56 PHYSICAL GEOGRAPHY. 



and its waters are known to be very turbid far toward its 

 sonrce. Two collections of its waters have been made from 

 its channel at Council Bluffs, and the solid contents deter- 

 mined by Prof. Emery. One parcel was collected at low 

 water on November 9th, 1868, and the other on July 5th, 

 1868, when the river was just bank-full. The amount of 

 sediment filtered out of the water in both instances is as 

 follows : 



Low-water, .462 grams in one liter=52 grains in one gallon. 



High-water, 5.672 grams in one liter=4Q4 grains in one 

 gallon. 



It will thus be seen that the amount of suspended sediment 

 at times of high water, is more than twelve times as great as 

 it is at low water. 



The amount of solid matter held in solution in the same 

 water, was also determined by Prof. Emery, and will be 

 found on another page of this report. The turbidity of the 

 water is at all times so great that no object can be seen 

 beneath its surface. The amount of solid matter brought 

 down by the river has always been very great, as will be 

 explained under the head of Bluff Deposit, but it seems 

 necessary here to anticipate the description of that deposit, 

 so far as to say that it is composed of fine sedimentary 

 material, similar to that which the river now deposits 

 from its waters, and consists of material which the same 

 river really did deposit, in a broad depression, in the surface 

 of the drift that formed a lake-like expansion of that river 

 in the earliest period of the history of its valley. That 

 lake, as shown by its deposit which now remains, was 

 about a hundred miles wide and more than twice as long. 

 The water of the river was muddy then as now, and the 

 broad lake became completely filled with the sediment which 

 the river brought down, before its valley had deepened 

 enough in the lower portion of its course to drain it. After 

 the lake became filled with the sediment, the valley below 

 became deepened, by the constant erosive action of the 

 waters, to a depth more than sufficient to have drained the 



