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. . . GENERAL FIELD AND OFFICE PROCEDURES FOR INDIRECT DISCHARGE MEASUREMENTS 11 High-Water Marks High-water marks are the evidence of the highest stage reached by the flood. There are many different types of marks, and the proper identification of them is that part of the work which requires the most experience~ For this reason the most experienced man in the field party should act as rodman and locate the high- water marks. High-water marks tend to disappear rapidly after the flood peak, particularly in humid regions where rain is frequent. For this reason start the work of surveying as soon as possible after the peak. If enough field parties are not available, locating the high-water marks at the desired sites before making the complete sur- veys may be worth while. Identify the marks by means of stakes, cloth tags, paint, paint sticks, nails, or crayon. Make field sketches showing the approximate locations of these marks for the benefit of the survey party. Be- cause it is difficult to stake out sufficient marks in this manner, the field party should attempt to survey all additional marks necessary to define the profiles well. Locate many high-water marks on both banks through the reach and for a considerable dis- tance above and beI.ow, in order to aid in inter- pretation of the profiles. The slope as deter- mined by these marks is probably very nearly parallel to that of the water surface prevailing at the time of the erest stage. Select high-water marks on surfaces parallel to the line of flow so that they represent the water surface and not the energy grade line of the stream. However, there may be times when ponded elevations representing the total energy head are desirable, sUch as in dllJll, bridge, or culvert computations. High-water marks on the ground where wave action and runup from surge are at a minimum are generally prefer- able to those.in bushes and trees as defined by debris which has been carried up, by wave ac- tion or the velocity of the current, to a level above the prevailing water surface. Even along the banks, the upstream sides of projections into the stream will tend to show higher marks be- cause of runup or velocity-head recovery, where- as embayments may have lower elevations. Under such conditions, obtaining of more close- ly spaced marks is advisable, to show the shape ( f the water's edge and to aid in interpretation. Surge The effect of surge on the high-water marks found on the banks is an important point to be considered. Observation and photographs of floodflow in natural channels show that, al- though there may be extensive wave action in the middle of a fast-flowing stream, at the sides, velocities are low and the water surface quiet. Although there undoubtedly is some effect from surge, the high-water marks should be used as found and no adjustments attempted for surge. Any adjustments would necessarily be subjec- tive and would lead to questionable results. This is justified by the fact that roughness values as determined from "verification" studies are determined from high-water marks on the banks, and any effect of surge is contained in the n values determined; if similar n values are applied for like conditions using the sllJIle meth- ods, then the effect of surge would be minimized. Identification and rating of high-water marks In the field notes, describe the type of high- water mark, such as "drift on bank," "wash line," "drift on upstrellJll side of tree." Also rate each mark as "excellent," "good," "fair," or "poor." All this information will help in interpreting the high-water profile. Types of high-water marks Many kinds of material which float, chiefly vegetative, are left stranded at the high-water line (and at lower eleVations) when the water subsides. The finer material produces more definite and better marks and is apt to repre- . sent the highest elevation that the water at- tained than would some scattered clumps of large drift. Leaves or cornstalks are apt to become waterlogged, and at the very edge in slow velocities they will not rise with a slight rise of the water surface. In this manner, a