A STUDY OF LAKE BASINS
=Freshwater and saline lakes.=—Lakes require for their existence a
basin within which water may be impounded, and a supply of water more
than sufficient to meet the losses from seepage and evaporation. If
there is a surplus beyond what is needed to meet these losses, lakes
have outlets and remain fresh; their content of mineral matter is
then too slight to be detected by the palate. If, on the other hand,
supply is insufficient for overflow, continued evaporation results in a
concentration of the mineral content of the water, subject as it is to
continual augmentation from the inflowing streams.
As we have seen, there are in areas of small rainfall special
weathering processes which tend to bring out the salts from the
interior of rock masses, these concentrated salts generally first
appearing as a surface efflorescence which is ultimately transferred
through the agency of wind and cloudburst to the characteristically
saline desert lakes.
Lake basins may be formed in many ways. Depressions of the land surface
may result from tectonic movements of the crust; they may be formed by
excavating processes; but in by far the greater number of instances
they result from the obstruction in some manner of valleys which were
before characterized by uniformly forward grades. In relatively few
cases loose materials are heaped up in such a manner as to produce
fairly symmetrical basins.
[Illustration:
FIG. 430.—Map and diagram to bring out the characteristics of newland
lakes.]
=Newland lakes.=—On land recently elevated from the sea, basins of
lakes may be merely the inherited slight irregularities of the earlier
sea floor, in which case they may be assumed to be largely the result
of an irregular distribution of deposits derived from the land. Lakes
of this type are especially well exhibited in Florida, and are known as
newland lakes (Fig. 430). Such lakes are exceptionally shallow, and are
apt to have irregular outlines and extremely low banks. Under these
circumstances, they are soon filled with a rank growth of vegetation,
so that it is sometimes difficult to properly distinguish lake and
marsh.
[Illustration: FIG. 431.—View of the Warner Lakes, Oregon (after
Russell).]
[Illustration: FIG. 432.—Schematic diagrams to illustrate the
characteristics of basin-range lakes.]
=Basin-range lakes.=—Newland lakes may be said to have their origin in
an uplift of the land and sea floor near their common margin. A lake
type dependent upon movements of the earth’s crust but within interior
areas has been described as the basin-range type and is exemplified
by the Warner Lakes of Oregon. In this district great rectangular
blocks of the earth’s crust, which in their upper portions at least
are composed of basaltic lavas, have undergone vertical adjustments
in level and have been tilted so that the corresponding corners of
neighboring blocks have been given a similar degree of down-tilt (Fig.
431). Lakes formed in this way are of triangular outline, are bounded
on the two shorter sides by cliffs, but have extremely flat shores on
their longest side. From this shore the water increases gradually in
depth and attains a maximum depth at or near the opposite angle. Such
lakes naturally betray a tendency to appear in series (Fig. 432), and
are unfortunately much too often illustrated on a small scale after a
shower by the tilted blocks of imperfectly made cement sidewalks.
[Illustration:
FIG. 433.—Schematic diagrams of rift-valley lakes, and the rift valley
of the Jordan with the Dead Sea and the Sea of Galilee as remnants of a
larger lake in which their basins were included.]
=Rift-valley lakes.=—Another type of lake basin which has its origin
in faulted block movements is known as the rift-valley lake, and is
best exemplified by the great lakes of east Central Africa. In this
type a strip of crust, many times as long as it is wide, has been
relatively sunk between the blocks on either side so as to produce a
deep rift, or what in Germany is known as a _Graben_ (trench). Such
a basin when occupied by water yields a lake which is long, straight,
deep, and narrow, and is in addition bounded on the sides by steep rock
cliffs. At the ends the shores are generally by contrast decidedly
low. If the hard rock at the bottom of the lake could be examined, it
would be found to be of the same type as that exposed near the top of
the side cliffs. The valley of the Jordan in Palestine is a rift of
this character and was at one time occupied by a long and narrow lake
of which the Dead Sea and the Sea of Galilee are the existing remnants
(Fig. 433).
[Illustration:
FIG. 434.—Map showing the rift valley lakes of east Central Africa.]
One of the most striking examples of a rift valley lake is Lake
Tanganyika, while Albert Nyanza, Nyassa, and Rudolf in the same region
are similar (Fig. 434).
[Illustration:
FIG. 435.—Earthquake lakes which were formed in the flood plain of the
lower Mississippi during the earthquake of 1811 (after Humphreys).]
