This module is part of a series designed to be used by life science students for instruction in the application of physical theory to ecosystem operation. Most modules contain computer programs which are built around a particular application of a physical process. This module deals specifically with concepts that are basic to fluid flow and streams. The objective is to be able to describe the types of flow, as well as the velocity and discharge of particular streams. These characteristics are used in comparisons of streams, stream classification, flood control, control of sedimentation, and gauging of available water for downstream use. A method used for determining discharge and velocity for a section of a stream, the concept of continuity, is presented. This concept also forms the basis for understanding the relationships of aquatic organisms to running water. Basic concepts included in this module are developed and used in later modules. (Author/CS)