Localised control of aquatic weeds in rivers - 2 years on

Summary

Study 1

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.

 

Study 2

In 1996, a trial was set up along two sections of the River Thames in southern England, to assess the long-term effectiveness of localised herbicide 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, and also to improve the amenity value of the river. (See also related Cases 258 and 260).

Study site: The River Thames in the area selected to be treated with herbicide was approximately 20 m wide and more than 1 m deep. Prior to treatment, this section of the river was filled with a dense stands of common club-rush Schoenoplectus lacustris, mixed with branched bur-reed Sparganium erectum along the shallower riparian zone.

Herbicide application: In the summer of 1996, a 4-m wide club-rush-dominated swathe was carefully sprayed (at manufacturers recommended dose) using a knapsack sprayer along the centre of two sections of the river, the first 100 m and the second 75 m in length. In addition, three access points were created for the benefit of anglers by spraying the dense beds of Sparganium and Schoenoplectus which were growing between the bank and the sprayed channel.

Consequences: Assessments in 1997 showed that the herbicide treatment had eradicated all emergent plants from the sprayed channel and the three access points. By August 1998, there had been very little spread of emergent plant growth from the margins into the cleared channel which was still approximately 4 m wide. Some recolonisation had occurred in the three access points where the velocity and depth of water were lower than in the main channel, although these were still usable by anglers. Healthy beds of water crow-foot Ranunculus spp. had also become established in the open water of the cleared channel. Although accurate depth measurements were not taken, wading across the river revealed that the water in the sprayed channel was noticeably deeper than in the reed beds on either side. It as decided that no additional treatments were required to maintain the clear channel in 1998.

Conclusions: This study showed that glyphosate effectively controlled emergent Sparganium and Schoenoplectus both in the central channel and up to the banksides. This in turn led to accelerated water flow, scouring and deepening of the channel. Observations in 1998 showed that a single spray application was enough to maintain a channel, clear of emergent plants for at least two years.

 

Study 3

In 1996, a trial was set up along a section of the River Evenlode in southwest England, to assess the long-term effectiveness of localised herbicide control of dense growths of emergent water 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 reliant upon higher water velocities and more open river channels, and also enhance amenity value.

Study site: A short section of the River Evenlode, Oxfordshire, approximately 40 m long by 5 m wide was selected for herbicide treatment. This section of the river contained a dense growth of emergent branched bur-reed Sparganium erectum and common club-rush Schoenoplectus lacustris. During low-flow conditions the water in much of this area was less than 10 cm deep and, during the summer of 1997, some of the channel dried out completely.

Herbicide application: in the summer of 1996, the bur-reed and club-rush were carefully sprayed with glyphosate (at the manufacturers recommended concentration) using a knapsack spayer.

Plant recolonisation: By the autumn of 1996, the glyphosate treatment had effectively killed the area of Sparganium and Schoenoplectus. However, low water levels in 1997 allowed initial recolonisation by annual broad-leaved weeds, and by the summer of 1998, Sparganium and Schoenoplectus had also started to reinvade the sprayed area. This rapid recovery of emergent plant growth slowed the velocity of the water and re-stabilised the river bed sediment. It would have been possible to re-spray the area but this was not considered worthwhile because of the very shallow water depth along this section which would have encouraged further rapid plant colonisation of the site, especially if low flow conditions persisted. In addition, this stretch of the river was too shallow for angling and there did not appear to be any other environmental advantage in attempting to keep it clear of emergent vegetation. It was therefore decided not to re-spray the area and thus simply observe how long plants would take to recolonise.


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