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Big rains hit Kawakawa Island! ... in which we learn about the principles of fluvial geomorphology

We have had about 80 mm rain in the last few days, so we ventured down to the gully with deep anticipation as to how this may have reshaped the stream at Kawakawa Island.
Pippa surveys the high flows at Kawakawa Island

Though far from an expert, I guessed that the thick carpet of tradescantia each side of the stream would be interfering with the natural pattern of bedload movement and depositation that the stream would do in more natural conditions. The weed was clearly trapping sediment but by holding fast to the ground material underneath, was preventing any renewal or movement of the streambed itself.

So what might have happened with that restraint gone? Quite a lot as it turns out.

In the photo below, a stony 'beach' has now replaced the silt-trapping mass of tradescantia that once grew there (see below for a 'before pic'). The pate (Schefflera digitata) at centre whose roots were being eroded away by the stream as it teetered precariously on its edge is now a comfortable distance away from its currents.

The stream course shifted about 2 metres to the left and created an expansive 'beach'

A 'before pic' - what this area looked like before our assault on tradescantia (there is now a huge mountain of tradescantia where I stood to take this shot)

And what about all the kawakawa, titoki, maire seedlings we have planted over the last weeks? They have retained their root-hold, at a safe distance from the stream's new meandering freedoms.

My one worry was the kōura we discovered in our last post - the spot we found them is now part of a beach of gravel, and the stream has shifted a metre or two sideways. Well, we have been reassured by a freshwater scientist friend that they are likely to have scuttled to higher ground during the turbulent peak of the downpour and stream realignment and have now no doubt settled into their new home.

And in other good news, alarming though they may look, the light brown blobs of foam that we were finding in eddies and along the sides of the stream after periods of high rainfall is unlikely to be caused by contaminants but more likely to be caused by the natural decomposition of leaves and other organic substances (we found this useful guidance of these and other naturally occurring substances that can be confused with pollutants).

This foamy stuff is likely the product of decomposing vegetation

A scene at Spurs Bend, upstream of Kawakawa Island

And so, what is fluvial geomorphology? Fluvial geomorphology is the study of how rivers shape the world. It is fundamentally the study of the interaction between sediment, water and vegetation throughout a stream or river catchment. And this interaction is exactly what we have seen, in miniature form, in our little gully stream over the last few days.


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