THE ANCIENT OUTLET OF LAKE MICHIGAN. 221 



it is next to be noted that the small lateral streams which now 

 enter the old river course find it much too large for their vol- 

 ume. This is especially true'near the divide across which the old 

 channel was cut. It is only at some distance down the channel 

 that enough water has entered through lateral streams to form a 

 considerable river ; yet all along the channel maintains about the 

 same width. Evidently, therefore, it was not cut out by the ex- 

 isting drainage. In consequence of the small volume of the lon- 

 gitudinal streams now occupying old channels, they are frequently 

 more or less obstructed by the alluvial fans built at the entrance 

 of lateral tributaries ; thus swamps or long, narrow, lakelike ex- 

 pansions of the rivers are produced up stream from the fans. 

 This was first noticed by Warren, and since then the list of ex- 

 amples has been greatly increased by Upham and others. In the 

 last of Upham's papers referred to above, he describes a number 

 of lakes of this kind on the Qu'appelle and Pembina Rivers and 

 elsewhere. Long Lake in Assiniboia is about fifty miles in length, 

 but only one or two miles wide. Lac qui Parle and Lake Trav- 

 erse, in the old channel at the head of the Minnesota River, are 

 of this kind. The sluggishness of the Minnesota and of the 

 Illinois Rivers just above their junctions with the Mississippi has 

 been attributed to the same cause, and this would indicate that at 

 the time when the channels of the Minnesota and the Illinois were 

 occupied by the large rivers which once flowed through them, 

 these held the place of main streams, while the Mississippi came 

 into them with smaller volume as a tributary. 



There is on this account a curious contrast to be noted between 

 the excavation of the late glacial channels that were cut out dur- 

 ing the closing stages of the Glacial period by the overflow from 

 glacial lakes, and the clogging of the preglacial valleys that were 

 commonly filled with sands and gravels by streams that came di- 

 rectly from the retreating ice front without delay and filtering in 

 lakes, as in southeastern Ohio. Both of these kinds of valleys 

 mark the courses of " constrained " drainage near the end of the 

 Glacial period. At the ice front the water supply in both cases 

 was doubtless surcharged with detritus ; but the waters that had 

 to accumulate in lakes marginal to the ice front before flowing 

 away as rivers must have been nicely filtered, so that they issued 

 clear and blue from the lake outlets ; while the others had to 

 carry their detritus down stream for many miles, and must have 

 been of gray and turbid color for long distances. The plentiful 

 clear waters of the streams of the first class ran down the valleys 

 that led from the lowest pass in the lake rims and cut down 

 their channels to a moderate grade, oftentimes so moderate that 

 the present river occupants of the valley are unable to keep 

 them clear of the alluvium that is brought in by tributaries ; 



