Rolling stones gather no moss …

Back in early July I mused on how rivers changed over time (see “Where’s the Wear’s weir?”) and reflected on how this shapes our expectations about the plants and animals that we find.  In that post, I compared a view of the River Tees today with the same view as captured by J.R.W Turner at the end of the 18th century.   The photograph above is taken about 40 kilometres further upstream from Egglestone Abbey and shows the River Tees as it tumbles along in a narrow valley between Falcon Clints and Cronkley Scar.   I’ve written about this stretch of river before (see “The intricate ecology of green slime” and “More from Upper Teesdale”) and it is an idyllic stretch.   It all looks, to the uninitiated, very natural, almost untouched by the hand of man.

However, a couple of kilometres beyond this point we turn a corner and are confronted by a high waterfall, Cauldron Snout, formed where the river cascades over the hard Whin Sill.   Scrambling up the blocky dolerite is not difficult so long as you have a head for heights but, on reaching the top, a wall of concrete comes into view.  This is the dam of Cow Green Reservoir, constructed between 1967 and 1971 and highly controversial at the time.  The purpose of the reservoir was to regulate the flow in the River Tees, in particular ensuring that there was sufficient flow in the summer to ensure a steady supply for the industries of Teeside (most of which have, subsequently, closed).  My first visit to Cauldron Snout was in the early 1980s on a Northern Naturalist Union field excursion led by David Bellamy.  As we scrambled down Cauldron Snout, Tom Dunn, an elderly stalwart of the NNU, told me how much more impressive Cauldron Snout had been before the dam was closed.

Now look back at the picture at the top of this post.   The dark patches on the tops of the boulders emerging from the water are growths of the moss Schistidium rivulare, which thrives on the tops of stable boulders that are occasionally submerged.    The old adage “a rolling stone gathers no moss” is, actually, true, leaving me wondering how much less of this moss an walker beside this river in the mid-1960s might have seen.   How many more powerful surges of storm-fuelled water would have there been to overturn the larger boulders on which Schistidium rivulare depends?   Bear in mind, too, that two major tributaries, the Rivers Balder and Lune, also have flow regimes modified by reservoirs and the potential for subtle alteration of the view that Turner saw at Egglestone increases.   I wrote recently about how differences in hydrological regime can affect the types and quantities of algae that are found (see “A tale of two diatoms …”).   I may have stood at exactly the same place where Turner had sat when he drew the scene at Egglestone, but I was looking at a very different river.

The dam of Cow Green Reservoir looming above the top of Cauldron Snout in Upper Teesdale National Nature Reserve, Co. Durham, July 2017.  The picture at the top of this post shows the Tees a couple of kilometres downstream from Cauldron Snout.

Trevor Crisp from the Freshwater Biological Association showed that the consequences of Cow Green Reservoir on the River Tees extend beyond alterations to the flow.  Impounding a huge quantity of water in one of the coolest parts of the country also affects the temperature of the river, due to water’s high specific heat capacity.  This means that there is not just a narrower range of flows, but also a narrower range of temperature recorded.   The difference between coolest and warmest temperatures in the Tees below Cow Green dropped by 1 – 2 °C, which may not seem a lot, but one consequence is to delay the warming of the river water in Spring by about a month, which delays the development of young trout.  However, Crisp and colleagues went on to show that any reduction in growth rate due to lower temperatures was actually offset by other side-effects of the dam (such as a less harsh flow regime) to result in an increase in the total density of fish downstream.   Others have shown significant shifts in the types of invertebrate that he found in the Tees below Cow Green, with a decrease in taxa that are adapted to a harsh hydrological regime, as might be expected.   Maize Beck, a tributary which joins just below Cauldron Snout, and which has a natural flow regime, shows many fewer changes.

One conclusion that we can draw from all this is that healthy ecosystems such as the upper Tees are fairly resilient and can generally adapt to a certain amount of change, as Trevor Crisp’s work on the fish shows us. The big caveat on this is that the upper Tees is relatively unusual in having no natural salmon populations, as the waterfall at High Force presents a natural obstacle to migration.  Had this not been present, then all potential spawning grounds upstream of the reservoir would have been lost.   A second caveat is that there is still a lot that we do not know.   The studies of the river that followed the closure of the dam focussed on lists of the animal and plant species found; a modern ecologist might have put more effort into understanding the consequences for ecological processes, the “verbs” in ecosystems, rather than in the “nouns”.  Who knows how different energy pathways are now, compared to the days before regulation, and what the long-term consequences of such changes might be?  Schistidium rivulare is a good example of the limitations of our knowledge: its presence offers insights into the hydrology of the river, but we know relatively little about the roles that these semi-aquatic mosses play in the river ecosystem.   Knowing that there is much that we do not know should, at least, keep us humble as we struggle to find the balance between preserving natural landscapes and their sustainable use in the future.


