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River adjustment to changes in sediment load: The effects of tin mining on the Ringarooma River, Tasmania, 1875–1984
Author(s) -
Knighton A. D.
Publication year - 1989
Publication title -
earth surface processes and landforms
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.294
H-Index - 127
eISSN - 1096-9837
pISSN - 0197-9337
DOI - 10.1002/esp.3290140408
Subject(s) - aggradation , sediment , hydrology (agriculture) , geology , alluvium , channel (broadcasting) , bed load , river regime , drainage basin , period (music) , environmental science , sediment transport , structural basin , geomorphology , geotechnical engineering , fluvial , geography , cartography , engineering , electrical engineering , physics , acoustics
The mining of alluvial tin in the Ringarooma basin began in 1875, reached a peak in 1900–20, and had virtually ceased by 1982. During that time 40 million m 3 of mining waste were supplied to the main river, quickly replacing the natural bed material and requiring major adjustments to the channel. Based on estimates of sediment supply from more than 50 widely scattered mines and the frequency of flows capable of transporting the introduced load, the river's transport history is reconstructed using a mass‐conservation model. Because of the lengthy time period (110 years) and river distance (75 km) involved, the model cannot predict detailed change but it does reproduce the main pattern of sediment movement in which successive phases of aggradation and degradation progress downstream. Peak storage is predicted in that part of the river where braiding and anastomosis are best developed. Aggradation was most rapid in the upper reaches close to major supply points, becoming slower and later with distance downstream. Channel width increased by up to 300 per cent where the valley floor was broad and braiding became relatively common. Bridges had frequently to be replaced. While bed levels were still rising in lower reaches, degradation began in upper ones, notably after 1950, and by 1984 had progressed downriver over 30 km. Rates of incision reached 0·5 m yr −1 , especially in the early 1970s when record high flows occurred. As a result of degradation the bed material became gravelly through either reexposure of the original bed or lag concentration of coarser fractions. Also a narrower unbraided channel has developed. The river is beginning to heal itself and upper reaches now have reasonably stable beds but at least another 50 years will be required for the river to cleanse its channel of mining debris.