126 UNIVERSITY OF COLORADO STUDIES 



in volume of water the percentage of solid material contained, 

 especially in suspension, is high, since the plains soil is easily washed. 

 In mid-summer the streams become clear but very shallow so that 

 the relatively small quantity of water is easily heated by the sun. 

 As compared with the fairly constant temperature of the mountain 

 streams the fishes of the plains streams are subjected to a wide range 

 of temperatures; the water in a small pool in the South Platte at 

 Julesburg, from which several hundred specimens of Fundulus zebrinus, 

 Notropis scylla and Semotilus atromaculatus were taken on July 19, 

 191 2, was heated to 84° F., although connected by a small stream of 

 running water with the main channel. The periodic change of stream 

 level and the arid climate limit, indirectly, the fish fauna through the 

 elimination of the shore zone of aquatic vegetation. Plants so com- 

 mon along streams in the Mississippi Valley are almost entirely lacking 

 along the plains streams in eastern Colorado. Such shore vegetation 

 would be killed by drying, when the water recedes in the summer, since 

 the margin of the stream does not remain moist as it does in more 

 humid regions, or if there were any such vegetation it would be swept 

 away by the current during the high water of the spring. The 

 importance of this shore vegetation comes from the fact that it shelters 

 small crustaceans, insect larvae and other forms which are food for 

 Centrarchids and various species of minnows. It is interesting in 

 this connection to note that at Wray and Ovid — localities in the 

 eastern "rainbelt" of Colorado — some of this aquatic shore vegeta- 

 tion was found along the streams in more favored places. Here, 

 as expected, it contained Centrarchids, Percids and large numbers 

 of minnows. 



The fish fauna of the true plains streams in eastern Colorado is 

 strikingly different from that of the mountain streams. The suckers, 

 the top-minnows, Fundulus floripinnis and Fundulus zebrinus, and the 

 various species of true minnows, particularly Notropis scylla, Notropis 

 piptolepis and Semotilus atromaculatus, are the forms regularly found. 

 In the larger streams may be added the catfish. 



In the plains lakes and reservoirs are found the optimum conditions 

 for Sunfishes, Perch and Catfishes, since the rather constant water 



