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Caithness Field Club Bulletin
THE WILD PLANTS OF THURSO RIVERSIDE
The first mile of the Thurso riverside, stretching from the sea to the southern end of the Salmon Pool, is a fine recreation area for the town, and at the same time has the remnants of a very good northern river valley flora, which could be completely lost if it is not better appreciated. This detailed description will be useful to people who walk in the area, and may help others to realise its value. It is a valley cut into clay till which is some 30 metres deep, so that the river runs at bedrock level and most of the vegetation is on the alluvial valley floor or on the steeply sloped rich calcareous clay banks.
The river arises deep in the Caithness peat-moors which drain to Loch More. At Westerdale the ground becomes fertile clay till which is intensively farmed, so that fertiliser products wash into the water over the rest of the river's length, enriching it with nitrogen, phosphorus, potassium and calcium. This water pervades the alluvial floodplain in times of spate helping to maintain a fertile valley floor. The clay slopes also provide nutriment by way of underground springs which wash mineral salts out of the rich clay.
An important feature is the shelter from winds and salt spray provided by the valley, allowing a shrub and tree community to develop when it might not do so on the open plain.
I have divided the valley into sections bounded by the bridges across the river, and given the bridges obvious names: starting at the sea the "seaward footbridge" is followed by the "main bridge", then the "mill footbridge" and finally the "cemetery footbridge". Upstream of the cemetery footbridge is a low weir at the inlet to the mill stream and upstream from the weir the "salmon pool" extends for 500 metres to the bend in the river which marks the end of the area under consideration.
The English names of plants correspond with those found in popular books and those used in the Field Club booklet "Wild Flowers of Caithness".
The Banks of the Lower Reaches
The River, the Mill Stream and the
The mill stream is slower flowing than the river and has a muddy bottom supporting a thick tangle of vegetation. There is branched and unbranched bur-reed, the pondweeds Potamogeton berchtoldii, P. natans and P. x nitens, with watercress, water forget-me-not and occasional flag iris.
The mill stream provides the water supply for the boating pond and then the small pond by the main bridge. The boating pond is scraped clean quite frequently but the least bur-weed and the pondweed Potamogeton x nitens have nevertheless become established there. The small pond usually has a community of bulbous rush and toad rush of variable extent.
The wettest areas are often flooded, especially where the mill stream spills over its banks. The dominant species are reed canary-grass, meadow-sweet, flag iris, common marsh-bedstraw, and great willowherb forming a tall-herb community. Wet areas without tall herbs have marsh marigold, water-mint, marsh pennywort, water forget-me-not, jointed rush, marsh horsetail, brooklime, bottle sedge, lesser spearwort, bog stitchwort and common valerian.
The drier parts of the floodplain form a herb and grassland community of considerable diversity. There are areas dominated by few species such as the yorkshire fog and tufted hair-grass patches, but mostly mixed groups occur with silverweed, sheep's fescue, common sedge, clover, crested dog's-tail, knapweed, northern marsh-orchid, common mouse-ear, goosegrass, creeping bent, hedge parsley, hogweed, lesser stichwort, cocksfoot, ribwort plantain, bush vetch, tufted vetch, daisy, false oat-grass, creeping thistle, sorrel, yellow rattle, birdsfoot trefoil, greater plantain, pignut, creeping buttercup and milfoil. One special plant of this area is the shady horsetail (Equisetum pratense) on the east bank in its only known locality in the county.
At the south-east corner of the salmon pool there is a fragment of floodplain which is rather different from the rest in having mare's-tail, amphibious bistort, water horsetail, unbranched bur- reed and marsh ragwort as well as the commoner species. It also has a hybrid foxtail (Alopecurus pretensis x geniculatus) which is rare in Britain, and which was first noticed by the riverside more than 100 years ago. It is surviving quite robustly in spite of severe grazing and trampling.
The river banks in the floodplain areas are similar to the wet or dry floodplains (depending on the height of the bank), but there are some extra plants which favour the banks, particularly globeflower, water avens, and the monkey flower (Mimulus). One might wonder about the identity of the monkey flower, for it produces no seed and has to propagate long the river by pieces breaking off and taking root downstream. It seems to be the hybrid Mimulus guttatus x luteus which has been cast out from gardens. The marsh ragwort and the rampant hybrid woundwort Stachys x ambigua (= Stachys palustris x sylvatica) both prefer the riverbank.
The Railside Wood
The Clay Slopes
The drier slopes have more complex vegetation, for they are by nature scrub woodland but restrained by grazing, burning and trampling. On the west bank, just upstream of the cemetery bridge, the first slope lacks a distinct spring, and has a rich coating of vegetation. Shrubs are dominant, especially hazel, eared willow, gorse and creeping willow. Herbs include hawkweed, lady's bedstraw, fragrant orchid, false brome, water avens, milkwort and slender St. John's Wort. The second slope upstream has a very thick canopy of shrubs and trees, mainly aspen, hazel, gorse, sycamore, burnet rose and honeysuckle. There are flushes in the centre and short dryer vegetation on the side slopes. The herb community includes harebell, yellow rattle, clover, ribwort plaintain, birdsfoot trefoil and meadow vetchling. The third slope upstream has some small shrubs in the lower part, while the upper part is thickly covered with cinders from the railway. The cinders are invaded by common toadflax, which is common on railway cinders further south, but rare in the county.
The other large extent of clay slopes is on the east bank and spans the distance from the mill to the south end of the salmon pool. From the mill it extends along the side of the mill stream and this slope is either cultivated or dominated by gorse, so lacks interest. Beyond the mill stream entrance the slopes have a short turf with low shrubs similar to that of the first and second slopes of the west bank, with dominant gorse, yellow rattle, cocksfoot grass, knapweed, lesser stitchwort, harebell, tufted hair-grass, lady's bedstraw, bracken, devil's bit scabious with occasional agrimony and columbine. One part has dense hazel scrub with hawthorn, aspen, burnet rose, false brome and Smith's pepperwort.
Ken Butler's Plants
Nature & Environment