• In the Strathbogie Ranges, most wetlands occur where groundwater comes to the surface and where soils are seasonally or permanently saturated.
  • Though healthy wetland habitat is now rare, it was once widespread and covered roughly 15-20% of the Strathbogie Tableland.
  • Extensive clearing of wetland habitat occurred by the early 20th C, enabling agricultural production, but has had a high environmental cost.
  • Re-establishing wetland habitat has multiple benefits.

Wetland vegetation on the Tableland (variously known as spring-soaks, swamps, bogs, tea-tree scrub, sphagnum moss beds) occurs where soils are reliably damp, or even water-logged, for most of the year. This includes much of the riparian (natural watercourse) zone. Most streams on the Tableland are flanked by groundwater discharge areas (that deposit groundwater almost directly into the channel), making it impossible to say where the riparian zone ends and the wetland zone begins (more on this below). In this article, I’ll refer to this wetland-riparian mosaic simply as ‘wetland’.

Nature doesn’t always respect the way humans want to divide her up. Here’s a typical Strathbogie Tableland scene, a drainage line flanked either side by groundwater discharge areas. Are our concepts of riparian and wetland zones useful in this circumstance?

Not that some wetland sites don’t dry out occasionally, or even regularly/seasonally, but the distinctive vegetation that makes up these areas is specialized to cope with saturated soils, often clay or humus-rich, that have developed where ground water discharges – seeps out of the soil (as distinct from wetlands on the plains which usually occur where water pools, from run-off, at low points in the landscape). In the Ranges, these wetland sites are referred to as ‘ground water dependent’ ecosystems, because they only occur where groundwater reliably seeps out of the ground. These wetlands are considered to be a particularly important environmental and cultural feature of the catchment, but the lack of accurate mapping of these wetlands has been considered a hindrance to management (Strathbogie Groundwater Management Area LMP, GMW, 2013, p. 10.).

The Victorian Wetland Inventory maps many significant wetland areas in the Seven Creeks catchment, however substantial wetland areas are completely absent from the mapping. See below for discussion of the VWI.

This image gallery shows the variety of wetlands found on the Tableland, many of which are now fragmented and degraded.

  • Strathbogie perched bog wetland

As is abundantly clear when you drive through the Strathbogie landscape, most of the native vegetation has been cleared and replaced with farm land. It’s not always easy to say where wetlands once occurred, though there are often tell-tale signs. Rushes, reeds and tea-tree indicate a wetland, even if most of it has been cleared. Most streams and drainage lines are fed by and flanked by wetlands, where you can still find Mountain Swamp Gum (Eucalyptus camphora) and Blackwood Wattle (Acacia melanoxylon). It’s even more obvious from space! Wetlands also have characteristic clay soils (see below). If you know what to look for, satellite images are an ideal tool for identifying wetlands, even if the original vegetation is gone.

Here’s a Google Earth satellite image of the Seven Creeks catchment above Polly McQuinn’s. The degree to which native vegetation has been lost is quite startling – more than 80% of the native vegetation in the entire catchment has been cleared. This figure increases to about 95% if we look only at private property (excluding the non-arable, mountainous public land) – that’s a staggering degree of clearing and makes all native habitat in the catchment extremely valuable.

Seven Creeks catchment above Polly McQuinn’s Weir; yellow line is the catchment boundary above Polly’s (15,500 ha), blue lines are the major streams, native vegetation is dark green. Note the near complete elimination of large patches of native vegetation except around the margins of the catchment.

The type of vegetation a wetland can support depends on several factors: the extent and duration of water-logging, the type and depth of soil, site history, surrounding topography and geology. Some sites support trees, others are far too wet for trees and are dominated by water-loving shrubs (like tea-tree), sedges and rushes. These different types are technically described as, for example, Spring-soak Woodland (EVC 85), Perched Boggy Shrubland (EVC 185) and Swampy Riparian Woodland (EVC 83) (The floristics of wetlands in the Strathbgie Ranges (p. 2)). As often as not, these vegetation types intermingle and exist as a patchwork, or mosaic. When wetland vegetation is cleared, all this diversity and complexity, and the habitat itself, disappears. It’s little wonder that animals like the Long-nosed Bandicoot (Perameles nasuta) that rely on dense ground-layer vegetaton, now only occurs in isolated pockets and is on the verge of local extinction (personal observation).

So, how much wetland and riparian vegetation once occurred on the Tableland? The image below maps all groundwater discharge and riparian areas in the catchment above Polly McQuinn’s Weir – the total comes to some 2700 ha! [notes on mapping, below] So, wetland and riparian vegetation once comprised a staggering 15-20% of this landscape – it must once have truly been a Wetland Wonderland!

Mapped wetlands, groundwater discharge zone, on the Strathbogie Tableland.
Wetlands of the Seven Creeks catchment; blue polygons represent the minimum extent of wetland habitat (ground water discharge areas), where the soil is reliably, seasonally saturated – a total of about 2,700 ha.

Much of the focus of this Bogies and Beyond project has been on monitoring how groundwater levels change seasonally and from year to year (eg here) and better understanding what influences groundwater in the ranges, but it’s important to recognize that this unique wetland habitat (eg here and here), though once widespread and quite common, has been largely cleared and most is now significantly degraded.

Historically, these wetlands, acting like sponges, held onto water, slowed its passage through the landscape – storing water and releasing it slowly throughout the drier months, ensuring that the Seven Creeks at Strathbogie and Euroa had a year-round flow.

Today, perhaps as little as 500 ha of the original 2700 ha of wetland-riparian habitat remains. Whilst there are still some fantastic wetland sites, much of what remains, as already stated, is fragmented and degraded.

