Geneva, it is turbid and full of silt, but when it emerges, it is clear and without sediment. Lakes also act as reservoirs for the water that pours into them at the time of freshets. Rivers emerging from lakes of considerable size vary little in the height of their water atdifferent seasons of the year. They are without floods. The St. Lawrence illustrates this. On the other hand the Ohio with its frequent and terribly destructive floods shows the effect of unrestrained run-off.
Lakes often form most valuable internal waterways, as in the ease of the Great Lakes and the Caspian Sea. Lakes are also most beautiful objects on the landscape and their rippling waters give joy and pleasure to thousands.
In some regions the rainfall is so small that the depressions never fill up sufficiently to overflow their rims. The water is evaporated from the surface as fast as it runs into the lake. Thus all the salt and other soluble substances which have been extracted from the land and brought into the lake by the rivers remain there, since only pure water is evaporated. In this way lakes without outlet become salt. Great Salt Lake in Utah is an example of this. Some salt lakes, like the Caspian Sea, were probablyLAKE DRUMMOND.
A lake in Dismal Swamp, Virginia, which is being filled by vegetable growth.
once a part of the ocean, so that they have always been salt.
As time goes on, more salt is brought to these lakes without outlets, and they become more and more salty. Great Salt Lake has something like 14 or 15% of solid material in its water and the Dead Sea about 25%. An effort to swim in these waters gives one an exceedingly queer sensation. The buoyancy is so great that a large part of the body is out of water, and one finds oneself bobbing around like a cork.
Depressions that are very shallow and are largely filled with vegetablegrowths are called swamps.
155.The Work of Running Water. -Running water has the power of carrying solid materials. If it is moving slowly, this power is not great; if moving swiftly and in great volume, it is tremendous. The carrying power of a stream increases very rapidly if its velocity is in- creased. A stream having its velocity doubled will carry several times as much material as before. Thus it happens that water running over a surface sweeps loose material with it, the amount varying with the ra- pidity and volume of the flowing water.
As this loose material sweeps over solid surfaces, it cuts them down. Thus flowing water is continually wearing down and sweeping away the surface over which it moves. This sort of work is called water erosion.
GULLIES BEING CUT BY RUNNING WATER.
When running water is concentrated into a stream, the work of erosion is also concentrated and the wearing down of the stream bedbecomes comparatively rapid. This cutting down goes on irregularly, being greatest at time of flood and least when the flow is slight. It is estimated that the solid material carried by the Mississippi River from its basin lowers the basin about one foot in 5000 years; but the material which is dissolved increases the amount carried away, so that theTHE BAD LANDS OF DAKOTA.
Running water has so dissected this land as to render it valueless.
basin is lowered a foot in from 3000 to 4000 years.
The channels of some of the streams in this basin are cutting down with far greater rapidity than this. We see gullies cutting down little troughs for themselves several inches deep in one rainstorm. The rapidity of cutting depends upon the material, the slope and the quantity of water. That "the waters wear the stones" was noted even in Job"s time.
When rain falls upon a sloping surface of fine textured, easily eroded material not covered thickly with vegetation, this will beDIVIDES BETWEEN STREAMS.
The ridge in the center of the picture separates two streams flowing in opposite directions.
deeply and fantastically sculptured. An excellent example of this kind of sculpturing is found in the Bad Lands of Dakota. Here travel is exceedingly difficult. It was in these natural fastnesses that the Sioux Indians made their last ineffectual stand against the white man"s civilization.
156.Divides. -If we carefully observe the drainage of a region,we find that the areas from which different streams gather their water are usually so distinctly separated from one another that a line could be drawn so that wherever water falls the rivulets on one side would flow into one stream and on the other side into another. Such a line of the highest land between the drainage areas of neighboring streams is called a divide. The line may be very distinctly marked, as on mountain ridges, or it may be difficult to determine, as in a flat country, but if the drainage is well established, it will be apparent.
If the drainage is not well established, areas may be found which at one time drain in one direction and at another time in another. A singular example of the shifting flow of a drainage area is found in Yellowstone Park where Two-ocean Creek shifts from one side to the other of a fan it has built, and at one time delivers its drainage into the Atlantic Ocean and at another time into the Pacific.
Near the dividing line between two drainage areas, swamps sometimes occur, which have streams flowing from them in two directions so that part of their water goes to one stream and part to another. But as these swamps become better and better drained, each stream will carry off its definite part of the water. Divides are irregular in their height, so that roads and railways in passing from one drainage basin to another usually seek out the lowest part of the divide. In mountain regions these low places are called passes.
Divides do not always remain in the same place, as the river onone side may from some cause become able to carry off the drainage more easily than the river on the other side. It will thus push back its headwaters and shift the divide back until the divide becomes adjusted to the abilities of the two rivers.