164
7.1.5 Seasonal Variations
There are seasonal variations in both rate of
entry of water into a lake and the rate of loss in
almost all lake basins, thus, producing an annual
variation in levels. However, in the temperate
regions, evaporation is generally highest during
summer. On this seasonal evaporative cycle, a
seasonally determined period of melting of ice
and snow in the spring is often superimposed.
There is also almost always a seasonal variation
in rainfall. On the other hand, in a number of
lakes, both the presence of ice and snow and the
seasonal incidence of rainfall determine the variation, which may vary from year to year. For
example, in Lake Constance, rain falling in the
summer may greatly augment the meltwater and
cause fl oods during June or July, followed by a
smaller autumnal maximum.
Contrary to the above, the nature of the drainage basin is of considerable importance in determining the form of the seasonal variation. It could
be because in forested areas or other regions in
which the soil can take up much water, the amplitude of fl uctuations would, perhaps, be reduced
and their period increased. In this context, Day
( 1926 ) had opined that the lag between precipitation and its effects on lake level is about nine
months for Lake Superior and about a year for
Lake Michigan. Thus, it could be said that the lag
was greater during the nineteenth century than in
the twentieth, a result not unexpected owing to
extensive deforestation. Concomitant to this, the
variations of the levels of the Caspian Sea have
been the subject of continuous discussion since the
early works of Bruckner ( 1890 ) and Huntington
( 1907 ). The latter believed that geomorphological,
historical and archaeological evidence existed,
around 600 BC, for a relatively high level, about
40 m above that prevalent in the early twentieth
century. However, this view was criticised by the
later workers (Berg 1934 ). In South Central Asia,
the longest record seems to relate to Lake Manas
Sarovar and to Rakas Tal in which the former lake
fl ows and which can itself discharge into the Sutlej
(Hedin 1917 ). Further, far to the northeast, on the
northern border of the arid centre of Eurasia, Lake
Gusinoye, just north of Lake Baikal, appears to
had undergone a series of changes in level
comparable to those of Pangong Tso.
Notwithstanding the above, good amount of
information is available for Lake Albert and Lake
Victoria (Walker 1936 ), Lake Nyasa (Kanthack
1941 ) and Lake Tanganyika (Tison 1949 ). It was
reported that both Lake Victoria and Lake Albert
had oscillated in an irregularly periodic manner
since about a century. The history of Lake
Tanganyika is complicated by the fact that the
lake rose from the time of its discovery in 1854
until 1878. In Lake Nyasa, the main variation had
been a slow fall since 1895 or before, to a minimum level in 1915, followed by a rise lasting at
least to 1940 (Kanthack 1941 ). On the other
hand, little information is available on Lake Chad
which appeared to have been high in the eighteenth and early nineteenth centuries ( Tilho
1919 ).
The Lake George in New South Wales (NSW)
in Australia was high early in the nineteenth century, dry or low from 1835 to 1863, relatively
high through most of the later part of the nineteenth century and very low in the fi rst two
decades of the later part of the twentieth century
(Walker 1936 ).
Information available from western North
America indicates very low levels around the middle of the nineteenth century (Bowmann 1935 ).
Thus, there is a good amount of evidences to
suggest worldwide low levels in the middle of the
nineteenth century and rather widespread low
levels also at the end of the fi rst quarter of the
present century. However, there is no unequivocal evidence that variations in lake levels exhibit
any objectively signifi cant regular periodicities.
7.2
Distribution and Zonation in
Various Kinds of Lakes
Fassett ( 1930 ) briefl y described the fl ora of three
small glacial lakes in NE Wisconsin, namely,
Weber, Crystal and Clear Lakes. In these lakes, he
had found sandy bottoms, very high transparency
(8.0–10.7 m) and soft water (pH 5.8–6.8). Almost
all the angiosperms in the lake proper exhibited
rosulate or comparable form.
