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We are all aware of the rapid increase in weeds on the Ashley-Rakahuri River and the link between the loss of bare gravel areas and the apparent decline in bird numbers. Hence the interest in the recent flood and what difference that might have made. The flood got up to 235cm3 on April 6, which was the best since the one in May, 2014 (480cm3). So it was a decent one.
Looking from the Cones Road bridge as I drive over, it is obvious that there has been some clearance of weeds, but I imagined that generally all it would have done was to ‘rearrange the furniture’. However, a couple of days ago, Geoff and I had a wider look at our core sites, and we were pleasantly surprised that the clearances had actually removed a lot of furniture. What’s more, the cleared areas appeared greatest where weeds had already been removed – either by gravel extractors or by our island works last year. This makes sense, as in those locations there would not have been the vigorous woody weed cover to slow down the flow (tops) and hold the shingle together (roots). Also, the gravel extractors often leave chains of pools alongside the main channels, and floods quickly enter these and sweep down them, creating new flow paths.
Therefore, it may not take as much time and funds to implement our intended clearances this winter. We had four locations in mind. Most of the flood changes have occurred at the Hillcrest site, where about 70% of our islands have disappeared. Off Groyne 1, where there was best breeding success last season, the low cleared island has been well swept and enlarged. At the other two locations, Groyne 2 and Smarts, the weed removal has not been so extensive.
The above is the good news. The not-so-good news is how the flood has highlighted new problems associated with the recent changes in weed species composition. The yellow lupins offer little resistance to a strong current due to their poor rooting habit – besides which they are naturally short-lived pioneers anyway, and in many of the higher areas they were dead or dying (accelerated by drought). But, as expected, the more recently arrived gorse, broom and willow seedlings (all shrubby, osier species, not the tree crack willow) have proven far more resistant. At the moment, few of the gorse and broom are over ankle height (generally due to trimming by hares) and they are not obvious, as the flood has left them covered by a mantle of fine woody debris. This will not hold them back at all, in fact the debris and silt may well promote better growth next season. The willow seedlings are closer to the waterways, where they have been either partially buried or bent over. So right now, although wide spread, they are not visually obvious. However, they too will come back with a vengeance next season. The shallow water/shingle borders were also being smothered in herbaceous weeds such as water cress and monkey-musk. The flood has cleared many of these margins, but some patches just have a covering of new shingle and will regrow.
So what does all this mean for our intended weed clearances this winter? They will certainly go ahead (conditional to consents and funding etc), and there are now better opportunities to do a good job. But we must pay greater attention to getting on top of the more persistent woody weeds in the fairway, such as the gorse and broom, and in particular the shrubby willows.
In summary, the recent flood has reinforced the historical importance of a good flood for weed clearances. However, in my mind this does not remove the urgent need for artificial interception if we are to retain sufficient bare gravel areas to maintain viable bird populations on our local river.
– Nick Ledgard