34:2 C. A. FISHER GIANT SPRINGS AT GREAT FALLS, MONTANA 



sheet extended into the region from tlie southeast, damming up Missouri 

 river and its tributaries, forcing the former to abandon its channel in 

 many places, some of which were not reoccupied on the retreat of the ice. 

 The abandoned channel, a portion of which is shown on the accompan3dng 

 map (figure 1), extends from the mouth of Sand coulee, 4 miles south of 

 Great Falls, nortlieastward in meandering course to the mouth of Belt 

 creek, where it unites with the present Missouri. Missouri river, being 

 obstructed by glacial material, Avas diverted to the north at the mouth of 

 Sand coulee, passing up Sun Eiver valley for a short distance and then 

 flowing northeastward over Cretaceous sandstone and shale, developing 

 a new channel, along which tliere are a number of large cataracts or falls. 

 Prior to this diversion, liowever, while the river channel was occupied by 

 ice. an extensive lake existed in front of the ice-sheet, and boulders, prob- 

 al)ly deposited by floating ice, are now found high up the plateau slopes 

 to the south and west. After the ice had receded, the terminal moraine 

 acted as a barrier for a lake of much, smaller dimension. Sediments were 

 brought into this lake by the Missouri river and its tributaries, filling all 

 of the preexisting valleys of the district, and these filled valleys are a con- 

 spicuous physiographic feature at the present time. The relation of the 

 pregiacial to the present channel of Missouri river in the vicinity of Great 

 Falls is shown on the map. 



Amount of Flow 



The fact that the water of the Giant springs issues from rocks at the 

 water's edge and in the bed of the river renders it difficult to measure 

 their exact flow. In order to ascertain this amount, stream gaugings 

 were taken by E. S. Xettleton^ of the total flow of Missouri river above 

 and below the springs. The difference between these two measurements 

 is assumed to be the quantity furnished to the river l)y the springs. The 

 total flow of the springs thus obtained is approximately 638 cubic feet per 

 second — an amount which, converted into gallons, is the equivalent of 

 over 400,000,000 gallons every 24 hours — a veritable underground river. 

 Tt is readily seen from these figures that the springs rank among the 

 largest in the United States. The water boils up with consideral)le force, 

 is clear, blue, and relatively pure. It contains no more dissolved salts 

 than the average well water of the region, and differs in composition from 

 Missouri River water chiefly in sulphate content, which can be accounted 

 for by the large amount of gypsum in the shale of the Kootenai forma- 



■ E. S. Nettleton : Op. clt., p. 77. 