=Earthquake lakes.=—The complex adjustments in level of the surface
of the ground at the time of sensible earthquakes are many of them
made apparent in no other way than by the derangements of the surface
water. This is at such times impounded either in pools or in broad
lakes, which inasmuch as they date from known earthquakes have been
called “earthquake lakes”, even though in a strict sense any lake
which has originated in earth movements might properly be regarded as
an earthquake lake. To avoid unnecessary confusion, the term must,
however, be restricted to those lakes which are known to have been
formed at the time of definite earthquakes (Fig. 435). Reelfoot Lake
in Tennessee, which in late years has acquired undesirable notoriety
because of the feuds between the fishermen of the district and the
constituted authorities, is a lake more than twenty miles across
and came into existence during the great earthquake of the lower
Mississippi valley in 1811.
=Crater lakes.=—The craters of volcanic mountains are natural basins
in which surface waters are certain to be collected, provided only
the supply is sufficient and seepage into the loose materials is
not excessive. Some craters, still visibly more or less active, are
occupied by lakes (Fig. 436).
[Illustration:
FIG. 436.—View of lake in Poas Crater in Costa Rica, a volcanic crater
more than half a mile across and with walls 800 feet deep. At intervals
there is an ejection of steam mixed with mud and ash after the manner
of a geyser (after H. Pittier).]
In the larger number of cases in which craters become occupied by
lakes, the evidence of continued activity is lacking, and it would
appear in such cases that the lava of the chimney had consolidated into
a volcanic plug, closing the bottom of the crater. Notable groups of
crater lakes are the _Caldera_ of the Roman Campagna (Fig. 437) and
the so-called _maare_ of the Eifel about the Lower Rhine. Crater lakes
are easy to recognize by their circular plan, their steep walls of
volcanic materials, and their considerable depth with a maximum near
the center.
One of the most remarkable of these water-filled basins is Crater Lake
in Oregon, which has a diameter of about six miles and is believed to
have resulted from the incaving of a great volcanic cone in the latest
stage of its activity. This remarkable feature has now been made a
national park and will soon be conveniently reached by tourists and
counted one of the greatest nature wonders of the Pacific slope.
[Illustration:
FIG. 437.—Diagrams to illustrate the characteristics of crater lakes.
The Roman Campagna is a plain formed of volcanic ash, with the crater
lakes of Bracciano, Vico, and Bolseno arranged on a line traversing it.]
=Coulée lakes.=—Far more important as lakes are those volcanic basins
which arise from the flow of a stream of lava across the valley of a
river so as to impound its waters (Fig. 438).
At the time of the great eruption under Skaptár Jökull in 1783 the
river Skaptár and many of its tributaries were blocked by the flow of
lava, which it is estimated exceeded in bulk the mass of Mont Blanc.
[Illustration:
FIG. 438.—View of Snag Lake, a _coulée_ lake with lava dam shown in
middle distance (after Fairbanks).]
=Morainal lakes.=—As we have learned, the obstruction of drainage,
due to the distribution of rock débris by continental glaciers, has
yielded lakes in almost countless numbers. Probably ninety per cent
or more of the known lakes have had this origin, and the type is so
common within the once glaciated regions that it forms perhaps the
best distinguishing mark of former glaciation. The hummocky surface
of morainal deposits is so characteristic that the lakes of this type
are never very large and are correspondingly irregular in outline.
They have often numerous islands, and their banks are formed of the
combination of rock flour and ice-worn materials known as till (Fig.
439). The smallest of the morainal lakes are mere kettles on the
marginal moraine, and these rapidly become replaced by peat bogs. In
contrast with pit lakes, morainal lakes lack the steep surrounding
slopes and the encircling plain.
[Illustration: FIG. 439.—Diagrams to illustrate the characteristics
of morainal lakes, and a sample map of such lakes from the glaciated
region of North America.]
=Pit lakes.=—The so-called pit lakes have their origin in continental
glaciation, and are found in groups within broad plains of glacial
outwash (mainly sand and gravel), which are for this reason described
as “pitted plains” (see p. 314). Those areas which lay between
neighboring lobes of the ice sheet were subject to particularly heavy
deposits of outwash material, and are, in consequence, particularly
likely to be occupied by pit lakes. As has been pointed out in an
earlier section, the water derived from surface melting within the
marginal portions of a continental glacier descends to the bottom
in the crevasses and thereafter flows in an ice tunnel under the
same conditions as water flowing in a pipe. Having in most cases a
considerable head at the outer margin of the ice, this water may rise
and issue well above the lower ice layers and so cover a portion of the
ice margin beneath sand and gravel (Fig. 440). Separated blocks, often
of massive proportions, are thus buried beneath nonconducting materials
by which they are long protected from further melting. Eventually,
however, with the approach of still milder climates they disappear,
thus causing the overlying sand and gravel to descend and form a pit of
steep walls similar to the sawdust pits over melted ice blocks within
our storehouses.
