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Saturday, February 28, 2009

Willows in braded streams

New Zealand's South Island has a rapidly rising range of mountains close to it's West Coast which is caused by the Australian plate in the West diving under the Pacific plate. In South Island, a great deal of material is shed from the mountains to be carried toward the east coast by streams and rivers. Wide plains have been formed by this material as braided rivers fill up, jump their banks, form new channels which in turn fill up and so on. The rivers work like a grouter spreading material back and forth across the land between the mountains and the sea.

All across the flood plains these braided rivers exist. They are seen most prominently as you land in Christchurch airport from the north. In many places, willows have been planted in the streams and they have spread upstream and down. There is considerable controversy with regard to the benefit or otherwise of the willows.

Some people feel that they cause flooding, that a flood can wash down trunks to pile up on the upper side of bridges and cause them to fail, that they take up water which could otherwise be used by farmers and that they are an eyesore. Some people seem unhappy that the streams aren't the way they were when they were children as if any change from the childhood situation is by definition bad.

There is a committee to try to enable various uses of the river, sometimes conflicting, without one interfering too badly with another. Some of these uses include:
a) people with farms along the stream who own the steam bed to the centre of the stream,
b) rabbit hunters,
c) gun sighters,
d) people collecting wood for their wood burners,
e) hikers,
f) 4 wheel drivers and
g)fossil hunters.

On a recent trip spread over two Saturdays from the mouth of the river to near the top an interesting fact came to light.

On the trip we had farmers adjacent to the stream talk to us as we reached their farms. Many of them remember fondly around the 50's when the Waipara was an open braided river. Many of them mentioned that they felt that the willows endangered the bridges since washed out trunks could collect against abutments and lead to their failure with the pressure of water. Almost all noted that back around the 50's we had some serious floods but since then there haven't been any bad floods. This was attributed to a lack of serious rainfall events since then. I wonder.

It would be most interesting to collect rainfall figures from local farmers and to collate them for the past century. Some farmers have records going back even more than a century. I have a strong suspicion that we would find rainfall events just as serious over the past 50 years when willows expanded their territory to fill the streams as over the previous 50 when there were few if any willows. If that turns out to be the case, the only obvious explanation is that the willows have reduced flooding. Quite a reasonable hypothesis when you think about it.

Willows roughen the stream in a macro sense. With willows there is more friction for the flowing water. The water has to burble and swirl past tree trunks and as it slows, it spreads out over its flood plain. All this slows the flow and lowers and lengthens the flood peak. Could this be the reason that flood peaks have been much less serious in the past 50 years than in the previous 50 rather than the lack of rain. Only collating rainfall events will clear up this point. While we are at it, lets look at some other likely effects of the willows.

Just like when there are beaver dams in a stream which slow down the flow of the water to the sea, with willows, the water will be held longer on the land. The Canterbury plains are one great large alluvial plane which is ideal for holding ground water. The slower a stream flows, the more time there is for the recharge of this ground water. Beavers do this in aces. It seems reasonable that if the willows are slowing the flow of the water to the sea and spreading it over a larger area that they will have the same effect of directing more water underground. Incidentally, if this is a significant factor it may in part help to explain lower flood peaks. More water is going underground. Underground water, of course is very much slowed in its flow downhill and is protected from evaporation. All good for people using well water.

In New Zealand we still have a nature that can supply those of us who are willing to make the effort with some of our daily bread. One can still get a rabbit for the pot with a modest effort or a deer or pig with a bit more effort. The willow streams provide fire wood. Here there is an interesting dichotomy. The same people who decry the willows seem to resent someone who chops some down for fire wood. Go figure. Envy?? I don't know. Everyone on the trip I mentioned above seemed to be uncertain of the legal situation. Most thought that anyone could cut down whatever they wanted. Others thought that it was only OK to take dead trunks. Whatever the case, it is a great resource for rural New Zealand and willows coppice and grow very fast so a reasonable amount of harvesting could be done with very little reduction in the standing crop. An ideal situation one would think. How about the use of water by the willows. Willows are very thirsty trees which may help to explain why they do so well in stream beds.

In a Nor Wester, water which transpired from willows on the Eastern plain will most likely be swept out to sea and lost to New Zealand and in this situation, willows could use up some water that would otherwise be available for use. Perhaps balancing this is if the Nor western is humidified by the trees, there may be less evaporation from cultivated fields. However, I think we are clutching at straws with this line of argument.

When the wind is blowing from the East, by contrast, rising up over the alps and releasing its water, the situation is different. In this case all the additional water transpired into the westward moving air should be precipitated out to flow down the stream again. It would be interesting to quantify these effects to see if they are significant. How about the effect on fish.

Trout have been introduced to many of our streams and are found in pools all along the streams. Trout go into riffles but tend to live in pools. Anyone who has walked a stream with and without willows will know where the pools are. Clumps of willows hold the gravel together, the current washes around the roots and hollows out pools. A willow filled stream is a series of pools connected by riffles. A stream lacking willows in our area is virtually all riffle. The only place you find pools in a stream without willows is in the higher reaches in the foot hills where the river has hollowed out pools in the rock. On the out wash plain, there is virtually no deep slow flowing water until you get almost down to the tidal zone. A paralell situation exists in streams on both the east and west coasts of North America. Here it is the presence of large logs in the streams which has a similar effect of slowing the water and causing pools to form. It has been observed that streams with "big wood" in the water are far better for both spawning and rearing of salmon.

Willows have another interesting effect. Willow leaves have little red bumps on them. These are willow grubs. Fishermen know to tie lures which mimic these in the willow grub season. It is any ones guess how much food the willow grubs contribute to the trout in the stream. Another interesting subject for research and quantification.

As humans we have a tendency to notice the bad things that happen. We will note when a storm damages our property. We don't take any special note when no damage occurs along with a rainfall event. We notice when a stream floods and takes out a bridge. We don't really notice when there are no floods and the bridge sits there perfectly happily and does what bridges are supposed to do. We almost need a disaster to emphasize what is real. If, for instance, someone could wave a magic wand and eliminate all the willows from one of our river systems, we would then see what the results are. If we had severe flooding and damage to property, we might then appreciate the role of the willows. Baring such a magic wand, the only course we have is to use records of rainfall and flood peaks where they exist and to try to deduct the effect of the willows. This is much less sharp and clear and is always open to interpretation but it is what Humans do well if allowed to. Before we condemn willows, we should embark on a few studies to try to quantify what their role actually is.

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