Lake Glenmaggie is the major water-harvesting storage regulating Wirn wirndook Yeerung (Macalister River). Maffra Weir is a small diversion weir located further downstream in Maffra.
Before the construction of Lake Glenmaggie, Wirn wirndook Yeerung (Macalister River) would regularly receive high and medium flows in winter and spring. Although Lake Glenmaggie regularly spills, high flows are less frequent than natural because much of the water is captured by the storage. A notable impact of irrigation and water-harvesting is reversed seasonality of flows between Lake Glenmaggie and Maffra Weir. Summer flows through this reach are much greater than natural due to the delivery of irrigation water. Winter flows in this reach are lower than natural because a high proportion of the inflows are captured, and there are no irrigation demands over winter. Most irrigation water is diverted at Maffra Weir, and flow downstream of the weir is lower than natural year-round. The changed hydrology restricts fish migration, limits the growth and recruitment of in-stream and streamside plants and reduces the quality of in-stream habitat.
Water for the environment is stored in Lake Glenmaggie and released to Wirn wirndook Yeerung (Macalister River). The river is divided into two reaches for the purposes of managing environmental flows: Lake Glenmaggie to Maffra Weir (reach 1) and Maffra Weir to Carran Carran (Thomson River) (reach 2).
Maffra Weir is a major barrier to fish movement along the river, so delivery of water for the environment for migratory fish objectives mainly focuses on reach 2. All other objectives apply to both reaches 1 and 2.