A STUDY OF LAKES
Lakes are ephemeral features on the face of the earth. Compared to the
tens of millions of years of known earth history, lakes, even large
ones, are very short lived. They may, in truth, be regarded as merely
results of the temporary obstructions to drainage. Lake basins are
known to originate in many ways, and there are various means by which
they are destroyed. Not attempting an exhaustive, scientific treatment
of the subject, our present purpose may be well served by describing
and explaining some of the better known and more remarkable lakes of
the world.
Even a cursory examination of a large map of the world reveals the fact
that the regions of most numerous lakes are those which were recently
occupied by glaciers--either the vast ice sheets of the Glacial epoch
or mountain (or valley) glaciers. This is because more lakes of the
present time have come into existence as direct or indirect results
of glaciation than by any other cause. A considerable number of these
lakes occupy rock basins which have been eroded or excavated by the
direct action of flowing ice. Small lakes of this sort are commonly
found in the upper parts of valleys formerly occupied by mountain or
so-called Alpine glaciers, because there the excavating power of such
glaciers was especially effective. More rarely rock basins have been
scoured out by glaciers farther down their valleys. Many lakes occupy
rock basins excavated by ice in the high Sierra Nevada and Cascade
Ranges, in the Rocky Mountains from Colorado into Canada, in the Alps,
and in the mountains of Norway. Few, if any of them are, however,
large or famous. Other lakes, some of very considerable size, occupy
rock basins scoured out by the passage of the great ice sheets of
the Glacial epoch in North America and Europe, though they are less
common than formerly supposed. Some of the many lake basins of Ontario,
Canada, are quite certainly of this origin, as might well be expected,
because the power of the great ice sheet was there in general notably
greater than south of the Great Lakes where the tendency was to unload
or deposit the eroded materials as shown by the great accumulations of
glacial débris (moraines).
Where the ice walls of certain existing glaciers form dams across
valleys, waters are ponded, a small lake of this kind occurring
alongside the Great Aletsch Glacier of the Alps, where its wall is
slowly moving past a tributary valley. Lakes of this kind also occur
in Greenland and in Alaska, but none are of considerable size. During
the Great Ice Age, however, literally thousands of large and small
lakes were formed, both during the advance and the retreat of the
ice, wherever the glacier wall blocked valleys which sloped downward
toward the ice. New York State furnishes many fine examples of large
and small lakes of this sort. Thus, when the great glacier was melting
in northern New York, waters hundreds of feet deep and many miles long
were ponded between two ice lobes--one retreating eastward and the
other westward from the Mohawk Valley. An ice dam lake was also formed
a little later, when an ice wall blocked the northern part of the
Black River Valley just west of the Adirondack Mountains and caused a
lake covering about 200 square miles. One of the largest of all known
ice dam lakes has been called Lake Agassiz, which attained a maximum
length of over 700 miles and a width of 250 miles in the Red River of
the North region of eastern North Dakota, western and northwestern
Minnesota, and northward into Canada, most of its area having been
in Canada. It began as a small lake with southward drainage into the
Mississippi when the great northward retreating ice sheet formed a dam
across the valley of the Red River of the North. The retreating ice
continued to block the northward drainage until the vast lake, covering
a greater territory than all of the present Great Lakes combined,
was developed. Beaches, bars, deltas and the outflow channel of this
remarkable lake are wonderfully well preserved. Lake Winnipeg is a mere
remnant of great Lake Agassiz.
Many ponds and small lakes occupy basins formed by irregular
accumulations of glacial (morainic) materials. Still others lie in
depressions which formed by the melting of masses of ice which became
wholly or partly buried by ice deposits, or by sediments washed into
bodies of water which were held up by ice dams. Depressions of the
latter kind are commonly found as pits or so-called "kettle holes"
below the general level of sand flats or sand plains of glacial lake
origin.
Most common of all lake basins of glacial origin are those formed by
accumulation of glacial débris or morainic materials acting as natural
dams across valleys. This is, in fact, the most common of all ways by
which existing lake basins, some of them very large, have been formed.
Most of the thousands of ponds and lakes of Minnesota, Wisconsin, and
northern New York belong in this category.
