Vegetation and fauna in the Riverpark survive in a delicate balance with the water regime in the Swan and Canning rivers and their catchments. Significant change to water volumes, flow rates, water quality or the timing of water delivery can have a negative impact on the health of rivers, streams, pools and wetlands.
Environmental Water Requirements (EWR), commonly referred to as “environmental flows”, describe the water regime required to maintain the ecological values of water-dependent ecosystems at a low level of risk. Environmental Water Provision (EWP) describes a water release regime provided to improve or maintain these ecological values. EWPs are delivered, monitored and managed in some rivers according to water allocation plans negotiated by the Department of Water in consultation with a broad stakeholder group including Parks and Wildlife.
The flows in five of the Swan Canning estuary’s tributaries (Canning River, Wungong River, Helena River, Bickley Brook and Churchman Brook) are controlled to meet domestic water supply needs. Impoundments such as dams or reservoirs constructed for public water supply reduce the total volume of water flowing in the rivers downstream, impacting the seasonality, duration, extent and variability of flow in rivers. This can result in a disconnection between the river and its floodplain and wetlands. Impoundments can also disrupt the continuity of rivers by reducing the flow of organic material and sediments downstream, and by creating a barrier to the movement of fish and other fauna.
All these factors can combine to reduce the distribution and abundance of birds, fish, fringing and in-stream vegetation, frogs and other aquatic organisms. Reduced flows can also lead to degraded water quality as temperature increases and dissolved oxygen levels decrease, which can affect fish and invertebrate survival. Changing the frequency and extent of flows can cause changes to river geomorphology through bank slumping, sedimentation and colonisation by exotic and terrestrial vegetation which would otherwise not become established in an unaltered river.
Parks and Wildlife is addressing the need for greater environmental flows to be delivered to the major tributaries of the Swan and Canning rivers through its environmental flows program and supports the delivery of environmental water releases to the Canning River by the Department of Water. This program targets the Helena and Canning rivers.
Currently the Canning, Helena and Wungong Rivers receive environmental water releases.
In Western Australia, there has been a long term trend toward reduced stream flows, independent of the effect of impoundments. Reductions in freshwater flows to the Swan and Canning rivers have combined with the now permanent opening to the ocean at Fremantle to markedly alter the hydrodynamics, geomorphology and ecology of the estuary.
Climate change is expected to further reduce freshwater flows to the estuary and this will cause saline water from the ocean to penetrate further upstream. This will reduce habitat for freshwater fauna and flora and may affect breeding cycles of fish and other fauna that rely on consistent patterns of freshwater flow in the estuary to stimulate migration and reproduction.
Parks and Wildlife will work with other water management agencies to examine the feasibility of delivering environmental flows to other sections of river downstream of impoundments. In the short-term, the focus will be on the Canning and Helena rivers.