Assessment of Current Ecological Condition and Investigations for Potential Outcomes Resulting from Proposed Management Changes of the Tuppal Creek and Bullatale Creek Systems – M/BUS/106 DN184
SKM Technical Report (WCO3522)
MDFRC are sub-consultants to SKM on this project, for the NSW Department of Natural Resources. The work follows on from other work that MDFRC has done on these systems. MDFRC’s task in this project is to collect and analyse water quality and sulfidic sediment data, and assist in the assessment of the current ecological condition of the Tuppal and Bullatale Creek systems.
Sinclair Knight Merz were engaged by NSW Department of Natural Resources (DNR) to carry out a project to assess the outcomes of implementing proposed management changes to the Tuppal and Bullatale Creek systems. These changes are aimed at decreasing the frequency of unseasonal flooding in the Barmah-Millewa Forest caused by rainfall rejection flows by upstream users. A hydraulic assessment of Tuppal and Bullatale Creeks to define commence-to-flow thresholds and channel capacities of the offtakes has already been undertaken and a number of management options which could divert an increased volume of water through the creeks have been provided (SKM 2006a). Barmah Forest in Victoria and the Millewa group of forests in New South Wales, is the largest naturally occurring River Red Gum (Eucalyptus camaldulensis) forest in Australia (MDBC 2005). It covers approximately 66, 000 ha of floodplain between the townships of Tocumwal, Deniliquin and Echuca and contains a diverse range of wetland environments. The hydrology of the forests involves an intricate arrangement of inflow sources and drainage routes, the regularity, extent, duration and season of flooding of which is governed by flow in the River Murray. In excess of 50 water management structures are currently present throughout the Barmah-Millewa Forest (MDBC 2005). The Tuppal Creek and Bullatale Creek systems have been identified as a possible means of diverting rainfall rejection flows around the Barmah-Millewa Forest. The creeks are effluent streams of the Murray River, and carry flow from the Murray River just downstream of Tocumwal, to the Edward River near Deniliquin. The creek systems are approximately 100 km long and were naturally one large system that would have been connected through a series of overland flow paths during large flood events. Due to extensive management works in the past, the creeks are now two distinct systems. The Tuppal Creek system receives water from the Murray River near Tocumwal, although the offtake is presently blocked by a levee bank. The creek flows in a north-westerly direction away from the Murray, and is intercepted by several irrigation drains. Native Dog Creek is a tributary of Tuppal Creek that receives water from the Murray River. Tuppal Creek is considered to be in a degraded ecological condition and the local community is concerned about the health of Tuppal Creek (Vincent Kelly, NSW DNR pers. comm.). The Bullatale Creek system consists of the main stem and a number of tributaries, which drain in a north-westerly direction away from the Murray before turning north and entering the Edward River just upstream of Tuppal Creek. Forest streams that offtake from the Murray River supply the Bullatale Creek system; these streams include Deep Creek, Aratula Creek, Lower Toupna Creek and Aluminy Creek. Interaction between the Tuppal Creek system and the Bullatale Creek system only occurs during periods of high flow when floodplain channels may become active. Although Richardson, et al. 2005 considered Bullatale Creek was healthy and had maintained its connections with the Murray River the recent field assessment found that the condition of the creek varies along its length and a number of sections have unrestricted stock access and degraded riparian vegetation. These two systems have different management issues and consequently the proposed management objectives for the creeks vary. However, the aims of implementing the proposed management changes will contribute to meeting the proposed objectives for both Tuppal Creek and Bullatale Creek as well as the management objectives for the Barmah-Millewa Forest. The proposed management objectives for Tuppal and Bullatale Creeks are still in the development stage and are yet to be discussed with the local community. The recent field investigations have identified a number of flow related management issues in Tuppal and Bullatale Creeks. The main issues for Tuppal Creek are degraded water quality, problematic instream River Red Gum regeneration, spreading Cumbungi (Typha spp.) and a number of sites with the potential for acid sulphate soils. Key management issues for Bullatale Creek are degraded riparian vegetation and unrestricted