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12 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS .' mound of material, sometimes a foot or more in height, will form at the edge of the channel. Where this occurs, the elevation found by hold- ing rod on the top of the mound would be the proper high-water elevation if the material is consolidated; if the material is loose, the shore- ward toe would be the correct elevation. Much drift usually will be found on bushes or trees within the channel. Such marks are not generally as dependable as those on the banks. In swift water, varying amounts of pileup due to velocity will affect the marks at the upstream side of such objects. Marks at the downstream sides of large objects may be lower than normal. Brush in fast velocities often will be bent downstream by the flow, and drift will be caught on the upper limbs. When the veloci- ties slow down, the brush becomes erect once more, and the drift will appear to be at an ele- vation much higher than that of the actual water surface. In quiet water on overflow plains, the highest drift in brush or trees may be reliable. Often the SlIlall seeds of various plants will provide excellent high-water marks, remaining in the crevices of bark or in the cracks in fence posts or utility poles. The highest of such particles should be used. At times, seeds will adhere to smooth surfaces and encircle trees, poles, metal posts, or guy wires. When present, seeds are an excellent source of high-water data. In arid regions, or where sandy soil or steep- ness of banks prevent vegetative growth, the water surface may lap against bare banks. Soil will be washed away by the moving water and under some conditions will show "wash lines" which may be reliable high-water indicators. Good marks are indicators by the strr.ightness of the top of the wash line. Where the bank is steep or the soil unstable, the material may slough to elevations above the water surface. This condition may be recognized by the uneven ragged line at the top edge of the washing- such marks should be avoided. Usually wash lines are poor. Water carrying mud or silt will at times leave easily recognizable lines along banks, on trees, brush, rocks, and buildings. If there is only a slight difference in color, the mud line may be more readily visible from a distance. Foam lines are common on bridge abutments, wingwalls, riprap, poles, and trees. They may be affected by velocity head pileup. Buildings within the flood plain should be investigated; they sometimes are an excellent source of high-water marks. Even relatively clean water will leave stain marks within build- ings. Excellent marks may be found on windowpanes or screens. Use care to select marks that are not affected by velocity head, as are marks on the upstream side of buildings in an area where velocities were high. The ex- posure of flood-water entrances into buildings should be noted in order to judge drawdown or pileup. High-water marks on snow are not reliable. The flood debris may be deposited on snow which partially melts before a survey is made, leaving marks at a false elevation. Even though high-water marks around houses have been cleaned up or destroyed by rain, valuable information may be available from residents of the flood area. The information is usually i'tlliable where the water has come into a dwelling place, particularly if the family re- mained there at the time or returned shortly after. Information about flood heights away from dwellings, such as on trees, fences, or slop- ing ground, are frequently not reliable, par- ticularly if much time has elapsed or the facts are secondhand. All such data should be con- firmed independently, if possible. Photographs taken at time of flood crest by local residents may be helpful in guiding the search for flood. marks. .1 I Determination of gage height A series of high-water marks to define the water-surface elevation at the gage site should be obtained. Large differences, as much as 3 feet or more, between river elevations and those recorded in gage wells have been observed. This points up the necessity, in routine gaging-sta- tion operation, for establishing the relation between outside and inside gages at all stages, and the desirability of defining stage-discharge relations with reference to an outside gage, if at all practicable. High-water marks should also .