Further reading and background information on the Afon Conwy

Rivers: Afon ConwyFSC Rhyd-y-creuau Field CentreThe Afon Conwy, or River Conwy, is 55km long <strong>and</strong> is one of the major drainage systems inNorth Wales. The river begins up on the moorl<strong>and</strong> plateau of Migneint Moor to the south ofRhyd-y-creuau field centre. Its source is Llyn Conwy, a reservoir 450 metres above sea level(GR 782 457)North Wales <strong>and</strong> the Afon ConwyThe river flows North-east from theMigneint Moors before turning sharplyNorth-West after the small village of YsbytyIfan. The Conwy then continues North,cascading down the Conwy falls, flowingpast the settlements of Betws-y-coed <strong>and</strong>Llanrwst <strong>and</strong> eventually reaching the sea asa large estuary near the castled town ofConwy on the North Wales coast (GR 774795).The Afon Conwy has a relatively largedrainage basin of approximately 590 km²which drains some of the highest mountainsin the UK. Given the high rates ofprecipitation in the area, the catchment hasa high channel density with many largetributaries including the Afon Machno, theAfon Lledr <strong>and</strong> the Afon Llugwy.The Afon Conwycathment area

Rivers: Afon ConwyFSC Rhyd-y-creuau Field CentreYsbyty IfanLlyn ConwyAfon ConwyNant-y-brwynOS map extract of the upper courses of the Afon Conwy in the Migneint Moors. The main channel flows South-West to North-EastGeology <strong>and</strong> soils on the Migneint MoorMigneint Moor consists mainly of siltstones <strong>and</strong> mudstones from the Nant Ffranconsubgroup (Upper Ordovician), some of which have been metamorphosed into slate. Thisbedrock is of an acidic, impermeable character, directly influencing catchment dynamics.The cold conditions at this altitude <strong>and</strong> the saturated soils encourage the development ofpeat soils, especially after the large scale deforestation of the area. Peat soil ischaracterised by large accumulations of partially decayed plant matter due to a lack ofoxygen in the soil, delaying decomposition at that location. A report by the InternationalUnion for the Conservation of Nature (IUCN) has estimated that the UK's peatl<strong>and</strong>s <strong>and</strong>peatbogs lock in about 3 billion tonnes of CO₂, <strong>and</strong> are a far more significant carbon storethan the UK’s forests.The impermeable bedrock of the Migneints causes an increased rate of soil saturation insince water cannot percolate further down The saturated peat soils mean that infiltrationrates are very low, increasing the amount of overl<strong>and</strong> flow <strong>and</strong> reducing the lag time for theriver. The geology <strong>and</strong> soil combined with high rainfall <strong>and</strong> a lack of trees means that riverlevels of the Conwy in this upper catchment area can change very quickly after rainfallevents.

Rivers: Afon ConwyFSC Rhyd-y-creuau Field CentreA stratified sampling strategy is commonly used on the Afon Conwy to select sites forinvestigations. This sampling strategy uses Strahler’s stream order classification to choosesuccessive sites downstream.Strahler's (1957) stream order system is asimple method modified from Horton’sclassification system (1945). Streamsections are classified based on thenumber of tributaries upstream.Strahler’s stream order system 1957A stream with no tributaries is considereda first order stream. When two streamorders of n meet they are thencategorised as stream order n+1.The Nant y brwynThe first data normally collected by studentsis at approximately 400m above sea-level onthe small tributary called Nant y Brwyn withstream orders 1 & 2 (GR 792 453). This seriesof small tributaries lie at the bottom of asmall v-shaped notch <strong>and</strong> displays somecharacteristics typical of an upl<strong>and</strong> streamincluding the v-shape of the valley, narrowvalley bottom with no developedfloodplain, interlocking spurs, turbulentwater with step-pool sequences, steepchannel gradient <strong>and</strong> large, angular bedload.Small first order stream on the Nant y brwynFirst order channel on the Nant y brwynThese sites have good opportunities toobserve the bedrock geology with clearexposures left behind after the down cuttingof the river. In addition to the slate, whiteQuartz, an abundant mineral commonlyfound in Wales, is easily identifiable. TheQuartz mineral constituents (mainly Silica<strong>and</strong> Oxygen) were carried into the rockweaknesses in a heated liquid solution wherethey crystallised to form veins along joints orbedding planes.Second order channel on the Nant y brwyn

