Vegetation (Floating River Vegetation) - Office of Public Works

The primary riverbed substrate of watercourses favouring water crowfoot (Ranunculus) species
consisted of stone, gravel and sand. Channels with high levels of silt deposition were frequently
colonised by water parsnip or pondweed species in deeper waters. A combination of stone, gravel,
sand and some silt provided a habitat that could support a greater diversity of floating river
vegetation (section 4.3). Limestone bedrock is more favourable to the growth of floating river
vegetation in comparison to acidic environments. Care should be taken not to remove all silt
deposited in watercourses during arterial drainage works especially if floating river vegetation has
been identified upstream, on-site or downstream of works. Silt is a root medium, a source of
minerals and a potential seed bank for aquatic plants.
•Water quality
Factors affecting water quality include total suspended solids, turbidity and nutrient loading leading
to eutrophication. Water status is measured at a range sites by the EPA. A comparison between
water status and the presence of floating river vegetation (section 4.3) indicated that water classified
as slightly polluted can be tolerated by some species of water crowfoot. It has been reported in other
work that water crowfoot is less tolerant of turbid water (Hatton-Ellis & Grieve, 2003). It is
important; therefore, that arterial drainage maintenance work undertaken ensures that a minimal
amount of suspended solids is generated, thereby reducing the turbidity of waters supporting
floating river vegetation during summer months. It is recognised that floods cause naturally high
turbidity levels. A four to six year maintenance cycle should manage the level of turbidity and
impact positively on floating river vegetation plant communities.
•Rate of siltation
High rates of silt deposition are detrimental to the survival of aquatic vegetation. An increase in the
riverbed level reduces the aquatic nature of a watercourse by limiting water dynamics in terms of
flow, level and depth. Changing these factors allows the invasion of terrestrial plant species
resulting in the conversion from an aquatic to a terrestrial habitat through the rapid rate of
sedimentation. Controlling the level of silt build-up in watercourses under the Environmental
Drainage Maintenance (EDM) Programme (section 4.1) will ensure that floating river vegetation
can compete successfully in an appropriate aquatic environment.
•Shade/ sunlight availability
As discussed in chapters 3.5 and 4.4 shade has been found to reduce floating river diversity.
Riparian woodland habitats on the banks of large, wide watercourses do not appear to affect floating
river vegetation. Large hedgerows on smaller watercourses will cause shade and reduce floating
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