[Illustration: FIG. 440.—Diagram to show the manner of formation of
pit lakes.]
Pit lakes are thus easily recognized by their occurrence usually in
groups within a plain of glacial outwash and by their characteristic
banks inclined at the angle of repose of such materials (Fig. 441).
[Illustration: FIG. 441.—Diagrams to illustrate the characteristics of
pit lakes and a sample map from the glaciated region of North America.]
=Glint or colk lakes.=—It has been found to be true of existing
continental glaciers that where their mass has been held back by
a mountain wall, their current at the portals within this rampart
becomes greatly accelerated. Though the upper layers of the glacier
in the vicinity may move forward with a velocity of but an inch per
day, the current within the outlet may be as much as seven hundred
or a thousand times as great. In many respects these conditions are
similar to those about the raceway of a reservoir where the near-by
surface of the water is lowered by the indraught of the outlet and the
current in the raceway is so accelerated that, unless protected, the
bottom of the race is carried away and a basin excavated which extends
a short distance both above and below the position of the dam. In
Holland such basins hollowed out beneath breaks in the dykes are known
as colks. Basins which were excavated beneath the glacier outlets by
a similar process would not be open to our inspection until after the
ice had disappeared from the region; but it is most significant that
in Scandinavia, where the Pleistocene continental glacier, advancing
westward from the Baltic, was held in check by the escarpment at the
Norwegian boundary (the _glint_), lake basins have been excavated
in hard rock whose walls show the abrading and polishing which are
characteristic of glacial sculpture, and whose positions are such that
they lie beneath the former outlets partly above and in part below the
line of the escarpment. Their position in reference to the rampart and
to the former outlets is brought out in Fig. 442. The largest of the
glint lakes of this series is Torneträsk in northern Lapland (see p.
277 and Fig. 443).
[Illustration:
FIG. 442.—Diagram to show the manner of formation of glint or outlet
lakes where the continental glacier of Scandinavia issued from the
Baltic depression through portals in its mountain rampart.]
[Illustration:
FIG. 443.—Map showing a series of glint lakes which lie across the
international boundary of Sweden and Norway.]
=Ice-dam lakes.=—Whenever a continental glacier, either in advancing
its front or in retiring, lies across the lines of drainage upon their
downstream side, water is impounded along the ice front so as to form
ice-dam lakes. Such lakes are found to-day in Greenland and in the
southern Andes, and similar bodies of water of far greater size and
importance came into existence in Pleistocene times each time that the
continental glaciers of northern North America and Europe advanced
upon or retired from suitably directed river systems. Thus above the
Baltic depression, when the ice front lay to the eastward of the main
watershed, each easterly sloping valley was obstructed by the ice and
occupied by an ice-dam lake (Fig. 444), the beaches of which may all be
traced to-day (Fig. 445).
[Illustration:
FIG. 444.—Ice-dam lakes (in black) between the front of the late
Pleistocene glacier of northern Europe and the divide near the
Norwegian boundary (after G. de Geer).]
One side of each ice-dam lake is formed by an ice cliff at the glacier
front, and if the region is relatively flat, the remaining shores
are likely to be formed by a marginal moraine which the glacier has
abandoned in its retreat. In their smaller stages, therefore, ice-dam
lakes on prairie country have the form of a crescent, which is the more
pronounced because the waves by their attack upon the ice front flatten
the curvature of its outline (see Fig. 360, p. 330).
The life of an ice-dam lake is begun and ended in important changes
of glacier outline, and after the draining of lakes by this process
the land shores may be traced in beaches, and the ice margin by a
water-laid moraine of low relief (Fig. 359, p. 330).
A much smaller but in many respects similar ice-dam lake is to-day to
be seen at the side of the Great Aletsch glacier, a mountain glacier of
Switzerland. The traveler who makes the easy ascent of the Eggishorn
may look directly down upon this crescent-shaped lake with its ice
cliff on the glacier side (see Fig. 446).
[Illustration:
FIG. 445.—Wave-cut terrace at an elevation of 177.5 meters above sea
on the southern slope of the northern Dala valley north of Baggedalen
in Sweden. To the right in the foreground is a peat bog (after
Munthe).]
[Illustration:
FIG. 446.—View of the Márjelen Lake at the side of the Great Aletsch
glacier, seen looking directly down from the summit of the Eggishorn
(after a photograph by I. D. Scott).]
