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Under Mahakam watershed system, Pahu Sub Watershed is located at the upstream of it. Generally upstream area is characterized by steep topography, and used as conservation area. In fact that flood frequency at Pahu Sub Watershed increased year by year and long period of each event. The research is aimed to identify the bio-geophysics of Pahu Sub-watershed as an early information to overcome flood problem occurred in this area. It was conducted at Pahu Sub-watershed region, located at West-Kutai District. The study was completed by using a technique of Geographic Information System. The Pahu sub-watershed covers an area of 698.951 hectare, oval in shape; The region are dominated by Slightly slope; Soil type is dominated by organosol and podsolik; Main-stream length is about 162.85 km, the elevation differentiation between upper stream and down-stream is as much as 70 meters, riverbed slope 0.043%, river width is averagely 111.8 meters; The majority of land cover was secondary forest and bush. Index of vegetation land-cover is 0.976; River-flow discharge for the last ten years is in average of 471.94 m³/second. The frequency and duration of flood are affected by the high rainfall, the flat topography and land cover condition.

Keywords: flood, Bio-geophysics, Pahu Sub-watershed, Geographic Information System.