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Abstract

Benthic macroinvertebrate monitoring has been conducted in the lower Gordon River and associated reference sites over the duration of the Basslink monitoring program (Hydro Tasmania 2013) from 2001/02 to 2011/12. This monitoring has been extended for the two subsequent years 2012/13 and 2013/14.
Monitoring is planned to continue from the end of 2013/14 onward, but at a lower sampling frequency. The proposed frequency is as follows:
• Routine sampling once in spring every two years (‘biennial sampling’); and
• Event-driven sampling in either autumn or spring, triggered by antecedent flow conditions (‘triggered sampling’).
This design allows the impacts of specific flow events (see below) on the macroinvertebrate community to be evaluated in the context of any medium to longer term trends.
The Basslink monitoring program spanned a period prior to the commencement of Basslink operations (‘pre-Basslink’) and a period following commencement of Basslink (‘post-Basslink’). During this time a variety of flow conditions were observed in the lower Gordon, reflecting the pattern of releases from the Gordon power station.
However, not all flow conditions that are possible under the post-Basslink operating regime have been observed during the monitoring program to date. In order to fully evaluate the impacts of the post-Basslink operation of the Gordon power station, further, repeated sampling is required following periods of key flow conditions (‘events’) for which data is currently scarce or absent.
It is known that certain flow conditions have the potential to cause negative impacts on the benthic macroinvertebrate community of the lower Gordon. Three key flow features known to drive biological condition are:
• Low flows: Periods of low flow < 20 m3s-1.
• High duration of high flows: Prolonged or repeated periods of high flow, > 100 and >200 m3s-1;
• High frequencies of peaking: High frequencies of events where flows rise rapidly (within 2–4 hours) from low values (20 – 40 m3s-1) to high values (> 100 or > 200 m3s-1);
The role and management of low or ‘minimum environmental’ flows has been the subject of previous analysis and reporting and is not discussed here.
This report describes the results of review and analysis of previous monitoring information with the aim of identifying a minimum set of flow duration and peaking events to be used to trigger macroinvertebrate monitoring in the lower Gordon system.
The peaking events evaluated here are those where rapid rises in flow from low (ca. 20-40 cumec) to high values (> 100 or 200 cumec) occur in a slower short interval, and are not the same as those peaking events described in relation to bank erosion in the Gordon, which typically involve rapid flow changes in high flows in the range 100 to > 200 cumec.
The degree to which aspects of the flow regime drive biological responses varies across zones 1 and 2 within the river (zones 1 and 2 = upstream and downstream of the Denison confluence respectively), and across seasons (spring or autumn). Flow-event triggers need to address these differences where relevant

Item Type:	Report (Project Report)
Authors/Creators:	Davies, PE
Publisher:	Freshwater Systems
Additional Information:	© Hydro Tasmania 2014, © Freshwater Systems 2014
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