Perth is undergoing rapid growth, with the population doubling during the past 30 years and expected to reach 3.5 million people by 2050. Urban and rural developments throughout the catchment are placing increased pressure on the Swan Canning Development Control Area.

Without proper land use planning and development processes, increased sediment transport and mobilisation of nutrients and other contaminants into the Development Control Area can occur. With a drying climate there is also an increased need to maintain flows to the rivers as land uses change.

Major riverfront developments can also exert pressure on the Development Control Area and should be designed to enhance the quality if the river through the use of water sensitive urban design. Developments should also be built on the principle of enhancing public spaces along the waterfront to allow the community to continue to enjoy the highly regarded amenity of the rivers.

There is a need to balance development with the strong community desire to conserve and enhance the riverine environment, protect its landscape amenity and maintain and improve public access to the rivers for a range of recreation activities.

The Kent Street Weir is located on the Canning River in Cannington, next to the Canning River Eco Education Centre. 

The weir was first constructed from sand bags in 1911 to stop salt water affecting agricultural land upstream. A more permanent weir was completed in 1927. It has since undergone many changes to its design and function with its most recent update in 1989.

Since the completion of the permanent weir, a ‘weir pool’ has formed in the river channel upstream of the weir. This pool extends approximately 5km upstream, maintaining a relatively constant water level through the drier months. Because the pool’s water is usually fresh, the river environment has become dominated by freshwater-dependent flora and fauna, including native fish such as the western pygmy perch and western minnow, reptiles such as the oblong tortoise, crayfish such as gilgies and marron, in-river vegetation including Potamogeton species, and terrestrial vegetation like the flooded gum (Eucalyptus rudis) and freshwater paperbark (Melaleuca rhaphiophylla).

The Kent Street Weir is made from 17 concrete bays, each containing four removable stop boards. The boards are typically removed once there has been enough rainfall to push salt water downstream - usually in June or July - and replaced when river flows are reduced to stop salt water moving back into the weir pool - usually in October or November. This annual cycle of stop board removal and replacement allows annual flushing of the system, and facilitates the movement of fish and other animals upstream of the weir during winter and spring.

In 2010, for the first time on record, the boards were not removed. A dry year producing the lowest stream flows on record meant winter flows in the Canning River were not sufficient to allow their removal. In recent years, stream flows have tended to occur later in the season, meaning the weir boards are often not removed until July or August, and are typically replaced earlier to ensure that salt water is not ‘trapped’ behind the weir over the summer months.

While the principle aim of the modern day Kent Street Weir is to stop salt water from moving upstream, the primary justification has changed from protection of farm land, to protection of the freshwater-dependent flora and fauna now present in this stretch of the river.

The weir also provides footbridge passage across the river and allows recreational activities like kayaking and fishing during the summer months by maintaining water levels in the river upstream of the weir.

Recent inspections of the weir pylons have indicated structural weaknesses that need to be addressed. Refurbishments to the weir by the Department of Water, with assistance from Parks and Wildlife and City of Canning, are likely to occur in 2016-17. This refurbishment will extend the life of the weir by at least 50 years and address issues such as increasing water levels, increased public access and ensuring the structure remains safe.

Risks to the environment above the weir

The environmental values, and ultimately the social and economic values, upstream of the weir are at risk from three principal threats.

• Salt intrusion. The estuary downstream of the Kent Street Weir is saline, and occasional tide and storm surges have caused downstream water levels to exceed the height of the weir structure. This results in large volumes of salt water entering the weir pool, which can adversely impact native flora and fauna that is dependent on fresh water. Salt water is denser and therefore heavier than freshwater, and can sit at the bottom of the weir pool. This can reduce Parks and Wildlife’s ability to oxygenate the system, and increase the amount of nutrients, in particular phosphorus, being released from sediments.
• Nutrient enrichment/algal blooms. Large amounts of nutrients, predominantly nitrogen and phosphorus, enter the Canning River from its catchments, particularly after heavy rain. Much of this is available for algal growth, and under the right flow and weather conditions can lead to algal blooms. The availability of these nutrients is also increased under anoxic, or no oxygen, conditions that often occur after salt intrusion into the weir pool.
• Deoxygenation. Like most impounded water bodies, the Kent Street Weir pool is susceptible to low oxygen levels as a result of minimal water movement. This can affect the survival of the fauna in the weir pool, and interrupt natural processes like organic matter breakdown and nutrient cycling.

Management

The risks of nutrient enrichment, algal blooms and deoxygenation are managed through a range of initiatives.

