Effects of Weirs in the Mallee Tract of the River Murray

"August 2004".

Project Number: Monitoring and mapping the Euston Weir drawdown trial - M/BUS/5; MDBC Strategic Investigation Program - Rivers - Project R2131.

MDFRC item.

This individual Research Report has an additional 9 Research Reports, see project (Monitoring and mapping the Euston Weir drawdown trial).

The physical and biotic effects of weirs in the Mallee Tract of the River Murray were quantified from August 2001 - June 2003 by comparing weir pool and free-flowing sites. Effects of weirs on hydrology and hydraulics, water physico-chemistry, benthic sediments, phytoplankton, biofilm, macroinvertebrates and littoral vegetation were examined. The three weirs within the Mallee Tract have altered the hydraulics of flow and consequently the water physico-chemistry and biota within these reaches. The Euston (Lock and Weir No.15), Mildura (11) and Wentworth (10) weirs elevate the water level 4.75, 3.55 and 3.21 m, respectively, at a designated flow of 4,000 ML.d-1. The weir pools that these weirs create extend ca. 50-60 river km upstream. The weir pool reaches are characterised by greater depths, lower flow velocities and stable water levels relative to the free-flowing reaches. Complete short-term drawdowns of the Mildura weir pool in each year of the study period for weir maintenance were exceptions to this pattern. Flows throughout the study reach were low and remained less than 13,000 ML.d-1 at all times, and provide a backdrop against which some of the results of this study can be interpreted. Diurnal thermal stratification was a year-round phenomenon in the weir pools, with greater (and occasionally persistent) thermal stratification of the water column in the warmer months. The water column at the free-flowing sites remained well mixed throughout the year. Stratification of pH and dissolved oxygen was also a feature of the water column in weir pools but not the free-flowing sites. The low flow velocities in the weir pools resulted in a sequential deposition of suspended solids from the water column into the Euston (ca. 78 t.d-1), Mildura (46 t.d-1) and Wentworth (8 t.d-1) weir pools. As a consequence, turbidity levels and nitrogen and phosphorus concentrations in the water column progressively decreased in the weir pools of the Mallee Tract. The sediments on the riverbed of weir pools were correspondingly fine, nutrient rich and contained a high organic matter content compared with the typically sand-dominated sediments of the free-flowing sites. A total of 142 phytoplankton taxa were identified with Bacillariophyceae (diatoms), Cyanophyta (cyanobacteria) and Chlorophyta (green algae) being the dominant phytoplankton groups, together comprising over 99.4% of the phytoplankton abundance. Phytoplankton richness ranged from 31-62 taxa (mean 46.2 taxa) per sample. The phytoplankton communities along the Mallee Tract followed a seasonal pattern but underwent spatial shifts in community composition. Phytoplankton community composition remained similar along the 175 km of free-flowing reach examined, but changed within the ca. 50 km weir pool reaches where diatom abundance decreased and cyanobacteria abundance increased. These changes indicate that weir pools are providing favourable conditions for cyanobacteria (buoyancy regulators) whilst the diatoms (reliant on turbulent flows to maintain their position in the water column) are settling from the water column. These changes were also reflected in the chlorophyll measurements, where the total load of phytoplankton decreased in the weir pools and increased in the free-flowing reaches. Biofilm in the photic zone of the weir pools had a greater total biomass and periphyton biomass relative to the free-flowing sites, with both environments supporting greater biofilm development in the warmer months. The autotrophic index indicated that the biofilms from the weir pools were autotroph dominated and those from free-flowing sites heterotroph dominated. A total of 106 periphyton taxa were identified with Bacillariophyceae, Cyanophyta and Chlorophyta being the dominant phytoplankton groups. Each group was more abundant in the weir pools than free-flowing sites for a given time, with greater abundances during the summer period. Multivariate analysis showed that periphyton communities differed mostly between the summer and winter periods but also differed between weir pool and free-flowing sites. Diatoms were consistently the most abundant periphyton group at all sites, with Aulacoseira granulata comprising over 55% of the total periphyton cell count. Thirty-four snag-dwelling macroinvertebrate taxa were identified, with abundances on River Red Gum substrates 3½ times greater in the weir pools compared with the free-flowing reaches. The weir pool environment also supported a significantly greater number of macroinvertebrate taxa than the free-flowing sites. Chironomidae (midges) was the most abundant taxon, followed by Oligochaeta (segmented worms). Multivariate analysis of the macroinvertebrate communities revealed distinct seasonal differences in community structure, with different communities between weir pools and free-flowing sites. It is likely that the greater density of macroinvertebrates at the weir pool sites is due to the greater biofilm biomass providing increased food resources and habitat. A total of 29 macrophyte species were recorded in the littoral zone of the Mallee Tract. All 29 species were present in the weir pools compared with 10 in the free-flowing reaches. Species richness ranged from 1.4-5.4 species.quadrat-1 at weir pool sites to 0.1-3.5 species.quadrat-1 at the free-flowing sites. Percentage vegetation cover was also greater in the weir pools (12-76%) compared with the free-flowing sites (0.5-37%). Weirs have significant effects on hydraulic conditions, water physico-chemistry, benthic sediments, phytoplankton, biofilms, macroinvertebrates and vegetation in the Mallee Tract. For some species the free-flowing river is naturally a difficult environment in which to persist given the relatively high flow velocities and fluctuating water levels. The creation of weir pools has provided hydraulic conditions more akin to lentic systems, resulting in many species typical of wetlands becoming well established in the littoral zone. However, weirs have also altered riverine processes such as patterns of sediment and nutrient transport and phytoplankton dynamics. The ecological changes brought by weirs in the Mallee Tract further highlight the need to restore parts of the natural flow regime, and weir manipulation trials may partly achieve this. An examination of the ecological responses to weir manipulations is required to ascertain whether these types of trials generate positive changes to the riverine and floodplain environments. Several weir manipulation options are presented with consideration given to the potential costs and benefits of each. The data collected from this study would form important “before” data for any future weir manipulation within the Mallee Tract.