Mitchell–Thomson Rivers

In the highlands the outcropping Strzelecki Group bedrock and minor Quaternary alluvial valley fill are characterised by short, rapid groundwater flow paths that discharge to streams (Nicol, 2010). There is a general trend for high baseflow indices for streams within the highlands where bedrock is outcropping (Nicol, 2010).

In the low-lying areas of the geological Moe Swamp Basin, the watertable is elevated due to artesian discharge from the confined aquifers via vertical leakage (Brumley and Holdgate, 1983). This slow discharge is considered to have a role in the maintenance of the Moe Swamp (Brumley and Holdgate, 1983). Drains constructed within this river region now shed water into the Latrobe River.

Groundwater recharge has been identified from localised surface water leakage to groundwater in areas where the aquifers outcrop or subcrop (Fraser, 1980; Schaeffer, 2008; Walker and Mollica, 1990). For example, Brumley et al. (1981) (via stable isotope studies) identified the local recharge of the Morwell Formation where it outcrops along Wilderness Creek and Ten Mile Creek.

The incised river and creek course in the lower alluvial plains of the Avon River basin creates mostly gaining streams due to a relatively shallow watertable (Sinclair Knight Merz, 2008). During average rainfall conditions (pre-1997) the main stem of the Avon River is a strongly gaining system (Jones et al., 2009). In below average rainfall conditions (post-1997), the Avon River system has a reduced baseflow component, though it is still considered to be a gaining system (Sinclair Knight Merz, 2008).

For the Mitchell–Thomson rivers Geofabric river region boundary, Nicol (2010) noted the modified flow regime for the Thomson and Latrobe rivers and the subsequent difficulty in assessing natural baseflow. High baseflow indices have been calculated for the majority of the length along the Thomson and Latrobe rivers (Nicol, 2010).

Hofmann and Cartwright (2013) used a combination of major ions, stable and radiogenic isotopes to assess inter-aquifer mixing and groundwater flow paths. Study data indicate vertical hydraulic gradients are downwards in the west and upwards in the east towards the Gippsland Lakes, with instances of local artesian discharge. Lake Wellington is likely to be receiving groundwater discharge from the Boisdale Formation (Hofmann and Cartwright, 2013; Schaeffer, 2008). Isotope analysis indicates the ages and flow paths of groundwater are complex, suggesting preferential flow through faults and possible interactions with overlying surface water systems. Likewise, groundwater major ion chemistry is relatively consistent throughout all aquifers, indicating interconnectivity and mixing. Additionally, there are issues in attributing baseflow to discharge of groundwater from specific aquifers (Hofmann and Cartwright, 2013).

Hofmann (2011) undertook single-gauge hydrograph analysis which indicates that under low-flow conditions both the Avon and Mitchell rivers are gaining. However, under flood conditions, differential water balance assessment between two hydrographs showed the Avon River to be gaining (upper and lower river gauge comparisons) while the Mitchell River was losing (upper and lower river gauge comparisons) (Hofmann, 2011).The overall Mitchell River water loss in flood conditions is thought by Hofmann (2011) to be due to overbank flooding and river bank infiltration (i.e. reversal of head gradient). Both rivers have gaining and losing sections and vary between gaining and losing over time (Hofmann, 2011).

GHD (2013) investigated methods for quantifying regional baseflow using a digital baseflow filtering approach ’trained‘ to environmental tracer data – in this case, electrical conductivity (EC). This approach was termed the ‘EC mass balance’ (GHD, 2013). The EC mass balance approach was applied to the Lower Thomson–Macalister and the Lower Mitchell river systems. Due to the strong influence of Thomson Dam releases and irrigation diversions there is difficulty in applying the EC mass balance baseflow estimates to the Thomson–Macalister river system (GHD, 2013).

Minchin (2010) noted that leakage from the Mitchell River is a primary recharge process to the alluvial aquifer within the Wy Yung GMA and possibly in the Orbost GMA.

Last updated:
8 January 2018
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