252 THE GLACIAL LAKE AGASSIZ. 



VOLUME OF WATER RECEIVED AND DISCHARGED BY LAKE AGASSIZ. 



The present yearly discharge of water by the Nelson River is probably 

 equal to a depth of 10 to 15 inches upon its entire basin, which is approxi- 

 mately the same area that sent its drainage into Lake Agassiz and tlu'ough 

 the River Warren and the later outlets of this lake to the sea. A half or 

 larger part of the yearly rainfall and sno^^'fall on this basin is retui'ued to 

 the air by evaporation, leaving the volume stated to be carried into Hudson 

 Bay by the river. Through a few weeks in the spring the rapid snow 

 melting and accompanying rains raise all the streams and lakes which are 

 the feeders of the Nelson to their flood stage ; and, in contrast, many of the 

 watercourses are dry in summer, autumn, and winter, and the whole river 

 system is reduced to a small fraction of its previous maximum. 



But when the ice-sheet was being melted away and Lake Agassiz 

 received its drainage, the volume of water annually discharged from this 

 area was far greater than now. Considering first the rainfall and snowfall 

 of that time, we must doubtless suppose that they somewhat exceeded their 

 present amount, while the evaporation was less. It may be estimated, there- 

 .fore, that the water thus received and discharged by Lake Agassiz each 

 year was equivalent to a uniform depth of 20 or 30 inches over all its 

 hydi'Ographic basin. 



A still larger tribute was derived from the glacial melting, which in a 

 thousand years, more or less, dissolved from this area the greater part of an 

 ice-sheet 2,000 to 6,000 or 8,000 feet thick. The average yearly melting 

 would therefore be equivalent to an added depth of 3 or 4 feet of water, or 

 36 to 48 inches, making, with the supply from precipitation, approximately 

 5 to 7 feet upon all the basin. 



It thus appears that the inflow and outflow of Lake Agassiz were five 

 to eight times more than those of Lake Winnipeg. Instead of a flood dis- 

 charo-e dm-ing a few weeks in spring, the rapid ice-melting undoubtedly 

 maintained continuously tlu-ough the warm half of the year a larger out- 

 flow from Lake Agassiz than the spring stage of the Nelson River; but 

 during winters, when the glacial melting stopped, the discharge from the 



