420 G. K. Warren— Valley of the Minnesota and Mississippi. 
below the water surface. Explanations of the cause of this 
lake had been attempted by Long and by Featherstonhaugh, 
which did not seem to me satisfactory. 
Explanation of the cause of Lake Pepin and similar lukes.—The 
results of the levelings on the Minnesota and Mississippi Rivers 
made by me showed that just below the entrance of any consid- 
erable affluent there was an accumulation of deposit in the main 
stream brought by the tributary; that over this deposit the 
slope of the water was greater than the average slope, and that 
it was shoal and impeded navigation; that just above the afflu- 
ent the slope was less than the average, and the water deeper. 
So Zar, this was in seria with conditions which generally 
exist in rivers, and might be so even where the main stream 
was gradually wearing away and deepening its bed. 
But a marked peculiarity was exhibited at the junction of the 
Minnesota and Mississippi Rivers. Regarding the Mississippi 
as the affluent (which we might do from the comparative sizes 
of the two valleys), the rule here would be as it is elsewhere, 
steep slope and shoal water below the affluent and almost no 
slope and very deep water above. In this instance the effect is 
felt above for at least thirty miles. On the other hand, taking 
the Mississippi as the main stream (as the volume of its water 
has always caused it to be regarded), then we have the anomaly 
of the main stream filling up the valley and damming back the 
affluent so that the latter brings no coarse material whatever 
into the main valley or has any part in forming the shoal below 
the junction of the two streams. 
I called attention to this in my report published in 1867, and 
again made the anomalous condition a feature in my re ort on 
the Minnesota River,* where diagrams were given of the streams 
and valleys at the junction and of the Minnesota at its source 
in es Big-Sto e and Travers, which I here repeat as Dia- 
the Fe on : tie Minneso sp ee to, showed 
that the valley of the Misucosta River Tad i in 5 period sub- 
— to the glacial-drift e n og a much 
its volume were as ere in proportion to area as that of the 
ef of Engineers, 1875, Part I, page 387. 
+ Ibid, 1868, pp. 37 Sik. 