Twenty years ago, I would have recognised Schistidium rivulare, if not in the field, then at least after a quick check under the microscope.  Now, however, my moss identification skills are rusty and I had to turn to Pauline Lang to get this moss named.   I mentioned in “The Stresses of Summertime …” how the ecologist’s niche becomes the office not the field.  One danger is that we remain familiar with names (as I am with S. rivulare and other aquatic mosses) but, through lack of practice, lose the craft that connects those names to the living organisms.


Armitage, P.D. (2006).   Long-term faunal changes in a regulated and an unregulated stream – Cow Green thirty years on.  River Research and Applications 22: 957-966.

Crisp, D.T. (1973).  Some physical and chemical effects of the Cow Green (upper Teesdale) impoundment.  Freshwater Biology 7: 109-120.

Crisp, D.T., Mann, R.H.K. & Cubby, P.R. (1983).  Effects of regulation on the River Tees upon fish populations below Cow Green Reservoir.  Journal of Applied Ecology 20: 371-386.

Lang, P.D. & Murphy, K.J. (2012).  Environmental drivers, life strategies and bioindicator capacity of bryophyte communities in high-latitude headwater streams.  Hydrobiologia 612: 1-17.


Where’s the Wear’s weir?

There was minor excitement in Durham – and some consternation amongst the rowing fraternity – when river levels dropped rapidly overnight last week.  The river had been very low for some time but was 20 cm lower on Wednesday morning (28 June) due, we learned, to a failure in a sluice gate on the weir just below Prebends Bridge.   It does not look very dramatic in the picture above (a temporary – but not wholly effective – repair had been effected a couple of days earlier) but, as the hydrograph below shows, it was enough to alter the levels to a point where rowing becomes difficult.  The following two days were wet and miserable and the rain caused levels to increase again (note the steep rise on the evening of 29 June as floodwater washed down from Weardale) before gradually tailing off over the next few days.  My photograph was taken in the afternoon of 2 July when levels were back to normal.

“Normal” is, however, a tricky word to apply to any river, so diverse are the alterations to which they are subject.  For me, the ponded section of the Wear upstream of the weir is all I have known and the view of the cathedral looming over the Fulling Mill and its weir is the quintessential impression of Durham, immortalised in J.M.W. Turner’s paintings.  Without that weir there would be no rowing on the river – an important “ecosystem service” within the city (see “Bring on the dambusters …”) – yet that dip in the hydrograph on Tuesday morning offers us a rare glimpse into what the river would have looked like in summers in the far past.  Rowers would be not be very happy were this to pesist but perhaps canoeists would prefer faster-flowing water?  Maximising the ecosystem services that a river provides often involves a trade-off between competing needs.

River levels in the River Wear at New Elvet (NZ 272    ) from 27 June to 2 July.  The orange line indicates the point at which flooding may occur.   From:

I saw the opposite situation on the River Tees at Egglestone, just downstream from Barnard Castle.   Turner visited this location as part of the same trip that took him to Durham in 1797 and he sketched the view of Egglestone Abbey which he later worked up into a painting and engraving.   In his pictures you can see an old paper mill, what appears to be a weir across the Tees and open ground on the steep land in front of the abbey itself.   The view today is quite different: the mill is still there, albeit in a dilapidated condition and there is thick woodland on the river banks which completely obscures the view of the abbey.   There is also no sign of the weir but the mill race that diverts river water through the mill can still be seen, though water only flows through when the river is high.

I did wonder if this meant that the weir had been completely washed away since the mill had fallen derelict but another possibility is that the weir is artistic license on Turner’s part.  He made his sketch in 1797 but there is no obvious weir in the drawing that has survived.  The painting on which the engraving is based dates from about twenty years later, and it is possible that the weir was added to the composition, based on memories of other localities that he visited on that trip (including Durham).   The presence of a weir also cannot be confirmed from a painting by Thomas Girtin from about the same time but it is possible that he, too, worked up his watercolours some time after his sketching trips and relied on hazy memories.  And, as we know that Girton and Turner were acquainted, Turner may have fed off Girton’s interpretation of the scene, compounding the intital error.

Egglestone Abbey near Barnard Castle.  Engraved by T. Higham after J.M.W. Turner. 1822.   Image released under Creative Commons CC-BY-NC-ND (3.0 Unported)

The other possibility is that we are not looking at the same river as Turner or Girtin.   The river we look at today is downstream of major reservoirs at Cow Green and on two tributaries, the Greta and Balder, none of which were present when they visited.   Cow Green, in particular, was designed with regulation of the water supply in mind, in order to ensure that there was enough for the industries in Teesside.  One consequence is that there is more water in the Tees during the summer now than when Turner and Girtin visited.   Maybe a weir would have been necessary at that time to keep the water level high enough to feed the mill race during the summer?

So here, as in the Wear, “normal” is a difficult word to apply.   First impressions are that the river is now in a more natural state than two hundred years ago because an impediment to natural flow has been removed.  When we look more closely, however, we see that the river we see today is, in fact, a different type of “abnormal” to that which Turner and Girton sketched.   But we also need to remember that Turner and Girton’s interpretations are not entirely trustworthy guides to the past either.  There is much to be said for walking backwards into the future but occasionally this may mean that we trip ourselves up …

The view across the River Tees towards Abbey Mill and Egglestone Abbey from approximately the same place as Turner’s view.  The mill is just visible amongst the trees in the middle of the picture.


