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Kent Street Weir

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)