Floodplains

The following section on floodplain vegetation is taken from the preface to the Floodplain Flora: A flora of the coastal floodplains of the Northern territory, Australia (Cowie, Short & Osterkamp Madsen 2000).

“The coastal floodplains of the Northern Territory are the largest of their kind in Australia. They are seasonally flooded areas occurring between, and including, the rivers which enter into Blue Mud Bay in the east and the Moyle River in the west. This is a dynamic system: many places in the late dry season are devoid of water, with little vegetation, while in both the wet and early dry seasons they are full of water and burgeoning with plant and animal life. They are an important haven and breeding ground for numerous bird species, home to crocodiles and other reptiles, and their waters teem with an array of fish. About 300 species of vascular plants, make up the mosaic of vegetation that is so typical of these areas.

We consider the floodplain to be the wetlands area dominated by herbaceous species. Only a few woody shrubs and trees that sometimes encroach on the otherwise herbaceous plains or that occupy the immediate margins of floodplains are included. Examples of these woody plants are Barringtonia acutangula, Livistona benthamii, Melaleuca cajuputi and Pandanus spiralis.

The annually inundated floodplains of the Northern Territory lie within 100km of the coast and north of 15degrees south and cover an area of approximately 10,000 square kilometres. They extend in an arc from the Moyle River near Port Keats in the west to the small floodplains associated with the rivers and creeks running into Blue Mud Bay in eastern Arnhem Land. Most of the larger ones occur in the western half of the Top End. They vary greatly in size, from small systems of only a few hundred hectares such as the Howard River floodplain near Darwin, to almost 1,500 square kilometres (the Reynolds-Daly rivers floodplain system). Although there are extensive floodplains outside this zone, they are not annually inundated and do not develop extensive areas of aquatic vegetation. Scattered, floristically similar areas of small wetlands (isolated billabongs and seasonally flooded swamps) occur inland as far south as Lake Woods (latitude 18 degrees south), but their extent and similarity decrease as rainfall and flooding become less pronounced (Finlayson et al., 1988) . However, most of the flora found on the more southern swamps also grows on the northern floodplains.”

Recent evidence from pollen cores and radiocarbon dating suggest an age of 1,000-6,000 years for the riverine flood plains. In geological terms this is a very young landscape, in strong contrast to the sandstone and adjacent lateritic formations of the sub-coastal lowlands. For further information on geomorphic evolution of Top End wetlands, the following references are useful: Woodroffe et al . 1985, 1986).

As the flooding of the plains occurs during the wetter months, the strings of waterholes become reconnected with the major river channels. This is an exciting time in Aboriginal communities; people run with the water and stand in rows across channels as sheets of water wash across the land. Fish are so abundant they can be caught by hand as they are flushed out of the billabongs. The land drinks the water, the cracked black clay soil softens and allows the turtles, snakes, crocodiles and other creatures that have buried into the earth to roam freely again.

Song written by Diane Lucas during the season of Gudjeuk ( the seasonal name given to one of the wetter seasons by Gundjeihmi-speaking in Kakadu )

There’s rain in the hills

there’s rain on the plains

there’s rain through the trees …

Paperbarks flower across the land

when the rivers flood –

There’s rain in the hills

there’s rain on the plains

there’s rain through the trees…

Blue-grey clouds burst with rain

rain, rain, rain,

spear grass grows oh so tall

colours of green green-grey

There’s rain in the hills

there’s rain on the plains

there’s rain through the trees…

Jabiru stalk across the plains

spearing fish with their beaks

children play in the creeks

splashing through the puddles

There’s rain in the hills

there’s rain on the plains

there’s rain through the trees…