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save the Blue Tier

erosion & water catchment

- b. finlayson



The area to which these comments refer is on the Blue Tier in northeast Tasmania in the upper reaches of the George and Ransom Rivers. On the maps supplied to me it is described as the "10 year plan" where planned logging coupes have been mapped by Forestry Tasmania. I have not had the opportunity to visit this area though I have visited similar areas elsewhere in northeast Tasmania. This report is based on the 1:25,000 scale topographic map of the area; the Forestry Tasmania Provisional Coupes Series map, the geology map, Forestry Tasmania's Forest Practices Code 2000, the geomorphology manual of the Forestry Commission of Tasmania (Keirnan, 1990); and a report on low flow hydrology in the North Esk River catchment by Peel et al. (2002). For convenience I will refer to this area throughout this report as the Blue Tier.

I begin with a discussion of the topography of this site together with the geology and the soils derived from the underlying bedrock. Clearly topographic and soil conditions are of critical importance in the management of logging operations both from the point of view of operator safety and environmental damage. The vegetation cover plays a critical role in the catchment water balance. Typically the largest term on the output side of the water balance equation is evapotranspiration and any change in this caused by change to the vegetation cover will be reflected in the volume and timing of runoff. Finally, the standards of practice followed by logging operations are of major importance in determining the impact of the operations on environmental values and the aesthetic quality 9f the landscape and it is relevant to consider the standards applied by Forestry Tasmania in this context.

Topography, Geology and Soils

This area is erosional topography developed by fluvial processes on the Devonian granites of the Blue Tier Batholity. As assessed from the topographic map, this is steep topography with measured slope angles in some cases exceeding 50°. Slopes are convex in profile with the lowest slope angles at the slope crests and becoming steeper with increasing distance from the drainage divides. Along the channels of the major streams in the downstream reaches there are gentler slopes adjacent to the streams but the upstream reaches, and all the minor tributaries, have steep slopes abutting their channels In these topographic conditions, there is very good coupling of the valley side slopes with the stream channels. This means that disturbance on the valley side slopes is readily transmitted into the stream channels. Unless there are well developed stream side buffer zones, which are carefully protected from any disturbance, logging on these slopes can be expected to result in water quality deteriorating through high suspended sediment concentrations and aquatic habitat deterioration by the clogging of the substrate with fine sediment (Grayson et al., 1993).

Bedrock underlying most of this area is Devonian granite. On the eastern boundary, around Mt. Littlechild, the granites are overlain by olivine basalts of Tertiary age. Alluvial deposits in the river valley appear to be rare and are mapped only for a short reach of Sun Creek in the northern part of this area and within the Blue Tier Reserve. Most of the area to be logged is therefore underlain by the granites.

Granite is derived from the intrusion of acid magma into surrounding rocks at considerable depth below the ground surface. It cools slowly, thus producing large crystals of the individual minerals which precipitate from the magma. When the granite cools and solidifies, its volume is reduced and this causes fractures in the rock, commonly widely spaced and oriented in two major directions at right angles to each other. The influence of this joint structure can be seen in the orientation of the stream channels on the Blue Tier. Where granite appears on the surface, as in the case of the Blue Tier, it does so because the overlying material has been stripped by erosion. The effect of this is to reduce the pressure on the granite and this generates a further series of fractures in the rock at right angles to the direction of the original pressure.

The end result of these fracturing events in the history of a granite intrusion is that the rock mass becomes divided into larger blocks, separated by fractures or joints The blocks of granite are impermeable but water, which is the agent of weathering, enters the rock mass along the joint planes. The water hydrates the silicate minerals in the granite and enters into chemical reactions with them. This results in the physical destruction of the rock by a process called 'grussification' along the alignments of the joints while the core blocks remain unweathered.

The joints and the grus surrounding them are permeable and typically are saturated with groundwater. Where these joints intersect the ground surface, springs and seeps are common which may or may not be connected to the local drainage pattern by clearly defined channels. An assessment of forested granite terrain undertaken using air photos usually does not detect these springs and seeps and they only become obvious either by on-ground inspection or when the forest is cleared. This unique property of granite terrain needs to be taken into account when there are plans to introduce management activities such as logging.

