Lake Nillahcootie has a storage capacity that is about half the mean annual flow of its upstream catchment, so it fills in most years. The operation of Lake Nillachootie has modified the river’s natural flow pattern; winter/spring flows are less than natural because a large proportion of inflows are harvested, while summer/autumn flows are higher than natural because water is released to meet downstream irrigation demands. These impacts are most pronounced in the reach between Lake Nillahcootie and Hollands Creek. Downstream of Hollands Creek, the river retains a more- natural flow pattern due to the contribution of tributary inflows. The catchment has been extensively cleared for agriculture including dryland farming (such as livestock grazing and cereal cropping) and irrigated agriculture (such as dairy, fruit and livestock).
Water is released from Lake Nillahcootie to meet downstream demand and minimum-flow requirements specified under the bulk entitlement for the Broken River system. Releases from storage may be less than 30 ML per day as tributary inflows immediately below the storage (such as from Back Creek) can supply much of minimum- flow requirements specified in the bulk entitlement.
The upper Broken Creek is defined as the 89-km stretch of creek from the Broken River (at Caseys Weir) to the confluence with Boosey Creek near Katamatite. The upper Broken Creek flows across a flat, riverine plain and has naturally low run off from its local catchment. It receives flood flows from the Broken River, although the frequency of these floods has been reduced by earthworks and road construction.
Upper Broken Creek has been regulated for more than a century. Before 2007, water was diverted into upper Broken Creek at Casey’s Weir to meet local demand, but recent water-savings projects have reduced the demand on the creek. There are now low flows throughout the year between Caseys Weir and Waggarandall Weir. Flows downstream of Waggarandall Weir are mainly influenced by rainfall and catchment run off. These changes have reduced the amount of permanent aquatic habitat.
Delivery of water for the environment to the Broken River is primarily constrained by the availability of water. Usually, the available volume of water for the environment is well short of the volume required to deliver the desired flow components. Deliveries of water for the environment to the upper Broken Creek are also restricted by channel capacity and by the need to avoid flooding low-lying adjacent land.