Upland streams
Mike Dilger heads for the hills to enjoy dippers, mayfly and brown trout
Upland watercourses are among Britain’s wildest and wettest places. They are very different beasts from the slow, sluggish, meandering rivers further downstream. Living in such a dynamic habitat poses unique challenges, and so upland streams support a quite different selection of wildlife to the lowland watercourses into which they will ultimately morph.
Rising in mountainous areas, upland streams are formed from innumerable small rivulets, trickling across ancient rocky terrain, percolating through peaty moorland soils or nourished by groundwater springs. Gravity sees these different water sources coalescing into the streams, rivers, ghylls or becks that are such a common feature across large swathes of upland Britain. The origins of the water in upland streams make them not only relatively cold and acidic, but also comparatively low in nutrients. On the plus side, however, the physics of cold water make it capable of holding far higher quantities of dissolved oxygen, which can turn this tough aquatic environment into an attractive proposition for any animals able to cope in such chilly conditions.
Subject to steep gradients along large sections of their course, upland streams can be at the mercy of huge fluctuations in water flow, which may change even from hour to hour. With levels often rising rapidly following heavy
rains or a swift snowmelt – causing immense volume of water will quickly fall again once the tap has effectively been turned off.
The enormous variations in flow rate and the amount of water tumbling downstream create an environment where mosses and liverworts, rather than taller plants, tend to flourish on protruding boulders and along the banks. The aquatic invertebrates that dominate this habitat also have specialised adaptations to help them cope with
the ever-changing conditions. Many of the nymphs of mayflies and stoneflies, for example, have flattened bodies so they can crawl under rocks and boulders, while caddisfly larvae protect themselves with ‘body armour’ made from gravel and other debris. This not only gives protection from predators, but also anchors them to the stream bed.
Slow and steady
The erosion of hard, upland rocks is a relatively slow process, but the impact of previous ice ages has left a huge resource at the disposal of many upland watercourses. Boulders, pebbles and gravels, as opposed to mud and silt, are thus the defining feature of many upland stream beds. Since moving
water is such a powerful force (one cubic metre weighs a tonne), these beds can often look like a jerky conveyor belt.
Both along and across a stream’s channel, all manner of different microhabitats will additionally be created, such as bars, riffles and cascades. Clean gravel beds, for example, are a vital substate for spawning salmon and brown trout.
All naturalists should take particular care when exploring this remote upland habitat. Slippery boulders, unstable terrain and quickly rising water levels are just a few of the hazards competing to trip you up or give you an unexpected bath. It’s essential that you check the weather ahead of your trip, as well as being sure to stick to well-established paths or official viewpoints.
Rocks in midstream are the most obvious places to scan for dippers, while easily spooked goosanders will frequently be encountered fishing above the well-oxygenated gravel beds. Meanwhile, adult mayflies – as their name suggests – should be most easily observed on the wing this month, whereas stream-side rocks and bank-side vegetation are the best places to look for the weaker-flying stoneflies.
Both along and across a stream’s channel, all manner of different micro-habitats will additionally be created.