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Bank erosion has been identified as a major contributor of silt to this river system, impacting on the aquatic flora and fauna.
Rivers and streams follow the lie of the land, their direction dictated by the local topography. In the uplands they are swift and shallow, while in the lowlands they are slower and more powerful. A typical river channel is sinuous, with erosion on the outer bank and deposition on the inner bank. However, the likelihood of erosion increases where soil is bare or poached and is exposed to heavy rainfall or floods. Erosion in the Allow catchment has been greatly enhanced due to tall and steep river banks, not suitable for vegetation growth.
River bank erosion can lead to a number of problems through the physical and chemical alteration of the river channel. This in turn can have a negative effect on its biological composition, that is not exclusive to the species that inhabit the streams and rivers, but also on the species associated with the riparian zone that are dependant on an ecologically functioning freshwater ecosystem.
The intensification of agriculture in Ireland has resulted in increased pressure on watercourses. Unrestricted access of livestock to the river has resulted in poor water quality and destabilised river banks. Compacted and poached soils in unfenced areas lead to greater run-off and potential for loose material to enter the water body where it can be detrimental to all fish and other habitats.
Unrestricted access to watercourses by livestock also results in inputs of plant nutrients via dung. Increased nutrient levels in a river can result in increased growth of filamentous algae, which can result in trapping of a layer of silt on the surface of the riverbed affecting oxygen levels in the gravels, with severe consequences for pearl mussel, salmon and other river life.
Himalayan balsam is an attractive, non-native invasive terrestrial plant species. Since it was introduced, it has spread to most parts of Ireland. The species is particularly frequent along the banks of watercourses, where it often forms continuous stands. This tall, fast-growing, invader grows in dense clumps that prevents shorter native plants from getting enough light to grow underneath it. As a result Himalayan balsam can take over large areas. Each plant produces about 2,500 seeds which fall to the ground, and with several parent plants close together, seeds can occur at a density of between 5000-6000 seeds per square metre. The seeds float, making watercourses a prime route for dispersal of the species. Seeds can also begin to germinate in water on their way to new sites.
As well as causing problems for native species, Himalayan balsam also increases the risk of riverbanks washing away because it stops the more long-lived plants such as grasses, which bind the soil with their roots, from growing. Himalayan balsam is one of the highest risk non-native invasive species in Ireland. This is largely due to its impact on native waterside vegetation within designated sites. Since the species is rapidly expanding its range, a major concern is Himalayan balsam will dominate waterside vegetation and damp ground, at the expense of native species across Ireland.
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