Many salt lakes, such as the Dead Sea and Great Salt Lake, are descended from fresh-water ancestors, while others, like the Caspian and Aral Seas, are probably isolated portions of the ocean. Lakes of the former class have usually become salt through a decrease in the humidity of the region where they occur. The water begins to be salt when the aridity is such that evaporation from the lake exceeds its inflow. In this case the inflowing waters bring in small amounts of saline and alkaline matter, which is concentrated as evaporation takes place. The concentration may go on until the point of saturation is reached, or until chemical reactions cause precipitation. In general the least soluble minerals are precipitated first. Thus gypsum begins to be deposited from sea-water when 37% of it has been evaporated; but the saturation-point for salt is not reached until 93% of the water has been evaporated (see [p. 375]). The relations in lakes are similar, and gypsum deposits often underlie those of salt. Deposits of salt and other mineral matters once in solution are making in some salt lakes at the present time, and considerable formations of the same sort have been so made in the past. Buried beneath sediments of other sorts, beds of common salt or of other precipitates are preserved for ages. Lime carbonate has been precipitated in quantity from some extinct lakes ([Fig. 333]).

The lakes which originate by the isolation of portions of the sea are salt at the outset. If inflow exceeds evaporation, they become fresher and may ultimately become fresh; otherwise they remain salt. If evaporation exceeds inflow they diminish in size and their waters become more and more salt or bitter.

Indirect effects of lakes.—Lakes tend to modify the climate of the region where they occur, both by increasing its humidity and by decreasing its range of temperature. They act as reservoirs for surface-waters, and so tend to restrain floods and to promote regularity of stream flow. They purify the waters which enter them by allowing their sediments to settle, and so influence the work and the life of the waters below.

Composition of lake-waters.—The accompanying table[191] shows the composition of various inclosed lake-waters, and gives some idea of the wide range, both in kind and quantity, of the mineral matter held in solution by them. It is to be noted that the table shows the composition of the waters of exceptional, rather than common, lakes. The waters of fresh lakes do not depart widely from those of rivers ([p. 107]).

