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The first accounts of underground drainage in the Gunung Mulu National Park were given by Wilford in his 1964 publication ‘The Geology of Sarawak and Sabah Caves’.  In 1977-1978, during the RGS expedition to the National Park, aspects of the hydrology were investigated by several members of the research team but it was the series of water tracing experiments carried out during the Mulu’80 expedition which produced a comprehensive description of the catchment of Clearwater Resurgence. During this expedition, Hidden Valley was shown to drain to Nasib Bagus (Good Luck Cave), while leakage from the Melinau Paku valley occurred into both Clearwater resurgence and Cave of the Winds. A hydrological link was also proved between the Melinau Gorge and the Clearwater system.

Several different dye detection methods were used during the ’80 expedition with the aim of concluding the most satisfactory technique for use in exploratory tracing work in tropical karst areas. Concentrated fluorescent dyes were introduced into the water, and were observed in resurgence waters, either visually or with activated charcoal detectors.

Because of the very sudden changes in flow conditions which can occur following heavy tropical rainstorms, large quantities of dye were used and visual detection of the dye at the resurgence was therefore often possible. Flow-through times give an indication of the nature of the intervening passage, typically rapid flow-through is indicative of vadose passage, whilst slow flow-through indicates deep phreas.

Green dye placed in the Hidden Valley river sink in 1980. Photo © Jerry Wooldridge

The results of the 1980 dye tracing from the Hidden Valley to Nasib Bagus resurgence concluded that that the passage is generally vadose. Subsequent exploration upstream from the resurgence proved this to be the case, as the stream was followed for 1000 m before being lost under the boulder floor of Sarawak Chamber.  Beyond the Chamber the conduit is again a vadose streamway for a further 600 m to where it emerges from a sump.  As this point is only 200 m from Hidden Valley, but some 95 m lower than the sink and it is likely much of this distance is also vadose. This test involved placing fluorescein in the stream in Hidden Valley, with the result that on the following day the Melinau Paku was “brilliantly green” and charcoal detectors installed at Nasib Bagus resurgence and in the Melinau Paku were also positive when analysed. In addition, visible observation indicated that water from Cave of the Winds and from Clearwater Cave was also coloured, with the colour more intense at Cave of the Winds

A subsequent trace involved the injection of dye into the Melinau Paku, just downstream of the Nasib Bagus resurgence.  This dye was also detected at both Cave of the Winds and Clearwater resurgence with the combination of test results suggesting that there was no direct Hidden Valley to Clearwater link, but that leakage from the Melinau Paku was responsible for the observed results, with a substantial flow from the Melinau Paku to Clearwater occurring during flood. This predicted connection was confirmed during the Mulu ’84 expedition when an entrance was located in the Paku Valley which led to the streamway in Clearwater 3.

The final major hydrological link was established by a 1980 test confirming that dye injected into a small stream sink in the floor of the Melinau Gorge was subsequently detected at the Clearwater 2 sump and the main resurgence. This trace confirmed that the Clearwater resurgence drains the major portion of G.Api through a well-developed conduit. Furthermore, flow times suggested that much of this would be vadose passage, suggesting that there were excellent prospects for significant caves in the northern part of G.Api.

A succession of expeditions confirmed this conclusion. This process began with the discovery of Blackrock Cave in 1988, in which the main river was relocated. A dye trace was undertaken during the following (’89) expedition when fluorescein dye was placed in a sink in the Melinau Gorge, approximately 1km east of Camp 5. Positive traces were detected in Blackrock Cave at Black Magic, at the Clearwater 2 sump and at the Clearwater Resurgence.

In 2003 Whiterock Cave began to be explored and, in 2009, the northern extremity of the river in Whiterock was surveyed to within meters of Camp 5 and the Melinau. Regrettably, there is, as yet, no surface connection – it’s a long way back to Camp5 for a beer and a bath!

The drainage pattern along the foot of the west slope of G. Api is very complicated, as is a similar complex marginal drainage system along the northern margin of the Deer Cave Massif in the Melinau Paku valley.  Complex marginal drainage systems are also well developed in the northern hills of G. Buda and G. Benarat, and appear to be a characteristic feature of the hydrology of the area.

For further detail, see:

An Assessment of the Methods and Results of Water-tracing Experiments in the Gunung Mulu National Park, Sarawak.  P. L. Smart and H. Friederich in Cave Science, Transactions of the British Cave Research Association, Vol 9 Number 2, June 1982

Map showing confirmed dye traces Gunung Api.