12 2 UNIVERSITY OF COLORADO STUDIES 



it is replaced by Yarrow's Dace, Agosia yarrowi. Omitting the Dulcis 

 Minnow and the Sculpin, the fishes of the Platte and Arkansas drain- 

 ages are species of the mountain front region or with eastern affinities, 

 many ranging rather generally throughout the Mississippi Valley, or 

 being closely related to Mississippi Valley forms. Although the 

 majority of the species of eastern Colorado belong to the Cyprinidae, 

 the Minnows, the higher fishes are represented by both native Cen- 

 trarchidae and Percidae (see p. 120). 



Relations to Altitude 



Considering the fishes of the state as a whole, two distinct groups 

 of species may be recognized, those of the mountain streams and those 

 of the plains streams. In the foothill region both are found, the local 

 conditions determining the relative abundance of each. 



The 6,500-foot contour on the east side of the Continental Divide 

 and the 7,000-foot contour west of the Divide bound roughly a central 

 area the streams of which are quite difterent as fish habitats from the 

 streams of the lower eastern and western portions of the state. The 

 streams thus included in the central portion of the state are the 

 mountain streams, those popularly known as "trout streams," while 

 the streams of the lower flatter areas are the plains streams. 



Mountain Streams 

 The exact altitudinal boundary of the mountain streams varies 

 locally to a considerable extent but even in the San Luis Valley, which 

 is the largest area of high flat land in the state, all of the streams above 

 the 8,000-foot contour are clearly of this class. The mountain streams, 

 regardless of the river system to which they belong, are characterized 

 by several features. Fed the year round by melting snow, they are of 

 clear cold water, the annual thermal variation being rather small. 

 Flowing over igneous rock for the most part the percentage of con- 

 tained solid, either in solution or suspension, is very low. Since these 

 streams have their sources high up in the mountains their fall is very 

 abrupt, and this taken with the usual rocky stream bed makes them 

 turbulent, at the same time giving maximum opportunity for aeration 

 of the water. Mountain streams are subject to sudden and consider- 



