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 metre 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 replenishment and delivery of silt, as well as 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
To better understand changes in water quality a two-year 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. A summary report of key findings is published here.
Given the high value ecosystems in the upper estuary and the declining water quality from the catchment, one of the DEP’s strategic objectives is to further investigate and encourage optimal catchment land and water management practices to maintain the health of the river and estuary.