The basic piece of data obtained at a station is the stage, which is the height of the eater surface above a reference elevation. If the stage of the streambed is known and is subtracted from the water-surface stage, then the result is the depth of the water in the stream. Although stage of a stream is useful in itself in planning uses of flood plains, most users of streamflow data need to know the discharge of the stream. Discharge is defined as the volume of flow passing a specified point in a given interval of time and includes the volume of the water and any sediment or other solids that may be dissolved or mixed with the water. The units of discharge usually are measured in cubic feet per second. Discharge is derived from the stage data through the use of a relation between stage and discharge. The stage- discharge relation for a specific stream location is defined from periodic discharge measurements made at known stages. Measuring Discharge The most practical method of measuring the discharge of a stream is through the velocity- area method. This method requires the physical measurement of the cross-sectional area and the velocity of the flowing water. Discharge is determined as the product of the area times the velocity. Velocity is measured by using a current meter. The meter consists of a propeller that is rotated by the action of flowing water. The rotation depends on the velocity of the water passing by the propeller. With each complete rotation, an electrical circuit is completed and recorded in some fashion. Given the number of revolutions in a given time interval, velocity can be determined for the location of the current meter. Measuring the average velocity of an entire cross section is impractical, so the method uses an incremental method. The width of the stream is divided into a number of increments; the size of the increments depends on the depth and velocity of the steam. The purpose is to divide the section into about 25 increments with approximately equal discharges. For each incremental width, the meter is placed at a depth where average velocity is expected to occur. That depth has been determined to be about 0.6 of the distance from the water surface to the streambed when depths are shallow. When depths are large, the average velocity is best represented by averaging velocity readings at 0.2 and 0.8 of the distance from the water surface to the streambed. The product of the width, depth, and velocity of the section is the discharge of the river. |
 Looking upstream on the McKenzie River just east of Walterville,OR Photographer, Ryan Beck  McKenzie River just east of Walterville, OR Photographer, Ryan Beck  Looking upstream on the McKenzie River just west of Vida, OR Photographer, Ryan Beck |
