Changes of In-Channel Vegetation Following Two-Stage Channel Construction on a Small Rural Clay River

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Abstract
(1) Aquatic vegetation growing under, or emerging from, dry-weather water level along 5 km of the River Roding, near Abridge, Essex [England, UK] was monitored during 1979-82 to assess floristic changes following two-stage channel construction. (2) Aquatic plant communities remained stable in the absence of river engineering. (3) In-channel vegetation cover, species richness and diversity were maintained when the undisturbed lowermost portion of river was incorporated into a two-stage channel. (4) Vegetation cover increased for at least 2 years after flood berm excavation. Species richness increased in the short-term but subsequently declined as invasive species, notably Sparganium erectum L., proliferated. Saggitaria sagittifolia L. and Sparganium emersum Rehmann rapidly became more abundant in the short-term but Nuphar lutea (L.) Sm. and Schoenoplectus lacustris (L.) Palla responded slowly and declined in relative abundance. (5) Increased light and reduced scour along the channel edge created ideal conditions for the growth of tall emergent vegetation, and S. erectum, Phragmites australis (Cav.) Trin ex Steudel and Typha latifolia L. proliferated along reaches bordered by low-level flood berms, in particular. (6) Two-stage channel construction is ecologically preferable to channelization but appropriate measures to counter excessive vegetation growth must be incorporated if the short-term benefits are to be maintained in the longer term.