Distribution of Lakes.—Although there are few countries where lakes are entirely absent, still it requires little study to see that they are much more thickly grouped in some places than in others. Of the larger lakes, for instance, we have the remarkable group in North America, which together form the greatest extent of fresh water in the world. A similar group of immense lakes is found in Central Africa:—Lakes Victoria Nyanza and Albert Nyanza, whose overflow waters go to form the Nile; Lake Tanganyika, at the source of the Congo; and Lake Nyassa, on a tributary to the Zambesi. In Asia the largest fresh water lake is Lake Baikal, on the upper waters of the Lena. All these freshwater lakes of great size are at the sources of large and important rivers; the salt lakes in which Asia also abounds are at the mouths of large rivers, as the Caspian at the mouth of the Volga, and Aral Sea at the mouth of the Oxus.
Passing from the consideration of these larger lakes, which from their size may be considered inland oceans, and which therefore necessarily occur in small number, we find large numbers of lakes of comparatively small dimensions, and when we consider them attentively we find that they are reducible to a small number of species, and, as in the case of plants and animals, the distribution of these species is regulated chiefly by climate, but also by geological conditions. Perhaps the most important and remarkable species of lakes is that to which the Scottish lakes belong. They are generally characterized by occupying long narrow depressions in the valleys of a mountainous country in the neighbourhood of the sea, and in a temperate climate. On the sea-coast, lakes of this character are found in Norway, Scotland, Newfoundland, Canada, the southern extremity of South America, and the south end of the middle island of New Zealand; somewhat removed from the sea we have the Alpine lakes of Switzerland and Tyrol, and the great Italian lakes, all of which display the same features as those of Scotland or of Norway. In many flat countries lakes are extraordinarily abundant, as for instance in the north part of Russia and Finland, in the southern part of Sweden, in the northern parts of Canada, and on a small scale in the Hebrides.
Lagoons, found on all low sandy coasts, owe their origin to the shifting of the sand under the influence of the wind and tide. They are found at the mouths of large rivers, as on the Baltic and at the mouth of the Garonne.
In volcanic regions lakes are not uncommon, generally of a more or less circular form, and either occupying the site of extinct craters or due to subsidences consequent on volcanic eruptions; such are the Maare of the Eifel in Germany, and many lakes in Italy and in the Azores.
Lakes are not only widely distributed in latitude and longitude, they also occur at all elevations. Indeed, as a certain elevation above the sea produces an effect as regards climate equivalent to a certain increase of latitude, we find lakes existing in the centre of continents, and on high plateaus and mountain ranges, in latitudes where they would be speedily dried up if at the level of the sea. Many of the lakes in Scotland (as Lochs Lomond, Morar, Coruisk), of Norway, of British Columbia, and of southern Chili are raised only by a few feet above the level of the sea, and are separated from it often by only a few hundred yards of land, while in the Cordilleras of South America we have Lake Titicaca 12,500 feet, and in Asia Lake Kokonor 10,500 feet above the sea. Many lakes whose surface is raised high above the level of the sea are so deep that their bottom reaches considerably below that level.