=Glacier lobe lakes.=—Upon the sites of the former lobes of the
Pleistocene glacier of North America are found the basins of the
Laurentian River system, the largest freshwater lakes in the world.
There has been much controversy concerning the manner of formation
of these lakes, but the view which has seemed to have the largest
following is that they were excavated by the eroding action of the
continental glacier over the drainage basins of former rivers. It is
but one phase of the long controversy between opposing schools, which
have advocated on the one hand the efficiency of glacier ice as an
eroding agent, and upon the other its supposed protection from the
weathering processes. The positions and the outlines of the several
lakes of the series sufficiently proclaim their connection with the
former glacial lobes, and the name which we have adopted leaves the
exact manner of their formation a still open question. The recognition
of the importance of the glacial anticyclone, in giving shape to the
glacier surface and in effecting a transfer of snow from the central to
the marginal portions, has had the effect of emphasizing the relative
importance of erosion under the marginal and lobate portions. Thus
the importance of ice lobes has been greatly accentuated, though this
applies only to the shaping of the basins and not in any important
way to the impounding of the present waters. The present Laurentian
Lakes owe their existence to the elevation by successive uplifts of
the country to the northward and eastward, since the glacier retired
from the lake region. When the ice front lay to the northward of the
Ottawa River, the discharge of the upper lakes was by a channel through
Nipissing River and Lake and thence down the Ottawa River to a gulf in
the lower St. Lawrence. The uplift of the land has had the effect of
raising a barrier where the former outlet existed, and diverting the
waters to a roundabout channel by way of Detroit and Lake Erie (see
Fig. 365, p. 335).
[Illustration:
FIG. 447.—Diagrams to illustrate the arrangement and the characters of
rock-basin lakes, together with a map of such lakes from the Bighorn
Mountains in Wyoming.]
=Rock-basin lakes.=—The reversed grades which develop in a valley
deepened by mountain glaciers—the back-tilted treads of the cascade
stairway (see p. 376)—furnish a series of basins hollowed in rock which
are strung along the course of the valley like pearls upon a thread,
or, far better, like the larger beads in a rosary (Fig. 447). This
characteristic arrangement accounts for the name “Paternoster Lakes”
which has sometimes been applied to them in Europe. Their positions in
series within U-shaped mountain valleys, and their rock shores with
characteristically smoothed and striated surfaces, make them easy of
determination. In the higher portions of the valley, where the treads
of the cascade stairway are relatively narrow, such lakes are often
approximately circular in outline, but in the lower levels and upon
wider treads they may be ribbon-like, though lakes of this type are to
a large extent replaced in the lower levels by the valley moraine type
or a combination of the two.
[Illustration: FIG. 448.—Convict Lake, a lake behind a moraine dam
within a glaciated valley of the Sierra Nevadas, California (after a
photograph by Fairbanks).]
=Valley moraine lakes.=—The recessional moraines which mark the halting
stations of mountain glaciers, while retiring up their valleys,
form dams in the later river and so produce a type of lake which is
in contrast with the morainal lakes which result from continental
glaciation. They may, therefore, be distinguished by the name _valley
moraine lakes_. Their positions on the bed of a U-shaped mountain
valley, and the glacial materials which compose the dams, are
sufficient for their identification (Fig. 448). Moraine Lake and Lake
Louise in the Canadian Rockies are typical examples. Rock basin and
valley moraine lakes may occur in alternation or combined in mountain
valleys.
[Illustration:
FIG. 449.—Lake basins produced by successive slides from the steep
walls of a glaciated mountain valley (after Russell).]
=Landslide lakes.=—The sheer-walled valleys which are carved by
mountain glaciers are too steep to long retain their perpendicularity
when the support of the glacier has been removed. Aided by the ever
present joint planes, which admit water to the rock, they succumb to
frost action, and further give way in avalanches whenever the rock
is of sufficiently porous material to become saturated with water.
Landslides sometimes occur successively until the original valley wall
has been replaced by a terraced slope. The treads of the steps in
this terrace have generally a backward-sloping grade, so that basins
are formed to be filled by relatively long and narrow lakes or by
successions of small pools (Fig. 449 and plate 23 B).
[Illustration:
FIG. 450.—Lake Garda, a border lake upon the site of a piedmont apron
at the margin of the Alpine highland (after Penck and Brückner).]
