We are all familiar with the beach, where the height above sea level reduces to zero. But the same situation occurs on the edge of tidal wetlands, where the effect of the tide is very weak. This is the Hull River in tropical north Queensland, which winds its way through mangroves and wetlands.
Hull River
The Highest Astronomical Tide, or HAT, is the highest predicted tide (excludes storm surge and floods) in the period 1900 to 2100. Tidal heights usually use the Lowest Astronomical Tide as their datum (zero point). In this locality, HAT is 4.18 metres above LAT.
On 23-25 December 2007 there were 3 king tides of 3.78m, 3.84m, 3.78m and no rain. A few hours after the third high, water appeared at the HAT line.
This water doesn't seem to flow from anywhere - it just seeps up from below.
Within 24 hours of the above photos being taken, we had 108 mm of rain and the situation looks like this :
Beyond the HAT line, the lower-lying areas are a tall, closed mangrove forest.
Mangroves often have prop-roots that help support the trees in the wet mud. This is Rhizophora mucronata.
At the margin the dominant species is Hibiscus tiliaceus, which is a tree that is able to fall over and keep growing, prostrate and scrambling over itself and other trees.
The yellow flowers develop a red tinge before falling, and it gets redder with time.
The fern Acrostichum speciosum is common in the understorey as it can handle the water-logged ground. The fertile fronds have spores all over the topmost pinnae, and less of an arching habit.
|
The Mangrove Lily Crinum pedunculatum is also common, but many specimens don't
look very healthy, and I haven't seen any flowering. The same species also grows on
exposed beach-front sand dunes. |
The Alexandra Palm, Archontopheonix alexandrae will spend a long time in the mid-storey before breaking through.
When the wet season (January-April) gets going, the water-table rises and stays high for several months.
These photos and many more are also available through Dave Kimble's Rainforest Photo Catalog
Dave Kimble December 2007
You can support my not-for-profit causes by making a
donation
by secure on-line transaction. Thank you.