Individual study: Herbicide treatment to create open channels in areas choked with common club-rush Schoenoplectus lacustris and branched bur-reed Sparganium erectum, the River Thames, Oxfordshire, England
Barrett P. (1999) Localised control of aquatic weeds in rivers - 2 years on, in
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.
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