When the avalanched material is so disposed as to dam the valley, much
larger lakes of this type come into existence. During an earthquake
which occurred on January 25, 1348, there was a landslide within the
valley of the Gail, Carinthia, which destroyed seventeen villages and
produced a lake which even to-day is represented by a great marsh.
=Border lakes.=—Whenever mountain glaciers push out their fronts
beyond the borders of the mountain range by which they are nourished,
they spread upon the foreland in broad aprons about which morainic
accumulations are particularly heavy. This elevation of morainal
walls about the margins of the aprons yields natural basins that are
occupied by lakes so soon as the glacier retires its front within the
valley. Because such lakes are found at the borders of upland districts
they have been called _border lakes_. The beautiful Lakes Constance,
Lucerne, Maggiore, Lugano, Como, and Garda (Fig. 450), on the borders
of the Alpine highland, are all of this type.
┌───────────────────────────────────────────────────────────────────┐
│ PLATE 23. │
│ │
│ [Illustration: _A._ View of the American Fall at Niagara, showing │
│ the accumulation of rocks beneath (after Grabau).] │
│ │
│ [Illustration: _B._ Crystal Lake, a landslide lake in Colorado. │
│ (_Photograph by Howland Bancroft._)] │
└───────────────────────────────────────────────────────────────────┘
=Ox-bow lakes.=—The cutting off of a meander within the flood plain of
a river yields a lake which is of horseshoe (ox-bow) outline and lies
generally with low banks within a plain composed of river silt. Before
separating from the parent stream the meander had begun to silt up,
especially at the ends. Ox-bow lakes are, however, relatively deep near
the convex shore and correspondingly shallow toward the concave margin
(Fig. 451).
[Illustration: FIG. 451.—Diagrams to bring out the characteristics of
ox-bow lakes, together with a map of such lakes from the flood plain of
the Arkansas River.]
[Illustration:
FIG. 452.—Diagrammatic section to illustrate the formation of
saucer-like basins between the levees of streams flowing in a flood
plain.]
=Saucer lakes.=—As we have learned, a river meandering in its flood
plain has banks which are higher than the average level of the plain,
for the reason that at flood time the main current of the stream still
persists in the channel, thus allowing the burden of sediment to be
dropped in the relatively slack water upon its margin. Because of these
natural embankments or levees, tributary streams are often compelled
to flow long distances in nearly parallel direction before effecting
a junction. Between the trunk stream and its tributaries, likewise
bounded by levees, and between streams and the valley walls, there thus
exist low basins which are more or less saucer-shaped (Fig. 452). At
flood time, when the levees are overflowed or crevassed, water enters
these depressions, and an additional supply may be derived from the
walls of the valley. Good illustrations of such lakes are furnished
by the flood plain of the former river Warren near the banks of the
present Minnesota River (Fig. 453).
[Illustration: FIG. 453.—Saucer lakes upon the bed of the former river
Warren (from the Minneapolis sheet, U. S. G. S.).]
=Crescentic levee lakes.=—As we approach the delta of a river, the
size and importance of the levee increases, and here a new type of
levee lake may develop in series (Fig. 454). At flood time the levee
is breached near the point of sharpest curvature on the convex side
(Fig. 454 _a_). When the waters are subsiding, the current is kept
away from the old channel by the rising grade of the levee as well as
by the inertia of the current, and an entrance to the old channel is
first found below the next change in curvature of the meander, since
here scour becomes effective in cutting through the levee. The new
channel is thus established in the form of a loop inclosing the old
one, and the process of levee building now erects a wall about the
territory newly acquired by the meander. This territory has the form
of a crescent, and when occupied by water produces a crescentic levee
lake often joined to its neighbors in series. The abandoned channel now
closed at both ends by levees may be occupied by water to produce a
subordinate ribbon type of curving trench (Fig. 454 _b_, _c_).
The importance of levees in obstructing drainage to form lakes is only
beginning to be appreciated. It has quite recently been shown that
when trunk streams are greatly swollen and burdened with sediment
while flowing from a receding continental glacier, they may build such
high levees as to aggrade their tributary streams above the junctions,
even producing reversed grades and so impounding the waters to form
extensive lakes. During the “ice age” lakes of this type were formed
in Illinois and Kentucky rivers just above their junctions with the
Ohio. The old lake floor with its eastern shore line and its protruding
islands is easily made out upon the new topographic maps of Kentucky.
[Illustration:
FIG. 454.—Levee lakes developed concentrically in series within
meanders of a stream tributary to the Mississippi and flowing upon its
delta plain. _b_ and _c_ are examples of the ribbon type of levee lake
due to occupation of the abandoned river channel. The larger number of
lakes, of which Sip Lake and Texas Lake are examples, have the form of
crescents and lie between abandoned levees (from recent map of U. S. G.
