Vegetation (Floating River Vegetation) - Office of Public Works

6.0 Conclusion and summary
•Arterial drainage works, which involve excavation of the riverbed, are likely to cause a
significant negative impact on floating river vegetation and their habitats. If hydraulic
conditions are suitable floating river vegetation may recolonise the watercourse.
•Arterial drainage maintenance carried out in accordance with the Environmental Drainage
Maintenance (EDM) Programme causes a minor negative impact or a minor positive
impact on habitats supporting floating river vegetation. The mitigation measures proposed
will tend to lead to a minor positive impact. In certain cases excessive floating river
vegetation growth may occur and additional maintenance may be required in these
circumstances.
•Floating river vegetation was present on most Office of Public Works main channels. No
floating river was evident on the River Maigue. It is uncertain whether floating river
vegetation was present on the Owenriff River due to flood waters observed during the
survey.
•Parameters influencing the diversity of floating river vegetation include shade, water
depth and water flow. Other parameters that affect growth are water quality, adjoining
land use, riverbed substrate and habitat encroachment by terrestrial plant species.
•In general terms deep waters were inhabited by Pondweed species. Fast water flow
favoured some water crowfoot species; slow water flow favoured other water crowfoot
species, starworts, and some pondweed species. Shallow water supported aquatic moss
and sections of watercourses with silt deposits were colonised by water parsnip.
•Although no confirmed identification of R.fluitans was recorded during the course of this
survey, the true species identification of Ranunculus spp. could not be undertaken due to
the absence of flowering structures.
53