72 



hccaiise of the chemical composition of the soil, iuid imrtly i)ecause it is so 

 tliin. The only method for the prevention of erosion of tlie sides of tlie val- 

 ley is forestration. 



The form of the valley gives a deep narrow impounding hasin, which is 

 well protected from wind. This results in the complete thermal stratifica- 

 tion of the water early in the summer. The leaves from the wooded slopes 

 of the valley are blown into the water producing a very considerable 

 amount of oxydizable material. The result is that during the latter part 

 of the summer the oxygen is exhausted from the lower levels of the water. 

 This oxygenless region increases in thickness until the autumn "turnover" 

 and reaches the level of the intalce early in August. As a result of this 

 the water is very unpalatable during most of August and a part of Septem- 

 ber. Of course a better flavor could be obtained by placing the intake above 

 the thermocline. but the water is carried directly to the taps without icing 

 and hence it is a marked advantage to take it from the cooler water below 

 the thermocline. 



The apparatus consists of an ordinary compression piuup such as is sold 

 by the trade for use in garages. To this is coupled a % inch air line which 

 is carried over the dam and down to a point near the bottom of the reser- 

 voir, and just below the intake. This point is 9.75 M. below the top of the 

 dam. At this end was attached a crow-foot formed of three % inch pipes 

 7 feet long, and jointed at right angles to the line and to each other. The 

 ends of these pipes are capped and small holes (% inch) are drilled on the 

 under side. These holes are 1 inch apart, placed alternately in two rows 

 (JMg. 1). The pump is operated by a belt connecting it to the flywheel of 

 one of the oil engines which operates the water pump. The pump usually 

 runs eight hours in each twenty-four. 



The pressiire at the air outlet is slightly more than one ad<litional atmos- 

 phere. The temperature was about 17 ('.. while at the surface it varied 

 from 22 C. to 2G C. It was 22 C. on September 25, 1019. The increased 

 pressure and the lower temperature would of coiirse make the rate of gas 

 absorbtion more rapid per unit of surface than it would be at the surface 

 of the I'eservoir. By discharging the air through the small openings, small 

 bubbles were produced which increased the surface per unit of air volume. 



It was hoped that this operation would introduce enough oxygen to main- 

 tain a potable quality of water below the thermocline. The quality of the 

 water was improved markedly, but the improvement was due. in part at 

 least, to another set of factors which had not been considered when the 

 apparatus was put into operation. These were the convection currents 

 caused by the rising bubbles. The friction of these bubbles set up a vertical 

 current that was approximately 100 sq. ft. in cross section. The rate of 

 flow of this upward current was not measured but it was sufficient to rai.se 

 the level of the surface some 2 or 3 cm. 



The amount of dissolved oxygen was determined at various levels at the 

 intake and at the deepest point in a cross section 150 feet above the dam. 

 In the upper 8 M. at the upper station (the water was 1.75 M. below the 

 top of the dam) there was more oxygen at every level than at the intake. 



