This is a characteristic edaphic formation found in the bottom of valleys which is subjected to inundation throughout the year. This is a unique forest type found exclusively in the plains and low elevations of the southern most part of the Western Ghats. These swamps are more localized and are seen only in the poorly drained regions with a very long rainy season. It is restricted to the sluggish streams as fringing forest below 300m elevations.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’. These forest types are found in areas like Mankuthu, Onnam mile, Choodal, Pappamkuzhy and Vilakkumaram.
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. The dominant trees are Myristica fatua var. magnifica M.dactyloides, M. malabarica, Gymnacranthera farguhariana, 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.
These swamps, because of their location in low altitude, are under trmendous biotic pressure and their conservation is a challenging task and subjected to heavy degradation in various ways.