DISPERSION OF FRESH-WATER FISHES. 113 



the range of most of the larger fishes of the river, 

 but the streams above it have their quota of Dart- 

 ers and Minnows. It is evident that the past his- 

 tory of the stream must enter as a factor into this 

 discussion, but this past history it is not always 

 possible to trace. Dams or artificial waterfalls 

 now check the free movement of many species, 

 especially those of migratory habits ; while, con- 

 versely, numerous other species have extended 

 their range through the agency of canals. 1 



Every year fishes are swept down the rivers by 

 the winter's floods ; and in the spring, as the spawn- 

 ing season approaches, almost every species is 

 found working its way up the stream. In some 

 cases, notably the Quinnat Salmon 2 and the Blue- 

 back Salmon, 3 the length of these migrations is 

 surprisingly great. To some species rapids and 

 shallows have proved a sufficient barrier, and other 

 kinds have been kept back by unfavorable condi- 

 tions of various sorts. Streams whose waters are 

 always charged with silt or sediment, as the Mis- 

 souri, Arkansas, or Brazos, do not invite fishes ; and 

 even the occasional floods of red mud such as dis- 

 figure otherwise clear streams, like the Red River 

 or the Colorado (of Texas), are unfavorable. Ex- 

 tremely unfavorable also is the condition which 

 obtains in many rivers of the Southwest; as for 

 example, the Red River, the Sabine, and the Trin- 

 ity, which are full from bank to bank in winter and 



1 Thus, Dorosoma cepedianum Le Sueur, and Clupea chrysochloris 

 Rafinesque, have found their way into Lake Michigan through 

 canals. 



2 Oncorhynchus tschcnvytscha Walbaum. 



3 Oncorhynchus nerka Walbaum. 



