But there is another way in which mountains regulate the supply of water, and prevent it from running away too fast,—one not so effective as the freezing process, but still very useful, because it applies to the lower hills below the line of perpetual snow. This may be well illustrated by the state of some of the Scotch hills in the middle of summer or autumn, when there is little if any snow resting upon them.
Any one familiar with these hills will have noticed how full of water their sides are. Tiny threads of streams trickle slowly along everywhere; peat-beds are saturated with dark-brown water; even the grass and soil are generally more or less wet, especially under pine forests. One can generally get a cup of water somewhere, except after a long dry summer, which is exceptional. Then there is the dew forming every night. Forests with their undergrowth of soil—moss and fern—also help very considerably to check the flow of water. We have often asked ourselves when watching some swift-flowing river, "Where does all this water come from? Why does it not dry up in hot weather?" The answer came fully after we had climbed several mountains, and seen with our eyes the peat-beds among the hills, and heard the trickling of the tiny rivulets hurrying along to feed some neighbouring burn, or perhaps to run into some mountain tarn or loch, and noticed the damp, spongy state of the soil everywhere,—not to mention the little springs which here and there well up to the surface, and so contribute their share.
The rivers and streams of Scotland assume various tints of amber and dark-brown, according to the amount of rain which has recently fallen. These colours are due to organic matter from the peat. Compare Scott's description of the Greta:—
"In yellow light her currents shone,
Matching in hue the favourite gem
Of Albion's mountain diadem."
The waters of some Scotch rivers after heavy rain look as black as pitch.
Nor must we omit the lakes which abound in most mountain regions, and serve as natural reservoirs for the rivers, besides giving a wonderful charm to mountain scenery.
The largest lakes in mountainous regions are found on the courses of the rivers; and there is good reason to believe that they were formed, not by any process of subsidence, but by the same operations that carved out the valleys. In many cases they are due to the damming up of a stream. But in some countries the streams dry up during summer,—in Palestine or Sinai, where there is but little soil on the hills, and consequently hardly any vegetation. Such barren hills cannot hold the continual supplies which pour gently forth from the mountains of higher latitudes.
The Alps feed four of the principal rivers of Europe. We cannot do better than quote Professor Bonney, whose writings on the Alps are familiar to all geologists. In his "Alpine Regions of Switzerland" the following passage occurs:—