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The Belait and Tutong rivers exhibit wide variations in discharge.
Discharge records are available for the Belait and Tutong rivers. The records for the Tutong river are more comprehensive (Watson, 1979). The mean annual discharge of the Tutong is approximately 35 m3/sec, with a standard deviation of 4.5 m3/sec. Over the period 1967-1974 the largest recorded mean daily discharge (5/12/66) is estimated at 147 m3/sec, while the lowest gauged discharged (7/3/73) is 0.5 m3/sec. The few gauged records available for the Belait river (Allersma and Marchand, 1994) are strongly influenced by tidal effects. The highest flow gauged on the Belait (in 1977) is 284 m3/sec. Higher flows are not properly gaugeable as the river banks are overflowing. The mean annual discharge at Badas (63 km from the mouth) probably lies around 110 m3/sec and around 145 m3/sec at the mouth.
The river regimes are generally very variable, with frequent and rapid rises and falls in response to rainfall on the hills of the catchment area. During flood events the rivers are no longer confined to the channel itself and they overflow onto the surrounding alluvial plain. Tributary streams also overflow as they are ponded back from the main channel. Large volumes of water are distributed over the alluvial plains. Together with drainage from resultant groundwater, the flooded alluvial plain act as a buffer that evens out the discharge. Flood events on the Belait river may be as common as twenty per year. Hydrographs from the Tutong river indicate a total flood duration of the order of 45 days per year.
Discharge data for the entire Belait/Baram catchment (21,500 km3) compiled by Staub and Esterle (1994) list an average discharge of 1,590 m3/sec compared to 3,550 m3/sec for the large Rajang delta in Sarawak (catchment area 50,000 km3).
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