C.4.3  Shaping the cross profile of a stream channel

Another milestone in the implementation of the LIFE13 NAT/PL/000009 LIFEDrawaPL project “Active protection of water-crowfoots habitats and restoration of wildlife corridor in the River Drawa basin in Poland /Czynna ochrona siedlisk włosieniczników i udrożnienie korytarza ekologicznego zlewni rzeki Drawy w Polsce”.

On 26th October 26 2018, the Regional Directorate for Environmental Protection in Szczecin made the final acceptance of works carried out as a part of the C.4.3 task “Shaping the cross profile of a stream channel” entitled “Constructing the riffle-pool system in the Drawa channel and the bottom ramp in the Sucha channel”

The total value of the completed task: PLN  215.250,00

Piles have obscured the reason for this part of the action, but it is easy to notice what the difference in height is between the culvert’s outlet under the road and the bottom of the Sucha River. The fault caused by sudden rainwater flows from a new road has eroded about one metre high hole in the bottom of the stream, by the way undercutting the bank and covering one of the better-assessed patches of water-crowfoots.

 

The action consisted of:

  • restoring ecological connection of the Sucha channel by building a bottom ramp from a rip-rap;
  • completing the riffle below the mouth of the Sucha River in the Drawa channel by introducing a gravel-stone mixture, which initiates the riffle-pool sequence on this section of Drawa.
After filling up the gravel, the river regained its character which was typical for most of the Pomeranian rivers flowing from the moraine systems left by the glacier.

After filling up the gravel, the river regained its character which was typical for most of the Pomeranian rivers flowing from the moraine systems left by the glacier.

The Drawa, like many other Pomeranian rivers, flows from moraine hills, flowing through some lakes, where it loses its impetus, and breaking through the moraine where it often reaches falls similar to submontane rivers. For thousands of years in lots of such places gravel bed sections have been formed, where the energy of water have formed characteristic patterns of riffles and pools. On such sections people located mills, but they also supplied themselves for centuries with the building material for their homes. Due to the use of the river for the transport of wood, larger boulders were removed from the bed and shallowings were eliminated on riffles’ crests on which floating logs stopped. In case of mountain rivers during one flood water can reconstruct the system, but there are no such floods on the rivers of Pomerania. Therefore, there is a need to supplement the diversity of the channel’s morphology in order to restore the good ecological status of the river. Without such interventions there is a gradual clogging of sediments and disappearance of habitats characteristic for water-crowfoots rivers and this was the type of the majority of Pomeranian rivers. For the last two decades on the Drawa, due to the low level of precipitation in the catchment and the stabilization of flows through two hydroelectric plants in the lower and middle course, the disappearance of water-crowfoots have been observed, and with them a weaker condition of many species of water fauna typical for water-crowfoots rivers. For this reason, a large part of the activities planned in the project is connected with supplementing the gravel-stone substrate. One of the first heaps was restored below the mouth of the Sucha River, as an action improving the condition of the water-crowfoots patch. These reconstructed riffles are an important place for the native ichthyofauna, including the Atlantic salmon.

The bottom ramp has restored the ecological connection of the Sucha River with the Drawa. Thanks to this, small fish will be able to hide in case of danger in a smaller stream.

The bottom ramp has restored the ecological connection of the Sucha River with the Drawa. Thanks to this, small fish will be able to hide in case of danger in a smaller stream.

A month after the gravel was supplemented on the riffle some traces of autumn spawning fish appeared.

A month after the gravel was supplemented on the riffle some traces of autumn spawning fish appeared.

The reproduction place of numerous species of fish and lampreys is one of riffles’ many functions in a gravel-bottom river. The systems of gravel shallowings are water’s energy stabilizers, habitats of many species involved in the process of water purification from carried matter, from bacteria to animals. These are also extremely important areas for the safe development of juvenile fish and lampreys, alongside the larvae of many insects’ species. To this day, science cannot say that all the functions of these systems have been recognized, but it can certainly be said that the importance of diversifying of the rivers beds’ morphology is one of the key elements of their good ecological status.

The critical part of the bottom ramp. At this point, the river had no passability for migration due to a bottom fault. The ramp compensated for the difference in bottom levels.

The critical part of the bottom ramp. At this point, the river had no passability for migration due to a bottom fault. The ramp compensated for the difference in bottom levels.