Spring ecosystems of the Surat and southern Bowen Basins
Springs in the Surat and southern Bowen basins are predominantly fed by Triassic and Jurassic aquifers including the Clematis Sandstone, Precipice Sandstone, Boxvale Sandstone Member of the Evergreen Formation, Hutton Sandstone, Gubberamunda Sandstone and Bungil Formation. In addition to these sedimentary aquifers, springs are also associated with unconsolidated Cenozoic sediments and the fractured rock aquifers of the Tertiary Volcanics. Individual springs can occur due to one or more of three basic mechanisms:
The wetlands supported by spring discharge vary considerably. Although it is important to understand the mechanism and source aquifer supporting a wetland, it is the receiving environment which will influence the way a wetland will respond to a change in the groundwater regime. For this reason, a wetland typology has been developed for spring ecosystems of the Surat and southern Bowen Basins which identifies four types of wetlands. The key attributes used to differentiate the wetland types are their landscape setting, geomorphology, the nature of the groundwater flow system, the regolith and water regime.
The four wetland types are:
Type 1a Wetlands located in off-stream environments, within floodplains
Type 1a wetlands occur in palustrine landscape settings, within topographic lows, gently sloping landscapes and occasionally on floodplains. The wetlands are supported by groundwater inflows from both regional and local groundwater systems, and predominantly occur over deep regolith profiles. The permanent supply of groundwater has enabled the development of peaty wetland soil and mounding.
The wetland water budget is dominated by diffuse discharge and evapotranspiration. During cooler months (outside of summer) when evapotranspiration is low, wetlands may free-flow and pooling can occur depending on the wetland geometry.
The permanent supply of groundwater and deep regolith have enabled a dense coverage of wetland vegetation, including large woody vegetation. The wetlands are characterised by a central core of aquatic vegetation that corresponds to the subsoil development of peaty wetland soil. The immediate area of the wetland is confined by the low hydraulic permeability of the surrounding regolith, contributing to the formation of discrete wetlands.
The wetlands are influenced by seasonal changes in climate, longer-term changes in climate and changes in groundwater pressure. During dry periods, the central core of the wetland remains saturated and is dominated by aquatic vegetation. Over this period, the extent of saturated soil and aquatic vegetation is very similar. During cooler and wet periods, wetland discharge increases, which results in an increased wetland area that may result in free-flowing water inundating downslope areas.
Short-term, seasonal changes result in the formation of an outer rim of dried and pugged soil that is often coated in salt efflorescence and iron staining. Long-term changes are expressed as multiple generations of woody vegetation, both living and dead, small ephemeral discharge zones and extensive ephemerally saturated discharge tails.
Type 1 wetlands form around a central core of saturated wetland soil. Eight focal zones have been identified primarily on the basis of wetland vegetation. The zones represent the variability across the wet and dry phase of the wetlands, driven by the groundwater regime. These are:
Type 1b Wetlands located at the interface between floodplain and riverine environments and influenced by surface water flows
Type 1b wetlands share very similar conceptual model and wetland processes with Type 1a, however the areas where they occur within the landscape are different. Type 1b wetlands occur at the interface between the floodplain and riverine setting. As a result, Type 1b wetlands have two important differences to Type 1a wetlands:
Type 2 Semi-permanent brackish, palustrine wetlands with minor wetland soils and minor vegetation cover, mainly connected to regional groundwater systems
Type 2 wetlands occur in palustrine landscape settings, located within topographic lows or gently sloping landscapes. The wetlands occur on highly weathered regolith profiles, often sodic soils and receive groundwater inflows predominantly from regional groundwater systems. The wetland water budget is dominated by diffuse groundwater discharge. These wetlands are not permanently connected to the groundwater system.
Type 2 wetlands are relatively small wetlands and are dominated by terrestrial vegetation, with little to no free water. The semi-permanent connection to the groundwater system has not enabled the development of distinct wetland soils. It is likely that during extended dry periods, there is no physical discharge from these wetlands. The wetlands are often associated with a broader saline discharge zone that is actively eroding and is characterised by ‘flowing sands’ that create small non-vegetated mounds.
Six focal zones in the wetland vegetation have been identified that represent variability across the wet and dry phases of the wetlands, driven by the groundwater regime:
Type 3 Permanent to semi-permanent riverine wetlands with minor wetland soils and moderate vegetation cover, sourced from local and regional groundwater systems and significantly influenced by surface water flows.
Type 3 wetlands generally occur within active watercourses on exposed sandstone bedding planes. The wetlands have minor substrates, with the wetland soil forming through the accumulation of riverine sediment or bed sands, rather than exclusive development of wetland soil over time. The wetlands receive groundwater contributions from both regional and local groundwater systems and are predominantly actively discharging wetlands, with little soil water storage potential or evapotranspiration demand. The wetlands contain only minor wetland vegetation (due to lack of soil substrate).
