The term 'BLOCK THE FLOW' can be summarized by two crossword solutions: DAM (3 letters) and DAMUP (5 letters). A DAM is a structure built across a river or other body of water to control the flow of water. It is usually constructed from concrete, earth, or other materials. A damup is an act of blocking the flow of water with a DAM. This can be done for various reasons, such as to create a reservoir for irrigation, to generate hydroelectric power, or to prevent flooding.