If the proposed gaging station is to be established for purely hydrologic purposes, unconnected with the design or operation of a project, the general location for the gage will be the stretch of channel between two large tributary or confluent streams. The same consideration will apply in the sense that the gage should be far enough downstream from the upper tributary so that flow is fairly uniformly established across the entire width of stream, and far enough upstream from the lower stream confluence to avoid variable backwater effect. Those limits often provide a reach of channel of several miles whose hydraulic features must be considered in selecting a specific site for the gage installation. The total flow is confined to one channel at all stages, and no flow bypasses the site as subsurface flow. The streambed is not subject to scour and fill and is free of aquatic growth.
Banks are permanent, high enough to contain floods, and are free of brush.
A pool is present upstream from the control at extremely low stages to ensure a recording of stage at extremely low flow, and to avoid high velocities at the streamward end of gaging-station intakes during periods of high flow. A satisfactory reach for measuring discharge at all stages is available within reasonable proximity of the gage site. The site is readily accessible for ease in installation and operation of the gaging station. Rarely will an ideal site be found for a gaging station and judgment must be exercised in choosing between adequate sites, each of which has some shortcomings. Often, too, adverse conditions exist at all possible sites for installing a needed gaging station, and a poor site must be accepted.