JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, F01S02, doi:10.1029/2006JF000622, 2008 Click Here for Full Article The depositional web on the floodplain of the Fly River, Papua New Guinea Geoff Day,1 William E. Dietrich,2 Joel C. Rowland,2 and Andrew Marshall3 Received 1 July 2006; revised 28 June 2007; accepted 31 July 2007; published 20 March 2008. [1] Floodplain deposition on lowland meandering rivers is usually interpreted as either lateral accretion during channel migration or overbank deposition. Previous studies on the Fly River in Papua New Guinea suggest, however, that floodplain channels (consisting of tie channel and tributary channels) play an important role in conveying sediment out across the floodplain. Here we report the results of an intensive field study conducted from 1990 to 1998 that documents the discharge of main stem water from the Fly River onto its floodplain and maps the spatial pattern of sediment deposition on the floodplain (using as a tracer elevated particulate copper introduced into the system by upstream mining). An extensive network of water level recorders demonstrates significant hydraulic heads from the main stem out the floodplain channels. For the monitoring period 1995–1998, net water discharge into the floodplain channels was about 20% of the flow. Another 20% is estimated to spill overbank from the main stem in wet years. Annual floodplain coring from 1990 to 1994 obtained over 800 samples across the 3500 km2 Middle Fly floodplain for use in documenting temporal and spatial patterns of sediment deposition. Early samples record the rapid spread of sediment up to 10 km away from the main stem via floodplain channels. Later, more intensive coring samples documented a well-defined exponential decline in sediment deposition from the nearest channel (which differed little between floodplain and main stem channels). Deposition, averaging about 6–9 mm/a, occurred in a 1 km corridor either side of these channels and effectively ceased beyond that distance. About 40% of the total sediment load was deposited on the floodplain, with half of that being conveyed by the over 900 km of floodplain channels (equal to about 0.09% sediment deposition/km of main stem channel length). Levee topographies along the main stem and floodplain channels are similar but cannot be explained by the observed exponential functions. Channel margin shear flow during extended periods of flooding may give rise to the localized levee deposition. Our study demonstrates that tie and tributary floodplain channels can inject large volumes of sediment-laden main stem waters great distances across the floodplain where they spill overbank, forming a narrow band of deposition, thereby creating a depositional web. Citation: Day, G., W. E. Dietrich, J. C. Rowland, and A. Marshall (2008), The depositional web on the floodplain of the Fly River, Papua New Guinea, J. Geophys. Res., 113, F01S02, doi:10.1029/2006JF000622. 1. Introduction ment discharge to oceans (and addressing biogeochemical cycling issues), and practical matters of river management. [2] A simple question that can be asked about any reach An important distinction must be made between sediment of river bordered by floodplain is: what controls the load diverted to the floodplain that contributes to net proportion of a river’s sediment load that is deposited on deposition or aggradation versus sediment that is deposited its floodplain? Answers to this question are crucial to on the floodplain, but is eventually re-entrained, typically understanding floodplain evolution, interpreting the strati- by lateral bank erosion [e.g., Dunne et al., 1998]. graphic record that floodplains preserve, estimating sedi- [3] The two primary mechanisms of floodplain deposition are lateral shifting (and bar accretion), and overbank depo- 1Rio Tinto, London, UK. sition during flood events [e.g., Wolman and Leopold, 2Department of Earth and Planetary Science, University of California, 1957]. Generally, lateral shifting leaves coarse bed material Berkeley, California, USA. (sand or coarser), while overbank deposits tend to be 3Andrew Marshall and Associates, Sydney, New South Wales, dominated by mud (except for local splay and levee Australia. deposits rich in sand) [Bridge, 2003]. A balance of processes may arise such that no net accumulation occurs [e.g., Wolman Copyright 2008 by the American Geophysical Union. 