This is a characteristic edaphic formation found in the bottom of valleys which is subjected to inundation throughout the year. The characteristic feature of this type is the abundance  of  species  of  Myristicaceae  family,  particularly  two  species which are not common under other conditions viz.  Gymnacranthera farguhariana  and Myristica fatua var. magnifica. These species have very dense stilt roots, some of which sprout 6m above the soil. The floor of this swamp is covered by looped knee-roots of Myristica species. The soil in which this association stands is alluvium brought down from the surrounding slopes   and containing a large proportion of humidified sand.  The whole association is evergreen in character and occurs in midst of west coast   tropical evergreen forest and west coast tropical semi-evergreen forests.  The association forms a fairly dense forest with a completely closed canopy. There are no distinct storeys among the trees. The height of the forest is usually about 25 to 30m and the trees have clean and  comparatively slender  boles.  This type of forest  was  first  reported  by  Krishnamoorthy (1960) in the valleys of Shendurney,  Kulathupuzha and  Anchal ranges of Travancore. Champion and Seth  (1968) have classified them into the subgroup ‘tropical fresh water swamp forests’.

From conservation point of view these fresh water swamp forest (4C/FS) described by Champion and Seth (1968) are unique in its biotic composition. These swamps are water logged six months in a year from July to December. Consequently only those plants capable of surviving the anaerobic rhizophore. climate prevail in this habitat. So trees of these fresh water swamps are characterized by the presence of knee-roots. These forests are otherwise referred to as Myristica swamps as they are dominated by the tree species belonging to Myristicaceae family. Relics of Myristica Swamp forest are found in Kattilappara, close to the settlements. The dominant trees are Myristica fatua mangifera var., M.dactyloides, M. malabarica, Gymnacranthera canarica, Syzygium travancoricum, Hydnocarpus pentandra, Lagerstroemia hirsuta, Lophopetalum wightianum, Mastixia arborea ssp. Meziana, Knema attenuata, Glochidion zeylanicum, Calophyllum apetalum, Persea macrantha, Pandanus thwaitesii,  etc.

Forests of Thenmala Forest Division have been subjected to heavy extraction of timber in the past. Forests were also cleared for raising rubber (Rehabilitation Plantations), oil palm (Plantation Corporation) and sugarcane (State Farming Corporation) in the Government sector. Tea plantations have been tried in the past at places near Dharbhakulam. Tea trees have almost become naturalized here. People from the reservoir area of Kallada Dam have settled at Rosemala. There are about 877ha of enclosures inside the sanctuary. Further, there is an area of 450ha of vested forest within the limits of sanctuary (Vignarajan, 1990).