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17 Reservoir and Tailwater Levels For most dams, it is important to know the water level in the reservoir and downstream pool or channel. This information can help correlate other monitoring data for a better understanding of the response of the structure at varying pool levels as well as provide useful information for documenting typical operating conditions for each dam. Typical instruments and tools used to determine reservoir levels include staff gauges, slope stakes, or a tape measure. A graduated staff gauge can provide accurate and repeatable water level measurements for reservoir pools, tailwater, and within weirs or flumes. They can be permanently mounted on any flat surface and bolted directly to a structure. The gauge should be made with indelible graduations and markings, so it is resistant to sun bleaching, rusting or other forms of deterioration. Staff gauges can be mounted on a vertical surface such as a pier or post or on an inclined surface such as the embankment slope. The gauge should be carefully surveyed to accurately mark the elevation graduations. A typical staff gauge is shown in Photograph 1. Photo 1: Staff Gauge Slope stakes are another common method of measuring reservoir level. These consist of stakes that are installed in a line along a slope with a consistent grade and can be used to interpolate reservoir level. Typically, the slope stakes cannot be installed on the embankment, as wave protection is often present. The water level measurement is recorded relative to the known position of the stakes and using a correlation table to interpret the reading. If the potential for movement is detected or suspected, regular resurvey of the stakes is required to maintain an accurate water level reading. An example of a typical slope stake system table is shown in Figure 1. Figure 1: Slope Elevation Storage Table The third and obviously simplest means of measuring reservoir and tailwater level, is by dropping a weighted tape measure from a reference mark with a known elevation. Similar to the consideration for locating a permanent staff gage, the measurement location should consider effects of drawdown caused by entrance flow into a gate, structure, or weir. For this reason measurements should not be performed into a drop structure or near a spillway with an open gate as the head losses at the inlet can cause significant error. Seepage Flow Measurements Weirs are frequently chosen to measure seepage rates, monitor turbidity, and monitor sediment transport. Common weir shapes are square, trapezoidal, and V- notch. The appropriate shape and size of a weir depends mainly on the volume of flow to be measured. Some weirs are more capable and accurate at