In the Adirondack Mountains, for example, most of the lakes, like the
well-known Lake Placid, Saranac Lakes, Long Lake, and Schroon Lake,
have their waters ponded by single dams of glacial débris across
valleys. In some cases a series of such dams blockades a valley
and forms a chain of lakes like the well-known Fulton Chain in the
Adirondacks. Less commonly the lake may have its waters ponded by two
natural dams of glacial débris, one across a valley at each end of a
lake. A very fine, large scale example of the last-named type is the
famous Lake George in the southeastern Adirondacks. It is over 30 miles
long and from 1 to 2-1/2 miles wide. It lies in the bottom of a deep,
narrow mountain valley, mountain sides rising very steeply from a few
hundred feet to 2,000 or more feet above its shores. There are many
islands, especially in the so-called "Narrows," thus greatly enhancing
the scenic effect. The valley itself has been produced by a combination
of faulting and erosion. There was a preglacial stream divide at the
present location of the "Narrows." This divide was somewhat reduced by
ice erosion when the deep, narrow body of ice plowed its way through
the valley during the Ice Age. During the retreat of the ice heavy
morainic accumulations were left as dams across the valley at each end
of the lake.
Another remarkable body of water, similar to Lake George in its origin,
is Chautauqua Lake of western New York, famous for its Chautauqua
assemblies. It lies 1,338 feet above sea level, with its northern end
near the edge of the steep front of the plateau overlooking Lake Erie.
Chautauqua Lake really consists of parts of two valleys, one sloping
north and the other sloping south, each dammed by glacial deposits.
The famous Alpine lakes--Garda, Como, and Maggiore--have resulted from
deposition of glacial morainic materials under conditions different
from those above described. In these cases great mountain or valley
glaciers once flowed down the valleys and spread out part way upon the
Italian plain. Great accumulations of glacial débris took place around
the borders of the glacier lobes, and, after retreat of the ice, the
glacial deposits acted as dams ponding the waters far back into the
mountain valleys.
The origin and history of the Great Lakes constitutes one of the most
interesting and remarkable chapters in the recent geological history of
North America. Most of the salient points have been well worked out and
they may be very briefly summarized, as follows: Before the Ice Age the
Great Lakes did not exist, because the region, prior to that time, had
been land subjected to erosion for millions of years--a time altogether
too long for any lake to survive. Their sites were occupied by broad,
low, stream-cut valleys which were quite certainly locally somewhat
deepened by ice erosion during the Ice Age. Ice erosion is, however,
altogether insufficient to account for the great closed basins.
The two most important factors entering into the formation of the
basins of the Great Lakes were doubtless the great glacial (morainic)
accumulations acting as dams along the south side, and the tilting
of the land downward on the north side of the region. In support of
this explanation it has been established that the great dumping ground
of ice-transported materials from the north was in general along the
southern side of the Great Lakes and southward. It has also been well
established that, late in the Ice Age, the land on the southern side
of the Great Lakes region was lower than at present, as proved by the
tilted character of beaches of the well-known extinct glacial lakes
which were the ancestors of the present lakes. Such a down-warp of the
land must have helped to form the closed basins by tending to stop the
southward and southwestward drainage of the region.
[Illustration: Fig. 27.--Sketch map showing a very early stage in the
history of the Great Lakes when two relatively small lakes in front of
the ice wall separately drained into the Mississippi River. (Drawn by
the author from map by Taylor & Leverett.)]
[Illustration: Fig. 28.--Lake Whittlesey stage of the Great Lakes
history when the ice had retreated far enough to allow the eastern and
western ice margin waters to join with a single outlet past Chicago.
(Drawn by the author from a map by Taylor & Leverett.)]
[Illustration: Fig. 29.--The Algonquin-Iroquois stage of the Great
Lakes when their whole area was ice-free, and all their waters drained
through the Mohawk-Hudson Valleys of New York into the Atlantic Ocean.
(After Taylor, published by New York State Museum.)]
