Localised control of aquatic weeds in rivers - 2 years on


In 1996, trials were set up, along the River Glyme in southwest England, to assess the long-term effect of localised control of dense growths of emergent plants. The objectives were to part-clear channels in order to re-instate increased water flow and in so doing improve habitat quality for species such as brown trout Salmo trutta, reliant upon higher water velocities and more open river channels.

The initial results of the trials showed that in 1997, one year after spraying, a distinct channel was created through previously densely vegetated areas, by an accurate application of glyphosate herbicide onto the emergent vegetation. In response, the water velocity in the cleared channel increased, resulting in scouring of the silt on the river bed and deepening of the channel, while a corresponding reduction of velocity in those areas where the emergent weeds had been deliberately retained, resulted in sediment deposition. Treated sections were revisited in 1998 to determine whether the channels originally sprayed in 1996 continued to perform adequately and,if not, to decide if further localised spraying would be beneficial. (See also related Cases 259 and 260).

Study site: Two sections of the River Glyme, Oxfordshire, that had been treated in 1996 were revisited in 1998. Each was approximately 100 m x 8 m and less than 0.5 m deep. Both sections had been choked with a dense growth of branched bur-reed Sparganium erectum prior to initial glyphosate treatment in the summer of 1996.

Herbicide application in 1996: A 3 m wide swathe of branched bur-reed Sparganium erectum was sprayed with glyphosate (at the manufacturers recommended dose) using a knapsack sprayer, down roughly the centre of the river. The Sparganium was effectively controlled by this treatment and the channel remained clear of emergent weed throughout 1997. However, by mid-summer 1998, regrowth of plants on either side of the cleared channel had reduced the effective width to 2 m.

Effects of herbicide treatment in 1998: When surveyed in summer 1998 (two years after the initial treatment) the remainder of the open channel was considered to still carry a large proportion of the water discharge. However, it was thought probable that bur-reed would have regrown across much of it by the end of the 1998 growing season that autumn. In addition, the reduced channel width combined with tall bur-reed growth on both sides, made access for angling almost impossible. Because of this and at the request of the local fishing club, on 2 August, a 1 m wide band of bur-reed, either side of the open channel, was sprayed with glyphosate to widen the channel to approximately 4 m. The effect of the herbicide (dead and dying vegetation) was visible by 9 October 1998.

Conclusions: Observations in 1998 showed that a single spray application maintained a channel clear of emergent Sparganium for at least two years. These results suggest that, where emergent Sparganium growth has become a problem, it is possible to use a careful herbicide application to create an open channel and thus promote water-flow and scouring, at both minimal cost and minimal environmental disturbance. The deeper the water becomes due to scouring, the more difficult it is likely to be for emergent weeds to recolonise due to a finite depth tolerance and the inability to establish and root securely to the river bed at increased flow velocities.

Note: If using or referring to this published study, please read and quote the original paper. Please do not quote as a case as this is for previously unpublished work only.

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