Development of Experimental Design and Monitoring for a Weir Drawdown Trial to Determine the Nature of Ecological Benefits

"April 2001".

Project Number: Monitoring and mapping the Euston Weir drawdown trial - M/BUS/5; Strategic Investigation & Education Program, MDBC Project R 10010.

MDFRC item.

Riverine flows along most lowland sections of the Murray-Darling Basin (MDB) are intensively regulated (e.g. Close, 1990). Weirs are one of the major control devices, and play an important role in irrigation, domestic water supply and recreation. However, weirs exert a considerable impact on the in-stream physical and biological environment (Thoms and Walker, 1993; Walker and Thoms, 1993; Walker et al., 1994). These effects have been extensively studied in the lower Murray River, which has been converted into a series of stable-height weir pools. Acting in concert, weirs and upstream dams have facilitated diversions and extractions from the Murray River. This has reduced the total flow volume in lowland reaches and resulted in a reduction in the incidence of flooding. Weirs have also created weir pools that have altered the channel morphology and reduced the seasonal variability in stage height (Walker and Thoms, 1993; Thoms and Walker, 1993). In turn, changes in flow variability and channel morphology have affected the distribution of littoral plants (Walker et al., 1994, Blanch et al., 1999,2000) and the composition of biofilms (Walker et aI., 1992; Mullen, 1998). Furthermore, weirs isolate large sections of the river by preventing fish migration, and reduce the downstream transport of organic matter and sediment (Thoms and Walker, 1993). River managers are exploring ways of minimising the negative impacts .of weirs whilst retaining some, if not all, of the services they provide to the wider community. For example, fish ladders have been installed to facilitate the movement and migration of native fish (Mullen-Cooper and Brand, 1992). Other initiatives have been aimed at the restoration of elements of natural flow regimes in weir pools as part of programs to manage environmental flows (Blanch et al., 1996). One of the major impacts of weirs has been to reduce the seasonal variation in stage. It has been suggested that a sustained drawdown with controlled rates of rise and fall may ameliorate some of these impacts and provide significant ecological benefits to the river (Thoms et al. 2000). The present study commenced in response to a call through the Murray-Darling Basin Commission's Strategic Investigations and Education (SI&E) Program for the design, conduct and monitoring of a trial weir pool drawdown. The specific objectives of the project are to: (1) Scope the types of beneficial weir manipulations, potential locations, the experimental design of a drawdown trial, and the potential environmental, social and economic issues associated with a trial; (2) Select a location to conduct a trial weir drawdown exercise and design a program to assess ecological, social and economic implications; (3) Perform the trial drawdown, and examine the responses, the benefits and impacts (social, economic and ecological) of these manipulations; and (4) Develop guidelines for future weir pool manipulations. This Issues Paper addresses the first of these objectives and provides a platform on which to select a location for a trial manipulation. Tube key (asks of this first objective are to identify the: (A) types of manipulations likely to be beneficial to the environment (B) likely ecological responses to these manipulations (C) social and economic impacts or implications of these manipulations (D) locations where these manipulations can be made within the Murray Darling Basin (E) constraints and advantages arising from each location.