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benckmark. This procedure is called recheck or level loop closure. To determine the allowable error of closure, equation 20 may be used: Maximum Error of Closure = 0.05 4M_ (20) where M = length of level loop in miles (Bouchard and Moffitt, 1965). WATER SURFACE ELEVATION Measurement of the water surface elevation requires a special pro- cedure for the rodman, and. good coordination between rodmanr and level- man. The rodman should dip the rod to the water-surface and lift it again as soon as the rod forms a meniscus with the water. The levelman should read the high rod reading repeated the most often. The rodman can help by loudly indicating "touch" when the rod touches the water surface. This technique has proven much more effective and accurate than standing the rod at water's edge. The water surface elevation should be measured on both sides of the river at each transect. If the elevations from both sides of a transect are not equal, an average should be calculated for the transect. One should expect water surface elevations to be unequal at the inside and outside edges of meanders. The degree of inequality is a function of the radius of curvature of the meander and the velocity of the water. Water surface elevations should be measured to the nearest 0.01 foot or 0.5 cm. It is- a good idea to survey water surface elevations at the start and end of the day to determine the amount of change in stage which has occurred during data collection. A temporary staff gage placed at water edge at the beginning of the day may be read to deter- mine change in stage. BED ELEVATIONS A profile of each cross section should be measured from headstake to headstake. The profile is defined by a series of elevations and horizontal measurements, starting at some predefined zero point.- (NOTE: when measuring horizontal distances across a transect, do not attach tag lines or chains to the headstake. This practice may change the headstake elevation. The headstake is normally used as the zero , point, with the tagline anchor behind the headstake.) Horizontal distances should be measured to the nearest foot (30 cm). Elevations should be measured to the nearest 0.1 foot (5 cm if in metric). Ouring the profiling, the rodman needs only to make measurements where there is an obvious break in slope of the bed, or where a change in substrate material is encountered. The substrate should be described at each measurement point. Substrate descriptors are based on the size classes listed in the modified Wentworth scale shown as Table 7. Estimated percentages of each substrate class should be included in the substrate descriptions. Bed elevations may be determined by sounding if use of a stadia rod is impractical. (03