Effects of Forests Upon Run-off in the Rockies 71 



off is collected in the basin and allowed to stand for some time, 

 so that the silt held in suspension may settle. It is thought 

 that a period of two hours is a sufficient time for the sediment 

 to settle, and necessarily the basin should be large enough to 

 hold the discharge for this length of time. If it is found that 

 the water does not pass out of the basin perfectly devoid of 

 sediment, samples of the water as it passes out should be 

 I a ken and the silt found therein should be figured for the 

 total fiow and added to that collected in the basin. One cor- 

 ner of the basin should be lowered to facilitate the measure- 

 ment of the silt. As a means of diverting the water when 

 it becomes necessary to measure the silt in the basin, a large 

 pipe with a screw cap should be placed in the dike just below 

 the opening into the basin. When this cap is removed the 

 water is diverted through the pipe and the water level brought 

 below the opening into the basin. To empty the basin a siphon 

 or drain pipe may be used. 



To measure the actual waterftow a \veir, forming the 

 outlet, should be placed in the lower end of the basin. A 

 weir of triangular form is recommended in this case and is 

 merely a right-angled notch cut in a steel plate, through which 

 the water flows. A weir of this form seems best for two rea- 

 sons; first, it most easily provides for the lowest possible flow 

 and also for the maximum which may equal from twenty to 

 thirty times the average, and second, the water passing through 

 tlie weir has the same general shape at all heights. The 

 height of the water in the w r eir is all that is necessary to cal- 

 culate the discharge as will be seen from the following formula 

 in w r hich Q equals the flow expressed in cubic feet per second 

 and h the height of the water in the weir, the number being a 

 constant for this particular form of weir: 



Q = 2.63r>h 5/2 



The water heights should be obtained by hook-gage read- 

 ings at intervals to check the waterstage register which gives 

 a continuous record of the water height in the weir. The hook- 

 gage consists of a hook attached to the lower end of a sliding 

 bar in a stationary frame. To measure the height of the water 

 in the weir the hook-gage is dropped below the surface of 

 the water, then by means of a tangent screw is slowly raise J 



