Freshwater swamp forests are found on in valleys and along water courses that are subjected to periodic inundation by fresh water. These forests are usually the next forest type to develop behind the mangrove swamps. Over very short distances, changes both in the structure and composition of the forest is evident.
Most of the fresh water swamp forests can be found in the Belait District's small rivers and along the Belait River from Kuala Balai all the way up to the border. It can be found in Tasek Merimbun and Sungai Medit in Tutong District. It is estimated that there are 12,668 hectares of this type of forest in the country.
Locally called 'empran', freshwater swamp forests on levee alluvium generally occur as a narrow strip (100 to 200 meters) along the Belait River with a high concentration in some tributaries, especially in the Labi Hills Forest Reserve. The forest has moderately dense high canopy dipterocarps and other tree species. Typical tree include two 'kawang' species ( Shorea macrophylla and S. semins ) as well as S. bracteolatus. The valuable timber species 'belian' ( Eusideroxylon zwageri ) and 'merbau' ( Intsia palembanica ) also occur very occasionally. Epiphytes and climbers are abundant and there is a rich ground flora of small palms and gingers.
This forest occurs along rivers and streams, frequently as abroad zone behind the heavy alluvium. The vegetation is non-arboreal, comprising a dense mass of sedges (Mapania spp.) with clumps of the fan-leafed palm, 'silad' (Licuala spinosa) on the margins and a dense shrub growth of 'ubah' (Syzygium densiflorum).
This forest is similar to the non-arboreal forest. What differs is that sub-arboreal forests comprise a moderately dense stand of small trees, which are generally of poor form. Many have buttresses, stilt roots or pneumatophores, and include 'pendarahan' (Horsfieldia irya), 'sial menaun' (Pternandra coerulescens), and 'gurah' (Shirakiopsis indica), as well as many species of figs or 'kayu ara' ( Ficus spp. ). Though the ground surface is more or less level, mounding of soil at the base of trees can be observed due to alluvium accumulation.