Abstract

An outburst flood from Ape Lake in October 1984 severely affected channel and floodplain morphology of Noeick River. This event, estimated to have exceeded 103 m3∙s−1, was as much as 2.4 times greater than the probable maximum meteorological flood. Storage of water in ponds, lakes, and floodplain margins resulted in attenuation of the flood peak as it progressed downstream. As a result, flood effects were most evident in the alluvial reaches of upper Noeick River, below Ape Lake between km 10 and km 23. Here, the channel zone widened from a preflood average of 75 m to almost 200 m, the area of eroded valley bottom increased by about 2.5 times, and a cumulative length of 2.5 km of valley-side colluvial fans was eroded. Transported sediment stored in the floodplain and along valley margins resulted in aggradation of up to 0.75 m, except in restricted reaches where limited sediment supply and increased stream power caused degradation of up to 3 m. A second, and fractionally larger, outburst flood in August 1986 caused mobilization of sediment and some channel infilling but, overall, further morphological changes were minimal. The first flood in this sequence can be characterized as "catastrophic" because it destroyed the normal (climatic) regime of the river. The event effected an abrupt change in the regime of the river, resetting it to be in equilibrium with channel-forming events of order 103 m3∙s−1, such as the second outburst event. There have been no further outburst floods, resulting in a return to disequilibrium conditions. Ultimate recovery, measured in morphological terms as adjustment to accommodate normal meteorological floods or, in sediment transport terms as a balance between boundary shear stress and available sediment sue, will take on the order of decades to a century. Establishment of riparian vegetation will be an important determinant of the extent and timing of channel and floodplain recovery.

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