6 Natural and Anthropogenic Lakes of River Valleys
93
Division by the origin
The oxbow lakes are the most numerous group of floodplain lakes and generally have
a relatively large surface area and basin capacity. They are formed in the valleys of
meandering rivers. They are always located in the meandering belt of the channel
zone of the river valley bottom. They are a fragment of an old river bed, cut off
from the present one with a near-bed embankment. The flow of flooding waters
through lower and higher flood terraces is the basic factor shaping the surface of
inter-levee lakes. Erosion, transport and deposition of sediments directly determine
the possibility of such reservoirs creation and functioning. Lithologically diversified
forms of dumping, of different durability and size, are most often created under
the conditions of free-flowing or standing water. Meander drops are accumulation
forms of the largest size and high stability. Lakes can be formed in places where
embankments separate depressions, with favorable arrangement of supplying them
valley waters. Flows of water in the out-of-bed zone determine the recharge of the
basins of inter-splitter lakes during potamophase. The recharge of the lake basin
results from the occurrence of erosive forms created during flooding flows. The shape
of the lakes refers to the form of an inter-levee depression, which was filled with water.
Inter-levee lakes are characterized by a clear loss of water in the limnophase period,
a small maximum depth and a relatively small area.
Avulsion occurs in fragments of the valley where a typical free migration of
meanders is replaced by the process of abandoning of longer sections of the river
bed. Sections of valleys, which are characterized by an increase in the longitudinal
slope of the river bed, are predisposed to the occurrence of avulsion. Especially in the
gorges zones, before the topographical obstacle of water outflow, the longitudinal
slope of the river decreases, but after its crossing, it increases significantly. It is
the increase in the gradient of the longitudinal slope during flooding that favors
the abandonment of the old bed. Increased slope of the water table also favors an
increase in the role of deep erosion at the expense of side erosion, which results in
a weakening of the river’s tendency to meandering. The modeling processes then
took place within abandoned, straightened, narrower and deeper beds. Under the
conditions of existence of stabilized channel drains delimiting the shoreline zone,
alternating outflows and depressions occurred in the longitudinal profile. A system
of linear cascades of basins was created, which were formed in the deep zones and
isolated from each other by shallows. The underground recharge may also be the
factor favoring the preparation and stabilization of the deepest zones of the basins
of the avulsion lakes. Especially in gorges areas, water-bearing layers, the drainage
of which allows to maintain full watering of the lake basins throughout the year, can
be often intersected.
Anastomotic lake basins were shaped by the processes resulting from the adaptation of the flow of flooding waters to the decreasing slope of the valley bottom (most
often sections located before its narrowing or topographic obstacle). The lower transport efficiency of the bed (resulting from the lower velocity of flowing water) led to
swelling of water on the floodplain terrace during flooding. At that time, alternative
(supplementary) beds were created. Such organization of the outflow, in conditions
93
Division by the origin
The oxbow lakes are the most numerous group of floodplain lakes and generally have
a relatively large surface area and basin capacity. They are formed in the valleys of
meandering rivers. They are always located in the meandering belt of the channel
zone of the river valley bottom. They are a fragment of an old river bed, cut off
from the present one with a near-bed embankment. The flow of flooding waters
through lower and higher flood terraces is the basic factor shaping the surface of
inter-levee lakes. Erosion, transport and deposition of sediments directly determine
the possibility of such reservoirs creation and functioning. Lithologically diversified
forms of dumping, of different durability and size, are most often created under
the conditions of free-flowing or standing water. Meander drops are accumulation
forms of the largest size and high stability. Lakes can be formed in places where
embankments separate depressions, with favorable arrangement of supplying them
valley waters. Flows of water in the out-of-bed zone determine the recharge of the
basins of inter-splitter lakes during potamophase. The recharge of the lake basin
results from the occurrence of erosive forms created during flooding flows. The shape
of the lakes refers to the form of an inter-levee depression, which was filled with water.
Inter-levee lakes are characterized by a clear loss of water in the limnophase period,
a small maximum depth and a relatively small area.
Avulsion occurs in fragments of the valley where a typical free migration of
meanders is replaced by the process of abandoning of longer sections of the river
bed. Sections of valleys, which are characterized by an increase in the longitudinal
slope of the river bed, are predisposed to the occurrence of avulsion. Especially in the
gorges zones, before the topographical obstacle of water outflow, the longitudinal
slope of the river decreases, but after its crossing, it increases significantly. It is
the increase in the gradient of the longitudinal slope during flooding that favors
the abandonment of the old bed. Increased slope of the water table also favors an
increase in the role of deep erosion at the expense of side erosion, which results in
a weakening of the river’s tendency to meandering. The modeling processes then
took place within abandoned, straightened, narrower and deeper beds. Under the
conditions of existence of stabilized channel drains delimiting the shoreline zone,
alternating outflows and depressions occurred in the longitudinal profile. A system
of linear cascades of basins was created, which were formed in the deep zones and
isolated from each other by shallows. The underground recharge may also be the
factor favoring the preparation and stabilization of the deepest zones of the basins
of the avulsion lakes. Especially in gorges areas, water-bearing layers, the drainage
of which allows to maintain full watering of the lake basins throughout the year, can
be often intersected.
Anastomotic lake basins were shaped by the processes resulting from the adaptation of the flow of flooding waters to the decreasing slope of the valley bottom (most
often sections located before its narrowing or topographic obstacle). The lower transport efficiency of the bed (resulting from the lower velocity of flowing water) led to
swelling of water on the floodplain terrace during flooding. At that time, alternative
(supplementary) beds were created. Such organization of the outflow, in conditions