River restoration using simple decision support tools in the Lower Snowy River

Humans have extensively modified rivers throughout Australia. Such modifications can be direct, via changes to the natural flow regime, or indirect, such as removal of vegetation in the catchment, altering river geomorphology and changing sediment delivery to rivers. Past and ongoing human interventions have drastically changed the hydrology and geomorphology of the Snowy River, which has had a profound effect on the ecology of the system. Due to impoundments and water extraction, the Lower Snowy has seen a reduction of flows in the order of 55% (James 1989). However, environmental flows were introduced into the river in New South Wales at Jindabyne Dam in 2002. In the upper section, much attention has been given to the restoration of flows. In the Lower section, the Snowy has seen significant changes in geomorphology due to catchment and hydrology changes. Community concern at the observed environmental degradation resulted in the development of the Snowy Rehabilitation Project, a cooperative project involving Victorian state and regional bodies. One major outcome of this project was funding for rehabilitation works in the Lower Snowy. As a result, much effort was put into modelling work. The focus of this was on better understanding the behaviour of sediments within the river and defining the needs for restoring ecological functions, with a primary focus on providing fish passage. This type of information was intended to develop a way forward for rehabilitation works in the Lower Snowy. It is important to note that this section of the Snowy is surrounded by privately owned agricultural land. Any rehabilitation works also needs to consider how proposed changes in river behaviour will impact on surrounding land. From this work, a simple decision support system (DSS) was built. The DSS was designed to use modelled information and to assist in decision-making processes by linking management activities (or interventions) to outcomes. The tool uses a 'risk' approach to acknowledge the uncertainties that exist in our knowledge, including models, and the inherent variability of this natural system. The DSS ('The Snowy tool') user is the Victorian East Gippsland Catchment Management Authority (CMA). The purpose of the Snowy tool is to 'provide an assessment of the cumulative outcomes and risks due to different levels of management intervention'. The rehabilitation activities of interest are: riparian vegetation management; management of vegetation on in-channel benches; and installation of large wood in channel. The model aims to predict the likely outcomes of interventions to: scour holes for fish habitat and migration; occurrence of overbank inundation; avulsion likelihood; and bench and bank stability. The Snowy tool is a probabilistic model (Bayesian network) that incorporates data from the hydraulic model, HEC-RAS, as well as expert opinion, and a set of ecological response models developed previously. The model was evaluated within an expert workshop. The tool highlighted flaws in past studies, and could only partially address the needs of the decision-makers. The outcome of the project stresses the need for decision-making tools to be designed early in the project development, in order to better guide process modelling and data collection exercises. Without being designed upfront, much of the data collected can be well intentioned but poorly targeted at addressing the key needs for the river system