The Wimmera River commences in the Pyrenees Range near Elmhurst and receives flows from several tributaries including the MacKenzie River and the Mount William, Burnt and Bungalally creeks. All of these can receive environmental water, as can the Wimmera River downstream of lower Mount William Creek. Just east of Mt Arapiles the Wimmera River swings to the north and continues through Dimboola and Jeparit to Lake Hindmarsh, one of Victoria's largest freshwater lakes. During exceptionally wet periods, Lake Hindmarsh overflows into Outlet Creek and on to Lake Albacutya, an internationally recognised Ramsar-listed wetland extending to the Wirrengren Plain in the southern Mallee.
Water in the Wimmera system is stored in three on-stream reservoirs — Lake Wartook on the MacKenzie River, Lake Lonsdale on Mount William Creek and Lake Bellfield on Fyans Creek — and in several off-stream storages: Taylors Lake, Lake Fyans and Toolondo Reservoir. The channel system enables water movement between storages and from the Glenelg to the Wimmera system. Inter-basin transfers of water can occur from Rocklands Reservoir (in the Glenelg system) via the Rocklands–Toolondo Channel and from Moora Moora Reservoir via the Moora Channel to the Wimmera system. Water from the system is also delivered to towns and several Wimmera–Mallee wetlands in the Loddon, Avoca and Mallee catchments.
Passing flows are provided to the Wimmera River and to Mount William and Fyans creeks. Where possible, environmental water releases will be combined with passing flows, unregulated flows and the delivery of consumptive water en route, to optimise environmental outcomes.
Priority reaches for environmental watering in the Wimmera system are the Wimmera River reach 4, MacKenzie River reaches 2 and 3, upper and lower Mount William Creek, upper and lower Burnt Creek and Bungalally Creek, due to the range of environmental values they support (see below).
Yarriambiack Creek, a distributary of the upper Wimmera River, has historically received some flows during high-flow events in the Wimmera River. However, the creek now receives more flows due to modifications to the offtake. This reduces the effectiveness of environmental water deliveries to the high-priority reaches of the Wimmera River. In line with past practice during dry years, flows entering the creek may be blocked to ensure watering objectives in the Wimmera River are not compromised.