7 Lake Hydrology
7.1.5 Seasonal Variations
There are seasonal variations in both rate of
entry of water into a lake and the rate of loss in
almost all lake basins, thus, producing an annual
variation in levels. However, in the temperate
regions, evaporation is generally highest during
summer. On this seasonal evaporative cycle, a
seasonally determined period of melting of ice
and snow in the spring is often superimposed.
There is also almost always a seasonal variation
in rainfall. On the other hand, in a number of
lakes, both the presence of ice and snow and the
seasonal incidence of rainfall determine the variation, which may vary from year to year. For
example, in Lake Constance, rain falling in the
summer may greatly augment the meltwater and
cause fl oods during June or July, followed by a
smaller autumnal maximum.
Contrary to the above, the nature of the drainage basin is of considerable importance in determining the form of the seasonal variation. It could
be because in forested areas or other regions in
which the soil can take up much water, the amplitude of fl uctuations would, perhaps, be reduced
and their period increased. In this context, Day
( 1926 ) had opined that the lag between precipitation and its effects on lake level is about nine
months for Lake Superior and about a year for
Lake Michigan. Thus, it could be said that the lag
was greater during the nineteenth century than in
the twentieth, a result not unexpected owing to
extensive deforestation. Concomitant to this, the
variations of the levels of the Caspian Sea have
been the subject of continuous discussion since the
early works of Bruckner ( 1890 ) and Huntington
( 1907 ). The latter believed that geomorphological,
historical and archaeological evidence existed,
around 600 BC, for a relatively high level, about
40 m above that prevalent in the early twentieth
century. However, this view was criticised by the
later workers (Berg 1934 ). In South Central Asia,
the longest record seems to relate to Lake Manas
Sarovar and to Rakas Tal in which the former lake
fl ows and which can itself discharge into the Sutlej
(Hedin 1917 ). Further, far to the northeast, on the
northern border of the arid centre of Eurasia, Lake
Gusinoye, just north of Lake Baikal, appears to
had undergone a series of changes in level
comparable to those of Pangong Tso.
Notwithstanding the above, good amount of
information is available for Lake Albert and Lake
Victoria (Walker 1936 ), Lake Nyasa (Kanthack
1941 ) and Lake Tanganyika (Tison 1949 ). It was
reported that both Lake Victoria and Lake Albert
had oscillated in an irregularly periodic manner
since about a century. The history of Lake
Tanganyika is complicated by the fact that the
lake rose from the time of its discovery in 1854
until 1878. In Lake Nyasa, the main variation had
been a slow fall since 1895 or before, to a minimum level in 1915, followed by a rise lasting at
least to 1940 (Kanthack 1941 ). On the other
hand, little information is available on Lake Chad
which appeared to have been high in the eighteenth and early nineteenth centuries ( Tilho
1919 ).
The Lake George in New South Wales (NSW)
in Australia was high early in the nineteenth century, dry or low from 1835 to 1863, relatively
high through most of the later part of the nineteenth century and very low in the fi rst two
decades of the later part of the twentieth century
(Walker 1936 ).
Information available from western North
America indicates very low levels around the middle of the nineteenth century (Bowmann 1935 ).
Thus, there is a good amount of evidences to
suggest worldwide low levels in the middle of the
nineteenth century and rather widespread low
levels also at the end of the fi rst quarter of the
present century. However, there is no unequivocal evidence that variations in lake levels exhibit
any objectively signifi cant regular periodicities.
7.2
Distribution and Zonation in
Various Kinds of Lakes
Fassett ( 1930 ) briefl y described the fl ora of three
small glacial lakes in NE Wisconsin, namely,
Weber, Crystal and Clear Lakes. In these lakes, he
had found sandy bottoms, very high transparency
(8.0–10.7 m) and soft water (pH 5.8–6.8). Almost
all the angiosperms in the lake proper exhibited
rosulate or comparable form.
7 Lake Hydrology