The most common mineral in granite is quartz, which is stable in the weathering environment and provides the sand component of the soils that result from the weathering of the granite. Other silicate minerals in the rock (biotite, feldspars, etc.) decompose in the weathering environment and produce clay as a secondary mineral. The clays produced by the weathering of these minerals (unlike those which would be produced from the basalts in the east of this area) are typically clays of low cohesion such as kaolinite. Granite soils are thus rich in sand which is non-cohesive and readily transported by water given that the grain size is small, usually less than 1 mm diameter. There is relatively little clay present to provide resistance to erosion by cohesion and the clays that are there have low cohesion anyway.

The information on which this discussion of the weathering of granite is based is widely available in the geomorphological research and textbook literature and is well known to all geomorphologists. Not surprisingly, the Geomorphology Manual, published by the Forestry Commission of Tasmania (Kiernan, 1990) draws attention to the problems of erosion of granite soils, especially in high rainfall areas, and specifically mentions the granite soils of northeast Tasmania (i.e. those on the Blue Tier and nearby areas) as being highly prone to erosion. The combination of very steep slopes, high rainfall and granite soils on the Blue Tier should give cause for extreme concern to anyone contemplating logging operations.


One of the more alarming features of recent developments in forestry in northeast Tasmania under the Commonwealth Regional Forestry Agreements is that major vegetation change is being carried out with no prior assessment of the consequences for catchment water yield. Prior to carrying out these changes on such a major scale, it would be best practice to first conduct experiments to determine the impacts. There are good precedents for this. In the United Kingdom, when plans were being developed in the 1950s to reforest upland areas which had been deforested since the Iron Age, the Institute of Hydrology was established to determine what the impact of this would be on water yields from upland catchments. The Coweeta forest hydrology experiments in the United States were also established to determine experimentally the impact of forest management on other forest values, especially water yield. In Victoria, the Melbourne and Metropolitan Board of Works (MMBW, now Melbourne Water) established forest hydrology experiments in the Yarra Valley in the late 1950s in order to assess the impact of forest management strategies on water yield into the Melbourne supply system (Watson et al., 1998). That the Tasmanian Government allows substantial and widespread changes to forests of the kind currently being carried out in the northeast without first requiring a major assessment of impact is incomprehensible.

The Launceston City Council is obviously concerned about possible impacts on their water supply system. In the absence of experimental data and without the opportunity to collect experimental data before the forest changes are put in place, they have contracted a group of hydrological modelers to assess the likely impacts (Peel et al., 2002). While this modeling is the best available, it is a model and not reality. A review of the outcomes of the experimental work from the Institute of Hydrology in the UK, the Coweeta project in the USA and the MMBW in Victoria shows that these experiments made substantial contributions to the development of knowledge about the hydrology of those specific forested catchments that could never have been known from models alone The Tasmanian case is unique. There are no experimental data which shows what the hydrological effects will be of converting the range of existing forests (and associated vegetation) in northeast Tasmania to single species plantations over such a wide area. The modeling of Peel et al. is the best possible under the circumstances but it is not necessarily what will actually happen.

Launceston sources its water supply from the North Esk and St. Patricks Rivers. Water is taken directly from the rivers and there are no storage structures. In these circumstances, it is the summer low flows that are critical to the reliability of supply and it is precisely these flows that are likely to be most affected by changes to the vegetation cover of the catchments. The North Esk and St. Patricks Rivers lie to the east of Launceston and the upper ends of their catchments are only some 30 to 40 km west of the Blue Tier. The modeling results of Peel et al. provide the best estimate currently available of the likely impact of plantation establishment on the water yield of the Blue Tier.

Peel et al. modeled a number of forest management scenarios in the North Esk and St. Patricks catchments. The one which would appear to most closely match what is planned for the Blue Tier is the logging and replanting of the Eucalyptus regnans forest at the rate of 5% per year which is a 20 year rotation and the conversion of pasture to plantations of E. nitens, also logged and replanted at 5% per year. The model was run for two rotations where the first rotation represented the impact of the logging rotation policy and the second rotation modeled the impact on long term water yield once the rotation policy is in place. In the modeled catchment E. regnans forest covered 73% of the area and the remainder was covered by pasture converted to E. nitens. To assess the impact of the vegetation management the model was run using the climate data of an average year. In this way only the impact of the vegetation change is assessed and the effects of climate variability are eliminated.

