214 The American Geologist. April, i904. 
ing- conditions is readily brought to light by a study of the 
causes kown to have been in operation. Stated comprehensive- 
ly, these are : ( i ) the rapid melting of the ice over a large area 
tributary to the Missouri river, thus furnishing an ample supply 
of water; (2) the absence of any corresponding increase in the 
water supplied to the Osage river, which lies entirely outside 
of the glaciated region; (3) a probable, or at any rate, possible, 
diminution of the grandient of the Missouri river by reason 
of a differential depression of the land towards the north ; (4) 
a reduction of the gradient of the Missouri river by reason of 
corresponding high water in the Mississippi; (5) the narrow- 
ness of the gorge of the Missouri below the Osage river, which 
would cause an abnormal rise of water above that point for a 
brief period each year. 
(i) At the present time it is estimated by Humphreys and 
Abbott,* that the Missouri river annually discharges about 28 
cubic miles of water, which is but 15 per cent of the total rain- 
fall. Letting this stand as a constant quantity, we can readily 
see that the entire amount of water furnished by the melting of 
the ice during the closing stages of the Glacial period would 
have to be added to this amount, since the evaporation is as 
much now as it ever was. Bearing upon the supply of water 
this furnishes professor Todd supplies the following facts and 
estimates : 
"Vast quantities of water were available for the purpose of 
depositing the loess. The amount may be estimated as fol- 
lows : At the time of the first or Altaniont moraine the Mis- 
souri received the melting from 130,800 square miles of ice 
sheet in the United States, that is one-half of Dakota, one-third 
of Minnesota and one-sixth of Iowa, besides there was probably 
as great an area in Canada to be added, because the upper Sas- 
katchewan at that time doubtless drained into the Missouri 
river. The ablation of certain Alpine glaciers as estimated by 
Desor is ten feet a year, by Forbes over twenty feet, and even 
as high as two and one-half inches a day has been observed. 
It seems not unreasonable to count most of this area in the zone 
of ablation at as much as ten feet for the United States alone, 
♦Report on the Physics and Ilydraulios of the Mississippi River, liy O.irx. 
A. A. Humphreys and Lieut. II. L. Abbott, Professional Papers of the 
Corps of Engineers, U. 8. Army. No. ^?•> (1801) : reprinted with additions in 
1876, p. 84. 
