ebb and flow

Short definition from Wiktionary

Noun

1. 1
   The flowing out and in of the tide.
2. 2
   A large flowing out and in, or any waxing and waning of fortunes or activity.

Verb

1. 1
   To alternately ebb and flow, as: To recede and advance.
2. 2
   To alternately ebb and flow, as: To wax and wane.