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Environmental Impact of Dams, Weirs & Pumping

A river is said to be regulated when the flow of the river is regulated by large dams or weirs. The types of environmental impacts that may occur within regulated systems are shown in the table below.

Water resource management operation	Ecologically significant component of flow regime that can be impacted downstream	Probable impact on river health
Groundwater bores (depending on volume extracted)	• reduction in summer baseflow in rivers • decrease in water depth in wetlands	Loss of critical summer habitat resulting in loss in biodiversity
Catchment dams (depending on degree of development in catchment)	• reduction in summer baseflow • reduction in winter baseflow • reduction in freshes	As above plus: • loss of opportunities for movement between habitats • reduction in breeding cues
River diversions (depending on volume of water extracted) • summer	• reduction in summer baseflows • considerable reduction or increase in daily variability of summer baseflows (depending on the pumping regime)	• loss of critical summer habitat resulting in loss in biodiversity • loss of opportunities for movement between habitats
• winter	• reduction in winter baseflow • reduction in freshes	• loss of opportunities for movement between habitats • reduction in breeding cues
Small on-stream dams	As above	As above plus: • physical barrier to movement
Large regulating weirs (depending on their function)	As above plus: • reduction in winter baseflows • general reduction in freshes • loss of daily variability in baseflows • change to the seasonality of flows (e.g. increased summer flows for irrigation) • rapid rise and fall of water levels	As above plus: • loss of breeding cues • erosion of river banks and deepening of river channel (this may impact on the groundwater table) • loss of bank habitat and species • conditions may favour blue-green algae • loss of natural sediment transport • stillwater habitats may favour exotic species (e.g. Carp)
Diversion/irrigation channels	• alteration to natural flow paths and patterns	• spread of exotic species • transport of native species into irrigation areas – loss of biodiversity
Large dams (capacity < mean annual run-off)* • generally built to supply irrigation and town water supply • often run at high flows during the irrigation season	• reduction in frequency and duration of 1 in 2 year floods • increase in summer baseflows • decrease in cease-to-flow events • reduction in winter baseflows • general reduction in freshes and flushes • loss of daily variability in both summer and winter baseflows • increase in unseasonal summer flooding	As above plus: • loss of lower areas of floodplain and wetland habitat • loss of breeding cues and opportunities with subsequent loss of biodiversity • loss of trigger flows may favour reproductive success of exotic species (e.g. Carp) • reduction in organic exchange between floodplain and river resulting in reduction in ecological processing • changes in water temperature resulting in reduction of breeding of fauna • erosion of river banks and scouring of river beds • loss of bank habitat and species • changes in river bed with loss of instream habitat • loss of nutrients, sediments and organic matter downstream • reduction of organic debris and sediment input into floodplains and estuaries • changed frequency/duration of estuary closure
Very large dams (capacity > mean annual run-off)* • if built to supply irrigation and town water supply - generally releases at high flows during the irrigation season - may have a constant release pattern	As above plus: • reduction in frequency and duration of all categories of flood events	As above
* if run for hydroelectricity generation	• may have greatly increased daily variability	• bank slumping • bed scouring and deepening of river channel resulting in loss of habitats and species