S.).]
=Raft lakes.=—Within humid regions the flood plains of our larger
rivers are generally forested, and as the river swings from side to
side in its perpetual meanderings, the timber which grows upon the
convex side of each meander is progressively undermined by the river
and felled upon its bank. The prostrate trees remain upon the banks
during the low water of the summer season, to be gathered up at the
time of flood in the next spring season. It is log jams thus acquired
which so generally block the main channel of a river and turn the
current across the neck of the meander when cut-offs occur with the
formation of ox-bow lakes. When the mass of timber thus gathered up
by the river is excessive, as, for example, within the flood plain of
the Red River of Arkansas and Louisiana, huge log rafts are produced
which dam up the river so effectively as to produce temporary lakes.
The impounded waters soon find an outlet over the levee at some point
higher up the river, and the waters flowing off through the timbered
bottom lands, other logs are caught by the standing timber as in a
weir. A second dam is thus formed which is separated from the initial
one by open water, and in this way the driftwood dam acquires enormous
proportions as it gradually moves up the river. After a period of
perhaps a century or more, the lower sections of the jam become decayed
and dislodged so as to float down the river.
[Illustration:
FIG. 455.—Raft lakes along the banks of the Red River in Arkansas and
Louisiana at their fullest recorded development (after A. C. Veatch, U.
S. G. S.).]
In the lower Red River a great raft of alternating jams and open water
reached a length of about one hundred and sixty miles and moved up
the river at the average rate of something less than a mile per year.
Within the limits of the dam all tributary streams were blocked, so
that secondary lakes were formed in a double fringe about the main
river (Fig. 455). The great raft which formed here in the latter part
of the fifteenth century has now at the beginning of the twentieth
been largely removed and measures have been adopted to prevent its
re-formation.
[Illustration:
FIG. 456.—The Swiss lakes Thun and Brienz, formed by deltas at the
junction of streams tributary to a steep-walled valley.]
=Side-delta lakes.=—It is characteristic of river drainage that the
tributary streams enter the main valley on steeper gradients than the
trunk stream at the point of junction. Wherever the difference in
velocity of the two streams at the junction is large, and the side
stream is charged with sediment, a delta will be formed at the mouth
of the tributary stream. Such deltas push out from the shore and
may eventually block the main channel so as to form a more or less
sausage-shaped expansion of the river—a side-delta lake. Traverse and
Big Stone Lakes in the valley of the Warren River in Minnesota have
been formed in this way (Fig. 354, p. 326). Lakes Thun and Brienz in
the Swiss Alps are of similar origin, the beautiful city of Interlaken
being built upon the delta plain over the valley of the earlier river
(Fig. 456). The Mississippi has similarly been expanded to form Lake
Pepin above the delta at the mouth of the Chippewa River.
[Illustration:
FIG. 457.—Delta lakes formed at the mouth of the Mississippi through
the junction of the levees of radiating distributaries with the shore
of the estuary (after Berghaus).]
=Delta lakes.=—A somewhat different type of delta lake has been
formed in Louisiana, where the “father of waters” discharges into the
gulf. Here the various distributaries radiate from the main channel
to produce the “bird-foot” delta type and the toes in this foot by
their junction with the banks which outline the ancient estuary, have
separated in succession a series of basins that before were in direct
connection with the sea (Fig. 457). Lake Pontchartrain is the largest
of this series, while the so-called Lake Borgne is in process of
separation.
Where large deltas push out from the shore into the open sea, the
levees which border the individual distributaries are attacked by the
waves and their materials are transported by the shore currents and
built into barriers. These barriers cut off the re-entrants between
neighboring distributaries so as to produce lagoons or lakes (Fig. 458).
[Illustration:
FIG. 458.—A type of delta lakes formed by levees in part destroyed
and built into barriers on the margin of the delta of the Nile (after
Supan).]
A type of delta lake, which more resembles the side-delta lake above
described, has formed at the mouth of the Colorado River, where it
enters the Gulf of Lower California. The Imperial Valley lying to the
north of this delta is the desiccated floor of the earlier Gulf of
Lower California which has been captured from the sea by the delta
of the Colorado. The rampart of mountains, by which this valley is
surrounded, has cut it off from any water supply derived from clouds,
and its waters being no longer renewed from the sea, the region has
passed through a period of desiccation which has left the Salton
Sink as the only existing remnant of the earlier lagoon. It will be
remembered that careless operations in diverting distributaries of the
Colorado recently reversed this process so that the waters rose in the
valley, and expensive emergency operations were necessary in order to
again turn the waters of the Colorado into their accustomed channels.
