Effects of weirs in the Snowy Mountains – Master’s student project

Thesis title:  Macroinvertebrate community changes along channels bisected by prospective barriers: a paired case study in Snowy Mountain headwater streams

Student: Jared Polkinghorne

Degree: Master of Science (Geography)

Funding: The University of Melbourne, plus logistic support from Snowy Hydro and NSW National Parks

Downstream of Island Bend Dam on the Snowy River

Downstream of Island Bend Dam on the Snowy River

Weirs and dams are recognised as sources of significant disruption to freshwater streams. A comprehensive understanding of these structures, their impacts as well as the underlying causes of these impacts, is necessary in order to best manage catchments with weirs. Currently, there are two models which are potentially able to explain the  impacts of weirs as well as changes to community assemblages more generally. One model suggests that dispersal processes are key to determining community structure while the other advocates that environmental conditions are key. The relative role of each of these models is not fixed and varies based on the nature of the focal organisms and the ecosystems inhabited. This study investigated the efficacy of these two models in determining the structure of macroinvertebrate communities in headwater streams using two comparable streams in the Snowy Mountain region.

Looking downstream below the weir on Munyang/Whites River - no water escapes except during floods

Looking downstream below the weir on Munyang/Whites River – no water escapes except during floods

The two streams are Munyang/Whites River and Dicky Cooper Creek. Munyang/Whites, which is a tributary of the Snowy River, has numerous small off-takes on almost all first-order tributaries, plus a weir that removes all flow from the creek. Dicky Cooper Creek, which is in the catchment of the Murray River, has no weirs although it has a series of waterfalls that may act as a natural barrier to dispersal. The streams flow in different directions from the same high point at Schlinks Pass and thus share similar territory at their highest points.

Dicky Cooper Creek

Dicky Cooper Creek

Analysis suggests that the impact of the weir on Munyang/Whites caused the fauna in the river below the weir to look more like that of the headwater sections. Nevertheless, the downstream fauna of Munyang/Whites was indistinguishable from that of  downstream areas of Dicky Cooper. This suggests that second-order tributaries flowing into Munyang/Whites below the weir may ameliorate its effects.  Unexpectedly, the fauna of the upstream sections of these two streams were dissimilar to each other despite the ability of terrestrially based adult insects to fly the short distances across the catchment boundary.  This suggests we have much to learn about the role dispersal plays in community structure in headwater streams.