Williams, R.J. and Thiebaud, I., 2007. An analysis of changes to aquatic habitats and adjacent land-use in the downstream portion of the Hawkesbury Nepean River over the past sixty years. Final report to the Hawkesbury-Nepean Catchment Management Authority. NSW DPI – Fisheries Final Report Series No. 91. 97pp.

Summary

The catchment of the Hawkesbury River below Warragamba Dam has been subject to urban development, agriculture and recreational use for many decades. Some degradation of terrestrial and aquatic habitats has occurred because of these pressures. For example, there has been a marked input of nutrients to this stretch of the river, and because up to 90% of the flow has been diverted for use by the Sydney metropolitan area, a number of changes would have been expected to have occurred to aquatic habitats from nutrient enrichment.

To assist in understanding the nature and extent of these changes, a series of historical aerial photographs was analysed. Thirteen fixed locations were established from Warragamba Dam to the entrance, and within each location, six permanent sites were established. Features of interest in relation to disturbance of aquatic habitat over the past 60 years were identified in a geographic information system (GIS). The choice of locations and sites was biased towards situations where large scale change could be measured, and hence are not representative of the river as a whole. Fieldwork allowed the collection of data for habitat features not visible in the photographs.

The GIS analysis revealed that, except for the upper portion of the study area, natural vegetation was stable at most locations. In the locations closest to Warragamba Dam, natural vegetation increased markedly. Some loss of agricultural land was seen, with reduction at 12 of the 13 sampling locations. Complete extinction of this landuse occurred at Pittwater, but small losses were seen also in the middle stretch of the river. A doubling in residential area occurred at one location and a multifold increase occurred at five others. Area developed for industrial activity increased at the upper river locations. Increase in formal passive recreation area was seen at seven locations.

With regard to aquatic features, there was a marked overall change in the distribution of features. Seagrass was lost at three locations, stable at a fourth and increased substantially at a fifth location. Cover of mangrove increased. The cover of saltmarsh, recently declared a Threatened Ecological Community, decreased at all locations.

This study is the first of its kind to assess decadal scale change in landuse in relation to temporal change in aquatic habitat. It paves the way for further investigations of cause and effect relationships between changes in cover on the land and concomitant disturbances in intertidal and submerged macrophytic vegetation.