[Illustration: FIG. 459.—Diagrams to illustrate the characteristics of
barrier lakes, with an example from the southern coast of the Island of
Nantucket.]
=Barrier lakes.=—The Salton Sink illustrates a type of lake which is
formed at the border of the sea through the erection of some kind of
barrier which captures a small area of the ocean’s surface. Though such
lakes may be properly described as strand lakes, it is usually at the
mouth of a river that the process becomes effective. The common type
of _barrier lakes_ is found developed on most ragged coast lines where
the shore currents have formed first bars and later barriers at the
mouths of the estuaries (Fig. 459). Such embankments are usually gently
curving or crescent shaped and are composed of sand or shingle which
presents a steep landward and a gradual seaward slope.
[Illustration:
FIG. 460.—Dune lakes on the coast of France (after Berghaus).]
=Dune lakes.=—Within the narrow strips of shore in which all the fine
soil that could be available for plant life has been washed away by
the waves, beach sand is exposed to the direct action of the winds. In
time of storm the sand is picked up and after drifting in the wind is
collected in long ridges parallel to the shore. Constantly traveling
along shore, these dunes block the mouths of rivers and thus produce a
series of lakes such as are indicated in Fig. 460.
=Sink lakes.=—Another class of lakes are due either directly or
indirectly to the work of underground waters. In districts which are
underlain by limestone, the surface water descending along the joints
of the limestone may widen these passageways through solution of the
rock and at lower levels flow on the floors of caverns eaten out by the
same process on bedding planes of the formation. At the intersections
of joints, more or less circular shafts known as “swallow-holes” go
down to the caves from the surface. Locally, also the cavern roofs
give way so as to choke the galleries with rubble and leave a basin at
the surface which has an irregular but generally a more or less oval
outline. If sufficiently clogged at the bottom by finer rock débris,
these basins become occupied by small lakes which are known as sinks,
and constitute one of the best surface indications of a limestone
country.
[Illustration: FIG. 461.—Sink lakes in Florida, with a schematic
diagram to illustrate the manner of their formation (map from U. S. G.
S.).]
=Karst lakes—poljen.=—In the limestone country to the north and
east of the Adriatic Sea—the so-called Karst region—there are many
interesting features which are directly traceable to the solution
of the country rock. Here all the surface water descends in certain
districts along the widened joint planes so that the drainage is
largely subterranean. The so-called _dolines_ or sinks of very regular
and symmetrical forms resembling deep bowls cover a large part of the
surface.
The entire country is, moreover, faulted in the most intricate fashion
into many rift valleys. The drainage being so largely subterranean,
these downthrown blocks of crust, the so-called _poljen_, become
flooded at certain seasons of the year when the subterranean passages
become choked or are too small to carry away all the water. A seasonal
lake of this character is the Zirknitz Lake (p. 189).
=Playa lakes.=—It is the law of the desert that the arid region be
walled in by mountains. This encircling rampart forces the clouds to
rise, and by robbing them of their moisture leaves the desert dry and
barren. Those waters which fall upon the inner margin of the ranges
drain toward the interior of this pan-like depression and are not
returned to the sea—the desert is without an outlet. Infrequent though
they be, the desert rains are of the cloudburst type and in the hills
develop torrents whose waters, emerging upon the desert floor, develop
lakes in the space of a few minutes or at most hours. In the hot and
dry atmosphere the waters of these shallow basins may be sucked up in
the space of a few hours but reappear in the same basins at the time
of the next succeeding cloudburst. Such ephemeral lakes are known as
playas.
=Salines.=—Desert lakes more favored in their supply of water may be
relatively long lived and persist for periods measured in years or
centuries. Such lakes are, however, extremely sensitive to climatic
changes (see p. 198).
For the reason that they have no outlet the waters of desert lakes
become salt through continued evaporation. They are, therefore, spoken
of as _salines_. Lake Bonneville, so long as it discharged its waters
over the sill of the Red Rock Pass, must have remained fresh; but when
the level of its waters had fallen below this outlet, its waters became
salt and the content increased as the volume diminished.
The shallow basins upon the floors of desert lakes may have come into
existence in various ways; but it would appear that the irregular
removal of the soil by the winds, modified as this is by differences
in composition of the rock materials and by vegetable growth, and the
deposition of sand by the same agent, are by far the most important.
Many of the types of tectonic and volcanic lakes which have been
described are characteristic of humid and arid regions alike.