We shall now very briefly trace out the principal stages in the
history of the Great Lakes during the final retreat of the vast ice
sheet. This may best be done by the aid of maps which need only brief
explanation. When the ice sheet had retreated far enough northward to
uncover the very southern end of the Lake Michigan basin and a little
beyond, a small glacial lake (Lake Chicago) developed against the ice
wall. Its outlet was through the Illinois River and thence into the
Mississippi. At the same time a larger glacial lake, held up by the ice
wall, developed over the western part of the Erie basin and beyond.
Its outlet was through the Wabash River. With further retreat of the
ice a large lake (Whittlesey) covering considerably more than the area
of Lake Erie developed, with outlet westward across Michigan into the
enlarged Lake Chicago which continued to drain into the Illinois River.
During a still later stage of ice withdrawal the remarkable set of
three glacial lakes existed--Lakes Duluth, Chicago, and Lundy. Each of
these large lakes had its own outlet. Lake Duluth covered about half
of the Lake Superior basin and drained through the St. Croix River
into the Mississippi. Lake Chicago expanded to cover nearly all of the
Michigan basin and continued to drain through the Illinois River. Lake
Lundy covered not only more than the area of the Erie basin, but also
considerable territory north of Detroit, and drained eastward alongside
the ice lobe of the Ontario basin through the Mohawk and Hudson
valleys of New York, and into the Atlantic Ocean. Just after the ice
completely withdrew from the area now occupied by the Great Lakes, but
still blocked the St. Lawrence Valley, the vast body of water called
Lake Algonquin more than covered the sites of the present Superior,
Michigan, and Huron. At this time the land was distinctly lower toward
the northeast than at present, causing the outlets to the west to
be abandoned. The great Lake Algonquin poured its waters eastward
through the Trent River channel of Ontario, Canada, into glacial Lake
Iroquois, which was the great ancestor of Lake Ontario. Lake Iroquois,
in turn, had its outlet eastward through the Mohawk and Hudson Valleys
of New York. For part of the time at least, Lake Erie maintained a
separate existence discharging into Lake Iroquois near Buffalo. During
the Algonquin-Iroquois stage the combined area of all the lakes was
notably greater than the present area of the Great Lakes. The volume
of water discharged by the lakes through the Mohawk Valley of New York
was doubtless greater than that which now goes over Niagara Falls.
Gradually, as the St. Lawrence ice lobe waned, the outlet waters of the
lakes began to move alongside the ice through the St. Lawrence Valley.
Finally the ice withdrew far enough to free the St. Lawrence Valley and
the waters of the Great Lakes region dropped to a still lower level,
bringing about the Nipissing Great Lakes stage not greatly different
from the present. East and northeast of the Lakes the land was low
enough to allow tidewater (the so-called Champlain Sea) to extend
through the Hudson, Champlain, and St. Lawrence Valleys, and possibly
into the Ontario basin, as proved by the occurrence of marine beaches
and fossils. The waters in the Erie and Ontario basins covered about
the present areas, while the Nipissing Lakes, which covered a little
more than the present areas of the three upper Great Lakes, had their
outlet through the Ottawa River channel into tidewater (Champlain
Sea). Postglacial warping of the land has brought the whole region to
the present condition.
[Illustration: Fig. 30.--Map showing next to the present stage of the
Great Lakes history when the land was lower on the north and the upper
(Nipissing) lakes drained through the Ottawa River Valley into an arm
of the sea (Champlain Sea) which reached through the Champlain and
Hudson Valleys. (After Taylor, published by New York State Museum.)]
Many lakes, including some remarkable ones, occupy basins which are
directly due to movements of the earth's crust--either faulting or
warping. An example of a lake occupying part of a fault basin is the
famous Dead Sea of Palestine. This lake lies in the lowest part of the
Jordan Valley, which has geologically recently come into existence
by the sinking of a long, narrow block of earth for several thousand
feet between two great earth fractures (faults). The Dead Sea covers
about 500 square miles and its surface lies about 1,300 feet below
sea level, which makes it the lowest lake in the world. Almost equally
remarkable is the fact that its depth is about 1,300 feet, so that the
lowest part of the lake basin is 2,600 feet below sea level. The lake
contains approximately 24 per cent salt, mostly common table salt,
causing it to be a thick brine in which there is neither plant nor
animal life--hence the name "Dead Sea." At one time, probably just
after the Ice Age, the lake was much larger and deeper, when it filled
a considerable part of the Jordan Valley and had an outlet to the
south. During the high-level stage the water was fresh, but gradually,
as the climate became drier, evaporation was greater than intake,
the outlet was abandoned, and the mineral matter (mostly chloride of
magnesia and common table salt) carried by the streams in solution
into the shrinking lake steadily accumulated until the high degree of
salinity of the present time has been reached.