0148-0227/08/2006JF000622$09.00 and Leopold, 1957]. Bank erosion during lateral shifting F01S02 1of19 F01S02 DAY ET AL.: DEPOSITONAL WEB F01S02 Figure 1. A 1990 satellite image of upper Middle Fly. Flow is from top to bottom, average channel width is about 320 m, and horizontal distance from top to bottom of the scene is about 11 km (north is at top of image). River entering from east is the Binge (see Figure 3). Note the tie channels connecting the main stem to oxbows and blocked valley lakes. may return coarse material to the channel, balancing bar Amazon above Manaus divert and capture on their beds as accretion on the opposite bank, and undermine overbank much as 37% of the total annual load. These channels shift sediments, thus offsetting overbank deposition elsewhere. and rework the floodplain surface, are active through a On aggrading river systems, however, channel bed level may range of discharge, and are not associated with avulsion rise with floodplain deposition [e.g., Bridge, 2003], elevating processes. the near-channel deposition zone relative to more distal [5] Dietrich et al. [1999] describe two distinct types of regions of the floodplain, and thereby creating the potential channels that disperse sediment great distances across the for significant flood currents from the channel across flood- floodplain of the Fly River in Papua New Guinea. Tie plain. This may lead to avulsions and the formation of channels, first described by Blake and Ollier [1971], are transient multiple channels that split and rejoin, forming an small (typically 0.1 the width of the main stem) channels anastomosing pattern [e.g., Makaske, 2001, Bridge, 2003; which form when sediment laden flow from the main stem Slingerland and Smith, 2004]. Slingerland and Smith [2004] enter the still water of oxbow and blocked valley lakes [e.g., argue that deposition associated with avulsion dominates Rowland et al., 2005; Rowland and Dietrich, 2006]. The floodplain formation of aggrading alluvial rivers. These second channel type is the low-gradient tributary channel deposits will tend to be mostly overbank mud [Makaske, from adjacent uplands. Flow reversals, i.e. flow both to the 2001]. The question of what controls the rate of deposition of floodplain from the main stem and from the floodplain to a river’s load on its floodplain raises questions about the the main stem channel, are common and apparently essen- controls on rates of channel bed aggradation as well as on tial to channel maintenance of tie and tributary channels. rates of lateral and vertical accretion. It also points to the role Sediment-laden main stem waters have been observed to of multiple channels in spreading sediment across the travel over 40 km up tributary channels and spread out floodplain. overbank onto the Fly floodplain. Figure 1 shows a satellite [4] Anastomosing and anabranching (in the sense of image of the Fly floodplain where two tributaries connect to Nanson and Knighton [1996]) rivers create multiple path- the main stem, and tie channels join oxbow lakes. Dietrich ways down which sediment is carried and dispersed across et al. [1999] proposed that such channels play an important floodplains, leading to distinct patterns of deposition and role in delivering sediment to the floodplain. potentially large rates of sediment extraction [e.g., Bridge, [6] Multichannel pathways across floodplains, especially 2003; Slingerland and Smith, 2004]. There are other types low-gradient aggrading ones, appear to be common. De- of channels across floodplains that also contribute to sedi- tailed stratigraphic studies of deltaic plains channels reveal ment dispersion and extraction. Crevasse channels are most the complex interweaving of channel and overbank deposits commonly mentioned and are ephemeral; that is, they and a growing number of field studies have documented operate primarily during flood events [e.g., Bridge, 2003]. stratigraphy associated with avulsive evolution of alluvial Dunne et al. [1998] propose that floodplain channels on the floodplains [Slingerland and Smith, 2004]. 2of19 F01S02 DAY ET AL.: DEPOSITONAL WEB F01S02 [7] Here we make a distinction between avulsive and [10] Mining in the headwaters of the Fly (on the Ok Tedi anabranching floodplain systems, which generate primarily River) led to a large influx of mining waste to the river unidirectional flow down diverse channels that divide and beginning in 1985. This caused the sediment load to reconnect, and the tie and tributary system observed on the increase by about 4.5 times on the Middle Fly, leading to Fly River in which sediment-laden main stem flows inject progressive aggradation of the channel bed, increased flood- sediment great distances through nonmigrating floodplain- ing, and accelerated delivery of sediment to the floodplain. crossing channels. These channels typically experience It led to the delivery of copper-rich sediment across the bidirectional flow depending on the timing of main stem floodplain far above natural or background metal concen- hydrographs and rain in the lowlands. This injection process trations (in this initially pristine catchment), which enabled creates a distinct depositional web. sampling to trace the spread and rate of sediment accretion. [8] In this paper we build upon earlier work by Dietrich We will focus on floodplain sedimentation rates up to 1994 et al. [1999] to quantify for the first time the depositional and floodplain hydrology to 1998. At these times, aggrada- web created by tie and tributary channels across a flood- tion due to mine loading was concentrated near the Ok Tedi plain. We do so along the Middle Fly River, which has an junction and no discernable changes in process had occurred extensive floodplain channel system, and because of up- downstream.