Geoff Pickup1,*, Andrew R. Marshall2

1 Consulting Geomorphologist, 1538 Sutton Road, Sutton, New South Wales 2620, Australia

2 Andrew Marshall & Associates Pty Ltd., 43 Warrangarree Drive, Woronora Heights, New South Wales 2233, Australia

* Corresponding author. Tel.: +61 26238 3427;

E-mail address: pickup01@westnet.com.au

Abstract

The Fly River occupies a large humid tropical drainage basin in western Papua New Guinea. In its upper reaches, the river system occupies steep mountain country but further downstream, there is a 500-km floodplain reach that is backed up by the Strickland. This chapter examines the climate, hydrology, and geomorphology of the system. It also describes the changes that have occurred in response to disposal of tailings and waste rock from the Ok Tedi mine. These include extensive deposition, especially in the upper part of the system, and rising water levels that have had a major effect on the floodplain.

1.1 Introduction

The Fly River system drains an area of about 75,000 km2 in western Papua New Guinea (Fig. 1.1). There are three major rivers in the system: the Ok Tedi, which drains the Hindenburg Ranges; the Upper Fly, which drains the southern part of the Victor Emanuel Range; and the Strickland, which drains the Victor Emanuel and Central Ranges. The Upper Fly and the Ok Tedi meet at D’Albertis Junction to form the Middle Fly, which meanders down a 400-km-long floodplain with extensive scroll bar complexes, cutoffs, and blocked valley lakes. The Middle Fly and the Strickland meet at Everill Junction before entering the Fly Delta. The delta covers about 10,000 km2 and extends downstream for another 400 km before entering the ocean in the Gulf of Papua. The Ok Tedi receives waste rock and tailings from Ok Tedi while the Strickland receives sediment load from mining operations at Porgera.

This chapter covers the climate, geomorphology, and hydrology of the Fly. It examines rainfall distribution and basin hydrology. It describes the river system from the mountains to the lower end of the floodplain, including linkages between the river channel and the many off-river water bodies (ORWBs) in tributary valleys and on the floodplain. It concludes with a summary of mine-related impacts on the river system.

1.2 Climate

The Fly basin has a humid tropical climate. Average rainfall varies with elevation (Moi et al., 2001). Falls in excess of 10,000 mm/year occur at the Ok Tedi mine site (1,500–2,000 m), declining to about 8,000 mm/year along the upper and middle Ok Tedi. Further decreases occur downstream with values of 5,250 mm/year being recorded at Kuambit just downstream of D’Albertis Junction, 3,869 mm/year at Manda about two-thirds of the way down the Middle Fly floodplain, and 1,847 mm/year at Obo near Everill Junction. Heavy rainfall occurs throughout the year in the mountains, but there is a slightly less wet period from September to November. Seasonal variations are also stronger over the southern part of the floodplain, where rainfalls are lower than in the mountains.

Based on other stations in lowland Papua (McAlpine et al., 1975), annual evaporation is probably within the range 1,500–2,000 mm, which is less than annual rainfall in many areas.

Rainfall not only varies between seasons. The region experiences both El Niño and La Niña events, including severe drought episodes in 1972, 1983, and 1997. There may also be longer-term variations in rainfall. For example, Moi et al. (2001) report that rainfalls for 1999–2000 exceeded long-term averages across the region, with the largest percentage increases occurring in lowland areas.

The rainfall record for Tabubil, just below the mine site, shows some connection with the Southern Oscillation Index (SOI) (Fig. 1.2) but mainly during severe droughts. There is no obvious relationship at other times, making it difficult to use the SOI as a predictive tool.

1.3 River System Geomorphology

In its upper reaches, the Ok Tedi and its tributaries drain a heavily dissected ridge and ravine landscape. The ridges rise to over 2,000 m in the north, but most of the basin lies between 200 and 800 m. The eastern part of the basin is karst country, including the massive Hindenburg Wall escarpment, and contains large areas of landslide debris and old debris flow deposits. In the west, igneous rocks are exposed on high mountains, but much of the area consists of shales, limestones, and sandstones. Slopes are unstable in spite of a dense rain forest cover, and landslides and debris flows are common.

Mine waste is disposed of into two tributaries of the Ok Tedi: Sulphide Creek (which joins the Ok Gilor, and then the Ok Mabiong in its lower section) and the Ok Mani (Fig. 1.3). These tributaries are referred as the Mine Area Creeks. These systems occupy steep, narrow valleys and, prior to mining, were cut down to bedrock for most of their length. Sediment deposits were restricted to local boulder chokes and short reaches with thin layers of armored cobbles. Sulphide Creek contains a waterfall, as does Harvey Creek, a small tributary of the Ok Mani that now receives waste rock from dumps on the southern side of the mine. These creeks are typical of the supply zone (Table 1.1) and were transporting virtually all delivered load before mining operations started.

The Upper Ok Tedi is a fast-flowing river capable of transporting large quantities of coarse sediment, including boulders. For much of its length, it occupies a bedrock gorge less than 200 m wide at its base, although there are wider sections with a few islands developing in the lower sections. Debris flow deposits flank the upper reaches of the gorge, and bed levels have fluctuated over time in response to massive landslides from the Hindenburg Wall and other areas, including one of 7 km3 about 8,800 BP (e.g., Pickup et al., 1979; Blong, 1991). Prior to mining, this reach of the river was classified as a supply zone (Table 1.1). However, unlike the Mine Area Creeks, the bed had a veneer of coarse sediment in transit, much of which probably came from the 1977 Hindenburg Wall landslide.

Just upstream from Ningerum, the Lower Ok Tedi emerges from its gorge into a gradually widening valley filled with what are probably Pleistocene alluvial deposits. These include weathered and indurated sand and gravel deposits topped by distinctive red soils. Initially, these “red beds” form valley edges and confine the channel but, as the valley widens, form terraces or terrace remnants, and the Ok Tedi develops a currently active floodplain within them. A braided channel develops, and forested islands have formed on some of the more stable braid bars. Before mining increased sediment input to the river, this reach was classified as an armored zone merging into a gravel–sand transition zone (Table 1.1). Finer material, including a large volume of sand, is now being deposited in this area.

Prior to mining, the armored zone ended upstream of the junction with the Ok Mart where braiding ceases, and there was a distinctive gravel front. Beyond this point, there was a gravel–sand transition zone (Table 1.1). Flow in the gravel–sand transition zone was (and still largely is) concentrated in a single channel, and meandering begins to develop. However, although there are several meander cutoffs and some channel traces, the floodplain is confined to a narrow belt, 1–4 km wide within the alluvial piedmont, and shows some traces of minor incision. The outer sections of some meanders also cut into Pleistocene terrace remnants, and lateral movement is restricted by indurated and highly weathered soils developed on former fluvial sediments.

The Middle Fly extends for about 420 km from D’Albertis Junction to Everill Junction, where it meets the Strickland. There are two distinct reaches: a sand zone and a backwater zone (Table 1.1). Prior to mining, the sand zone had a riverbed consisting of well-sorted fine-to-medium sand and extended as far downstream as Manda. This reach has a steeper water surface slope than the backwater zone that extends from Manda downstream to Everill Junction. The pre-mining backwater zone had a bed composed of mainly fine sand but with an increasing proportion of silt clay particles further downstream until less than 20% sand was present. A localized body of medium sand occurred downstream from Bosset and is probably associated with the slightly steeper river long profile there. It may have originated from the catchment...