40 LAKES OF NORTH AMERICA. 



rence. During storms, all of the streams pouring into the upper Lauren- 

 tian lakes, from the surface drainage of the land, are brown and heavy 

 Avitli mud, ])ut the water rushing over Niagara remains of the same deep 

 greenish-ljlue tint season after season and year after year. Niagara river, 

 above the falls, and the St. Lawrence are surface streams, because their 

 clear waters have but slight power of corrasion ; it is for this reason that 

 during the centuries they have occupied their present channels they have 

 not materially deepened them. 



In the case of lakes fed b}* the turbid waters from glaciers, the part the}' 

 ])lay as settling basins is even more strikingly shown than in the instances 

 just cited. Lake Geneva, Switzerland, fed b}' the silt-laden waters of the 

 Rhone, is discolored for several miles from where the river enters, but 

 when the waters leave the lake and again start on their journey they are 

 wonderfully clear. An abundance of similar illustrations are furnished 

 by the glacial-fed lakes of the Sierra Nevada and Cascade mountains and 

 by some of the numerous lakes on the liead waters of the Yukon. 



The streams flowing from lakes are not always clear, however, as 

 exceptions occur Avhere the outlets are so situated that shore currents 

 may bring sediment to them. The construction of beaches and embank- 

 ments by shore currents may take place at the outlet of a lake so as to 

 obstruct the escape of its waters and initiate a struggle between the 

 waters tending to foim deposits and those escaping through the channel 

 of discharge. The outflowing waters may thus be rendered turbid and 

 have the mateiial supplied with which to erode their channels. A case 

 in point is thought to be furnished at the south end of Lake Huron, 

 where River St. Clair has its source, although definite observations on 

 the relation of the outlet to shore currents liave not been made. The 

 waters of River St. Clair are not of the transparent character that would 

 Ije expected in a stream starting from a large lake ; and a broad delta has 

 been formed in Lake St. Clair, into which the river empties after a short 

 course through low alluvial lands. The source of the material forming 

 the delta cannot be referred to the erosion of the banks of the stream, and 

 is not furnished by tributaries, but apparently comes from the action of 

 waves and currents on the shores of Lake Huron adjacent to its outlet.^ 



The rapidity with which lake basins in all parts of the world are 

 becoming filled with sediment is sufficient in itself to show that no lakes 

 fed by turl^id streams can be geologically old. 



1 These conclusions have recently bee confirmed by F. B. Taylor in an instructive paper 

 on "The Second Lake Algonciuhi," Am. Geol., vol. 15, March, 1895, pp. 171, 172. 



