A waterway barrier is any structure that limits fish movement along the waterway.

Waterway barriers have a significant impact on native fish numbers. Large waterway barriers such as dams and weirs prevent the movement of most fish species. Small waterway barriers such as road culverts and bridge pylons can also stop fish movement along a stream.

Waterway barriers can also change the natural river flow patterns, which in turn triggers fish movement. Examples of waterway barriers include:

• dams
• weirs
• bridges
• culverts
• tidal barriers
• fords
• causeways
• silt curtains
• any other barriers that restrict fish movement.

Waterway barriers reduce fish movement if:

• the water velocity is too high
• the water turbulence is too great
• the barrier acts as a physical barrier
• the barrier is too dark, too long or too narrow
• the water in or over the barrier is too shallow
• there is a drop on the downstream side of the barrier
• the barrier has been placed at too great a slope
• the barrier has not been maintained (i.e. is overgrown or full of debris)

The above waterway barrier effects can reduce native fish populations by reducing fish movement in streams.

Increased water velocity can affect fish by:

• severely depleting energy reserves
• increasing the time taken to reach feeding and/or spawning grounds
• reducing general condition and reproductive success
• leading to heavy mortality from predation.

Constructing or raising any barrier across a waterway (freshwater or tidal) requires a development approval under the Integrated Planning Act 1997 and the Fisheries Act 1994. Under this legalisation the application will be refused unless movement of fish across the waterway barrier is adequately provided for or an exemption is given due to no fish or habitat existing above the barrier. On large-scale infrastructure this may require a fishway that adequately provides for fish passage.