378 W. A. Johnston — Sedimentation 



%n 



winter, when the air temperature at times is as low as 

 — 40 degrees F. It does not flow out at the surface but 

 seeps through the drift dam at the lower end of the lake 

 and issues as springs. 



A delta and alluvial plain is being built at the upper 

 end of the lake by the stream flowing from Victoria 

 glacier. The delta extends into the lake about 400 feet 

 and the alluvial plain extends upstream about 1/3 mile, 

 thus indicating the amount of filling of the valley by sedi- 

 ment transported by the stream in post-glacial time. 



The temperature of the water in the stream at the exit 

 from the glacier is at or slightly above the freezing point. 

 At the delta it was 36 to 38 degrees F. in June, 1921 and 

 the temperature of the surface water at the lower end of 

 the lake was 6 to 8 degrees higher^ At a depth of 6 feet 

 below the surface the temperature was 39 degrees. It is 

 probable therefore the great mass of the water of the 

 lake remains at the temperature of maximum density 

 (39.2 degrees F.) through the year. 



Bate of Sedimentation. 



The stream flowing into the lake from the glacier 

 carries in suspension the maximum amount of sediment 

 at the time of the spring rains, when the snow at the 

 lower levels is rapidly melting. It also transports con- 

 siderable amounts during the summer but very little dur- 

 ing the winter. Sherzer found that the stream at the 

 exit from the glacier carried in suspension 506 and 230. 

 parts per million of sediment at times of maximum and 

 minimum flow in July, 1904. 4 Part of this material, how- 

 ever, is deposited on the flood plain of the stream and in 

 the part of the delta that is submerged only during high 

 water stages. Two samples of the water taken by the 

 writer in June, 1921, from the stream at the point where 

 it empties into the lake were found to contain an average 

 of only 90 parts per million of sediment. The samples 

 were taken at a time of approximately minimum flow of 

 water for the summer months. As the water in the 

 stream contains very little sediment during the winter 

 months, the average amount of sediment carried into the 



4 Opus cit. p. 28. 



