SEDIMENTOLOGY OF THE UPPER BURDEKIN RIVER OF NORTH QUEENSLAND, AUSTRALIA - AN EXAMPLE OF A TROPICAL, VARIABLE DISCHARGE RIVER

The Burdekin River is an example of a class of tropical streams which experience two to four orders of magnitude variation in discharge, in response to seasonal but erratic monsoonal rainfall. Floods of the Bur dekin rise abruptly, reaching peak discharges of up to 40,000 m(3) s(- 1) in less than 24 h; maintain peak flow for up to a few days, and rec ede exponentially. The geomorphology and deposits of these rivers refl ect the extreme discharge fluctuations, and have not previously been d escribed. A stretch of the upper Burdekin River comprising four bends and one straight reach was examined near the town of Charters Towers. The river bed is largely exposed for most of any year, with a small, m isfit perennial channel carrying low stage flow. Major geomorphic elem ents of bends include point bars with ridge-and-swale topography, thre e distinct types of chute channels, avalanche slipfaces up to 5 m or m ore high around the downstream edges of bars, and on the outer part of one point bar an elevated, vegetated ridge. Straight reaches are flat or gently inclined, sand- and gravel-covered surfaces. Much of the ri ver bed is covered by well sorted, in places gravelly, coarse to very coarse-grained sand with local accumulations of pebble to boulder grav el. Lower parts of the river bed are periodically draped by mud which is desiccated on exposure. Dunes and plane beds are the most commonly occurring bedforms, with local development of gravelly antidunes. Most bank tops and upper, vegetated bars are covered by silt and fine-grai ned sand. The river bed also hosts a low-diversity but locally high-ab undance, flood-tolerant flora dominated by the paperbark tree Melaleuc a argentea, which plays an important role in controlling the distribut ion of sediment. The gross geomorphology of the river bed and most of the sedimentary features are interpreted as having formed during major (bankfull or near bankfull) flows, which have a recurrence of about 1 8 years (based on 65 years hydrographic data). The initial rapid drop in discharge following flood peaks appears to preserve flood peak feat ures on upper bars more or less intact, whereas lower areas are subjec ted to variable degrees of modification during falling stage and by mo re frequent, non-bankfull discharge events.