22 W. UPHAM ICE AND RIVER EROSION IN SAINT CROIX VALLEY 



lake level. Probabl}^ the highest part of the swamp now forming the 

 watershed in the channel has been filled 20 to 25 Feet since the lake for- 

 sook this mouth, which was thus lowered by erosion some 100 feet, from 

 1,150 to 1,050 feet, approximatel.y, above the present scale vel. 



Origin of Lake Saint Croix and Lake Pepin 



Since the ice barrier which caused the glacial lake Agassiz and the 

 Western Superior lake disappeared, the Minnesota valle}^ and that of 

 the Mississippi below their confluence, and also the Saint Croix valley 

 below Taylors Falls, carryina^ only a small fraction of their former vol- 

 ume of water, have become considerably filled by the alluvial gravel, 

 sand, clay, and silt, which have been brought in by tributaries, being 

 spread for the most part somewhat evenlv along these valleys by their 

 floods. Tlie changes produced b}' this postglacial sedimentation have 

 been pointed out and abl}' discussed by General G. K. Warren, Avho thus 

 added much to our knowledge of the geologic history of these rivers. 

 Lakes Traverse, Big Stone, and Lac Qui Parle occupy hollows in the 

 outlet of lake Agassiz due to inequalities of these recent deposits. At 

 the mouth of the Minnesota river, the Mississippi has brought more sed- 

 iment than its branch, which is thus dammed for a distance of 30 miles, 

 to Little Rapids, with a depth of 20 to 25 feet at low water. In the same 

 way the Mississippi vallc}^ at the mouth of the Saint Croix has become 

 more filled by ])ostglacial deposits than its tributary, which is thus held 

 as back-water 20 miles, to the head of lake Saint Croix, which is 25 feet 

 deep. 



Lake Pepin, having a depth of about 60 feet, according to General 

 Warren, lies in the continuation of the valley which was deeply chan- 

 neled by the outflow from these glacial lakes, because it has become 

 unequally filled below b}^ the deposition of alluvium from the Chippewa 

 river. The depths of lakes Saint Croix and Pepin, however, are only a 

 partial measure of the channeling of the Saint Croix and Mississip})i 

 rivers during and shortly after the departure of the ice sheet. The 

 greatest depth of the Saint Croix river, stated by Dr Berkey to be 160 

 feet near Angle Rock of the Upper Dalles, was worn down probably by 

 the river when it flowed, at the end of the Ice age, along all its lower 

 course, like the lower part of the Minnesota river and the Mississippi 

 thence southward as well, about 100 to 150 feet beneath the present 

 river bed. 



Preglacial river erosion in the great valleys had reached far below their 

 present depths, and additional deepening may have occurred during the 

 early and greater part of the Ice age for the Mississippi in the Wisconsin 



