January 29, 1892.] 



SCIENCE. 



59 



amined. It is, however, a permanent addition to the vast 

 mining' territory of the Rocky and trans-Rocky Mountain 

 country, and when considered in connection with previous 

 discoveries, it suggests the probability that the mineral de- 

 posits of the State of Washington exceed in quantity and 

 value those of any other State. 



W. H. RUFFNEE. 

 Lexington, Va., Jan. 23. 



THE EVOLUTION OF THE LOUP RIVERS IN 

 NEBRASKA. 



The most casual inspection of a map of central Nebraska 

 might suggest that the hydrography of the region has proba- 

 bly undergone radical changes. It looks as if the three 

 Loup rivers, and the smaller creeks running parallel to them, 

 had once been separate tributaries of the Platte, all indepen- 

 dent of each other, as roughly indicated by the dotted lines 

 on the map (Fig. 1). The Platte is the great central trunk 

 of the drainage, and these streams all seem to be headed for 

 it like branches, and would join it directly if they had not 

 been somehow turned eastward and united to form the Loup 

 River. 



It is the fate of such impressions to fade out in the light 

 of accurate knowledge, but there are some survivals, and this 



waters of the South Loup one is impressed with the apparent 

 continuity of its valley with that of Wood River, rather than 

 ■with that of the South Loup itself below Callaway. It is ob- 

 viously an instance of the lower, more easterly stream cutting 

 through the divide and drawing to itself the headwaters of 

 the higher one. 



This series of captures by lower tributaries is exhibited on 

 a grand scale and in a mature form in the Loup system. 

 Another example on a smaller scale, and in its incipient 

 stages, is shown in Fig. 3. The streams a and b have each 

 captured the headwaters of some streams lying westward, 

 and a thi'eatens to capture the headwaters of b. 



In this case, on the Republican River, the slant of the 

 country is directly east, and is due to the Rocky Mountain 

 upheaval, which gave an eastward tilt to the great plains. 



S.W 



■yre 



In the Loup region there is also, in addition to the eastward 

 slope, a pitch to the north-east, which has a more local 

 origin, but is, none the less, an important factor in the evo- 

 lution of the Loup, system. The last great tertiary lake 

 (Cheyenne) submerged the Loup and the Republican com- 

 pletely, hut left the upper Platte a vigorous mountain stream, 

 bringing down silt at a rapid rate. This silt, quickly sub- 

 siding in the still lake waters, formed a succession of bars 

 o£E the mouth of the river, as the shore line shifted east and 

 west in the vicissitudes of climate, and of upheaval and sub- 

 sidence. There was no permanent point of discharge, and 

 consequently no permanent single bar, but a general distri- 

 bution of silt in and along the channel of the Platte, which 

 accumulated to such an extent as to raise the level of this 

 river above that of the Loup on the north and the Republican 



bids fair to be one of them. I have, I think, verified it by 

 field work, and I will briefly recount some of the topographic 

 and geological data which tend to confirm the first impres- 

 sion. 



The Loup rivers flow in channels excavated from fifty to 

 two hundred feet in soft tertiary marls. Taking them in 

 succession from south-west to north-east each stream is lower 

 than the preceding one. A profile on a line at right angles 

 to the general south-east course of all the streams of the 

 Loup system, would have the general character roughly repre- 

 sented in Fig. 2. 



This general north-east slant of the country gives a great 

 advantage in rapidity of erosion to all ravines on the south- 

 west side of each stream. They become longer, deeper, carry 

 more water, and are cut down more rapidly than those on 

 the north-east side of the next higher stream, because they 

 run vnth the slope of the country and have a lower outlet. 

 Thus the space between the streams is captured by the more 

 vigorous headwater erosion of the north-easterly tributary. 

 Presently a branch more vigorous than the rest captures the 

 headwaters of its neighbor lying to the south-west. This 

 imparts still greater vigor of attack, and the succeeding 

 captures in the same direction are hastened. 



The latest robbery in. the Loup system is that of the head- 

 waters of Wood River. Journeving down from the head- 



on the south. This is true in the case of the Loup, notwith- 

 standing it is a tributary of the Platte, and the anomally is 

 explained by the lower gradient of the Loup. The tributary 

 is at the same level as the parent stream at the point of con- 

 fluence, but the Platte falls 7.1 feet per mile and the Loup 

 only 5.6 feet per mile. This brings the Platte rapidly above 

 the Loup in following them upwards from the point of con- 

 fluence. It is true that this lower gradient of the Loup is 

 itself anomalous, so that I have only explained one anomaly 

 by another. The full explanation of the second would require 

 another article. 



The natural result of excessive deposition along the Platte 

 would be to crowd the mouths of its tributaries eastward and 

 obliterate their old channels. Not only would they be turned 

 to the east by the mass of silt in their former path, but 



