300 A. G. LEONARD DRAINAGE CHANGES IN NORTH DAKOTA 



It will be noted that the Knife, Heart, and Cannon Ball rivers, to- 

 gether with several of their tributaries, now occupy parts of this old 

 Pleistocene valley of the Missouri and Yellowstone rivers. The valley is 

 clearly much too large to have been formed by several of the streams 

 which today flow through it, such as Curlew, Elm, or Louse creeks, and 

 some portions now have no stream. Throughout much of its course the 

 old valley has a broad, flat bottom one-half to one mile and more wide, 

 with gently sloping sides. 



Pleistocene Valleys of the Yellowstone Eiver 



The lower 50 miles of the Yellowstone Valley was blocked with ice 

 during the Glacial period and the river was forced to seek a new channel. 

 Its waters flowed east to the valley of the Little Missouri and formed at 

 least two broad, flat-bottomed valleys connecting these streams. The 

 most northerly of these old valleys, which is 28 miles long, is now occu- 

 pied by the northwestward flowing Benny Pierre Creek, a tributary of 

 the Yellowstone, and by the eastward flowing Hay Draw Creek, a tribu- 

 tary of the Little Missouri (figure 2, plate 14). 



The second valley has a northeasterly course, is about 32 miles long, 

 and joins the first about 10 miles above its junction with the Little Mis- 

 souri. That portion of the Benny Pierre-Hay Draw Valley floor which 

 forms the low, flat, almost imperceptible divide between the Yellowstone 

 and Little Missouri drainage systems has an elevation of 185 feet above 

 the latter river, or about 2,209 feet above sealevel. The valley is bor- 

 dered on either side by high, steep bluffs and the level plain forming its 

 broad bottom is nearly a mile wide. This great trench was clearly occu- 

 pied at one time by a stream many times larger than those now having 

 possession of it. 



Preglacial Valley of the Little Missouri Eiver 



The Little Missouri, as well as the Yellowstone and Missouri rivers, 

 was forced out of its preglacial valley by the ice-sheet. The lower 55 

 miles of this valley was filled with ice, so that a lake was formed back of 

 the glacial barrier, the water rising and overflowing to the east by way of 

 the old valley previously described as occupied by the Missouri and Yel- 

 lowstone rivers during the Pleistocene period. 



The abandoned valley of the preglacial Little Missouri was first men- 



