4 BULLETIN OP THE UNITED STATES FISH COMMISSION. 
Most of tbe streams of Colorado rise in springs in or above the mountain meadows, 
many of them having their origin in banks of snow, which the clear weather of sum- 
mer is not sufficient wholly to melt. 
These streams are clear and very cold. In their descent from the snow-banks 
they are brawling and turbulent, often so much so as to be unfitted for fish life. In 
their course through the mountain meadows (very similar to the Alp” pastures of 
Switzerland) the streams are usually of gentle current, with many windings and with 
occasional deep holes beloved of trout. Lower down most of them pass to the valleys 
through deep canons, some of them very deep and with many rapids. Vertical falls 
are, however, very rare in Colorado, and most of these canons form no obstacle to trout. 
Below the caiions, the stream, still clear and cold, enters the valley, where the flat 
bottom is usually covered deep with sediment which the streams bring down. 
Here the water grows warmer, the fine silt renders it more or less turbid, and at 
last it becomes unfit for trout and at the same time suitable for the suckers and chubs. 
In the winter and spring the water is cold and clear for some distance down the val- 
leys. In these seasons the trout extend their range to a corresponding degree. In 
the summer and fall they are more or less confined to the mountains or the caiions. 
Often the stream after entering the valley cuts its way through a moraine deposit. In 
that case its course is filled with boulders, and its waters are sometimes as brawling 
in a boulder-strewn valley as in the mountains. 
In some cases placer- mining and stamp-mills have filled the waters of otherwise 
clear streams with yellow or red clay, rendering them almost uninhabitable for trout. 
Parts of the upper Arkansas and Grand Elvers have been almost ruined as trout 
streams by mining operations. In a few streams the presence of iron springs seems 
to exclude all fishes. 
After reaching the base of the mountains the streams flow with little current over 
the ill-defined beds across the plains. They tear up the fine soil and shift it from place 
to i)lace. Occasional rains swell the dry beds of “ Saud-Arro^msp’ the stream be- 
comes more and more charged with clayey sediment, and in time not one of these 
rivers would be recognized as the crystal-clear stream which came down the mount- 
ains. The Platte spreads out broad and shallow over the plain, and its course is full 
of quicksands. Its banks arc rarely well defined. The Arkansas resembles the Platte, 
being even more muddy, however, and the Eio Grande is similar to it. The Colorado 
carries the peculiar erosion of the mesas to a still greater extent as it goes southward. 
The stream is large and swift, with treacherous currents and shifting bottom. As 
no rain-fall or frosts wear away its banks, it sinks deeper and deeper below the sur- 
face, until it forms the deepest gorge in the world, with banks which are vertical or, 
like stair-cases. 
In the progress of settlement of the valleys of Colorado the streams have become 
more and more largely used for irrigation. Below the mouth of the canons dam after 
dam and ditch after ditch turn off the water. In summer the beds of even large 
rivers (as the Eio Grande) are left wholly dry, all the water being turned into these 
ditches. Much of this water is consumed by the arid land and its vegetation ; the 
rest seeps back, turbid and yellow, into the bed of the stream, to be again intercepted 
as soon as enough has accumulated to be worth taking. In some valleys, as in the 
San Luis, in the dry season there is scarcely a droji of water in the river-bed that has 
