The Klias Association occurs:
in peatswamp notably in the Klias Peninsula,
in the Krah swamp near Kota Belud,
in the Sugut and Labuk estuaries, and
in the lower reaches and estuaries of the Kinabatangan and Segama.
The Manalunan swamp is the most extensive area and covers about 298 km². It is bounded by the Manalunan River in the east, the Segama River and the Manalunan in the southeast, low hills extending from the Segama to the Merah River in the west, the alluvia of Dewhurst Bay in the northwest and by coastal beaches in the northeast.
The soils are formed on deposits of peat, which overlie alluvium. Depths of up to 12 m of peat have been recorded.
The peat swamps, on which the association is mapped, are all associated with river floodplains with extremely sluggish external drainage. The Manalunan swamp is, in large part, a coastal backswamps where drainage to the coast has been blocked by a series of beaches, but to the southeast, it is continuous with the Segama backswamps where drainage to the river is blocked by river levees.
The Bilit swamps have developed in an area lacking effective drainage to the Kinabatangan and only minor streams cross the levees. The main drainage of these swamps is by the Menungal River (Ed. note: Menanggul?), a sluggish stream which is considered to occupy an old course of the Kinabatangan north of Bilit ridge.