Early farmers cleared land where they could and, in many instances, were required to clear a certain amount each year in order to maintain their selection, as well as create arable land for grazing and crops. By the early 20th Century, the Tableland came to resemble what it is today – a largely tree-less landscape. With most of the original forest and wetland vegetation cleared, the hydrological system has drastically changed and we are the poorer for it. The days of guaranteed summer flow in the Sevens at Strathbogie and Euroa are gone.

I’m happy to share the Google Earth .KMZ file – just get in touch.

Victorian Wetland Inventory

A detailed study of The floristic values of wetlands in the Highlands and Strathbogie Ranges provides invaluable information on a variety of aspects of these wetlands and is a key resource when considering management options. That study resulted in a considerable update of the Victorian Wetland Inventory (VWI) (Mapshare portal), mapping areas of both current and pre-European wetland extent.

The VWI layer appears to distinguish between groundwater discharge areas away from streams (mapped as wetlands) and groundwater discharge areas adjacent to the riparian zone (generally not mapped as wetlands), though there are exceptions.

The above comparison shows that VWI wetlands are generally a subset of the mapping conducted for this project and in this example coverage is similar. However, VWI wetland coverage in other parts of the Seven Creeks catchment shows the mapping is far from complete, likely demonstrating the limited scope of the source project/s that generated the data. Considerable caution should be exercised is using the VWI wetlands layer for decision making.

This image shows considerable difference between the VWI mapped wetlands (grey polygons) and wetlands mapped by this project (blue polygons).

Notwithstanding the gaps in the VWI mapping in the Seven Creeks catchment, the detailed classification and description of parcels in the ‘Current wetland’ layer improves our understanding these important habitats. The extensive coverage of the mapping for this project complements and expands the data available in the VWI.

What can a healthy wetland look like?

The value of plentiful, high quality groundwater on the Strathbogie Tableland can’t be underestimated – for nature and humans. ‘Spring-fed dams’ are present in almost every paddock and groundwater bores are becoming increasingly important for farming and domestic water security. The rub is, we still don’t really know that much about how the groundwater system functions, nor what will happen to water tables and wetland habitat as rainfall patterns change.

There have been a number of studies into the unique wetlands of the Strathbogie Ranges. Over the past 18 months, the Bogies and Beyond project, coordinated by the Goulburn Broken Catchment Management Authority and funded by the Victorian Government, has re-focused attention on groundwater and wetland habitat.

Why do wetland soils hold water?

Granitic soils are sandy and friable – they contain lots of quartzite/sand and silt, but relatively little clay (particle size <0.002 mm). Granitic soils in the Starthbogies are usually reddish-orange in colour due to the presence of oxidized metals like iron and aluminium. These soils, being friable and often deep, are pretty good for growing trees, but not at all good for building good dams. That’s why virtually all dams are built in or near wetlands, where the soil has a higher clay content.

Coates et. al. (2010) determined that the wetter a wetland site is, the higher the percentage of organic matter. Generally, where soil moisture is greater than 80%, soil organic matter is greater than 20% (p. 18).

Granitic soils prone to water logging turn from reddish, through yellow, to grey, or even white, in colour. This colour change is a result of reduced oxidation (of metals like aluminium and iron) and also indicates a higher proportion of clay particles in the soil. Physical and chemical weathering of feldspar and mica results in the formation of clay minerals (eg. kaolin), which have excellent water-holding capacity and are good dam building material. As the feldspar and mica breaks down, the tiny clay particles are carried by water to ever lower parts of the landscape. Some of the particles end up in streams and leave the catchment, but over the millennia, much of the clay has been deposited where groundwater discharges. Once deposited, these clay soils usually support wetland vegetation with extensive, fibrous root systems that bind the soil/clay particles and in some circumstance allow the build-up of significant amounts of organic matter as peat (see Coates et. al. 2010).

Mapping wetlands

Links to higher resolution images of the mapped wetland-riparian zone in the Seven Creeks catchment: North-west, north-east, south-east and south-west.

The Tableland’s wetland and riparian zone are so closely linked and grade into each other, that mapping them separately would be both difficult and somewhat arbitrary. Whilst wetland and riparian areas can be defined as distinct systems, there is substantial functional, ecological and practical overlap of these areas on the Strathbogie Tableland, as noted above. Here, I have mapped them collectively.

Mapping the previous extent of wetland habitat can be done by recognizing groundwater discharge sites from aerial images, based on the texture and colour of the vegetation. Polygons are then drawn to enclose the discharge areas, to calculate area. This technique does not discriminate between the different vegetation types that comprise (or once comprised) these wetlands – that would require site visits and considerable time and resource investment beyond the scope of this study. Coates et. al. (2010) conducted detailed mapping of many of the wetlands in their study (p. 29), but also concluded that remnant wetlands in paddocks where trees and shrubs had been removed accounted for 58% of all mapped wetland vegetation (p. 32).

Google Earth imagery allows areas of reliable groundwater discharge in paddocks to be fairly accurately identified. Utilizing the historical imagery available on Google Earth, in this landscape back to 2012, gives even greater accuracy.

Further Information- investigations into groundwater and groundwater dependent ecosystems:

Posts relating to wetlands and groundwater in the Strathbogie Ranges:

 The Bogies and Beyond Groundwater Monitoring Project is supported by the Victorian Government and the Goulburn Broken Catchment Management Authority.

For more information contact Bertram Lobert bertram.lobert@activ8.net.au 5790 8606, 0409 433 276 or go to: https://strathbogierangesnatureview.wordpress.com/?s=groundwater