From Tropical Topics newsletter, No. 68 June, July
2001, produced by Stella Martin from Queensland's Environmental
Protection Agency. Download the PDF to read the whole issue.
The importance of floods
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Floodplains on the lower Magela Creek in Kakadu
National Park, NT.
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Recently The Australian Geographic magazine published a
letter from a reader who suggested damming the large rivers of
northern Queensland and diverting the water into Murray-Darling
system. This, he argued, would serve to increase the flow of the
Murray and also help control flooding in the northern States.
Television news coverage of flooding emphasises only the
ill-effects on humans. It overlooks the fact that the natural
systems of the Australian tropical savannas have evolved with
weather extremes which constitute the climatic norm. Plants and
animals are adapted to cope with, and even flourish, with annual
inundations and droughts.
Each year, at the end of the dry season, floods bring life to
the savanna lands. Rivers break their banks, linking up to create
great sheets of water which spread out across the flood plains.
Water pours into drying swamps and waterholes, reconnecting
billabongs to their parent rivers, filling lagoons and travelling
on to provide water to areas which have had no direct rainfall.
Water tables are replenished, providing a reservoir of underground
water.
The first floods serve to cleanse the waterways. They scour
sediments out of river channels, maintaining the depth of
waterholes and keeping rivers navigable for boats. The rushing
water also flushes organic matter out of billabongs and lagoons
where a build up of nutrients might otherwise provide ideal
conditions for potentially toxic algal blooms. Sediments and
organic matter are carried downstream and deposited over the flood
plain, replenishing nutrient levels and revitalising these areas
for another growing season.
The arrival of water triggers breeding among many animals.
Crustaceans, frogs and insects breed quickly, in turn attracting
predators and fuelling a surge of growth right up the food chain.
Floods not only stimulate spawning in fish of the savanna
waterways, but also provide them with the means to migrate as they
link up the rivers and lagoons. The bigger the flood the further
they go, resulting in better genetic mixing and colonisation of new
areas.
Although sometimes destructive to human endeavours, floods bring
immense benefits. Floodplains, replenished by nutrients and water,
provide feed for cattle, while temporary and permanent floodplain
water allows the pastoral industry to make full use of grazing
lands. Underground water is drawn on by farmers and urban dwellers
throughout the Dry. Good floods, with large numbers of fish
spawning, mean good catches in subsequent years.
Even the marine environment benefits. Estuaries are kept
healthy, nutrients introduced by the floods providing food for many
young marine fish and prawns; breeding for many species is timed
accordingly. There is a direct correlation between big floods and
large catches of banana prawns. Sediments washed to the coast also
go on to nourish our beaches.
We need our floodwaters in northern tropical Australia. They can
cause major disruption at times, but they are part of the natural
system which, without them, would be in trouble.
Drying out
How does the drying out of a wetland lead to an increase in
ducks? It doesn’t seem very logical, but a chain of related
events creates a connection.
As the area dries, it is colonised by grasses and other
terrestrial plants. However, when water returns, these plants
drown. The dead grasses, along with the remains of aquatic plants,
fish and invertebrates which perished during the dry period, form a
rich organic layer. This, in turn, leads to a population explosion
of invertebrates which feed on this nutritious detritus. Notable
among these are rice midge larvae, which have been recorded in
densities as high as 15,000 per square metre. This ready food
source encourages the adult ducks to breed. Then, by the time the
ducklings hatch, the adult midges have emerged to form dancing
clouds on the water surface, just at duckling bill
height — an easy catch for these hungry learners.
The extreme variations in rainfall patterns in Australia mean
that many wetlands dry out on a regular basis. This benefits not
only ducks. Studies have shown that these ephemeral wetlands are,
biologically, among the most productive and most diverse.
A variety of water levels suits a variety of plants. As one type
of vegetation is succeeded by another it leaves behind tubers and
seeds. Many of these can withstand long dry periods, ready to
sprout as water levels rise. On the other hand, others may respond
to increased light, available when water levels drop.
High water levels, which never drop, are detrimental to many
plants. River red gums, which grow close to water and can withstand
flooding, will nonetheless die if high water levels not fall.
Seedlings of paperbarks and other waterside trees need a period of
low water levels to become established while the seeds of cumbungi
(Typha spp), produced in thousands from the bull-rush-like
flower heads, can only germinate in moist mud which has become
exposed. Similarly, the female flowers of ribbonweed
(Vallisneria gigantea) must be able to protrude above the
water surface, when levels are low, for pollination to take
place.
A number of wetland animals also require dry periods as part of
their lifecycle (see Surviving the Dry). Various small freshwater crustaceans
lay eggs which are not only resistant to drought but, in some
cases, actually require a period of desiccation before the eggs
will develop. Some of these eggs are so small they are dispersed by
wind during dry periods. Some long-necked turtles lay eggs below
the water, development only beginning when falling water levels
expose the eggs (see Waterholes).
It is a mistake to think that a wetland must always be wet.
Studies have shown that the cycle of flooding (see Threats to Wetlands) and drying is probably the single
most important factor in maintaining wetland health. If water
levels are held constant (or if a wetland is drained) the natural
diversity and productivity suffer.
