THE LIFE OF DEVILS LAKE 15 



tween the 15th and 25th of November, while it is usually free from 

 ice between April 20 and 25. The ice commonly reaches a thickness 

 of one metre, a few centimetres of which are frozen snow. 



The comparatively low relative humidity (table 2) and the 

 characteristically high winds of the region, are mainly responsible 

 for the rapid evaporation, rather than the occasional brief extremes 

 of temperature. 



TABLE) 1 

 Showing mean maximum (A) and minimum (B) monthly and annual tempera- 

 tures (C°) at Devils Lake City from 1900 to 1920. 



Wind is one of the most important factors, not only in lowering 

 the level of Devils Lake ; but in moulding its shores, thru ice and 

 wave action, and in determining its currents, the distribution of its 

 inhabitants, its dissolved gases, and other chemical constituents, its 

 temperatures and turbidity. 



The average total annual wind movement at Devils Lake for 

 13 years was 168531 km., maximum velocities of 97 km. per hour 

 being occasionally reached. The prevailing directions are N W. and 

 S. E. 



The average annual precipitation at Devils Lake is 44.6 cm., 

 while during a period of 14 years (1907-1920) the level of the main 

 part of Devils Lake fell from 435.1 to 433.1 m, or 2.0 m, an average 

 annual fall of 0.15 m. 



The amount of water represented by this drop can only be 

 estimated since no accurate survey of the lake has been made since 

 1883, and the shore lines are constantly changing. According to 

 the report of the state engineer (Atkinson, 1912) on the survey 

 for a proposed diversion of the Mouse River into Devils Lake, the 

 amount of water needed to make good the annual excess of evapora- 

 tion is 24,635, 790 eu. m. Basing an estimate on the above data 

 and allowing for the total rainfall reaching the lake, plus 50% 

 additional in run-off and seepage, the average amount of evapora- 

 tion for the past 15 years may be estimated at 0.8 m. or 131,390,880 

 cu. m. This figure is of course the merest estimate, based on the 

 average rise from melted snow in spring, when most of the run-off 

 occurs and amounting to about 35% of the annual rainfall, with an 

 additional 15% for run-off during the rest of the year and for 



