In the Netherlands large numbers of dams, weirs, and sluices, which contribute to the management of the water level, present barriers for migrating fish by blocking or restricting connections between adjacent water bodies. The FishFlow fish ladder is a facility that restores or improves the possibilities of upstream migration past such barriers.

The siphon fish ladder explained

The FishFlow siphon fish ladder is based on the principle of a poorly functioning siphon; i.e. a siphon with an air bubble. This bubble is an essential element of the construction because the volume of the bubble defines the flowrate through the system. The siphon fish ladder is equipped with a vacuum pump that controls the volume of the air bubble to prevent change of flowrate due to import or export of gas from the water. An aeration valve is provided as a safety precaution to make sure that the siphon can not start running at its maximum flowrate in case the vacuum pump would malfunction and pump uninterruptedly. The water that is being siphoned consequentially flows over a conventional fish ladder to the downstream section of the river or stream.

The presence of this type of fish ladder offers upstream migrating fish the possibility to pass barriers at their own convenience by swimming against the gentle current at any time. The FishFlow fish ladder is suitable for all sizes of fish and is equipped with small slopes in front of each partition to enable bottom-dwelling species to pass the fish ladder as well.

The FishFlow siphon fish ladder is a flexible system that can be fit to serve as a passage for weir, dams, and dikes. The system can also be used to provide a migration route between tidal waters and inland waters.

  • adjustable flow-rate
  • low costs of construction
  • low energy requirement
  • flexible positioning
  • no need to acquire lands
  • maintenance friendly
  • flexible design
  • adjustable for integration in the landscape, for decorative or educational purposes
  • special attributes enable bottom-dwelling species to use the passage
  • made from glass fibre reinforced composite material
  • standard diameters are 800, 1000, 1200 and 1600 mm
  • diameter up to 2400 mm possible for special purposes
  • length of the siphon fish ladder is determined by the height difference of the adjacent water bodies
  • flow rate adjustable from 10 to 50 litres per second at a maximum flow speed of 1.5 metres per second.
  • equipped with boxes at either end fish use as hideout
  • energy supply optional from solar panel or windmill
  • shape is determined per location

Realised projects

The FishFlow siphon fish ladder can be placed at weirs, sluices, barrages,  dams or other man-made or natural obstructions. The first FishFlow siphon fish ladder was realised in May 2006 in the Hertogswetering in The Netherlands for Waterschap Aa en Maas. Since then more siphon fish ladders have been placed in The Netherlands and Belgium. For instance in Wier en Oude Leije (Municipal  Leeuwarderadeel), het Ilperveld (Landschap Noord-Holland), de Kaweise Loop (Waterschap Aa en Maas) and de Rode Beek (Waterschap Veluwe).

Get a quotation

Do you have a situation where a siphon fish ladder would be an option? Send an e-mail to or use the contact form. We will contact you and propose our best solution.