fascinated by the ebb and flow of the Church's influence over the centuries. This expression alludes to the inward and outward movement of ocean tides. [Late 1500s]

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.