76
A. Choi´ nski and R. Skowron
dam reservoirs in comparison to the eastern part. The area south of the range of
the last glaciation occupies 197 631.4 km
2 , and the occurring dam reservoirs have a
surface area of 926.91 km
2 . The value constitutes 33% of the surface area of Polish
lakes and is equivalent to the index of cover with dam reservoirs of 0.47%. It is,
therefore, a very substantial value in comparison to the mean lake density of Poland
equal to 0.9%. The number of dam reservoirs in the analysed area can be estimated
for a dozen thousand. It should be mentioned that in the lake district zone, covered
by the range of the last glaciation, many dam reservoirs are also located. The areas
should be therefore considered in the total balance for the country.
Despite the known total surface area of dam reservoirs (approximately 1000 km
2 ),
it is difficult to adopt their mean depth due to high depth variability. Their total water
resources, however, are at a level of several km
3 and can constitute approximately ¼
of surface waters.
5.5 Changes in the Surface Area of Lakes and Their Water
Resources
Lake Districts in north Poland cover an area of approximately 110 thousand km
2 ,
which constitutes approximately 35% of the area of the country. They include 6 793
lakes (with an area of more than 1 ha), occupying approximately 2.77 thousand km
2
[1].
The decline of lakes is usually associated with a decrease in their surface area
and water volume. This results from the fact that lakes in Poland are usually small
and in the majority of cases shallow. The process of decline of lakes is determined
by two primary factors: lake water level fluctuations and aggradation of sediments
in the lake basin. Water level fluctuations in lakes are caused by climate changes,
deforestation of the direct catchment, incorporation of lakes to the system of surface
runoff, and various hydrotechnical works [14]. An increase in bottom sediments can
be a result of deposition of mineral sediments supplied by rivers, aeolian processes, or
those related to overland flow (deluvia) or mass movements, aggradation of the mass
of biogenic sediments, and precipitation of chemical compounds. Sedimentation
and sedimentation processes in lakes occur very rapidly, especially that over the
last centuries in northern Poland strong deforestation took place. A decrease in the
surface area of lakes has been very evident even over several decades. The process is
considerably affected particularly by the location of a given lake in the hydrographic
system, size of the (direct) catchment feeding the lake, surface formations (e.g.
permeability), land use in the catchment, and human activity.
Several factors have been responsible for transformations of lakes. The following
ones played the most substantial role: variable rate of melting of lumps of dead
ice, short- and long-term climate changes, variable time of incorporation of lakes
into the surface outflow system, catchment deforestation (concerning both the entire
and direct catchment), hydrotechnical measures (from the mid-eighteenth century)
A. Choi´ nski and R. Skowron
dam reservoirs in comparison to the eastern part. The area south of the range of
the last glaciation occupies 197 631.4 km
2 , and the occurring dam reservoirs have a
surface area of 926.91 km
2 . The value constitutes 33% of the surface area of Polish
lakes and is equivalent to the index of cover with dam reservoirs of 0.47%. It is,
therefore, a very substantial value in comparison to the mean lake density of Poland
equal to 0.9%. The number of dam reservoirs in the analysed area can be estimated
for a dozen thousand. It should be mentioned that in the lake district zone, covered
by the range of the last glaciation, many dam reservoirs are also located. The areas
should be therefore considered in the total balance for the country.
Despite the known total surface area of dam reservoirs (approximately 1000 km
2 ),
it is difficult to adopt their mean depth due to high depth variability. Their total water
resources, however, are at a level of several km
3 and can constitute approximately ¼
of surface waters.
5.5 Changes in the Surface Area of Lakes and Their Water
Resources
Lake Districts in north Poland cover an area of approximately 110 thousand km
2 ,
which constitutes approximately 35% of the area of the country. They include 6 793
lakes (with an area of more than 1 ha), occupying approximately 2.77 thousand km
2
[1].
The decline of lakes is usually associated with a decrease in their surface area
and water volume. This results from the fact that lakes in Poland are usually small
and in the majority of cases shallow. The process of decline of lakes is determined
by two primary factors: lake water level fluctuations and aggradation of sediments
in the lake basin. Water level fluctuations in lakes are caused by climate changes,
deforestation of the direct catchment, incorporation of lakes to the system of surface
runoff, and various hydrotechnical works [14]. An increase in bottom sediments can
be a result of deposition of mineral sediments supplied by rivers, aeolian processes, or
those related to overland flow (deluvia) or mass movements, aggradation of the mass
of biogenic sediments, and precipitation of chemical compounds. Sedimentation
and sedimentation processes in lakes occur very rapidly, especially that over the
last centuries in northern Poland strong deforestation took place. A decrease in the
surface area of lakes has been very evident even over several decades. The process is
considerably affected particularly by the location of a given lake in the hydrographic
system, size of the (direct) catchment feeding the lake, surface formations (e.g.
permeability), land use in the catchment, and human activity.
Several factors have been responsible for transformations of lakes. The following
ones played the most substantial role: variable rate of melting of lumps of dead
ice, short- and long-term climate changes, variable time of incorporation of lakes
into the surface outflow system, catchment deforestation (concerning both the entire
and direct catchment), hydrotechnical measures (from the mid-eighteenth century)