THE UPPER MISSOURI RIVER SYSTEM, 597 



low country the Missouri here possesses are generally to be found on 

 the north or left bank of that river (see Diagram No. I). 



Without attempting a description of those strange and interesting 

 mauvaises terres, which are the favorite theme of popular writers, I 

 shall endeavor to give some idea of the process by which the valleys 

 of these rivers have been formed and of the action of the rivers within 

 their present bed. It is quite evident that the entire configuration of 

 the land-surface of the region has been the result of erosion, and dis- 

 tinct breaks or even low cliffs sometimes occur, showing the edges of 

 the horizontal strata. At intervals of from five to ten miles small 

 streams or creeks fall into the river, often entirely dry in summer, 

 sometimes containing a small quantity of perfectly transparent water, 

 but so charged with alkali as to whiten the pebbles over which it flows, 

 and to render its use by man or beast almost impossible. These 

 creeks, locally denominated coulees — a name given them by the early 

 French explorers — have excavated valleys of different lengths and 

 widths, and between these occur narrow plains, or even mere ridges. 

 Of the immense volume of solid earth and rock that has been brought 

 down by the process of eroding these terraces, creek-valleys, etc., only 

 a minute fraction has been retained, but this has been deposited near 

 the river, forming an alluvial bottom of varying width. This alluvial 

 deposit it is the function of the river perpetually to wear away, while 

 at the same time laying down new matter, with which it is constantly 

 charged, to take its place. The result is, that throughout the lower 

 portions of these rivers, and also in the Missouri Yalley below their 

 junction, the bed of the river is perpetually shifting its position in 

 the general valley. When we contemplate the entire history of the 

 river, the valley must be regarded as due to this process, and its great 

 width relatively to that of the stream itself can only thus be accounted 

 for. But, if we contemplate it only at a given time, as the present, 

 the valley appears to consist of two quite distinct parts, viz., the river- 

 bed and the valley proper, raised above it and gradually sloping back 

 on one or both sides to the foot of the first terrace. If, in time of 

 low water, we compare these two parts, the latter will appear to be 

 stable, while the former will clearly show that it is unstable. There 

 was probably never a time in the river's history when these two dis- 

 tinct features did not exist much as now, though no one can say how 

 many times the river may have worn away the stable portion of its 

 valley on one side while it was forming anew on the other, and 

 afterward receded and carried off the last-formed valley, leaving its 

 previous bed to be again filled up until it has regained all the aspect 

 of permanence which it previously possessed. This crossing and re- 

 crossing by the river-bed of the general valley, proceeding simultane- 

 ously with the work of lateral erosion, have gradually lowered the 

 valley to its present position and are still lowering it. In a certain 

 sense this applies to all rivers and river-valleys, but nowhere perhaps 