Great Salt Lake, Utah, is a remarkable lake whose history has been
carefully studied. It occupies the lowest position of an extensive
basin which, in turn, forms but part of the whole great district of
Utah which has geologically recently sunk thousands of feet on the
west side of the great fault already described as occurring along the
western base of the Wasatch Mountains. At present the lake covers
about 2,000 square miles, but its area fluctuates considerably. It is
scarcely believable that this big lake has an average depth of only
fifteen feet and a maximum depth of only fifty feet. It lies 4,200 feet
above sea level, and it carries about 18 per cent salts in solution.
Most abundant by far is common table salt, of which there are no less
than 5,000,000,000 tons in solution. The waters also contain about
900,000,000 tons of other salts. Should the lake completely disappear
by evaporation, these salts would be deposited. Allowing for cars 40
feet long and of 40 tons capacity, a train more than 1,000,000 miles
long would be required to carry the salts. What has been the source of
these salts? Great Salt Lake is not, as supposed by some, a remnant
of an ocean once covering the region. Briefly, the explanation is as
follows: At one time, when the climate was moister, the basin now only
in part occupied by the lake was filled to overflowing with an outlet
north into the Snake and Columbia rivers. That great body of water
(called "Lake Bonneville") covered nearly 20,000 square miles and its
depth was about 1,000 feet deeper than now, the present depth being
very small. Because it had an outlet that lake was, of course, fresh.
Beaches and shore lines 1,000 feet above the present lake, and at
various lower levels, are still wonderfully well preserved. When, due
to climatic change, evaporation exceeded intake by streams, the outlet
was cut off. But slowly, as the lake shrank, streams (especially the
Jordan River) carried a little salt in solution, the percentage of salt
increasing until the present stage has been reached. In a real sense,
much of the salt was once in the sea, because it has been dissolved out
of strata which accumulated under sea water long before the basin of
Great Salt Lake came into existence.
Another famous lake, which also occupies part of a basin due to
faulting, is Lake Tahoe in the Sierra Nevada Mountains, near Truckee,
California. This lake, whose length is 21 miles, and width 12 miles,
lies 6,225 feet above sea level. On almost all sides steep mountains
rise several thousand feet above its waters. Its great depth of
1,635 feet makes it, so far as known, the second deepest lake in
North America, Crater Lake, Oregon, only outranking it. The water
is exceedingly clear. An experiment some years ago showed that a
white disk eight inches in diameter could actually be seen through
a thickness of 216 feet of its water. "The statement sometimes made
that 'Tahoe is an old volcanic crater' is not true. The region about
the lake shows evidences of volcanic activity of various kinds, and
the lake waters themselves have probably been dammed at times by
outpourings of lava. A lava flow appears to have temporarily filled
the outlet channel below Tahoe City. The lake, however, lies in a
structural depression--a dropped (fault) block in the earth's crust."
(U. S. Geological Survey.)
The basin of the largest lake in the world--the Caspian Sea--has
resulted from warping of the earth's crust. It has an area of 170,000
square miles, a maximum depth of 3,200 feet, and its surface is about
90 feet below sea level. The composition of its water and some of its
animal life indicate that it was once an arm of the sea. It has been
detached or cut off by an upwarp of the land between it and the Black
Sea region. If this great lake is a cut-off arm of the sea, with no
outlet, how do we explain the fact that its salinity is much less than
that of the ocean? Toward the north, where it is shallow and fed by so
much river water, it is, in fact, almost fresh water. Even the southern
one-half carries not over 1 per cent of salt. The explanation is that
a steady current passes through a narrow passageway into a gulf or bay
on its eastern side where evaporation is much greater than over the
general surface of the Caspian. The salt is, therefore, gradually
accumulating at the estimated rate of 350,000 tons per day in this
gulf, while the sea itself is becoming fresher.