• Strategic timing, based on monitoring data, for removal and replacement of stop boards to flush the weir pool and allow movement of fish into upstream habitat zones
• a long-term oxygenation program on the Canning River
• the occasional addition of Phoslock, a modified clay that can reduce dissolved phosphorus available in targeted areas of the river
• catchment activities designed to reduce nutrients entering the system from the catchment, including the Drainage and Nutrient Intervention Program and local water quality improvement plans (WQIPs)
   

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.

Impacts of dams and other impoundments

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.

Long-term stream flow trends

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.

The future

DBCA 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.

The Perth metropolitan area lies on the Swan Coastal Plain, a relatively flat sedimentary zone located between the Darling Scarp to the east and the Indian Ocean to the west. This zone superficially consists of mostly infertile sandy soils with limited nutrient holding capacity and a shallow groundwater table expressed as ephemeral wetlands.

Many wetlands have been cleared and drained for urban and agricultural purposes, with fertilisers and other chemicals applied to increase soil productivity. These pollutants, along with those contributed by other catchment sources, readily leach through the soil into groundwater, entering the drainage system through groundwater discharge and overland flow where they are transported into the river system.

This is compounded by the construction of Perth’s drainage network to convey floodwater and lower the shallow groundwater table through open drainage channels, piping and compensation basins. Without natural buffers in place, this drainage system readily conveys excessive nutrients and other pollutants to the rivers.

Under natural conditions most rain falling on a catchment soaks into the soil, tops up aquifers, is used by vegetation, or travels over or under the ground towards rivers, lakes, reservoirs or the ocean. Urban development creates impervious surfaces such as roofs, roads and pavements. This means rainfall no longer infiltrates where it falls. Large volumes of water must be artificially drained away.

Many urban drainage lines and sumps were once natural streams and wetlands, but these have been highly modified to cope with increased runoff caused by urban development. 

In rural areas of the Swan Canning Catchment, modified creek lines often still exist, however surrounding land and fringing vegetation is often degraded causing increased runoff, nutrients and sediment loads to be delivered to the rivers.

The urban drainage system includes:

• open drainage channels - mostly unlined, linear, trapezoidal shaped and excavated into the groundwater table so they flow all year round and are very efficient at conveying stormwater runoff to the receiving environment
• closed pipes and drainage structures - usually constructed of concrete, also very efficient at conveying stormwater runoff to the receiving environment with little water quality improvement other than gross pollutant removal
• compensating basins - sumps in a drainage system that allow high stormwater flows to be partially retained and infiltrated in the catchment ensuring that open drains and pipes do not flood
• Best Management Practice stormwater treatment measures - including biofilters, wetlands, swales and living streams which improve water quality, habitat and amenity

More recently, a greater focus on Water Sensitive Urban Design (WSUD) has incorporated water cycle management in the urban environment so environmental and social values are enhanced. This approach allows smaller annual rainfall events to be retained in the catchment and uses vegetation and soil to filter stormwater, improving water quality but also enhancing habitat and amenity.

The Department of Water’s Stormwater Management Manual for Western Australia, developed in partnership with the Swan River Trust (now Parks and Wildlife) and other stakeholders, advocates the use of WSUD where appropriate, providing high-level policies, planning principles and practical on-ground best practice advice to manage stormwater.

DBCA, through its Drainage and Nutrient Intervention Program, is working with partner organisations to remove nutrients from drains and tributaries in catchments that contribute high nutrient loads to the Swan Canning river system.

Acid sulphate soils

Acid sulphate soils occur naturally in Western Australia, including the Swan Canning Catchment, and are harmless when left in a waterlogged, undisturbed environment.

However, when exposed to air through drainage or excavation, iron sulphides in the soil react with oxygen and water to produce iron compounds and sulfuric acid. This acid can release other substances, including heavy metals, from the soil and into the surrounding environment, groundwater and waterways. Once mobilised, they can include kill vegetation and fish, damage fisheries and aquaculture industries, harm tourist resources such as fishing grounds, swimming areas and recreational waterways, and damage infrastructure including corroding concrete and steel pipes, building foundations and bridges

Activities with the potential to disturb acid sulphate soils must be managed carefully to avoid serious environmental harm. Proposed developments often require acid sulphate soils testing, and where appropriate management plans to prevent potential damage to the rivers or surrounding environment.

The Department of Environment Regulation has produced a series of fact sheets and guidelines to assist with the identification, investigation and management of acid sulphate soils in Western Australia.