Predicting potential impacts of environmental flows on weedy riparian vegetation of the Hawkesbury-Nepean River, south-eastern Australia

Remnants of native riparian vegetation on the floodplain of the Hawkesbury- Nepean River near Sydney, have significant conservation value, but contain a large component of weeds (i.e. exotic species that have become naturalize d). A proposal for the introduction of environmental flows required an asse ssment of potential impacts on 242 native and 128 exotic species recorded a long 215 km of the river. The likely effects of frequency, season, depth an d duration of inundation were considered in relation to habitat, dispersal season and tolerance to waterlogging. Overseas studies provided only limite d information applicable to the study area; however, comparisons with simil arly highly modified riparian habitats in New Zealand were instructive. Dep th and season of inundation appear to be the variables with the greatest po tential for differential effects on weeds and native plants. Because of lik ely spread of propagules and enhancement of growth under the present nutrie nt-enriched conditions, environmental flows that would cause more frequent flooding to higher levels of the riparian zone were judged to be of more be nefit to weed species than native species, unless supported by bushland man agement including weeding. Predictions were limited by incomplete data on H awkesbury-Nepean species, but two types of environmental flow were judged t o be potentially beneficial for native water-edge plants, and worth testing and monitoring: first, flows that maintain continuous low-level flow in th e river, and second, higher level environmental flows restricted to the riv er-edge habitat in autumn (the season in which a greater proportion of nati ve species than weed species are known to disperse propagules). In summary, the presence of environmental weeds in riparian vegetation constrain the p otential for environmental flows to improve river health. However, with ong oing monitoring, careful choice of water level and season of flow may lead to environmental flows that add to our knowledge, and benefit riparian vege tation along with other river system components.