Dimensions of Lakes.—The principal measurements connected with a number of lakes in different parts of the world, presented in the following table, will give a more precise idea of the size of the lakes than could be given by description alone:—
| Name of Lake. | Mean Lati- tude. |
Length. | Breadth (Max.). |
Depth (Max.). |
Height in Feet above the Sea of |
Tempera- ture of Water at Bottom. | |||||||||||||
| Surface. | Bottom. | ||||||||||||||||||
| Miles. | Miles. | Feet. | ° F. | ||||||||||||||||
| Superior | 47° | 45′ | N. | 350 | 100 | 978 | 627 | −351 | 38·8 | ||||||||||
| Michigan | 44° | N. | 320 | 80 | 840 | 594 | −246 | ... | |||||||||||
| St Clair | 42° | 30′ | N. | 18 | 22 | 20 | 570 | +550 | ... | ||||||||||
| Erie | 42° | N. | 220 | 48 | 204 | 564 | +360 | ... | |||||||||||
| Titicaca | 16° | 30′ | S. | 90 | 30 | 924 | 12,500 | −11,576 | 54·6 | ||||||||||
| Kokonor | 37° | N. | 91 | 42 | ... | 10,500 | ... | ... | |||||||||||
| Baikal | 53° | N. | 330 | 40 | 4,080 | 1,360 | −2,720 | ... | |||||||||||
| Balkash | 46° | N. | 280 | 25 | 238 | 72 | +166 | ... | |||||||||||
| Caspian | 42° | N. | 600 | 50 | 3,600 | −85 | −3,685 | 44·6 | |||||||||||
| Dead Sea | 31° | 30′ | N. | 45 | 10 | 1,308 | −1,272 | −2,580 | ... | ||||||||||
| Tanganyika | 6° | S. | 330 | 40 | 1,000 | 2,700 | ... | ... | |||||||||||
| Como | 46° | N. | 48 | 2 | · | 5 | 1,356 | 670 | −686 | ... | |||||||||
| Geneva | 46° | 25′ | N. | 45 | 8 | · | 7 | 1,092 | 1,218 | +126 | 41 | · | 7 | to | 43 | · | 5 | ||
| Constance | 47° | 40′ | N. | 35 | 8 | 394 | 1,300 | +906 | 39·6 | ||||||||||
| Lomond | 56° to 57° 30′ N.
|
20 | 4 | 630 | 25 | −605 | 41 | · | 4 | to | 42 | ||||||||
| Morar | 11 | 1 | · | 5 | 1,020 | 30 | −990 | 40 | · | 8 | to | 41 | · | 4 | |||||
| Ness | 23 | 1 | · | 3 | 774 | 50 | −724 | 41 | · | 2 | to | 42 | · | 4 | |||||
| Lochy | 10 | 1 | 480 | 93 | −387 | 42 | to | 44 | |||||||||||
| Katrine | 7 | 0 | · | 8 | 480 | 364 | −116 | 41·4 | |||||||||||
| Tay | 14 | · | 5 | 1 | 450 | 390 | −60 | 43·9 | |||||||||||
| Rannoch | 14 | · | 5 | 1 | 378 | 668 | +290 | 43·9 | |||||||||||
| Ericht | 14 | · | 5 | 0 | · | 8 | 330 | 1,153 | +823 | 44·7 | |||||||||
| Tummel | 2 | · | 5 | 0 | · | 5 | 120 | 450 | +330 | 45·5 | |||||||||
| Garry | 2 | · | 5 | 0 | · | 3 | 102 | 1,330 | +1,228 | 53·9 | |||||||||
From this table it will be seen that by far the largest continuous sheet of fresh water is the group of North American lakes, and of these Lake Superior is more than double the size of any of the others; this is principally due to its great breadth, as it is very little longer than Lake Michigan. Lake Superior communicates with Lakes Michigan and Huron, which are really branches of one and the same lake, by the St Mary’s river, the fall being 49 feet from Superior to Huron. Huron empties itself into Erie by the St Clair river, Lake St Clair, and finally the Detroit river. Lake Erie overflows by the Niagara river and falls into Lake Ontario, whence the water finally is conveyed to the sea by the St Lawrence. The area of the lakes together is in round numbers 100,000 square miles, and, if that of the St Lawrence and its estuary be added, the water area will be about 150,000 square miles, while the whole drainage area is only 537,000 square miles. Hence of the water conveyed by the St Lawrence to the sea, rather more than one-fourth falls on the surface of the water itself. Looking to their great extent, we should have suspected them to be much deeper than is found to be the case. The deepest, Lake Superior, is no deeper than Loch Morar in Inverness-shire. Comparatively shallow, however, as they are, the bottoms of them all, with the exception of Erie, are several hundred feet below the level of the sea. It has been supposed that in former times this chain of lakes formed an arm of the sea similar to the Baltic in Europe, and in support of this view we have the fact of the discovery of marine forms in Lake Michigan.
