ORIGIN OF LAKE BASINS. 7 



Sierra Nevada. In resfions \^'hel■e the conditions are most favorable 

 for the growth and preservation of alluvial cones, tliere is hut little 

 rain-fall, and the material dei)Osited in the valleys is apt to he porous 

 and of such a character that it al)sorbs water readily ; for this reason 

 lakes may be absent and the land remain desert-like and arid although 

 basins exist. 



A lack of close adjustment in the transporting power of streams may 

 sometimes be observed even in humid countries, and in regions of mild 

 relief. As described by G. K. Warren,^ the excess of material brought 

 by Chippeway river to the Mississippi, obstructs the main stream so as 

 to cause an expansion of its waters known as Lake Pepin. An api)i'oxi- 

 mation to the same conditions occurs where Wisconsin river and Illinois 

 river join the " Father of Waters"; but in these instances it is only in 

 the low water stages that the ponding becomes conspicuous. A tendency 

 in the same direction was noted by J. W. Powell while making his ad- 

 venturous journey through the canon of the Colorado ; dangerous raj^ids 

 were encountered at localities where lateral streams liad swept debris into 

 the main channel. 



Perhaps the best exam})les of lakes held by obstructions deposited 

 by lateral streams that can be cited, occur in valleys draining to the 

 Assiniboine, Manitoba. The liakes referred to, are situated in valleys that 

 were cut down to a gentle slope when the abundant drainage of glacial 

 lakes flowed through them ; but the weaker . modern streams are unable 

 to maintain such a faint grade, and are being silted up where tributaries 

 enter. Long narrow lakes are thus formed above delta-fans built by 

 streams having a higher grade than the main valley.- 



The separation of lakes Brienz and Tliun, Switzerland, has been cited 

 by Davis as an example of the i)artitioning of a valley by the union of 

 deltas from opposite sides. Interlaken stands on the beautiful alluvial 

 })laiii tlius formed. Several other similar examples in central Eurojje 

 have been described by various authors. 



Lakes retained by the deposits of lateral streams and In' alluvial 

 cones, pertain to young and immature streams, and are incident to their 

 work of erosion. As topographic development i)rogresses, these Avater 

 bodies are obliterated, but when streams reach maturity and old age, lakes 

 of another class appear along their courses. 



1 Am. Jour. Sci., val. l(i, .'M ser., 187^, p. 420. 



2 Warren Upham, " Report on Lake Agassiz," Canadian (ieol. and Xat. Hist. Smv., Ann. 

 Rep., vol. 4, 1888-8It, p. 22 B. 



