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.! 
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? and the Blue- 
back Salmon,’ 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 dise 
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 tschawytscha Walbaum. 
8 Oncorhynchus nerka Walbaum. 
8 
