Ch. 20] ECONOMIC SIGNIFICANCE 359 



up to 300 feet, correction for the inclination of the wire is set on 

 the distance wheel. 



Starting at the bank, the boat swings in an arc, keeping the 

 distance wire taut until the boat reaches the bank 50 feet distant 

 from the starting point. The distance wire is then extended another 

 25 feet to a total of 50 feet from the pivot point, and a reverse arc 

 is run if the sounding is taking place in still water. If there is ap- 

 preciable current, the soundings must all be made in arcs swinging 

 with the current. As the boat proceeds along each arc, the chart 

 operator presses the fix button and at the same time indicates to the 

 plane-table man his position by dropping a flag, using the red and 

 white flag system previously described. By successively increasing 

 the radial distance, the desired area is covered completely and ac- 

 curately. The plane-table man prepares the plane-table sheet before 

 setting up by drawing arcs at 25-foot intervals. As he follows the 

 sounding boat around each arc, he marks the position on each arc on 

 the plane-table sheet where a sounding fix is indicated. 



Density of Deposited Material 



In the utilization of the underwater sounding data from reservoir 

 resurveys, the density of the deposited material and the degree of 

 consolidation are particularly important (Iowa Institute of Hydraulic 

 Research, No. 9). 



Density of consolidated sediment should be known in order to trans- 

 late measurements of sediment moving in natural rivers into volume 

 that will be occupied by that sediment in a reservoir built at that lo- 

 cation. The state of consolidation is important in order to compare 

 volume occupied by sediment at any time with the volume that will 

 be occupied after ultimate consolidation takes place. Unless the de- 

 gree of consolidation at the time of a survey is known, erroneous con- 

 clusions may be drawn from the data with respect to rate of silting 

 and useful life of a reservoir. 



ECONOMIC SIGNIFICANCE OF RESERVOIR SEDIMENTATION 



Reduction of storage in reservoirs that serve vital regional or com- 

 munity purposes may eventually result in impairment of the function- 

 ing of the reservoir to the point where it will have a disturbing effect 

 on the economic life of the region or community. Stevens (1946) 

 illustrated this point when he sounded a warning to the interests 

 dependent on storage of water in Lake Mead that the useful life of 

 the reservoir would be relatively short owing to the filling of the 



