Indicators of Catchment Condition in the Intensive Land Use Zone of Australia – Impoundment density

It should be noted that this data is now somwhat dated!

Nation-wide measures that would reflect the changes to flow regimes in rivers were considered, for example changes in peakiness, and changes in volumes. However national long-term data sets approaching catchment scale detail were not available or were of dubious quality. The regulation of river flows has changed the natural flows regimes of rivers and this impacts on wetland ecosystems and aquatic biota. measure based on impoundments was considered to be the best available surrogate indicator for hydrological changes.

The Wild Rivers data set (Environment Australia, 1997) includes national data on impoundments to river flow. The data set has four classes: major structures (dams, reservoirs); weirs; locks/sluice gates; and minor structures (banks, farm dams on or immediately adjacent to stream). The Wild Rivers data set also includes flow diversions data, but these are not used in the indicator here.

The maps show the location of major irrigation schemes and the major dams for urban water supply. The Wild Rivers impoundments data were obtained from state point data sets, each of which were reclassified to a single four-class impoundment rating system. Collection and derivation of data are not consistent across the nation, and both point and polygon data have been included.

The polygon data has the effect of grossly inflating the number of impoundments as it is possible to have 400 x 250 m grid cells within a 5 km grid cell, all counted as individual impoundments. The data set is reliable at the Topo250K drainage layer scale (AUSLIG, 2000).

The most modified rivers and streams are in Tasmania (Derwent, Gordon, King Henty, Pieman, Mersey, Forth, and Tamar Rivers). Other areas with significant impacts on river flows are in the Mt Lofty Ranges around Adelaide (Fleurieu, Myponga, Onkaparinga, Torrens, and Gawler Rivers), the hinterland of Melbourne (Bunyip and Yarra Rivers) and in Queensland (Kolan, Ross, Barron and Tully Rivers). In Western Australia the rivers with the highest impoundment values were the Collie Harvey and Ord Rivers.

Data are available as:

• continental maps at 5km (0.05 deg) cell resolution for the ILZ;
• spatial averages over CRES defined catchments (CRES, 2000) in the ILZ;
• spatial averages over the AWRC river basins in the ILZ.

See further metadata for more detail.