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One pass across the river is referred to as a transect. and 10 Iransects detine a measuremenl. The pole. mounted directly over the depth-sounder transducer. was used as a reference guide along the kevlar line, One person in the boat watched the line and pole and used a switch attached to the depth sounder to activate a fixed mark on the graphical depth-sounder record when the pole passed under a flag (tig, 9A. step I), A second person in the boat made notes on the graphical record, Date. time. distance of end points. edges of water. and distance along the line of each fixed mark were noted on the graphical charts, Variability in the recorded depth at a given location is caused by measurement error of the instrument. changes in boaI position, and aCIual changes in the water-surface elevation during a measurement. Of these. the changes in boat position are by far the largest contributor to depIh variability between transects, The depIh sounder reported data to 0,03 m and had an accuracy of 0,5 percenI of the measured depth according to manufacIurer specifications (0.075 m at a depth of 15 m), Changes in boat position occurred because a tixed position under the \lagged line could not be maintained in the strong and variable current of the river. Because the river bed is irregular. small changes in boat position could yield large differences in the recorded depth, The method devised to minimize the uncertainty in depth caused by boat position was to measure the cross section 10 times in rapid succession and compute the mean of the 10 transects, The number of transects was selected to provide enough data for statistical analysis and to keep the measurement time sharI so that changes in water- surface elevation during the measurement would be insigniticant, This method resulted in a measurement duration of IOta 50 minutes, and changes in water- surface elevations during the measurements were typically small. Changes in river stage were documented by measuring Lhe vertical distance from a water-surface reference point. Iypically an "x" chiselled into a large boulder. to the waleI' surface before and after a cross- section measurement (10 transects) was completed, The water-surface data were used in processing the cross-sectional measurement data and enabled comparisons of cross-sectional change in arca between measurcment dates, A water-surface reference point was established for individual reaches. and Ihat reference point was typically used for all cross sections in that reach, In most cases, Ihe reference point was used throughout the data-collection period: however. because of changing \low conditions. some reaches had several water-surface reference points, Relations were established between the original and subsequent water- surface reference points, Data Processing The graphical record of each cross-section transect was digiIized by recording a point at a preset distance of horizontal cursor movement across the paper (tig. 9A. step 2), The interval between digitized points per nnit of paper was kept constant and was selected 10 give about two digitized points for each foot of distance along the river bed, Once Ihe data were digitized. the distance of each point from the left-bank end point (the left-bank monument) in inches of graph paper was converted to ground distance in meters using Ihe known locations of the fixed marks on the graphical record and assuming constant boat speed between marks, To provide depths at equal disIances from the end point for the statistical analysis of the data. points were selected or interpolaIed at 0,25-m intervals across the cross section (fig. 9A. step 3), The sonic-depth sounder recorded depths of the river bed below the water surface, Data were converted to bed elevation by subtracting the measured depth from a water-surface elevation measured at water- surface reference points (fig, 9A. step 4), Those water- surface reference points that were not surveyed were assigned an arbitrary elevation of 500 m, Water-surface elevation typically was measured before and after the 10 transects had been completed. If thcre was a difference beIween the before and after water-surface elevation measurements, a time-weighted average was used to represent the water-surface elevation. After the depths were converted to bed elevations at each 0,25-m increment for a transect, a mean bed elevation was computed at each increment using data frol11 all 10 transects (fig, 9A. step 5), This is referred to as the cross-section measurement. Each cross-section measurement was processed as described above, To calculate the cross-sectional change in area between two measurements at a cross section, the dilference in mean bed elevation was detennined for each 0.25-m increment (fig. 98). If the difference was deemed sIatisIically significant. the actual difference value was used in the change-in-area calculation, If the difference Wi:lS not st:Hislically significant. zero was '..,.. 010~' Methods 19