River metamorphosis and environmental change in the Macdonald Valley,New South Wales,since 1949

A significant metamorphosis of channel form occurred on the lower Macdonald River between 1949 and 1955; width and width-depth ratio increased and depth, sinuosity and weighted mean per cent silt-clay in the channel perimeter decreased. The channel has remained unstable since 1955, exhibiting frequent variations in bed elevation and a recent period of minor channel contraction followed by slight enlargement. Since 1946 there has been an increase in summer and annual rainfall and an abrupt, upward shift of the annual series flood frequency curve. As a result an increased proportion of the total sediment load of the river is now being transported as bed-material load. The change in sediment load is a result of greater stream competence and the consequent reworking of sandy sediment temporarily stored within the channel as benches. The observed channel changes are a quasi-adjustment to the increased flood and bed-material load discharges.     

Potential impacts of sand and gravel extraction on the Hunter River,New South Wales

A number of investigations have been undertaken by the New South Wales Water Resources Commission to determine the regional and compound effects of large scale extractive industries on the stability of the Hunter River, New South Wales. Sedimentologic studies of bed material suggest that the Hunter River upstream of Denman has an armoured gravel bed that is immobile during regulated releases from Glenbawn Dam but is disrupted by moderate but less than bankfull flows. Annual bedloadyields have been computed by the bedload rating-flow duration technique for five river gauging stations. Approximate sand and gravel budgets for selected reaches of the Hunter River demonstrate that the present annual extraction rate from temporary sediment storages within the channel greatly exceeds the transport rate upstream of Denman and is approximately equal to the transport rate downstream of Denman. River degradation is imminent if extractive industries continue operating in the channel of the Hunter River     

Morphological Impacts and Implications of a Trial Release on the Wingecarribee River,New South Wales

A trial, monitored release on the Wingecarribee River over 27 days had a peak discharge below the Wingecarribee Dam of 1090 ML/d, which had a flow duration of 0.95 per cent. Morphological impacts on 29 km of channel below the dam included bank erosion, concentrated neck overflow, bed degradation and channel widening at recent cutoffs, sedimentation in weir pools and overbank flow, particularly at discharges greater than 800 ML/d. Prolonged operational releases at bankfull discharge (400–800 ML/d) will have the greatest morphologic impacts and should be avoided. Pulses which dissipate stream power over a broad, well-vegetated floodplain should be incorporated in the operational release policy, provided such pulses are phased to facilitate channel and vegetation recovery.     

Water,Wind, Wood,and Trees: Interactions,Spatial Variations,Temporal Dynamics,and their Potential Role in River Rehabilitation

Recent Australian research has quantified the role of large wood (wood of any origin and length with a diameter greater than 0.1 m) in dissipating stream energy, forming pool habitats by local bed scour, protecting river banks from erosion, and damming rivers with long rafts causing avulsions. Large wood in Australian streams is sourced by a range of processes from the nearby riparian zone which has usually been degraded by post‐European settlement vegetation clearing. Large wood loadings within the bankfull channel are dependent not only on the type and quality of the riparian plant community but also on bankfull specific stream power, channel width, and the processes of large wood delivery to the stream. While bank erosion and floodplain stripping by catastrophic floods are obvious and important delivery mechanisms, treefall and trunk and branch breakage by strong winds during tropical cyclones and severe storms are also significant in the tropics. Furthermore, wood decay and downstream transport produce temporally dynamic large wood distributions. The longevity of natural large wood structures in rivers, such as rafts, debris dams, and log steps, requires determination. River rehabilitation programs need to not only include the reintroduction of large wood, but also carefully plan the spatial distribution of that wood, the most appropriate type and range of large wood structures, and, most importantly, the revegetation of the riparian zone to ensure a natural long‐term source of large wood. Exotic species management is an essential part of river rehabilitation.     

Hydrogeomorphic Effects of River Training Works: the Case of the Allyn River,NSW

Since 1955 the NSW Department of Water Resources has undertaken extensive river training works on many NSW streams. In the Hunter Valley alone over $23 million have been spent and more than 850 km of channel have been treated Published work suggests that river training works are usually undertaken in response to a pre-existing problem of river instability but can also induce adverse hydrogeomorphic effects. One of the treated streams, the Allyn River, had been supposedly degraded by the constructed works. It was alleged that vegetation clearing, channel excavations, alignment straightening and bank protection works had decreased roughness thus increasing velocity and flood frequency. As a result, what were thought to be relatively moderate floods were eroding the banks and destroying the floodplain. A critical evaluation of the available hydraulic, hydrologic and geomorphic data revealed that the river training works were a response to, rather than a cause of river instability.     

Rainfall,floods and river channel changes in the upper Hunter

Since 1946, there has been a significant increase in annual rainfall in the Hunter Valley, especially for stations in the western sector. Contemporaneous increases in rainfall intensities of frequent storms and flood peaks have also been recorded. From analyses of the data, together with a review of recent environmental changes in the Hunter Valley, it is concluded that human disturbance of the catchment has played no significant part in the increase in flood magnitude. Instead, the change in floods may be completely attributed to the variation in rainfall regime. The altered hydrologic regime combined with a decrease in sediment yield from extra-channel sources appears to be the primary cause of recent river channel changes.