TRIBUTARY AREA IN THE MACKENZIE BASIN. 63 



Hudson Bay. The upper half of the Nelson flows in a general direction 

 only a few degrees east of north, passing thrpugh Great and Little Play- 

 green, Pipestone, Cross, and Sipi-wesk lakes, to Split Lake; thence it turns 

 to the east for about 100 miles, passing through Gull Lake, and finally 

 takes a northeastward course along its lower 100 miles. According to Dr. 

 Bell's observation, Sipi-wesk Lake is approximately 570 feet above the sea, 

 or 140 feet below Lake Winnipeg; Split and Gull lakes are respectively 

 about 440 and 420 feet above the sea; and the descent in the next 48 miles, 

 •to the foot of Broad Rapid, is nearly 300 feet. The Nelson is navigable 

 from the sea about 90 miles to the First Limestone Rapid, where the eleva- 

 tion is probably about 50 feet above the sea-level. 



About four-tifths of the region drained by the Nelson, including the 

 basins of the Red River of the North, the Little Saskatchewan, and the 

 Saskatchewan, and the gi-eater part or possibly all of the basin of the 

 Rainy and Winnipeg river system, were uncovered /rom the ice-sheet and 

 were tributary to Lake Agassiz as early as the middle portion of the time 

 while it had its southward outlet. The waters of a large part of British 

 America were thus carried along the course of the Minnesota and the 

 Mississippi to the Gulf of Mexico. The basin of Lake Agassiz then 

 included approximately 350,000 square miles, of which nearly a third was 

 covered by the lake itself 



EXTENSION OF THE BASIN OF LAKE AGASSIZ BY GLACIAL LAKES 

 OUTFLOWING TO IT FROM THE REGION OF THE PEACE AND 

 ATHABASCA RIVERS. 



Furthermore, within the time after the ice-sheet had retreated beyond 

 the valley of the lower Saskatchewan, and before its melting u])on Hudson 

 Bay and the adjoining country permitted Lake Agassiz to gain an outlet to 

 the northeast, it seems certain that the ice must have been melted upon 

 a large region north of the Saskatchewan basin, where drainage now 

 passes east by the Churchill and north by the Mackenzie, but was then 

 pent up in lakes by the ice barrier and caused to flow to the south. Lake 

 Agassiz thus received the waters of the upper Churchill, and of the basins 

 of the Athabasca and Peace rivers, the great head streams of the Mackenzie; 



