Page:EB1911 - Volume 16.djvu/107

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LAKE
87


various stages of development a fairly good idea of the course they run can be obtained.

In humid regions two processes tend to the extinction of a lake, viz. the deposition of detrital matter in the lake, and the lowering of the lake by the cutting action of the outlet stream on the barrier. These outgoing streams, however, being very pure and clear, all detrital matter having been deposited in the lake, have less eroding power than inflowing streams. One of the best examples of the action of the filling-up process is presented by Lochs Doine, Voil and Lubnaig in the Callander district of Scotland. In post-glacial times these three lochs formed, without doubt, one continuous sheet of water, which subsequently became divided into three different basins by the deposition of sediment. Loch Doine has been separated from Loch Voil by alluvial cones laid down by two opposite streams. At the head of Loch Doine there is an alluvial flat that stretches for 11/2 m., formed by the Lochlarig river and its tributaries. The long stretch of alluvium that separates Loch Voil from Loch Lubnaig has been laid down by Calair Burn in Glen Buckie, by the Kirkton Burn at Balquhidder, and by various streams on both sides of Strathyre. Loch Lubnaig once extended to a point 3/4 m. beyond its present outlet, the level of the loch being lowered about 20 ft. by the denuding action of the river Leny on its rocky barrier.

In arid regions, where the rainfall is often less than 10 ins. in the year, the action of winds in the transport of sand and dust is more in evidence than that of rivers, and the effects of evaporation greater than of precipitation. Salt and bitter lakes prevail in these regions. Many salt lakes, such as the Dead Sea and the Great Salt Lake, are descended from fresh-water ancestors, while others, like the Caspian and Aral Seas, are isolated portions of the ocean. Lakes of the first group have usually become salt through a decrease in the rainfall of the region in which they occur. The water begins to get salt when the evaporation from the lake exceeds the inflow. The inflowing waters bring in a small amount of saline and alkaline matter, which becomes more and more concentrated as the evaporation increases. In lakes of the second group the waters were salt at the outset. If inflow exceeds evaporation they become fresher, and may ultimately become quite fresh. If the evaporation exceeds the inflow they diminish in size, and their waters become more and more salt and bitter. The first lake which occupied the basin of the Great Salt Lake of Utah appears to have been fresh, then with a change of climate to have become a salt lake. Another change of climate taking place, the level of the lake rose until it overflowed, the outlet being by the Snake river; the lake then became fresh. This expanded lake has been called Lake Bonneville, which covered an area of about 17,000 sq. m. Another change of climate in the direction of aridity reduced the level of the lake below the level of the outlet, the waters became gradually salt, and the former great fresh-water lake has been reduced gradually to the relatively small Great Salt Lake of the present day. The sites of extinct salt lakes yield salt in commercial quantities.

The Water of Lakes.—(a) Composition.—It is interesting to compare the quantity of solid matter in, and the chemical composition of, the water of fresh and salt lakes:—

  Total Solids by Evaporation
expressed in Grams per Litre.
Great Salt Lake (Russell) 238.12 
Lake of Geneva (Delebecque)     0.1775

The following analysis of a sample of the water of the Great Salt Lake (Utah, U.S.A.) is given by I. C. Russell:—

  Grams per Litre.   Probable Combination.
Na 75.825 NaCl 192.860
K 3.925 K2SO4 8.756
Li 0.021 Li2SO4 0.166
Mg 4.844 MgCl2 15.044
Ca 2.424 MgSO4 5.216
Cl 128.278 CaSO4 8.240
SO3 12.522 Fe2O3 + Al2O3 0.004
O in sulphate 2.494 SiO2 0.018
Fe2O3 + Al2O3 0.004 Surplus SO3 0.051
SiO2 0.018  
Bo2O3 trace  
Br3 faint trace  

The following analyses of the waters of other salt lakes are given by Mr J. Y. Buchanan (Art. “Lake,” Ency. Brit., 9th Ed.), an analysis of sea-water from the Suez Canal being added for comparison:—

  Koko-nor. Aral Sea Caspian Sea. Urmia Sea. Dead Sea. Lake Van. Suez Canal,
Ismailia.
Open. Karabugas.
 Specific Gravity   1.00907 ..   1.01106   1.26217   1.17500 .. 1.01800  1.03898
 Percentage of Salt 1.11 1.09 1.30 28.5 22.28 22.13 1.73   5.1  
Name of Salt. Grams of Salt per 1000 Grams of Water.
 Bicarbonate of Lime 0.6804 0.2185 0.1123 .. .. .. .. 0.0072
 Bicarbonate of Iron 0.0053 .. 0.0014 .. .. .. .. 0.0069
 Bicarbonate of Magnesia  0.6598 .. .. .. .. .. 0.4031 ..
 Carbonate of Soda .. .. .. .. .. .. 5.3976 ..
 Phosphate of Lime 0.0028 .. 0.0021 .. .. .. 5.3976 0.0029
 Sulphate of Lime .. 1.3499 0.9004 ..  0.7570  0.8600 .. 1.8593
 Sulphate of Magnesia 0.9324 2.9799 3.0855 61.9350 13.5460 .. 0.2592 3.2231
 Sulphate of Soda 1.7241 .. .. .. .. .. 2.5673 ..
 Sulphate of Potash .. .. .. .. .. .. 0.5363 ..
 Chloride of Sodium 6.9008 6.2356 8.1163 83.2840 192.4100  76.5000 8.0500 40.4336 
 Chloride of Potassium 0.2209 0.1145 0.1339  9.9560 .. 23.3000 .. 0.6231
 Chloride of Rubidium 0.0055 .. 0.0034  0.2510 .. .. .. 0.0265
 Chloride of Magnesium .. 0.0003 0.6115 129.3770  15.4610 95.6000 .. 0.0027
 Chloride of Calcium .. .. .. ..  0.5990 22.4500 .. ..
 Bromide of Magnesium 0.0045 .. 0.0081  0.1930 ..  2.3100 .. 0.0779
 Silica 0.0098 .. 0.0024 .. ..  0.2400 0.0761 0.0027
Total Solid Matter 11.1463  10.8987  12.9773  284.9960  222.2600  221.2600  17.2899  51.0264 

This table embraces examples of several types of salt lakes. In the Koko-nor, Aral and open Caspian Seas we have examples of the moderately salt, non-saturated waters. In the Karabugas, a branch gulf of the Caspian, Urmia and the Dead Seas we have examples of saturated waters containing principally chlorides. Lake Van is an example of the alkaline seas which also occur in Egypt, Hungary and other countries. Their peculiarity consists in the quantity of carbonate of soda dissolved in their waters, which is collected by the inhabitants for domestic and commercial purposes.

The following analyses by Dr Bourcart give an idea of the chemical composition of the water of fresh-water lakes in grams per litre:—

  Tanay. Bleu. Märjelen. St Gothard.
 SiO2 0.003  0.0042 0.0014 0.0008 
 Fe2O3 + Al2O3 0.0012 0.0006 0.0008 trace
 NaCl 0.0017 .. .. ..
 Na2SO4 0.0011 0.0038 0.0031 0.00085
 Na2CO3 .. .. .. 0.00128
 K2SO4 0.0021 0.0028 0.0044 ..
 K2CO3 .. .. 0.0003 0.00130
 MgSO4 0.006  0.0305 .. ..
 MgCO3 0.0046 0.0158 0.0008 0.00015
 CaSO4 .. .. .. ..
 CaCO3 0.107  0.1189 0.0061 0.00178
 MnO 0.001  .. .. ..