VARIATIONS IN LAKE LEVELS 405 



membered, fell continuously from 1886 to 1892, the total full 

 being about two and a half feet. 



The mean monthly variation, both in" lake levels and monthly 

 precipitation over the respective watersheds, is shown in the . 

 diagram on page 404. The full curve shows the average level in 

 feet of the surface of bakes Superior, Michigan, and Huron below 

 the plane of reference of the U. S. Lake Survey (high water of 

 1838). The dotted curve shows the average distribution of pre- 

 cipitation, in inches, throughout the year. The agreement be- 

 tween the two sets of curves is as close as could be expected, con- 

 sidering the nature of the data on which they are based and the 

 natural climatic differences between the regions under discussion- 



The climate of the Lake Superior watershed differs in several 

 respects from that of the region to the southward, partly by 

 reason of its geographic position and partly on account of the 

 lake itself. The precipitation of winter is generally in the form 

 of snow, and is derived for the most part from storms advancing 

 from the North Pacific or the Canadian Northwest. The snow- 

 fall is greatest on the south shore of the lake and is particularly 

 heavy from the Keweenaw peninsula eastward to and beyond 

 Wetmore, on the Duluth, South Shore and Atlantic railway. 



If the two curves showing the monthly rise in the level of 

 Lake Superior and the distribution of precipitation in its water-, 

 shed respectively be compared, it will be seen that the annual 

 rise in the lake begins coincidently with an increase of precipita- 

 tion. We should not be too hasty in placing these phenomena 

 in the relation of cause and effect. The rise in the waters of the 

 lake in the spring is doubtless due to the breaking up of the ice 

 in the rivers and the melting of the snow. Water from these 

 sources is fed into the lake during April and May more rapidly 

 than it is discharged through the St Marys river; hence the 

 surface level rises. The rains of June, July, and August, on the 

 average, equal about nine inches in linear depth, which amount, 

 plus the run-off from the watershed, should be set against the 

 loss by evaporation. The latter is, to a certain extent, an un- 

 known quantity, varying somewhat from year to year. Under 

 the most favorable conditions the loss by evaporation will not 

 greatly exceed the rainfall. The height to which the water of 

 the lake will probably rise, therefore, must depend greatly upon 

 the amount of water carried by the tributaries of the lake after 

 the breaking up of the ice, plus the amount conserved during 

 the spring and early summer in the swamps and forested areas 



