Channel modification in Tumut, Australia

Indicator: Channel modification

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What the results tell us for Tumut

See also: | Water Supply | Environmental Flows |

A number of weirs and dams are located on streams to provide a reliable water supply to towns within the Shire, and as integral components of the Snowy Hydro Electric Scheme. Details of these dams are given in Table 1. As a result of these structures, the major streams within the Shire are fully controlled, with associated downstream impacts on channel morphology, aquatic ecosystems (due to changes in temperature and impediments to fish passage). In particular, streams associated with the generation of hydro electricity are modified as a result of changes in the erosive power of water discharging through the system.

Table 1. Large dams in Tumut Shire
Dam	River controlled	Type	Gross reservoir capacity* (ML)
Blowering	Tumut	Rockfill	1 632 400
Batlow	Tumut	No data	120
Talbingo	Tumut	Rockfill	620 600
Jounama	No data	Rockfill	43 500

* absolute volume of water that dam can hold before spilling
(measured in megalitres)

Data from the Stressed Rivers Assessment Reports (DLWC 1999) provide an overview of the condition of streams and rivers across the Shire in relation to dams and in-stream structures (Table 2). Approximately 66% of Tumut Shire falls within unregulated subcatchments. Not all subcatchments listed in Table 2 lie wholly within the Shire. Subcatchments are of differing size, in terms of area and volume of water flowing through them.

Table 2. Stress ratings for in-stream and streambank structures, in unregulated freshwater subcatchments in Tumut Shire
Catchment or subcatchment name	Stress/Condition Rating and Comment		% of Subcatchment in the Shire	Area (ha) of Subcatchment in the Shire
Catchment or subcatchment name	Fish Barriers	Dams and Development	% of Subcatchment in the Shire	Area (ha) of Subcatchment in the Shire
Murrumbidgee Catchment
Adelong	Fair condition Some fish barriers, most passable.	Poor condition	100	29696.64
Bombowlee/Brungle/Adjungbilly	Good condition Few fish barriers, most passable.	Poor condition	99.71	53169.93
Gilmore/Sandy	Good condition Few fish barriers, most passable.	Very poor condition Extensive development has been undertaken in this subcatchment.	100	30402.86
Goobarragandra	Very good condition No barriers to fish passage in this subcatchment.	Fair condition	99.78	74133.58
Goodradigbee	Very good condition No barriers to fish passage in this subcatchment.	Good condition	0.2	224.48
Hillas	Good condition Few fish barriers, most passable.	Poor condition	100	49187.9
Tantangara	Very good condition No barriers to fish passage in this subcatchment.	Very good condition	11.23	5243.02
Tarcutta	Fair condition Some fish barriers, most passable.	Poor condition	100	6427.66

About the data

Data on the stress ratings for in-stream and streambank structures in unregulated streams have been made and collected by the NSW Department of Land and Water Conservation (DLWC). The data are reported in the Stressed Rivers Assessment Report (DLWC 1999) for the Murrumbidgee Catchment. Details about the data sources and assessment process are contained in each of these reports.

More information about the data can be found in About the data for the Australian Capital Region.

Reference

DLWC (1999) Stressed Rivers Assessment Report, Department of Land and water Conservation, Sydney.

Description: What does 'channel modification' measure?

Which data are collected?

number and type of in-stream and streambank structures constructed each year in each jurisdiction
tonnes of sand and gravel extracted annually in each stream reach

Why do we report this indicator?

The extent to which stream channels are actively being modified by human activities indicates the potential pressure on the structural integrity of the stream system. There is a range of human activities that can potentially modify stream channels. These include construction of new structures such as weirs and dams, and physical removal of sand and gravel from within the stream channel, and an indicator of the extent of these activities is of importance to State of the Environment reporting. Other activities, such as trampling by livestock and some recreational uses, can also impact of the channel structure but these are considered separately.

The impact of these modifications is felt primarily by aquatic biota due to the loss of habitat, changed flow regimes and changed water quality. Further, the construction of some in-stream structures, such as dams, provides a physical barrier to movement along streams, potentially interrupting breeding cycles.

The removal of sand and gravel can be beneficial in instances where deposits of sediments resulting from upper catchment erosion are being mined. However, this activity most commonly has the impact of significantly changing the sediment budgets of streams, potentially accelerating the rate of change of the channel structure.