stock access to the creek. Water quality in Bullatale Creek is generally considered good. These issues are the consequence of the current management of these three systems (i.e. Tuppal Creek has extended periods of low flow and the predominant water source is irrigation drainage). Increasing the flow in Tuppal Creek would improve the ecology of this system, but the impact of the timing and duration of such flows on the Edward and Murray Rivers should be considered and monitored. Both the Tuppal and Bullatale Creeks have undergone widespread clearance since European settlement, which in turn, has triggered a range of degradation processes including altered hydrology, weed invasion and salinity. Prior to settlement, the remnant vegetation of the Tuppal and Bullatale Creeks and surrounding environments was a mixture of forests, woodlands, grasslands and seasonal wetlands. Large areas of land have since been cleared and converted to irrigated and dryland agriculture. As a consequence, only fragmented, discontinuous and degraded examples of riparian vegetation and grassy woodland remain. The condition of remnant vegetation generally reflects the surrounding landuse and the extent of protection from disturbance. A number of hydrology management options that could divert rainfall rejection flows through the Tuppal and Bullatale Creek systems were investigated, modelled and discussed in SKM (2006a). The first option modelled was the Preliminary Option, which consisted of designing works for the Tuppal Creek and Bullatale Creek systems that could divert all flow in the Murray River between 10,600 ML/d and 15,600 ML/d at Tocumwal. The aim of this model was to better understand the physical constraints of the creek systems and likely costs of works involved in undertaking channel modifications. Options 1 to 4 of the hydrology management scenarios provided by SKM (2006a) are unlikely to improve or degrade the ecological condition of Bullatale Creek, as this creek is currently flowing for much of the time and is currently in good condition. Whereas the benefit of increased flows in Tuppal Creek is that they are likely to improve water quality and habitat condition as well as biodiversity the implementation of Option 5 may be of greatest benefit to Tuppal Creek, if it is also used to provide environmental flows down Native Dog Creek. This is because of the prolonged period that flows may be diverted into Native Dog Creek under the existing flow conditions in the Murray River. Further studies would be required to quantify the benefit to Tuppal Creek under this option. Flow diversions into Tuppal Creek under Option 6 would occur on average less than 25% of the time. The connection between Tuppal Creek and the Murray River under Option 6 would only be activated for relatively short periods when flow in the Murray River is greater than 18,100 ML/d. Although the impact would be infrequent, flows of this magnitude would be sufficient to freshen water quality and provide watering of bank vegetation in the upper reaches of Tuppal Creek. Once flow falls below the trigger level at Tocumwal, diversions would cease. The flow scenario options and the recent field investigations showed that any improvement in the ecological condition of Tuppal and Bullatale Creeks will only occur in channel and to some extent along the mid to upper banks. Diverting rainfall rejection flows from Barmah-Millewa via Tuppal Creek would occur in summer, a time when naturally stream flows are at their lowest or in the case of an ephemeral stream such as Tuppal Creek, a period of cease-to-flow. Consequently, any diverted flows to Tuppal Creek in the summer would also be unseasonal but may act in a similar way to a summer fresh. Given the high environmental values of the Barmah-Millewa forest and the degraded nature of Tuppal Creek, any flow down Tuppal Creek at that time of the year could be beneficial to water quality by flushing out pools and at the same time maintain the values of the Barmah-Millewa forest. This report provides a summary of the ecological conditions of Tuppal and Bullatale Creek as assessed during the field component of this project. The implications of the flow modelled scenarios (Options 1-6) on the current condition are discussed and a number of general recommendations have been provided for in channel flows to manage instream River Red Gum, Cumbungi, water quality and sulfidic sediments that will also assist in meeting the management objectives for the creeks and the Barmah-Millewa forest. An assessment of the monitoring program carried out is also provided.
MDFRC funding agency: Department of Natural Resources, NSW
MDFRC client: Sinclair Knight Merz
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Copyright (2006) Murray-Darling Freshwater Research Centre.