Rates of f loodplain accretion in a tropical island river system impacted by cyclones and large floods James P. Terry a, * , Sitaram Garimella b,1 , Ray A. Kostaschuk c,2 a Department of Geography, University of the South Pacific, Suva, Fiji b Department of Physics, University of the South Pacific, Suva, Fiji c Department of Geography, University of Guelph, Guelph, Ontario, Canada N1G 2W1 Received 5 September 2000; received in revised form 10 April 2001; accepted 12 April 2001 Abstract Fluvial processes, especially rates of floodplain accretion, are less well understood in the wet tropics than in other environments. In this study, the caesium-137 ( 137 Cs) method was used to examine the recent historical sedimentation rate on the floodplain of the Wainimala River, in the basin of the Rewa River, the largest fluvial system in Fiji and the tropical South Pacific Islands. 137 Cs activity in the floodplain stratigraphy showed a well-defined profile, with a clear peak at 115 cm depth. Our measured accretion rate of 3.2 cm year À 1 over the last ca. 45 years exceeds rates recorded in humid regions elsewhere. This is explained by the high frequency of tropical cyclones near Fiji (40 since 1970) which can produce extreme rainfalls and large magnitude floods. Since the beginning of hydrological records, large overbank floods have occurred every 2 years on average at the study site. The biggest floods attained peak flows over 7000 m 3 s À 1 , or six times the bankfull discharge. Concentrations of suspended sediments are very high (max. 200 – 500 g l À 1 ), delivered mainly by channel bank erosion. In the future, climatic change in the tropical South Pacific region may be associated with greater tropical cyclone intensities, which will probably increase the size of floods in the Rewa Basin and rates of floodplain sedimentation. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Floodplain sedimentation; Tropical cyclones; Floods; Fiji Islands 1. Introduction 1.1. General The hydrology and geomorphology of rivers on many volcanic islands in the tropical Pacific is not well understood compared to other regions, in part because of the difficulties in investigating remote and isolated island locations. Yet, Pacific island fluvial systems are likely to be highly dynamic owing to the wet climate with frequent and intense tropical storms, the highly weathered nature of the volcanic soils, steep topography, tectonic uplift (on some islands) promoting rapid rates of denudation, and the impact of humans in clearing the natural forest vegetation for timber and agriculture. As part of a wider programme of investigation of the characteristics and behaviour of rivers in the Fiji 0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0169-555X(01)00084-8 * Corresponding author. Fax: +679-301487. E-mail addresses: terry À j@usp.ac.fj (J.P. Terry), garimella À s@usp.ac.fj (S. Garimella), rkostasc@uoguelph.ca (R.A. Kostaschuk). 1 Fax: + 679-302548. 2 Fax: + 1-519-837-2940. www.elsevier.com/locate/geomorph Geomorphology 42 (2002) 171 – 182