With a catchment area of some 5000 square kilometres, the River Tweed is one of the largest rivers in the country. The combination of size, relatively low population base and an essentially rural character means that the catchment has remained relatively unaffected by land use changes and other developments. According to current methods of assessing water quality (EU Water Framework Directive), much of the catchment's freshwaters are in "good" or "excellent" condition, however, some are facing significant challenges arising from the impact of human activities.
The Tweed Habitat Transition
The Tweed shows a transition through three distinct habitat types:-
- the fast-flowing, nutrient poor, upper reaches above Peebles
- the more moderately flowing, slightly richer in nutrients, middle section between Peebles and Kelso
- the deep, slow flowing, more nutrient rich stretches, found below the confluence of the Teviot Water and River Tweed at Kelso
The Upper Reaches: Tweedsmuir to Peebles
The upper stretches of the Tweed and its tributaries, drain rocks with a relatively low content of base minerals, mainly Ordovician and Silurian greywackes and shales. The catchment at this stage is characterised by low-intensity extensive land use, mostly sheep grazing, heather moorland and significant coniferous forestry. Only in small areas of land, adjacent to the river, are soils of sufficient depth and fertility to support arable crops. From little more than an upland stream, the Tweed rapidly gains in size but retains the characteristics of an upland stream with predominantly fast current and shallow depth. Consequently, these physical factors are reflected in the types of vegetation the Tweed supports. In the upper reaches, mosses and liverworts are the dominant plant types, both within the channel, growing on rocks, and also the adjacent banksides. Over 90% of the moss species occurring in the river are restricted to this section. Larger plants also occur but are scarce, as most species, with the exception of water crowfoot (Ranunculus species), are confined to areas of slower water downstream. The number of groups of invertebrate species is lower compared with downstream sections of the river, with mayflies and stoneflies among the most abundant.
The Middle Reaches: Peebles to Kelso
The river by this stage has grown in stature. The underlying geology is subject to change, with a transition from the base-poor Ordovician and Silurian shales to richer, less acid, sandstones and volcanic rocks below St Boswells. This shift in geology, along with more intensive pastoral land use, results in comparatively richer water chemistry within the river, although forestry remains a significant land use in the upper part of this section. The river also becomes generally deeper and its rate of flow is reduced considerably from the upper stretches. This is reflected in the river's plant and animal communities; emergent vegetation on the river banks is now more apparent, characteristic species include reed canary grass and bur-reed. Downstream from Peebles, larger aquatic plants such as pondweeds, milfoil and water crowfoot, which are rooted in the river bed, become more abundant. Invertebrate fauna is increasingly diverse with leeches, molluscs, aquatic beetles and shrimps becoming more dominant. Stoneflies and mayflies are still plentiful, whilst blackflies and caseless caddis flies are also relatively abundant.
The Lower Reaches: Kelso to Berwick
Below Kelso, following its confluence with the Teviot, the river becomes much deeper and slower flowing. Consequently, the bottom of the river changes from a predominantly gravel substrate, to one including more silty material, typical of a depositing stretch within a river. The Tweed now lies in the bottom of a broad low-lying intensively farmed river valley. This type of land use increases the already naturally high nutrient status of the river within this section. Larger plants are found in a greater variety but not necessarily in such abundance as upstream. This may be due to the limiting effect of light penetration on plant growth. Typical plant species including those associated with slow-flowing, nutrient rich waters e.g. shining pondweed and spiked water milfoil, although water crowfoot is still plentiful. Invertebrates such as mayfly and stonefly species become less dominant in this stretch of the Tweed with snails, aquatic beetles, flatworms and shrimps becoming more abundant. As the river reaches the sea, certain species of invertebrates characteristic of estuarine and brackish water become apparent.
Some wildlife species, including invertebrates, are not constrained by the overall physical and chemical changes along the river, as it flows from source to mouth, and are widespread throughout the Tweed system. A good example of this is the otter, which enjoys statutory protection and may be found throughout the Tweed system. Despite this, the overall population density for otters, on some sections of the river, is lower than expected, considering the area of suitable habitat present. Nonetheless, the Tweed remains an important site for otters in a UK context. The river is also home to a diverse range of breeding and overwintering birds. The Tweed Estuary is of national importance for birds, especially mute swans, whose numbers peak at around 800 individuals, representing about 4% of the British population. The estuary is also important for goldeneye during the winter months. A variety of other ducks and wader species occur in smaller numbers. There is a large gull roost at Yarrow Slake. Other characteristic waterside birds include dipper, grey wagtail, common sandpiper, oystercatcher, black-headed gull and mallard.
The Riparian Zone
The riparian zone is the strip of land that immediately adjoins the river and is influenced by it. It is of considerable importance as one of the major sources of energy for the river's animal species. This energy is in the form of organic debris from plant or animal remains washed or fallen into the stream. The riparian corridor, besides providing this hugely important energy source, is sometimes the only fragment of semi-natural vegetation in a highly modified landscape.
Invasive Non-Native Species
An increasing threat to our naturally occurring plants and animals, invasive non-native species out-compete native species, causing them to decrease in abundance and, potentially, die out completely within a given area. Non-native species are able to out-compete native species because the natural checks and balances (for example predation) which native species are subject to, do not affect non-natives. Current non-native invasive species of concern are giant hogweed, Japanese knotweed, Himalayan balsam, skunk cabbage, monkey flower, rhododendron species, Australian swamp stonecrop, curly waterweed, Canadian pondweed, Nuttall's pondweed, bullhead, American signal crayfish and American mink. See the Tweed Biosecurity Plan for more information.