Drawdown of the Mildura Weir Pool 2001, 2002 & 2003: Water Quality Responses

"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 Mildura weir pool was fully drawn down in 2001, 2002 and 2003 to allow for maintenance work on the Mildura weir through the sequential replacement of weir trestles. The opportunity was taken to monitor these large amplitude and short duration drawdown events to measure hydraulic changes and the effects of drawdown on water quality and groundwater levels. The weir pool at Mildura was lowered by 3.43, 3.24 and 3.57 m from its normal full supply level of 34.40 mAHD during the weir pool drawdowns of 2001, 2002 and 2003, respectively. The different levels attained in each year reflected the different river flows occurring at the time of drawdown. For all events, drawdowns occurred over 7-9 days, the weir pool remained fully drawn down for 1-3 days, and refilling occurred over 7-10 days. It was estimated that the volume of water available for release from the weir pool through drawdown is approximately 11.5 GL. Longitudinal surveys of the weir pool when fully drawn down revealed a gradient of increasing bank exposure with distance downstream, and that the point at which the river ceases to become influenced by the weir is approximately 57 km upstream of the weir at a flow of 10,500 ML.d-1 (further during lower flow periods). A longitudinal survey with the weir pool at full supply level highlighted a gradient of decreasing flow velocity when progressing downstream due to the increasing cross sectional area of the river channel. Electrical conductivity increased significantly in the Mildura weir pool as a result of each drawdown. Electrical conductivities at the Mildura weir peaked at 525, 330 and 678 μS.cm-1 in 2001, 2002 and 2003, respectively. Salt loads were calculated for each drawdown event and the quantity of salt entering the river as a direct result of each drawdown was estimated to be 5,078, 3,436 and 9,266 tonnes in 2001, 2002 and 2003, respectively. Evidence of large changes in groundwater levels (maximum decrease of 1.8 m in one bore) and the absence of large backwaters along the Mildura weir pool demonstrated that the majority of salt entering the river during the drawdowns was sourced from groundwater. Longitudinal surveys of the weir pool in 2002 and 2003 whilst fully drawn down revealed sections of river where electrical conductivity increased substantially (up to 84 μS.cm-1 in one 2 km reach) in the absence of backwaters. The results demonstrate a strong hydraulic connection between the surface and groundwater systems and significant groundwater inflows to the river during low river levels when the hydraulic gradient is high. Turbidity and total suspended solids increased in response to each drawdown as a result of the greater flow velocities arising from the lower cross sectional area of the river. When the weir was reinstated flow decreased dramatically at the weir (flows were used to refill the weir pool) and suspended solid concentrations decreased through sedimentation. During the drawdown and refilling period in 2001 there was a total export of approximately 1000 tonnes of sediment from the Mildura weir pool. This is in contrast to the net deposition occurring prior to and following the drawdown/refilling event. The export of sediments from the Mildura weir during the drawdown event did not translate to detectable changes in the clay-dominated (and nutrient rich) soft sediments immediately upstream of the Mildura weir. For the Euston weir pool in 2001 there was a mean deposition of approximately 18 tonnes.d-1 of sediment (19% of which was organic matter) into the weir pool during this relatively low flow period. Total phosphorus concentrations increased in the Mildura weir pool in unison with the increased turbidity, because phosphorus is bound to sediment particles. In contrast, filterable reactive phosphorus (an estimate of the “bioavailable” fraction) did not change as a result of the drawdown and concentrations remained low (≤ 2 μg.L-1). Total nitrogen and oxides of nitrogen concentrations were consistently higher in the Mildura weir pool before and particularly during the drawdown/refilling event (relative to the upstream site). A potential but speculative source of these nutrients is groundwater inputs, particularly given the intensive irrigated horticulture occurring adjacent to the weir pool. Whilst biotic changes arising from the drawdown were not examined specifically as part of this study, observations of stranded river mussels (cover photograph) and the complete exposure of emergent and submerged macrophytes demonstrated potential impacts of large amplitude drawdowns on some biota. This biota appeared resilient to the short-duration drawdowns given their presence in 2003 following similar drawdowns in 2001 and 2002. A more detailed examination of effects of weir pool drawdown on biota is being considered by the MDBC as part of a larger project of which this study is one component.