The basin of Lake Champlain, about 100 miles long, was occupied by
tidewater geologically very recently (that is, since the Ice Age),
but it has been cut off by uplift of the land on the north, since
which time the waters of the lake have been completely rinsed out and
freshened.
Many lake basins directly result from volcanic action. In many parts of
the world lakes, usually of small size, occupy craters of volcanoes as,
for example, in the Eifel region of Germany, the Auvergne district of
France, and near Rome and Naples in Italy. Such a lake of exceptional
interest fills part of the great crater, several thousand feet deep,
which resulted from the explosion of Mt. Katmai, Alaska, in 1912. The
water of this lake, more than a mile wide and of unknown depth, is hot.
One of the most unique and beautiful lakes of the world is Crater Lake
in the Cascade Mountains of southern Oregon. It partly fills a great,
nearly circular hole, six miles in diameter, with a maximum depth of
about 4,000 feet, in the top of a mountain (Plate 11). The lake is
over five miles in diameter and nearly 2,000 feet deep, making it the
deepest in North America. Its surface is about 6,200 feet above sea
level. Precipitous rock walls rising 500 to 2,000 feet completely
encircle the lake, the main body of whose water is of a marvelous
deep, sapphire-blue color, while the shallow portions around some of
the shore are of emerald-green. Crater Lake has very little intake
except direct rainfall and snowfall, and its water is fresh. The great
hole was not produced by an explosion like that of Katmai, but rather
by the sinking of the top of a once much greater mountain. That the
mountain was once about the size and shape of Mt. Shasta is proved
by the fact that deep glaciated valleys lead up the slopes and end
abruptly at the very rim of the present mountain. Obviously these
valleys were scoured out in recent geologic time by glaciers whose
sources were several thousand feet up on a former cone-shaped mountain.
That the mountain top sank rather than exploded is proved by the
absence of volcanic débris over the sides and base of the mountain.
Still another way by which lakes are formed by volcanic action is
by streams of lava blocking valleys. The famous Sea of Galilee in
Palestine was thus formed by a stream of lava, which geologically
recently flowed down from the east into the Jordan Valley and across
it, where it cooled to form a dam ponding the waters of the Jordan
River. Because the river flows through the lake, its water is fresh.
One of the most remarkable facts about this lake is that its surface
lies nearly 700 feet below sea level. A number of lava-dam lakes are
known in the Sierra Nevada and Cascade Mountains.
A very interesting case of a lake basin, formed by cutting off an arm
of the sea without any movement of the earth's crust, is the Salton
Sink of southern California. This basin, many miles long and wide, lies
below sea level, its lowest point being 287 feet below tide. The Gulf
of California formerly reached much farther north and into California
where it covered the site of the Salton Sink. Gradually the Colorado
River, always loaded with sediment, built a broad delta deposit right
across the gulf, the northern end of which thus became cut off,
leaving a big salt lake. But the river flowed into the gulf, while in
the dry climate the evaporation was great enough to gradually dry away
the salt lake. This was the condition of things until 1904, when much
of the river at a time of flood got out of control and, following the
general course of a great irrigating canal, it flowed for several years
into the lowest part of the Salton Sink, partly filling it to form a
lake 45 miles long, 17 miles wide, and 83 feet deep. Since 1907 the
lake has been notably decreasing in size, and it may entirely disappear.
Other ways by which lakes, mostly relatively small ones, may develop
are by landslides blocking valley drainages; by streams cutting across
winding curves leaving so-called "oxbow lakes" which are common, for
example, along the lower Mississippi River; by wave and wind action
along shores of lakes or sea; by filling so-called "sink holes" which
result from dissolving or falling in of roofs of caves; and by beavers
through whose industry dams are built across valleys or streams.
Some of the most common ways by which lakes may be destroyed are the
following: by being filled with sediment carried in by streams, or by
vegetation, or by both; by cutting down outlets; by evaporation due
to a change in climate; by removal of the ice dam in certain types of
glacial lakes; and by movements or warping of the earth's crust.