In-stream wetlands and their significance for channel filling and the catchment sediment budget, Jugiong Creek, New South Wales

Evidence is presented here of recent and extensive in-filling of the incise d channel network of the Jugiong Creek catchment, SE Australia. The present channel network resulted from widespread stream and gully incision in the period between 1880 and 1920. Our survey shows that gully floors have been colonised extensively by emergent macrophyte vegetation since before 1944, forming continuous, dense, in-stream wetlands, which now cover 25% of the c hannel network in the 2175 km(2) catchment and have so far trapped almost 2 ,000,000 t of nutrient-enriched, fine sediments. This mass of sediments rep resents the equivalent of 4.7 years of annual sediment production across th e catchment and in some tributaries, more than 20 years of annual yield is stored within in-stream wetlands. Previous work on the lute Quaternary stratigraphy of the region has shown t hat there were repeated phases of channel incision in the past following wh ich the channels quickly stabilised by natural means and then filled with f ine-grained sediment to the point of channel extinction, creating unchannel led swampy valley floors. The current formation and spread of in-stream wet lands is interpreted to be the onset of the next infill phase but it is not known whether present conditions will allow complete channel filling and r eformation of the pre-existing swampy valley floors. Nevertheless, further spread of in-stream wetlands is likely to increase the sediment trapping ca pacity and further reduce the discharge of sediments and nutrients into the Murrumbidgee River. The in-stream wetlands may provide a significant capac ity to buffer erosion from gullied catchments of considerable size (up to 3 00 km(2)) as an adjunct to current riparian management options. They may al so assist the recovery of sediment-impacted channels downstream. (C) 2001 E lsevier Science B.V. All rights reserved.