Only minor seasonal changes occur to the spatial extent of the wetland as there is a constant supply from groundwater discharge. Long-term changes in wetland area will be influenced by stream erosion and deposition processes which are likely to alter the wetland area and location of discharge.
Discharge from the wetland is maintained all year and is either within, or adjacent to, permanent or semi-permanent watercourses. The shape and size of the wetland are controlled by the nature of the exposed sandstone and the geomorphology of the watercourse. The constant exchange of water due to discharge from the wetland and flow of low-salinity groundwater into the wetland generally maintains low-salinity water within the wetlands.
Only shallow wetland soils have developed at these wetlands. As a result, changes in groundwater discharge or a decline in evapotranspiration demand results in a change in the volume of discharge and minor changes to the wetland area. This is due to the wetlands having little regolith or wetland soil development and therefore limited water storage capacity to buffer the effects of an altered water balance.
Change to the wetland area is controlled by large-scale high flow surface water events rather than groundwater discharge. Within the current hydrogeological state, seasonal changes in groundwater discharge to the wetlands appear to have a negligible impact on the wetland. While it is expected that changes in groundwater discharge will have a negative impact on the wetlands, the condition of the wetlands is dominated by surface flow events.
These wetlands are characteristically wet and do not have a dry cycle. During the wettest phase, they are inundated to a degree by surface flow. During the lower flow periods, the wetlands are maintained by groundwater. Smaller wetlands located upstream of the main watercourse vary in vegetation type, with terrestrial and weed species occurring. In contrast larger wetlands are generally dominated by aquatic vegetation assemblages.
Type 3 wetlands have formed due to dissection of the landscape by riverine processes which results in the watertable being intersected within a sandstone aquifer. This creates zones of groundwater discharge (wetlands) adjacent to, or within permanently or seasonally flowing streams. Three focal zones have been proposed that represent the variability across the wetlands driven by the groundwater regime:
Type 4 Semi-permanent fresh riverine-to-palustrine wetlands with minor wetland soils and moderate vegetation cover, mainly connected to local groundwater systems.
Type 4 wetlands occur within active watercourses within deep sandy alluvial deposits. The wetlands receive groundwater inflows from local groundwater systems. Groundwater discharge from the wetlands is fresh and free-flowing. The lack of any clay substrate prevents any significant subsoil water storage and as a result, only very minor wetland soil can develop.
The wetlands contain an irregular distribution of wetland vegetation due to the lack of substrate and a dynamic landscape where the form of the wetland can be altered by stream bank erosion. Therefore, Type 4 wetlands are subtle features, only distinguishable from the surrounding landscape during dry periods due to greener vegetation. The fresh water and adjacent water bodies provide aquatic habitat suitable for sensitive macroinvertebrate species.
The wetlands fluctuate in extent and area and will be altered physically by bank erosion during flood events. They maintain free-flowing discharge all year round and are considered semi-permanent features of the landscape. The extent of the wetland area and associated flora appears constrained by the landscape position. However, the spatial distribution of terrestrial vegetation within the wetland varies, potentially reflecting changes in the distribution of saturation within the wetland.
Seasonal changes in groundwater discharge and climate appear to have little impact on the wetlands. There are no surface features that represent long-term or short-term changes in the wetland area and/or form.
Local groundwater flow from surficial aquifers discharges at the wetlands due to the presence of a low-permeability layer underlying the surficial sediments, forcing groundwater to flow to the wetland, and discharges in the banks of the watercourse. The source aquifer for the wetland is recharged seasonally during rainfall events into the sandy surface soils. Therefore, the discharge zone is responsive to short-term rainfall patterns. Three focal zones are described that represent the variability across the wetlands driven by the groundwater and surface water regime.
Type 5 wetlands are similar to Type 4. However, the wetlands occur within upper catchment drainage lines that have ephemeral surface flow. Their location often coincides with the headwaters of large, more defined watercourses. The significant difference to Type 4 wetlands is that there is no distinct channel within the drainage line, such that the wetlands occur as broad areas of saturation within only minor wetland soil development.
Seasonal changes in the wetlands occur when seasonal recharge increases discharge to the wetland, increasing the wetted area and wetland discharge rate. This is in contrast to Type 4, where any increase in discharge flows directly into the adjacent watercourse with only minor change in wetland area.
The wetlands have formed due to a local groundwater flow system overlying a lower-permeability weathered substrate. Lateral gravity-driven flow paths result in point discharge zones (wetlands) within the valley floors. Four focal zones are described that represent the variability across the wetlands driven by the groundwater regime.
Last updated: 18 December 2015
This page should be cited as:
Spring ecosystems of the Surat and southern Bowen Basins, WetlandInfo 2013, Queensland Government, Queensland, viewed 13 April 2017, .