LocalityAbert Lake, OregonBogdo LakeCaspian Sea. 2° W. S. W. of Pischina, at 15 feet depth, wind, W. S. W.Caspian Sea, near mouth of the VolgaDead Sea, Ras Dale, surface
Specific gravity1023.17..................1.0216
DateMay 3, 1883..................Mar. 20, 1864
AnalystTerreilF. W. TaylorGobelGobelH. Rose
ReferenceLartet Geological Exploration of Dead Sea, p. 278Fourth Ann. Rep. U. S. Geol. Survey, p. 454Lariet Geological Exploration of Dead Sea, p. 284Bischof’s Chemical Geology, Vol. I, p. 89Bischof’s Chemical Geology, Vol. I, p. 89
Sodium, Na2.83874.7001.4440.3081.885
Potassium, K10.8801.041.0398.474
Rubidium, Rb........................
Calcium, Ca......3.647.1854.12382.150
Magnesium, Mg.00213.777.4095.07284.197
Lithium, Li......
Iron, Fe........................Trace
Chlorine, Cl8.410163.3442.7376.457617.628
Bromine, br2.......043Trace.......167
Carbonic acid gas, CO24.653.......1382.3746Trace
Sulphuric acid, H2SO4.509.1981.3372.3109.202
Phosphoric acid, HPO4
Nitric acid, NO3
Boracic acid, H3BO3............
Silica, SiO2.064.006
Alumina, Al2O3..................Trace
Hydrogen in bicarbonates, H.0023.0062......
Ammonium, NH4......Trace
Organic matter........................Trace
27.357256.7506.29401.654025.709
LocalityDead Sea, near the Island, surfaceDead Sea, at 393 ft., between Ras Feschkak and Ras ZerkaDead Sea, at 656 ft., between Ras Feschkak and Ras ZerkaElton LakeElton Lake
Specific gravity1.16471.22251.2300............
DateApr. 7, 1864Mar. 15, 1804Mar. 15, 1864AprilAugust
AnalystTerreilTerreilTerreilGobelErdman
ReferenceLartet Geological Exploration of Dead Sea, p. 278Lartet Geological Exploration of Dead Sea, p. 278Lartet Geological Exploration of Dead Sea, p. 278Bischof’s Chemical Geology, Vol. I, p. 403–405Bischof’s Chemical Geology, Vol. I, p. 403–405
Sodium, Na22.40025.07125.10751.59029.300
Potassium, K3.5473.9904.5031.162
Rubidium, Rb........................
Calcium, Ca9.0943.7044.218.......106
Magnesium, Mg25.52941.30642.00629.97145.598
Lithium, Li..................
Iron, FeTraceTraceTrace............
Chlorine, Cl126.521166.340170.425159.498166.890
Bromine, br24.5684.8704.385.059......
Carbonic acid gas, CO2TraceTraceTrace.......272
Sulphuric acid, H2SO4.494.451.45913.32017.734
Phosphoric acid, HPO4
Nitric acid, NO3
Boracic acid, H3BO3..................
Silica, SiO2TraceTraceTrace
Alumina, Al2O3TraceTraceTrace
Hydrogen in bicarbonates, H..................
Ammonium, NH4TraceTraceTrace............
Organic matterTraceTraceTraceTrace5.080
192.153245.732251.103255.600264.980
LocalityElton LakeGreat Salt LakeGreat Salt LakeGreat Salt LakeHumboldt[192] LakeIndevak Lake
Specific gravity1.272881.1702.41.1021.007......
DateOctober18501869Aug., 1873............
AnalystH. RoseL. D. GaleO. D. AllenH. BassettO. D. AllenGobel
ReferenceBischof’s Chemical Geology. Vol. I, p. 403–405Stambury’s Expedition to Great Salt Lake, p. 410U. S. Geological Expl. 40th par. 1877, Vol. II, p. 435Amer. Chemist, 1874, p. 395U. S. Expl. 40th par. 1877, Vol. I, p. 528Lartet Expl. of Dead Sea, p. 284
Sodium, Na15.06085.33049.69038.3.2784294.050
Potassium, K1.2042.4079.9.06083.529
Rubidium, Rb..............................
Calcium, Ca......Trace[196].255.6.01257.123
Magnesium, Mg60.540.6363.7803.0.016485.076
Lithium, LiTraceTrace
Iron, Fe....................................
Chlorine, Cl171.936124.45483.94673.6.29545158.687
Bromine, br2Trace......
Carbonic acid gas, CO2.........................20126......
Sulphuric acid, H2SO442.56012.4009.8588.8.030403.065
Phosphoric acid, HPO4.00069
Nitric acid, NO3.......03250
Boracic acid, H3BO3TraceTrace
Silica, SiO2.......03250
Alumina, Al2O3
Hydrogen in bicarbonates, H
Ammonium, NH4......
Organic matterTrace.............................
291.300222.820149.936134.2.92800261.530
LocalitySoda Lake, near Ragtown, Nev., at 1 foot below surfaceSoda Lake, near Ragtown, Nev., at 100 feet below surfaceMono Lake, Cal., at 1 foot below surfaceUrmiah LakeOwen’s Lake, Cal.
Specific gravity1.1011.1011.0481.1551.051
Date............July 16, 1883............
AnalystT. M. ChatardT. M. ChatardT. M. ChatardHitchcockO. Loew
ReferenceAnte, p. 70Ante, p. 70Bulletin No. 9. U. S. Survey, p. 26Lartet Geological Exploration of Dead Sea, p. 284Appendix JJ Ann. Rep. Chief Engineers, 1876 p. 190
Sodium, Na41.63240.20618.10074.89021.650
Potassium, K2.2902.4251.1112.751
Rubidium, Rb..................
Calcium, Ca.............278.529Trace
Magnesium, Mg.245.245.1252.914Trace
Lithium, Li......Trace
Iron, Fe..............................
Chlorine, Cl41.49640.20611.610119.49613.440
Bromine, br2........................
Carbonic acid gas, CO215.650[7]18.058[7]14.465[7]......13.140
Sulphuric acid, H2SO411.77111.9436.5207.6719.362
Phosphoric acid, HPO4............
Nitric acid, NO3..................Trace
Boracic acid, H3BO3.285.287.153Trace
Silica, SiO2.275.281.268.164
Alumina, Al2O3......Trace
Hydrogen in bicarbonates, H......
Ammonium, NH4......
Organic matter........................Trace
113.644113.65149.630205.50060.507
LocalityPyramid Lake,[193] Nev.Sevier Lake, UtahWalker Lake,[194] Nev.Winnemucca Lake, Nev.Van LakeAral Sea
Specific gravity............1.0031.001............
DateAug. 18821872Sept., 1882Aug., 1882............
AnalystF. W. ClarkeO. LoewF. W. ClarkeF. W. ClarkeChancourtois......
ReferenceAnte, pp. 57 and 58U. S. Survey, W. 100 M., Vol. III, p. 144Ante, p. 70Ante, p. 63Bischof’s Chemical Geology, Vol. I, p. 94Roth Chemical Geology, p. 465
Sodium, Na1.179628.840.855351.29708.502[4]2.4512
Potassium, K.0733Trace.0686.246.0584
Rubidium, Rb.........................0022
Calcium, Ca.0089.118.02215.0196.......4581
Magnesium, Mg.07972.000.03830.0173.157[197].5965
Lithium, Li...........
Iron, Fe.......................Trace[197].0008
Chlorine, Cl1.430045.500.583751.69345.6933.8386
Bromine, br2.........................0029
Carbonic acid gas, CO2.4900[7].......47445[198].3458[198]5.267[195].0918
Sulphuric acid, H2SO4.18229.345.52000.13332.5553.3368
Phosphoric acid, HPO4.......0011
Nitric acid, NO3Trace
Boracic acid, H3BO3........................
Silica, SiO2.0334.00750.0275.180.0032
Alumina, Al2O3..................
Hydrogen in bicarbonates, H......
Ammonium, NH4Trace
Organic matter..............................Trace
3.486186.4032.501503.602522.60010.8416

CHAPTER VII.