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 Nillahcootie has modified the river’s natural flow pattern: winter/spring flow is less than natural because a large proportion of inflow is harvested, while summer/autumn flow is 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. Below Hollands Creek, the river retains a more natural flow pattern, due to flows from unregulated tributaries, although total annual flow is considerably less than natural. 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.
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. 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 river regulation, 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 is now low flow throughout the year between Caseys Weir and Waggarandall Weir. The flow below Waggarandall Weir is 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 due to the small volume of Water Holdings in the Broken system. Environmental water holders can trade it into the Broken system from other trading zones subject to relevant limits and conditions, to meet critical environmental needs.
The bulk entitlement for the Broken system held by Goulburn-Murray Water stipulates that minimum environmental flows — also known as passing flows — are to be maintained in the Broken River when there are natural flows into the system. The bulk entitlement also allows Goulburn-Murray Water and the VEWH to agree to reduce minimum flows and to accumulate the unused volumes for later releases that will provide a greater environmental benefit. In recent years, passing flows have been reduced, accumulated and delivered to maintain low flow (on days when there are no passing flows due to no natural flow into the system) and for freshes in the Broken River. In 2020-21, accumulated passing flows were used to supplement the flow in upper Broken Creek for the first time. Accumulated passing flows are the first volumes lost when the storage spills. Environmental flows in upper Broken Creek are restricted by the volume of available supply, channel capacity and the need to avoid flooding low-lying, adjacent land.