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).
To better understand changes in water quality a two-year monitoring program is currently underway with a focus on nutrients and sediments. Samples are collected monthly at five sites along the main stem of the River Derwent and at the lower end of the eight major tributaries, starting in September 2015. 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 will be published at the conclusion of this monitoring program, in early 2018.
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.
A Water Quality Improvement Plan for the catchment
In 2016/2017, the DEP and NRM South commissioned a scoping project to develop a Water Quality Improvement Plan (WQIP) for the River Derwent and upper estuary, with a focus on nutrient and sediments. Workshops were held in Hobart, Hamilton and New Norfolk for stakeholders with interests in the catchment, as well as a community forum, an online survey, numerous one-on-one consultations and a literature review.
The main stages of the plan development are:
- Consultation to identify catchment assets and values , their condition, and the nature and sources of potential pollutants to be considered in the plan
- Data analysis and development of a catchment water quality model
- Evaluation and prioritisation of management actions based on cost, feasibility and potential for impact
- Development of management modules for the various activities/industries in the catchment area.
- Review, feedback, revision
- Launch and promotion
When completed, the plan will provide a whole-of-catchment picture of water quality in the River Derwent catchment. Our understanding of the drivers of water quality issues and the levers we can use to address them will be greater. We will have a series of adoptable actions and nutrient load reduction targets to improve and protect water quality. The plan can be used to inform an investment strategy for the catchment to protect it into the future. A primary component of the plan will be a series of management ‘modules’ for each type of industry or activity in the catchment area. The full scoping report recommended that selection of the desired scope option and commencement of plan development occur as soon as possible. We are currently seeking funding to develop this plan. See WQIP Scoping Summary for details.
More detailed information about the River Derwent and its catchment are available in the 2015 State of the Derwent Report (Section 3.2), and the 2011 Derwent Catchment Review.