The Derwent Estuary catchment includes the River Derwent catchment (7500 km²), the Jordan River catchment (1250 km²) and other areas immediately adjacent to the estuary (375 km²). This is a region of varied relief, climate and vegetation; major land uses consist predominantly of natural areas (49%), followed by native production forestry and plantations (26%), agriculture (18%, predominantly grazing), water (5%) and residential (1%).
Derwent catchment area land use by percentage, 2013
The River Derwent starts at Lake St Clair and flows southeast through a series of dams, power stations and reservoirs until it joins the Derwent Estuary at New Norfolk, 190 km downstream. This is one of the largest rivers in Tasmania, with an average annual flow of about 90 cubic metres per second, and is characterised by highly modified flows, with the generation of hydro-electric power and irrigation causing significant changes to the natural flow regime. Flow modifications and dam infrastructure influence the estuary, and associated impacts include changes in water circulation patterns, oxygen depletion, sediment and silt from erosion from changing water levels, as well as impacts on the seasonal cycles and movement of migratory fish.
Recent water quality decline
Long-term monitoring at New Norfolk suggests a decline in water quality since the early 1990s, particularly with respect to nutrients and suspended sediments. Starting in 2015, significant taste and odour problems also started to affect the Hobart drinking water supply, attributed to growth of benthic blue-green algae in the river and several tributaries below Meadowbank Dam. Major filamentous algal blooms have also become increasingly prevalent in the seagrass beds of the upper estuary during the past five years. These observations have coincided with an increase or intensification of a variety of activities associated with agriculture (e.g. horticulture, dairy, grazing, irrigation), aquaculture (e.g. fish hatcheries), population centres (e.g. sewage and stormwater) and industry (paper production).
Water quality monitoring
Since 2022, the DEP is trialling novel in-situ real-time analyser technology which we hope will revolutionise catchment water quality monitoring in Tasmania. Current water quality monitoring programs in the State are sparse. The existing ones are driven by individual industries/organisations, they are costly, and measure at a low frequencies. This information does not capture incidents, variable pollutant sources, rainfall-runoff events and biogeochemical in-stream processes. With funds from stakeholders and the Ian Potter Foundation we have installed six Eco Detection real-time water quality monitoring systems to measure parameters such as nitrate, nitrite, phosphate, chloride, carbonate, sulphate and fluoride. The technology was developed by the University of Tasmania and commercialised by Melbourne-based company Eco Detection. The goal is to provide robust, autonomous and low cost systems that provide data in real-time allowing fast and smarter decisions regarding industry operations and waterway management.
The catchment, which provides 60% of Hobart's drinking water, has a history in land use changes, diverse 'diffuse' and 'point' sources of nutrients and is home to a variety of water-dependent industries. This is the first water quality monitoring network on a catchment scale using this innovative technology. With the infrastructure now in place (as of May 2022) in the River Derwent catchment and data being streamed to a data platform (6 systems, minimum of 4 measurements a day, 6 analytes, 5 external sensor measurements, totalling a minimum of 7,920 data points per month), this provides an unprecedented dimension of data availability compared to the typical ‘grab-to-lab’ approach (84 data points per month). The data will allow us to calculate more accurate nutrient budgets for the catchment, prioritise catchment management actions and help individual point source industries such as fish hatcheries and sewage treatment plants evaluate their nutrient discharges. The aim is to reduce / improve discharge levels. We are currently validating the data by comparing it with monthly grab-samples to ensure the analysers are performing accurately.
We are hoping that introducing this new technology to the River Derwent catchment in Tasmania will demonstrate its effectiveness and lead to broader water quality monitoring in the State and Australia-wide.
Prior to this project, a two-year water quality monitoring program (2015–2017) was conducted with a focus on nutrients and sediments. Samples were collected monthly at five sites along the main stem of the River Derwent and at the lower end of the eight major tributaries from September 2015 to September 2017. Most of these sites replicate a similar monitoring program previously carried out in 1996/1997, to evaluate how water quality has changed over a 20-year period. Download a summary report of key findings.