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A revised typology of Australian tropical rivers was applied to the complete channel network (named and major rivers) shown on 1:250 000 topographic maps for three large drainage basins in northern Australia (Daly River, NT; Fitzroy River, WA; Flinders River, Qld). Reach mapping and classification were conducted using the revised typology. The 12 major river types proposed were: (1) bedrock rivers; (2) bedrock-confined and -constrained rivers; (3) low sinuosity (straight) rivers; (4) meandering rivers; (5) wandering rivers; (6) anabranching rivers; (7) chains of ponds; (8) gullies; (9) floodouts; (10) lakes, swamps, billabongs and wetlands; (11) non-channelized valley floors; and (12) estuarine rivers. The 12 major river types were developed based on river reach mapping for more than 264 000 km2 of tropical Australian catchments. At scales larger than 1:250 000, subdivision of each major river type is recommended. In the Daly and Fitzroy catchments, confined and constrained rivers dominate, whereas in the Flinders and Fitzroy catchments, anabranching rivers dominate. The dominant river types need benchmarking with adequate numbers of control reaches so that channel changes induced by human and natural impacts can be measured by reference to the stability of these controls. Wandering rivers, floodouts and non-channelized valley floors were rare for the 1:250 000 channel network in northern Australia but need inclusion in national parks. [ABSTRACT FROM AUTHOR]

ABSTRACT In February/March 2007 extreme rainfall occurred over a four-day period in the 7000 km2 East Alligator River catchment in Arnhem Land, northern Australia. The resultant large flood caused extensive bank erosion, channel widening, stripping of point bars and floodplain, resulting in large amounts of sand transport. This sand was largely deposited in the downstream river channel as a sand slug, and as deep overbank sand splays where the valley abruptly widened immediately downstream of an island anabranching, bedrock-confined reach. Interpretation of a time series of aerial photographs and satellite images from 1950 to 2012 showed that there have been considerable channel changes along the study reach. The aerial photographs show that extensive sedimentation in the same reach as in 2007 also occurred in 1975, 1981 and 1984. Each time, the sand slug was reworked rapidly over succeeding years by subsequent smaller floods, and the channel deepened naturally as sand supply declined due to revegetation of the upstream riparian zone and the reformation and stabilisation of point and lateral bars. Sand slug formation at an intermediate floodout is an episodic process dependent on the supply of large volumes of sand by extensive channel erosion during extreme floods. A conceptual geomorphic model was developed to highlight the differential effectiveness of extreme versus moderate floods. [ABSTRACT FROM AUTHOR]

As a contribution to the estimation of extreme floods and rainfall, palaeofloods in the East Alligator River in tropical Australia were examined to derive estimates of palaeodischarges and their frequency. Nine extreme floods have occurred over 8400 years in a non-stationary series, the youngest five of which are stationary, and the youngest three of which occurred during wet periods produced by variations of the Southern Oscillation Index (SOI) modulated by the Pacific Decadal Oscillation (PDO). The magnitudes of these floods are similar, with one possible exception (in 2007 CE). The extreme floods in the region lie close to, or above, the Australian flood envelope curve showing that floods larger than the expected are possible. Their magnitudes also approximate the Probable Maximum Flood (PMF) but with a much higher frequency than expected. The annual exceedance probability (AEP) of the palaeofloods is 0.3%, which may also be the AEP for the rainfall. These results will be used as input to the design of an artificial landform at the Ranger uranium mine, within the East Alligator catchment, that has to last for 10,000 years. The 2007 CE flood was probably the most extreme flood (and associated rainfall) likely to occur in the vicinity of Ranger under the current climate and will occur in the future along with climate change. Because of the uncertainties associated with projections of future extremes the best design rainfall AEP for the Ranger landform is 0.3%. • Longest palaeoflood record in northern Australia of 8ka in the East Alligator River. • Eight of nine palaeofloods magnitudes above the Australian Flood Envelope Curve. • The large floods occur when La Niñas are frequent. • AEP for peak rainfall-generating floods of 0.3% is the same for PMF and PMP. • Design for 10ka uranium mine tailings can be based on this AEP. [ABSTRACT FROM AUTHOR]