David Hill (1996).  Turner and the North.   Yale University Press, New Haven and London.


The intricate ecology of green slime …


The first really warm weather of the year was a perfect excuse to visit Upper Teesdale and remind the world that spring gentians are not the only botanical wonders in this part of the world.   Our route to the gentians follows the course of the Tees up from Langdon Beck, alongside Falcon Clints (pictured above) and finally a scramble up beside Cauldron Snout to Widdybank Fell.  Along the way we saw birds eye primrose and butterworts growing alongside the path and, up on Widdybank Fell the spring gentians were approaching their best, and we saw some blue moor grass (Sesleria caerulea) too.  But my eyes were focussed beyond the terrestrial vegetation, and soon homed in on some green films in the river as we were sitting down to eat our lunch.   I had not come prepared for fieldwork today, so had to lean out precariously from the bank to grab a sample which I then had to store in an empty Twiglets bag for the journey home.


Sampling algae in the River Tees near Falcon Clints, May 2016

I had a hunch that I had found some more Ulothrix zonata (see “Bollihope Burn in close up …”) but could not be sure in the field.  The film of algae on the rock surface was slimy, and, as far as I could tell with the naked eye, unbranched, but there are other algae that share these properties and I could not know for sure until I had a specimen under my microscope.   This means that algal natural history lacks the immediacy that is associated with larger organisms and one of my campaigns this year is to see just how much can be achieved with field microscopes.  Today, however, was supposed to be a day off.  Hence the need to requisition a Twiglets packet.


Ulothrix zonata on the surface of a submerged boulder in the splash pool below Cauldron Snout, Teesdale, May 2016.   In contrast to my usual close-up views of the algal world, this was taken with a 300 mm telephoto lens.

The pictures below reiterate the comments I made in my earlier post about the problems we have appreciating the three-dimensional structure of microscopic algae when removed from their natural habitat: the left hand image was taken underwater with my Olympus TG2 camera in macro mode and shows the filaments supported by the fast-flowing water of the Tees; the right hand view shows them collapsed into a gungy green slime on top of a cobble that I have removed from the edge of the river.   It is equivalent to looking at a mass of wracks and kelp smothering a rocky shore at low tide and trying to imagine these as the vertical fronds of a “kelp forest” at high tide.


Ulothrix zonata in the River Tees, near Falcon Clints, May 2016.  Left: close-up view of filaments in situ, taken underwater with an Olympus TG2 camera; right: a cobble removed from the margins of the river.

Looking down the microscope, there were definite signs that this was late season Ulothrix, even if it was not so overgrown as the population I had seen at Bollihope Burn.   First, the chloroplasts were not in a particularly healthy state – earlier in the year they would encircle most of the cell, whereas now they seem to be shrivelled and fill less than half of the cell length.  Second, there were several cells in “reproductive” mode, forming zoospores. And, third, there was an abundance of epiphytes, particularly on those that had already released their zoospores.   Had I come earlier in the year, I would have expected to see filaments virtually free of attached algae.  Now, they were abundant.   Many of the epiphytes were diatoms, similar to those I described from Bollihope Burn, but there were a few groups of a different alga, which I initially thought was either Characium or  Characiopsis but which is more likely to be a germling of Ulothrix zonata that has grown from one of the zoospores.

There are nuances to the natural history of this nondescript green slime.   Studies in the North American Great Lakes have shown that it thrives in high light conditions, in the shallow littoral (less than a metre deep in the littoral) and grows best when the temperature is less than five degrees.   This gives it a window of opportunity in our northern England streams, thriving in the late winter and spring before tree cover reduces the light available.  It is not the only alga that likes cool temperatures, and the quid pro quo of this habit is that it needs to produce copious mucilage in order to stop epiphytes growing on the cell wall and blocking out the light that it craves.   As days get longer and the water becomes warmer, so the organism shifts to a reproductive mode, producing zoospores and, from those, zygotes, that can hunker down and survive the inhospitable conditions that we call “summer”.

Oh yes, we saw a ringed plover too.  Almost forgot.


Ulothrix zonata from the River Tees, near Falcon Clints, May 2016.  Left: vegetative cells with degrading chloroplasts; right: “dead” cells with epiphytic cells that are probably germlings produced by U. zonata zoospores.   Scale bar: 25 micrometres (=1/40th of a millimetre).


Auer, M.T., Graham, J.M., Graham, L.E. & Kranzfelder, J.A. (1983).   Factors regulating the spatial and temporal distribution of Cladophora and Ulothrix in the Laurentian Great Lakes.  Pp. 135-145.   In: Wetzel, R.G. (editor) Periphyton of Freshwater Ecosystems.   Dr W. Junk, The Hague.

Graham, J.M., Kranzfelder, J.A. & Auer, M.T. (1985).  Light and temperature as factors regulating seasonal growth and distribution of Ulothrix zonata (Ulvophyceae).  Journal of Phycology 21: 228-234.