133
located in the NE part of Armenia. Its largely triangular shape has elongations in the southwestern and northwestern directions. The lake is
75 km long, with a mean width of 19 km.
Lake Sevan is an example of a highly manipulated coldwater lake, the primary purpose of
which is the production of hydroelectric power
and, to a lesser extent, diversion of water for irrigation since 1938. Initially, the water level in the
lake was at 1,906 m above sea level and the lake
covered 1,416 km
2 . Later, water level decline by
18.5 m led to a 12.2 and 42 % reduction in the
lake surface area and lake water volume, respectively. The surface area in the 1980s was
1,244 km
2 (Parparov 1990 ). Consequently, the
maximum depth has been reduced from 98.7 to
42.2 m in the Minor Sevan and from 58.7 to
40.2 m in the Major Sevan, and the average depth
from 50.9 to 39.2 m, and from 37.7 to 23.6 m,
respectively. Further, drawing the water from the
lake eventually led to an almost disappearance of
the Sevan trout which became listed as an endangered fi sh species. The decline had two major
causes: desiccation of spawning grounds and
submersed macrophytes, and eutrophication of
the lake. The discharge of the lake water, the subsequent rise and level fl uctuations led to irreversible damage of spawning grounds of the endemic
trout, which, combined with eutrophication
(Oganesyan and Parparov 1983 ; Bagramyan
1984 ), led to major changes in the lake fi sh species composition.
Therefore, there had been plans to gradually
increase the water level in order to restore some
of the lost trout spawning grounds, increase the
dissolved oxygen levels and reduce eutrophication. Sevan could be raised by 6 m. This, in turn,
could change the eutrophicated lake back to an
oligotrophic character (Bagramyan 1984 ).
However, this may be too late for Sevan trout, the
formerly major fi sh species, to come back in the
same numbers as previously which supported a
lucrative fi shery (Meybeck et al. 1997 ).
Twenty- eight small rivers enter the lake, but there
is only one outfl ow, the Razdan. Spring fl oods
and snowmelt are the main sources of water.
Wind force is an important factor infl uencing
water mixing.
Temperature
Study of the lake water usually reveals average
annual surface water temperature 10–12 °C (the
minimum water temperature of 1–2 °C is reached
in January to April; maximum of 19 °C in
August). However, the water is not, generally,
stratifi ed during winter. The lake freezes over
every year. The lake stratifi es during summer
with the hypolimnion situated at 25–30 m depth.
From October to December, the lake water column is, generally, mixed from the top to the bottom, with almost the same temperature throughout
(Gezalyan 1979 ). It may be noted here that, in the
past, the lake used to freeze once in every 3–4
years; but later, it freezes almost every year and
the ice cover which may be 30–40 cm thick may
stay from the end of January until early April.
Under the ice, the water may show reverse stratifi cation, with the upper layers having a temperature close to zero and the bottom layer 1.2–2.3 °C
(Gezalyan 1979 ).
Dissolved Oxygen Concentration
DO ranged from 6.23 to 11.08 mg l
−1 in the littoral zone and 4.56 to 11.6 mg l
−1 in the central part
of the lake (Gezalyan and Khorlashko 1979 ).
Water Chemical Composition
Further comparison of the water quality parameters depicted changes from 1929 to 1983. The
lake had a pH of 8.4. However, there had been
no change in the concentration of Mg and moderate changes in concentrations of Ca, Na and
K. Bicarbonates dominated. Concentrations of
sulphates did not change, but there was a slight
increase in chlorides. There had been not much
indication of any changes in the concentration
of Si. However, the changes in dissolved oxygen and the increase in nitrogen and iron are
indicators of the eutrophication process in
Lake Sevan. The major causative factors
behind this are seen in the gradual lowering of
the water level.
Hydrobiological Characteristics
Since the 1960s, the lake has been undergoing
changes in the composition of phytoplankton,
zooplankton and benthos.
