1. the movement of a stream of water or other mobile substances from place to place 2. the movement of water, and the moving water itself; the volume of water passing a given point per unit of time —see discharge —note 1. base flow is the portion of stream discharge that is derived from natural storage —note 2. subsurface flow is that portion of the groundwater moving horizontally through and below the stream bed —note 3. surface flow is that part of water in a channel flowing on the surface of the substrate —see floodThis definition last updated 10/24/2008