Climate change and waterways

Changes to our climate, including rainfall, temperature, evaporation and extreme weather events, influence the water cycle, which is the primary driver of the hydrology of waterways.

As well as being dependent upon surface and groundwater flows and temperature, the condition and stability of waterways is a result of complex and dynamic network of interactions between bacteria, algae, plants and animals and the surrounding sediments, rocks, and chemicals.

We monitor the hydrology of our waterways and water resources, and the health of waterway ecosystems.

Our challenge is to understand how climate change will affect ecosystems in future, and take this into account in environmental planning, management and restoration to maximise the health and resilience of waterways.

Under future climate scenarios a 24 cm increase is projected by 2050 for the median case. As sea levels rise, low-lying coastal freshwater floodplain and wetland ecosystems are at risk as the incidence of inundation events increases and vegetation has less time to recover after more regular flooding by seawater.

Estuaries will be affected by increasing marine influence and changing habitat distribution and quality. Increase in saline habitats, altered sandbar dynamics, shoreline erosion and sediment redistribution may occur due to increased tidal influence and wave height, and changes to stratification within the water column. South-west WA’s estuaries are more likely to experience reduced flushing of sediments, nutrients and pollutants as they are affected by both sea level rise and reduced river flows.

Changes to rainfall patterns will result in altered salinity in many waterways, adding to ecosystem stress. 

In most south-west WA estuaries, reductions in freshwater inflows will lead to increased average salinity. Some areas of estuaries will be prone to hypersalinity where salt will concentrate in zones with restricted flows. Ecological consequences of hypersalinity are decreased phytoplankton diversity and restricted habitat for brackish and freshwater fish species.

In some areas outside south-west WA, rainfall has increased since the 1960s, with the highest increases recorded in the north-east parts of the state. Rainfall variability and extreme rain events are projected to become more intense, leading to more very wet and very dry years in these areas.

Many waterways in arid and semi-arid areas experience irregular large floods, with long dry periods between, where they exist as a series of disconnected pools or dry out completely. The aquatic and riparian fauna and flora that persist there are adapted to this extreme variability, so they may be more resilient to future changes compared with species in south-west WA.

Average temperatures in WA have increased by 1.3°C since 1910. Temperatures are predicted to continue to rise including an increase in the number of extremely hot days (above 40°C), and less cold nights.

Temperature affects many biochemical processes in aquatic ecosystems. Water temperature directly affects the distribution of aquatic organisms and the predicted extreme temperatures may exceed the tolerances of sensitive species, causing local extinctions. In rivers, areas where little riparian vegetation exists to shade pools are particularly vulnerable.

Increased temperatures may also disrupt lifecycles, for example affecting species that have a sex ratio dependent on temperature, such as crocodiles and freshwater turtles.

Higher temperatures will increase evaporation and transpiration rates, further reducing surface water and changing salinity. Disconnected river pools and wetlands, which are the critical habitat for WA species to survive the dry season, are at particular risk from accelerated drying. For instance, tadpoles may not have time to develop into frogs before habitats dry out again.

Higher water temperatures also result in less capacity of the water to uptake and hold oxygen, leading to greater risk of dissolved oxygen dropping overnight. This is the leading cause of fish kills in WA, particularly when coupled with increased nutrient levels. 

Shallow estuaries are highly vulnerable to warming conditions. Higher temperatures favour algal growth which affects the entire food web through related effects on oxygen consumption and production. Extreme heat waves also have negative impacts on some estuarine fauna and flora, such as the seagrass communities that are critical to maintaining dissolved oxygen levels, reducing nutrients and stabilising sediments.

More frequent and severe bushfires directly threaten riparian and wetland vegetation and habitats. Bushfires have indirect impacts on water quality due to ash and erosion after the removal of fringing vegetation.

Recent decades have seen higher rainfall through northern and central Australia and decreasing rainfall across south-west Western Australia.

The decline in rainfall in south-west WA has been larger than anywhere else in Australia. While the decreasing trend in rainfall is expected to continue across the region, the intensity of heavy rainfall events is expected to increase.

Reduced rainfall and fewer very wet years have resulted in lower and less frequent flows in south-west WA’s waterways. For more information on changes in flow, see surface water hydrology.

In some areas (such as around Perth), groundwater recharge and water tables have also fallen, affecting the waterways and other ecosystems that depend on groundwater during the dry summer months.

As this drying trend continues, waterways in south-west WA are likely to experience altered patterns of flow with perennial streams becoming seasonal and seasonal streams having longer periods without flow. Some wetlands may disappear over time.

Affect on our rivers

In rivers, high flows in winter have historically helped maintain river channels. The winter flows transport woody debris, organic matter and nutrients downstream, providing habitat and food.

Moderate flushing of pools maintains pool depth without negatively impacting fringing vegetation, which in turn results in a cooler refuge at depth during summer months. Shallow pools with higher summer temperatures and less shelter will add to ecosystem stress. A trend towards less frequent but more extreme rainfall events may also mean that when there is water available, it may contribute to rapid bank erosion and flooding.

Aquatic organisms will be affected by habitat changes related to overall reduced flow and shorter periods of inundation.

In rivers for example, these changes can impact both the availability of habitat as well the environmental cues for migration, breeding and foraging. Semi-aquatic and terrestrial species that depend upon food webs that start in waterways will also be affected.

Waterways, their habitats and flora and fauna in south-west WA are the most vulnerable to climate change, due to the steady and long-term decrease in rainfall and streamflow and added population pressures, including increasing demand for water and land development.

We take climate change and its influence on trends into account in the way it manages water resources, including:

• assessing water resources and modelling water availability
• making decisions about the amount of water that can be taken from surface and groundwater resources
• advising about water and land planning and inundation risk
• advising about management and restoration of waterways, their floodplains and estuaries.