Relatively simple relations between stage and discharge are presented in Chapter 4 and normally most streamflow records are based on this principle. By simply recording stage, therefore, and developing the stage-discharge relation, a continuous record of discharge can be obtained. Several factors, however, can cause scatter of discharge observations about the stage-discharge relation at some stations. Backwater is one of these factors whereby the velocity is retarded so that a higher stage is necessary to maintain a given discharge than would be necessary if the backwater were not present. Backwater is caused by constrictions such as narrow reaches of a stream channel or artificial structures downstream such as dams or bridges or downstream tributaries. All of these factors can increase or decrease the energy gradient for a given discharge and cause variable backwater conditions. If, however, the backwater caused by a fixed obstruction is always constant at any given stage, the discharge rating is a function of stage only. Constant backwater, as caused by section controls for example, will not adversely affect the simple stage-discharge relation. The presence of variable backwater, on the other hand, does not permit the use of simple stage-discharge relations for the accurate determination of discharge.