Derwent Estuary Coastal Catchment Initiative - Derwent River Environmental Flows Scoping Review: Issues, state of knowledge and management objectives

Abstract

The Derwent River is a cool-temperate river system in south-eastern Tasmania, with a catchment area of 8,900 km2. The Derwent's flow regime is highly regulated by hydro-electric storages and related infrastructure. The upper catchment of the Derwent River is mainly forested, but the middle and lower catchment is cleared for grazing and light agriculture. The estuary of the Derwent River is a salt-wedge estuary in a drowned river valley. The immediate foreshores and catchments of the estuary are highly developed, with the cities of Hobart and Glenorchy, and a number of other urban developments situated on its shores (Figure 1). Intensive hydroelectric development occurred throughout the Derwent catchment, primarily in the 1950's. This involved construction of a complex system of storages, dams, channels and weirs in the upper catchments, as well as a series of large impoundments in the middle and lower reaches of the main river channel. This system includes a substantial inter-basin transfer, exporting approximately 10% of upper catchment yield into the South Esk catchment (via Great Lake), coupled with a series of headwater storages which store and control the release of winter rain and snow melt into the middle and lower storages. These in turn, regulates the main-stem flow regime substantially through a series of low-head run-of-river lakes and power stations. As a result the flow regime of the lower Derwent is characterised by: - A reduced incidence of medium to large flows; - Seasonally altered and raised baseflows; - Reduced intensity of very large floods; - Flows fluctuating rapidly within a day . Significant takes of river water occur immediately upstream of the tidal limit for the majority of the urban water supply to the greater Hobart area (Hobart Water's pumping and treatment facility at Bryn Estyn), and for use in the Norske Skog paper mill at Boyer (pumped from the same reach). The latter water largely re-enters the upper estuary as a single waste stream, while the former contributes to wastewater releases in the middle and lower estuary. Davies et al. (2002) conducted an initial scoping of minimum environmental flows for the lower Derwent River by assessing requirements for riverine habitat maintenance and the biogeochemical status of the upper Derwent estuary. Prior to this no environmental flow provisions existed for this system. From 2004, the recommendations made by Davies et al. (2002) formed the basis of constraints imposed by state government on further licensed takes ('abstractions') from the lower Derwent. While hydroelectric development has been the single major cause of change to the flow regime of the Derwent, substantial localised changes in land use have also occurred, with approximately 30% of the catchment cleared since European settlement. The upper and upland sub-catchments of the Derwent have changed little, while sections of the central and lower sub-catchments of the Derwent river have been extensively cleared. There are no analyses of the influence of this on runoff and sediment yield, but a degree of change, especially in agricultural and urban lower sub-catchments is likely to be evident. The Derwent Estuary Coastal Catchment Initiative (CCI) project is largely focused on addressing knowledge gaps in the area of metal toxicity and sediment modeling in the estuary. This report is a product of the Derwent Estuary CCI project. It provides a scoping overview of issues, conceptual understanding and knowledge requirements associated with the effects of historical changes to the Derwent flow regime of the Derwent river and environmental flow needs. The current level of conceptual understanding and key knowledge gaps were identified in a workshop with relevant experts and by individual consultation. The report also proposes some initial objectives for environmental flow management.