Barriers to aquatic connectivity

Connectivity of waterways

The connectivity of a section of waterway to its floodplain (lateral connectivity) and to the waterway upstream and downstream (longitudinal connectivity) influences the movement of sediment, nutrients, carbon and animals through a river system and hence is important for the health of a waterway.

Seasonal flooding of the area next to a waterway (the floodplain) is important for the exchange of sediment, nutrients and carbon between the floodplain and waterway. For example, as water spills out of the waterway channel and onto the floodplain the speed of flow slows considerably –this results in any sediment being carried in the water to deposit on the flood plain, reducing the amount of sediment carried further downstream in the waterway channel.

For further information about sediment exchange see Sediment in streams (Water Note 17) and Stream channel and floodplain erosion (River Restoration Manual report no. RR18).

Floodplains provide an important habitat for fish and crayfish, frogs and wading birds. For some species seasonal flooding of these areas is crucial for a stage of their life cycle – many native fish species move into the floodplain and annual creeks during winter to reproduce in the flooded vegetation (Morgan et al 1998). The juvenile fish develop in these nursery areas before moving back into more permanent waterways.

The following documents have further information about the importance of floodplains: Waterway ecology and Habitat of river and creeks (Water Note 8). Goodga River fishway

The connection between a section of a waterway to the upstream and downstream waterway (longitudinal connectivity) is also important for the habitat requirements of aquatic animals. Fish move through river systems for many reasons including feeding, avoiding predators, migration for breeding/spawning, migration to nursery areas or new territory, movement to seasonal habitats and colonisation.

Natural and man-made barriers can restrict these movements, leading to increased competition for food and habitat, increased predation and the interruption of natural breeding/spawning cycles (Fairfull & Witheridge 2003). In addition, separating a population of a fish species into localised groups can affect the genetic diversity of a group and its resilience to predation and environmental changes.

Barriers to connectivity

Man-made barriers that have the potential to prevent movement of fish include dams, weirs, flow gauging stations, fords and culverts. The extent to which a structure forms a barrier to fish passage depends on a combination of factors including the structure's size, the flow regime of the waterway (which together determine how frequently water flows over the structure), the fish species present, their migration patterns and the location of the structures in relation to those patterns.

The following publications provide information and guidance about barriers and the movement of aquatic animals:

Barriers that prevent the movement of fish can also influence other aspects of waterway condition. Barriers can:

prevent the movement of organic material along waterways – this builds up in pools upstream from the barrier and can lead to poor water quality (e.g. increased nutrients and contaminants, reduced dissolved oxygen, shallower warmer water)
reduce the flow of water along a waterway leading to stagnant areas upstream from barriers - these areas tend to have poor water quality and provide an ideal habitat for the growth of nuisance species such as aquatic weeds, mosquitos, midges and introduced fish species
reduce the depth of water downstream of barriers. This can lead to increased water temperatures and lower dissolved oxygen; fish and macroinvertebrate species have different water quality requirements and some may be unable to tolerate the conditions in these shallow areas
increase the flooding of riparian vegetation upstream of barriers and decrease the water available to vegetation downstream from the barrier – this can change the species able to live in these areas and in some cases can lead to reduced vegetation cover which impacts waterway health (see aquatic and riparian vegetation for details).

The connectivity of a waterway is an important indicator of its health. The presence of barriers to longitudinal connectivity forms part of the assessment of waterways in the South West Index of River Condition.

References:

Morgan DL, Gill HS & Potter IC 1998, Distribution, identification and biology offreshwater fishes in south-western Australia: records of the Western Australian Museum, Supplement no. 56, Western Australian Museum, Perth.

Fairfull, S & Witheridge, G 2003, Why do fish need to cross the road? Fish passage requirements for waterway crossings,NSW Fisheries, Cronulla.