The Esk Rivers basin, including the Meander, Brumbys-Lake1, Macquarie, South Esk
and North Esk river catchments, occupies 10,205 km2 (16%) of the main island of
Tasmania (Figure 1.3). The Esk Rivers basin drains via the South and North Esk Rivers into the upper reach of the Tamar estuary, approximately 70 km inland from the estuary outlet at Bass Strait in the north. The Tamar estuary itself was discussed in detail in
Chapter 1. The immediate estuary catchment comprises 1122 km2, however its low-
lying reach is of little interest compared to the extensive riverine basin, which receives closer examination here.
The North Esk catchment occupies 1064 km2. The South Esk basin occupies 9141 km2
and includes the Meander (1555 km2), the Macquarie (2700 km2), the Brumbys/Lake
(1394 km2) and South Esk (3345 km2) river catchments.
Climates and topographic settings vary widely in the Esk Rivers Basin. It reaches south from the Tamar estuary and Launceston across the northern Midlands plains almost to Oatlands, and west from Meander and the Great Western Tiers to St Marys near the east coast. It includes much of the northeast highlands in the upper North and South Esk catchments and lies between sea level and 1570 m elevation. Continuous plains are limited to the northern Midlands between Launceston and Tunbridge (19 km north of Oatlands, Figure 1.3), occupying the Midlands graben and extending into the South Esk River valley (Davies, 1965).
1
Water diversion from the Great Lake for the Poatina/Trevallyn Hydro-electric power scheme
intermittently extends the Brumbys-Lake catchment onto the Central Plateau (Derwent catchment; Figure 3.5), however the natural catchment has been used in the project datasets.
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Tasmania lies partly in the path of the westerly zonal wind system known as the
Roaring Forties and has a marked west to east rainfall gradient, whereby precipitation is much lower in eastern lowland regions in the rain shadow of the western highlands (Langford, 1965; Jackson & Brown, 2005). Consequently, climates and storm response river flows also vary across the Esk Rivers basin. The western region and higher elevation northeastern regions have a winter-wet seasonal distribution of rainfall and higher annual precipitation, for example Deloraine (elevation 237 m) with an average annual precipitation 946 mm (Figure 4.2).
Figure 4.2: Precipitation distribution in the Esk Rivers basin. Source: Bureau of
Meterology (2013). Data are from extant rainfall stations with records since the 1880s, excepting Launceston (Ti-tree bend), with records from 1980-open. Note: Oatlands is just outside the catchment but no alternative long-term data from the distal extent of the northern Midlands plains are available.
The rainfall of the Midlands plains, including the broad plains of the South Esk River’s middle reaches, has a relatively even distribution throughout the year. However, it is the driest region of the basin, where potential evaporation exceeds precipitation (Langford,
0 20 40 60 80 100 120 140
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
M ean r ai n fal l (mm) Month Launceston (Ti-tree bend) Deloraine Lake Leake Oatlands Fingal
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1965). For example, the annual average precipitation of Oatlands (elevation 406 m) is 549 mm. At the pilot study catchment outlet, Fingal (elevation 237 m; Figure 1.3) the lowlands have an annual average precipitation of 610 mm, while in nearby alpine areas of the study catchment, where there are no extant weather stations, rainfall exceeds 1,500 mm per annum (Bureau of Meteorology, 2013), with a winter-wet seasonal distribution.
The most variable precipitation is in the east of the Esk Rivers Basin including the north-east highlands. For example, see St Marys in the South Esk catchment and Lake Leake in the Macquarie catchment (Figure 4.2). This region is subject to high
precipitation storm events from tropical troughs and cut-off cyclones, particularly in the late summer and early autumn months (Langford, 1965). Cyclic climatic conditions of drought followed by drought-breaking rains affect the Esk Rivers region, enhancing erosion processes associated with land degradation (Bobbi et al., 1996).
Within the South Esk River catchment, the pilot study area of 1016 km2 extends from the vicinity of the town of Fingal to include the St Marys district and much of the northeastern Tasmania highlands, from broad river valleys to some of the rugged mountain tops of northeast Tasmania. According to Land systems data, the elevation ranges from 220 m at the catchment outlet 2.3 km north of Fingal, at the confluence of the South Esk and Break O’Day Rivers, to 1570 m elevation on Legges Tor northwest of Fingal (Department of Primary Industries and Water, 1978-1989). Elevations are >700 m lower on the highest hills to the east of St Marys than the mountains of the upper South Esk catchment. According to GIS analysis within the study area, the upper
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South Esk sub-catchment occupies 794 km2 or 78% of the land area while the Break O’Day catchment occupies 222 km2 or 22% of the southeastern region of the study area. From digital geological data (Mineral Resources Tasmania, Undated), the inferred ages of the surface geology in the study area ranges from 440 million years to present, comprised of Ordovician-Devonian (sedimentary and acid igneous rocks) to Jurassic (basic igneous rocks) and Quaternary (alluvium and colluvium). From Vegetation of Tasmania (TasVeg) data, native vegetation (including regenerating native vegetation) co-dominates with silviculture plantations across the mid- and upper slopes of the catchment, while agriculture/exotic vegetation dominates lower elevations (Department of Primary Industries & Water & Department of Environment, 2002; Department of Primary Industry, 2013). The vegetation cover/land use mosaic exists in a complex topographic patchwork, particularly in the mid- to lower elevations.
The review in Chapter 2 considered the magnitude and rate of landscape change in the study basin over recent earth history and since the migration of the first people to Tasmania from the literature. However, to improve understanding of destabilisation processes today, landscape change and instability over time were analysed from paper maps and digital mapping, with particular focus on an upper catchment pilot study area of >1,000 km2. Digital data sources and other metadata are shown in Appendix 1.