134 THE IMPROVEMENT OF RIVERS. 



To illustrate the advantages and drawbacks of point and bend locations \M- will 

 quote two examples which have come under our notice. In one case the lock had been 

 located just below a sharp point, and towboats entering or leaving the upper entrance, 

 except in low water-season, had to put out lines to trees on the bank in order to escape 

 being carried out into the river and over the dam. Passenger boats usually made a 

 flying entrance, striking broadside on a large timber crib which guarded the upper end 

 of the approach, sometimes succeeding in entering and sometimes being swung out by 

 the current. In the latter case they had to turn in the middle of the river, where there 

 was a strong current toward the dam. This entrance was much troubled by the sedi- 

 ment deposited in the quiet water under the lee of the point. 



In another case, the lock had been located in a sharp bend, and boats had similar 

 trouble in entering or leaving, though there was of course much less current driving 

 toward the dam. In certain stages of water, when drift was running, it was swept 

 into the upper entrance and piled up till it lay in a solid mat eight or ten feet deep, and 

 much of it had to be cut into pieces before it could be removed. The deposit of sedi- 

 ment, however, was always slight. 



In the earlier constructions and in some later ones, the entire river was dammed 

 at or near the upper part of a sharp bend, and a cut-off or canal dug across the bend 

 from the pool thus formed. In this canal the lock was built, generally near the lower 

 end, to obviate its being filled with sediment from below. Another arrangement, 

 which has been frequently adopted in Europe, is to fix the location where one or more 

 islands divide the river, placing the lock in one arm and the dam in the others. The 

 disposition has generally been to place the dams near the head and the lock near the 

 foot of the island, but some very good works have been arranged the other way. The 

 selection of such a site for a lock and dam, whether at the upper or lower end, or in the 

 middle, and which arm shall contain the lock and which the dam, must be decided in 

 each particular case by a study of the locality and of the conditions affecting it. 



In this country the usual practice is to place both the lock and dam in the open 

 stream adjoining each other, but there are cases where a cut-off has been found best 

 suited to the situation. At rapids or long stretches of river or steep slope, it is neces- 

 sary to build canals connecting the quiet water above and below. 



Investigating Site. In the preliminary surveys necessary to determine a loca- 

 tion, soundings and borings should be made at all proposed sites, and after the final 

 one has been agreed upon, the whole area to be covered by the lock, the dam, and the 

 abutment, should be thoroughly gone over with drills, and the depth and character of 

 all materials determined. A dredge is sometimes used to assist in these investiga- 

 tions when the bed-rock is overlaid by a deposit. Test pits or wells should also be put 

 down at suitable points on the banks. Where there is no rock or where its depth is 

 great, borings may be made by driving lengths of pointed gas-pipe, fitting the 

 sections together as they are driven down, or an auger may be used, or in light material 

 a water jet. 



