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<br />12. To fill in the gaps, field measurements were made at various <br />sites in the study area. The sites were selected on the basis of prelimi- <br />nary air photo interpretation of the study area. Two fundamental criteria <br />were used: first, they represented what appeared to be commonly recurring <br />gap types; and second, they represented points or places in which no inter- <br />pretation could be made because the combinations of features were unfamiliar <br />to the interpreters. <br /> <br />13. The field data collection was designed to acquire the maximum <br />possible amount of basic data so that they would be as widely useful as <br />possible. For example, bed and bank configurations were recorded as a <br />vertical profile across the gap. From such data the various geometric <br />factors (such as gap width, bank angles, bank heights) can be readily <br />calculated. <br /> <br />Gap conditions as a function of time <br /> <br />14. A condition of the project was that the final description be such <br />that the gap conditions for any time of the year could be defined. To <br />achieve this, a method was developed and us,,;] for "pr"dicting" the condition <br />of every gap throughout a yearly cycle. <br /> <br />15. The literature data (i.e. the stream hydrograph data) commonly <br />included stage and discharge data, usually in such form that mean stage <br />and its correlative discharge by month could be readily obtained. With <br />these data, combined with a gap cross section, it was possible to compute <br />all of the pertinent geometric factors (water depth, water width, gap width, <br />bank angles, and bank heights) for any water level. Some, but not all, of <br />the literature sites also included current velocity data. In those instances <br />where such data were available, the current velocity for any water level could <br />also be computed. In those instances where current velocity data were not <br />included, it was necessary to calculate the velocities by indirect methods. <br />This was done by using the "Manning equation," with the required longitudinal <br />slope value ob tained from topographic maps. "Manning's n," the "roughness <br />number," was obtained by photo interpretation. The solution of the Manning <br />equation provides a mean velocity value which can then be adjusted to give <br />maximum velocity. Thus, at this point, all literature sites were represented <br />by complete hydrologic and geometric data (bank angles, bank heights, gap <br />width, water width, water depth, and maximum current velocities) for any <br />month or for any arbitrary water level. <br /> <br />16. The field data (i.e. that collected by the field team), of course, <br />recorded only one point in time, and thus no stage or water level data were <br />available except for the day on which the gap was measured. For these sites, <br />geometric data (gap width, water width, water depth, bank angles, bank <br /> <br />17 <br />