Please use wide screen for viewing data tables

Variable Mean Daily Runoff (ML)
Observed daily runoff 465
Modeled daily runoff 1st rotation 448
Modeled daily runoff 2nd rotation 310
Observed - modeled 1st rotation 3.6%
Observed - modeled 2nd rotation 33.3%

In this modeled scenario, the catchment is progressively converted to regenerated E. regnans and plantations of E. nitens during the first 20 years and the average mean daily flow over that period declines by nearly 4%. In the subsequent 20 year period, when the whole catchment has already been logged and replanted once, the mean daily flow declines by 33% relative to the present situation (Table 1). If only the lowest 40% of the flows are considered (and these nearly all occur in the critical summer period), during the second rotation flows are reduced by 25%. Clearly there is cause for concern about the impact of logging and conversion to tree farming on the water yield of catchments in northeast Tasmania.

Standards of Practice

An examination of the standards of practice achieved in areas already clearfelled and replanted in other parts of northeast Tasmania suggests that severe environmental damage will occur when the Blue Tier is logged. The Tasmanian Forest Practices Code 2000 is a weak document which fails to mandate appropriate standards of operation and leaves much to the operator's discretion. While the operations of Forestry Tasmania should be monitored by the Forest Practices Board, experience elsewhere in the northeast indicates that the Board consistently fails to insist that the appropriate standards of practice are met (see, for example, Finlayson, 2001).

While the Board publicises good practice, such as ploughing along the contour, the reality is that where the operations are on public land, the standards of practice are much lower (Figures 2, 3 and 4).

On the Blue Tier, with very steep convex slopes and highly erodible granite soils, the consequences of downslope ripping will be disastrous and the rip lines can be expected to quickly turn into eroded gullies. The sediment generated by the erosion of gullies will affect water quality and aquatic habitat in the receiving streams A more lasting effect will be the permanent loss of soil from these slopes. In most parts of the Blue Tier, the alternative of ripping along the contour will not be possible as the slopes are too steep for machinery to traverse across the slope.

The Forestry Tasmania Provisional Coupe Series map for this area shows coupes laid out across the Blue Tier without concern for steep slopes or for streamside reserves. Some areas are shown as "less important for wood production" due to factors such as steepness, stream reserve, accessibility etc., but note that they are not being excluded from wood production on these grounds. This failure to be prescriptive permeates the whole of the Forest Practices Code 2000. Here are some selected examples of relevance to the case of the Blue Tier.

The application of the Code is as weak as its provisions suggest. Streamside reserves are routinely either ignored or violated. Figures 6 and 7 is a Class 3 stream at Mt. Arthur. Note also the treatment of the Class 4 stream in Figure 4. The Code provides for a streamside reserve of 20 m for Class 3 streams and 10 m for Class 4 streams.

While the Code provides considerable detail on the construction of roads, the standards achieved in practice are also very low. The road at Mt. Arthur shown in Figure 8 crosses a running stream but no culvert has been installed, rather the stream has been diverted along the road and now occupies the table drain. It is well known that roads are a major source of sediment in forestry areas (Grayson et al., 1993) and in the case shown in Figure 8, virtually all rainfall events will cause a pulse of sediment from the road surface to enter this stream.

The examples I have used here to illustrate the poor standard of practice by Forestry Tasmania in the northeast are not rare cases that I have had to search hard to find. These illustrations are typical of what I observed during two visits to this area. Examples of good practice, where the provisions of the code had been adhered to, were very hard to find.

B. Finlayson, Ph.D.
Centre for Environmental Applied Hydrology
School of Anthropology, Geography and Environmental Studies
The University of Melbourne
Victoria 3010


Grayson, R.B., Haydon, S.R., Jayasuriya, M.D.A. and Finlayson, B.L., 1993, Water quality in Mountain Ash forests - separating the impacts of roads from those of logging operations. Journal of Hydrology, 150: 459-480.

Finlayson, B.L., 2001, report on the property of Mrs. Anne Gschwendtner - effects of proposed forestry operations on water supply, Upper Catchment Issues, Tasmania 1(1): 36-46.

Peel, M.C., Watson, F.G.R. and Vertessy, R.A., 2002, Modeling of Low Flows in the North Esk River using the Macaque Model. Report for the Launceston City Council, Cooperative Research Centre for Catchment Hydrology.

Watson, F., Vertessy, R.A., McMahon, T.A., Rhodes, B. and Watson, I., 1998, Effects of Forestry on the Hydrology of the Maroondah Experimental Catchment, Victoria, Australia, Cooperative Research Centre for Catchment Hydrology, Report 98/9

[Republished with permission of the author and editors of Upper Catchment Issues, which first published this in Vol.2#1A ISSN 1444-9560. A summary is also available online]

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