=Alluvial-dam lakes.=—Within the mountains upon the desert borders,
the alluvial fans which form at the mouths of valleys, because of the
characteristic cloudburst, sometimes obstruct a main valley at the
junction with its tributaries. By this process the waters of the main
river are impounded in essentially the same manner as are the rivers of
humid regions by the deltas of their tributaries.
=Résumé.=—The types of lakes which we have now considered are arranged
below in tabular form so as to show their relationship to important
geological processes. While not complete, the list includes the more
important classes, as well as others which, while not of common
occurrence, are yet of interest in giving further illustration to the
processes which have been treated in earlier chapters.
By giving careful attention to criteria which have been above
suggested, it should be possible in the greater number of instances at
least to determine whether any lake which is visited has had its origin
in one or another of the processes described.
CLASSIFICATION OF LAKES
_Tectonic Lakes_ _Volcanic Lakes_
Newland lakes Crater lakes
Basin-range lakes Coulée lakes
Rift-valley lakes
Earthquake lakes
_Continental Glaciation Lakes_ _Mountain Glaciation Lakes_
Morainal lakes Rock-basin lakes
Pit lakes Valley moraine lakes
Glint or colk lakes Landslide lakes
Ice-dam lakes Border lakes
Glacier-lobe lakes
_River Lakes_ _Strand Lakes_
Ox-bow lakes Barrier lakes
Saucer lakes Dune lakes
Crescentic levee lakes
Raft lakes
Side-delta lakes
Delta lakes
_Ground Water Lakes_ _Desert Lakes_
Sink lakes Playa lakes
Karst lakes—_poljen_ Salines
Alluvial dam lakes.
READING REFERENCES FOR CHAPTER XXIX
General:—
I. C. RUSSELL. Lakes of North America. Boston, 1895, pp. 125, pls. 23.
A. P. BRIGHAM. Lakes, A Study for Teachers, Jour. Sch. Geogr., vol. 1,
1897, pp. 65-72.
N. M. FENNEMAN. The Lakes of Southeastern Wisconsin, Bul. 8, Wis.
Geol. and Nat. Hist. Surv., 1902 (Rev. Ed., 1910), pp. 188, pls. 37.
A. DELEBECQUE. Les Lacs Français (with Atlas). Paris, 1898. (Work
crowned by the Society of Geology of Paris.)
H. R. MILL. Bathymetrical Survey of the English Lakes, Geogr. Jour.,
vol. 6, 1895, pp. 46-73, 135-166.
A. SUPAN. Grundzüge der Physischen Erdkunde. Leipzig, 1896, pp.
531-548.
H. BERGHAUS. Atlas der Hydrographie. Gotha, 1891, pl. 3.
R. D. SALISBURY. Physiography. 1907, pp. 292-327.
CHARLES RABOT. Revue de limnologie, La Géographie, Vol. 4, 1901, pp.
110-119, 172, 189.
I. C. RUSSELL. A Geological Reconnaissance in Southern Oregon, 4th
Ann. Rept. U. S. Geol. Surv., 1884, pp. 442-447. (Basin range lakes.)
ED. SUESS. The Face of the Earth, vol. 4, 1909, pp. 268-286. (Rift
valley lakes.)
J. S. DILLER. Crater Lake, Nat. Geogr. Mag., vol. 8, 1897, pp. 33-48,
pl. 1; Geology of Lassen Peak Quadrangle, California, Geol. Fol. 15,
U. S. Geol. Surv., 1895. (Coulée lakes.)
N. M. FENNEMAN. Lakes of Southeastern Wisconsin, _l.c._, pp. 4-6. (Pit
lakes.)
ED. SUESS. The Face of the Earth, vol. 2, 1906, pp. 340-346, pl. 7.
(Glint lakes.)
I. C. RUSSELL. A Preliminary Paper on the Geology of the Cascade
Mountains in Northern Washington, 20th Ann. Rept. U. S. Geol. Surv.
Pt. ii, 1900, pl. 14. (View of a rock-basin lake.)
E. W. SHAW. Preliminary Statement concerning a New System of
Quaternary Lakes in the Mississippi Basin, Jour. Geol., 1911, pp.
481-491. (New type of levee lakes.)
A. C. VEATCH. Formation and Destruction of the Lakes of the Red River
Valley, Prof. Pap. No. 46, U. S. Geol. Surv., pp. 60-62, pls. 29-33.
(Raft lakes.)
M. NEUMEYER. Erdgeschichte, vol. 1, pp. 595-596. (Poljen.)