Summary - City of Gosnells

Regional Setting 9Most of the rivers and creeks of the SCP (including Yule Brook and Bickley Brook) are heavily modified bythe deepening and straightening of existing channels and the construction of drains to increase conveyancecapacity and to lower superficial groundwater levels (Swan River Trust, 2009). The Yule Brook and BickleyBrook catchments in the MKSEA include a number of small creeks, but few of these have retained theirnatural form; most have been excavated and diverted to facilitate drainage (Figure 7). The area also has anumber of small-scale, discontinuous channel segments (that are evident in the stratigraphic sequences andvegetation of the area) that indicate the locations of natural surface drainage channels that in the past hadsimilar attributes to the currently active channel of Yule Brook (V & C Semeniuk Research Group, 2001).The Swan River Trust (2009) has prepared a Water Quality Improvement Plan (WQIP) for the Swan andCanning Rivers. The flow rates and nutrient levels of numerous watercourses (including Bickley Brook andYule Brook) that contribute to the Swan and Canning River system are regularly monitored (Swan RiverTrust, 2008a, 2008b). One of the main aims of the WQIP is ‘to set environmental flow objectives, protectwetlands and floodplains (mimic natural inundation and drying patterns) and to minimise the effects of damsand extraction of water on water quality’ (Swan River Trust, 2009).The environmental flow objectives for aquatic and riparian vegetation for watercourses on the SCP wereidentified in GHD (2008) and these are relevant to the MKSEA. These objectives were:• ‘Maintenance and restoration of wetland/riparian vegetation in winter-wet, pastured, floodplain regionsand along the periphery of drainage channels;• Inundation of wetland/riparian vegetation to restore and maintain these areas; and• Seasonal inundation of emergent and mid bank vegetation (for survival, germination and recruitment).’In the MKSEA there are some areas where deep and water-permeable Bassendean Sands overlie, or areemergent from, the Guildford Formation. These dunes and low ridges occur mainly in the southeast of theMKSEA, where they generally form localised perched water mounds. Rainwater infiltrates the BassendeanSands and is slowly discharged laterally (along the contacts of the dunes with the adjacent, largelyimpermeable, Guildford Formation clay layers) onto the adjacent palusplains. The hydrology of the smallscale,seasonally-waterlogged slopes (paluslopes) that are maintained by such seepages from low ridges inthe Greater Brixton Street Wetlands, and the artesian pressure heads in some aquifers associated with theBassendean Sands/Guildford Formation interfaces in this area, were described in V & C Semeniuk ResearchGroup (2001). These areas in the Greater Brixton Street Wetlands are the only examples of paluslopewetlands that have been described to date on the SCP. The general principles of paluslope formation areillustrated in Hill et al. (1996, Volume 2A, Figure 2.13).The boundaries between the Pinjarra Plain landscapes (that occur mainly in the northern and central area ofthe MKSEA) and the Bassendean Dunes landscapes (that occur mainly in the southeast of the MKSEA) areoften subtle and irregular. These boundaries effectively divide the wetland landscapes from the uplandlandscapes. In simple terms, one of the major boundaries of this type in the MKSEA in the vicinity ofVictoria Rd has been described as ‘a watershed’ (GHD, 2005). However, it is not simply the crest of a ridgethat sheds water uniformly down the surfaces of both of its slopes. This boundary is marked by a subtletopographical gradient but also complex stratigraphy and hydrology (where water recharge, storage anddischarge can be influenced by factors such as groundwater flow, seepage from perched aquifers, artesiansprings, and channelled surface flow). Thus an array of wetland types (including paluslopes, palusplains,floodplains, sumplands and damplands) has developed along this boundary. A number of small dams havealso been excavated along this boundary and this has further complicated the hydrological features of thearea. The boundary between the Bassendean Sands and the Pinjarra Plain in this area is also significant interms of biological values. The remnant native vegetation along this boundary is often atypical or uncommonin terms of other vegetation on the SCP and thus of high conservation significance and both the naturalwetlands and excavated dams in the area are significant habitats for waterbirds and probably other nativefauna. To achieve optimum environmental outcomes for the Greater Brixton Street Wetlands and theMKSEA, it is important that the Pinjarra Plain to Bassendean Dunes interfaces of the area are mappedaccurately at a fine scale. Decisions about development in the area (such as the determination of buffer zonesto protect the wetlands adjoining the Pinjarra Plain to Bassendean Dunes interfaces) should be based ondetailed field studies of topography, stratigraphy and hydrology that have been conducted over anTauss, C. and Weston, A.S. (2010). The flora, vegetation and wetlands of the Maddington-Kenwick Strategic Employment Area.A survey of the rural lands in the vicinity of the Greater Brixton Street Wetlands. Report to the City of Gosnells, W.A. Version 18.04.10