6.9 High Altitude (HA) Lentic Bodies
located in the NE part of Armenia. Its largely triangular shape has elongations in the southwestern and northwestern directions. The lake is
75 km long, with a mean width of 19 km.
Lake Sevan is an example of a highly manipulated coldwater lake, the primary purpose of
which is the production of hydroelectric power
and, to a lesser extent, diversion of water for irrigation since 1938. Initially, the water level in the
lake was at 1,906 m above sea level and the lake
covered 1,416 km
2 . Later, water level decline by
18.5 m led to a 12.2 and 42 % reduction in the
lake surface area and lake water volume, respectively. The surface area in the 1980s was
1,244 km
2 (Parparov 1990 ). Consequently, the
maximum depth has been reduced from 98.7 to
42.2 m in the Minor Sevan and from 58.7 to
40.2 m in the Major Sevan, and the average depth
from 50.9 to 39.2 m, and from 37.7 to 23.6 m,
respectively. Further, drawing the water from the
lake eventually led to an almost disappearance of
the Sevan trout which became listed as an endangered fi sh species. The decline had two major
causes: desiccation of spawning grounds and
submersed macrophytes, and eutrophication of
the lake. The discharge of the lake water, the subsequent rise and level fl uctuations led to irreversible damage of spawning grounds of the endemic
trout, which, combined with eutrophication
(Oganesyan and Parparov 1983 ; Bagramyan
1984 ), led to major changes in the lake fi sh species composition.
Therefore, there had been plans to gradually
increase the water level in order to restore some
of the lost trout spawning grounds, increase the
dissolved oxygen levels and reduce eutrophication. Sevan could be raised by 6 m. This, in turn,
could change the eutrophicated lake back to an
oligotrophic character (Bagramyan 1984 ).
However, this may be too late for Sevan trout, the
formerly major fi sh species, to come back in the
same numbers as previously which supported a
lucrative fi shery (Meybeck et al. 1997 ).
Twenty- eight small rivers enter the lake, but there
is only one outfl ow, the Razdan. Spring fl oods
and snowmelt are the main sources of water.
Wind force is an important factor infl uencing
water mixing.
Temperature
Study of the lake water usually reveals average
annual surface water temperature 10–12 °C (the
minimum water temperature of 1–2 °C is reached
in January to April; maximum of 19 °C in
August). However, the water is not, generally,
stratifi ed during winter. The lake freezes over
every year. The lake stratifi es during summer
with the hypolimnion situated at 25–30 m depth.
From October to December, the lake water column is, generally, mixed from the top to the bottom, with almost the same temperature throughout
(Gezalyan 1979 ). It may be noted here that, in the
past, the lake used to freeze once in every 3–4
years; but later, it freezes almost every year and
the ice cover which may be 30–40 cm thick may
stay from the end of January until early April.
Under the ice, the water may show reverse stratifi cation, with the upper layers having a temperature close to zero and the bottom layer 1.2–2.3 °C
(Gezalyan 1979 ).
Dissolved Oxygen Concentration
DO ranged from 6.23 to 11.08 mg l
−1 in the littoral zone and 4.56 to 11.6 mg l
−1 in the central part
of the lake (Gezalyan and Khorlashko 1979 ).
Water Chemical Composition
Further comparison of the water quality parameters depicted changes from 1929 to 1983. The
lake had a pH of 8.4. However, there had been
no change in the concentration of Mg and moderate changes in concentrations of Ca, Na and
K. Bicarbonates dominated. Concentrations of
sulphates did not change, but there was a slight
increase in chlorides. There had been not much
indication of any changes in the concentration
of Si. However, the changes in dissolved oxygen and the increase in nitrogen and iron are
indicators of the eutrophication process in
Lake Sevan. The major causative factors
behind this are seen in the gradual lowering of
the water level.
Hydrobiological Characteristics
Since the 1960s, the lake has been undergoing
changes in the composition of phytoplankton,
zooplankton and benthos.
6.9 High Altitude (HA) Lentic Bodies