Rivers: Afon ConwyFSC Rhyd-y-creuau Field CentreNant-y-brwyn.Stream orders 1 & 2Upper Conwy. Stream order 3OS map extract of the Migneint Moor with the Nant y brwyn <strong>and</strong> Afon ConwyThe next site for datacollection is usually located atapproximately 380 metresabove sea-level justdownstream of theconfluence between theNant-y-brwyn <strong>and</strong> the AfonConwy with a stream order of3 (GR 793 447).This site is unusual for theupper course of a river as itdisplays characteristics youwould more likely expect in arivers middle to lower course,such as a small but developedfloodplain <strong>and</strong> relativelylarge, regular <strong>and</strong> sinuousme<strong>and</strong>ers with river cliffs <strong>and</strong>graded slip-off slopes.This can be in part explained by theareas relatively recent past when largeaccumulations of glacial ice built up toform an ice cap centred over theMigneint Moors.This ice cap spilled outwards in alldirections filling a number of glacialtroughs in the area. Previously,scientists believed this ice cap was largeenough to overtop the mountains to theNorth but further study has refuted thistheory. Enough ice accumulated in themountain cwms to account for the icefeatures in these areas.Confluence between the Nant y brwyn <strong>and</strong> the Afon ConwyHowever given the significant accumulation of ice, relatively uniform siltstone geology <strong>and</strong>the lack of resistant peaks in the Migneints it is likely that the area was completely coveredduring the last glacial maximum giving the flattened, rolling appearance seen today withassociated river features.

Rivers: Afon ConwyFSC Rhyd-y-creuau Field CentreStudents on Migneint Moor sampling on the upper Afon ConwyGraded deposition on a slip-off slope. Afon ConwyEven though the features ofthe upper Conwy arecuriously located they dopossess great potential forobserving key river l<strong>and</strong>formssuch as well developedme<strong>and</strong>ers.Me<strong>and</strong>er on the Afon Conwy. Note the river cliff <strong>and</strong> slip-off slopeAfter heavy rain <strong>and</strong>deposition, slip-off slopesdisplay excellent gradingshowing how river energy<strong>and</strong> sediment size interact toorganise or ‘sort’ deposits ina fluvial environment.‘Grading’ or ‘sorting’ refers to sediment of similar size being deposited in the same locationdue to available energy. <strong>Further</strong> up the slip-off slope nearest the channel bank, watervelocity is low due to friction. Only small calibre sediment can be transported <strong>and</strong>deposited in this low energy environment. Lower down the slip-off slope, towards thechannel centre, friction decreases <strong>and</strong> velocity increases as the water gets deeper. Giventhe greater amount of available energy, larger bedload can be transported <strong>and</strong> deposited inthis location.

Rivers: Afon ConwyFSC Rhyd-y-creuau Field CentreThe Afon Conwy differs from a theoretical longprofile showing two distinct ‘knick points’ along it’scourse, the first is located 2-3km upstream of thevillage Ysbyty Ifan <strong>and</strong> the second is located 3-4kmupstream of Betws-y-coed at Conwy Falls.These knick points are believed to have formed afterlarge glaciers met from connecting valleys in eachlocation, rapidly scouring deeper troughs at thosepoints due to an abrupt increase in ice volume.Consequently, steps were left in the channel profile‘rejuvenating’ the river at these points.Part of the first knick point on the Afon ConwyYsbyty IfanConwy FallsLong profile of the Afon Conwy showing two distinct ‘knickpoints’ at Ysbyty Ifan <strong>and</strong> Conwy Falls. Source: MemoryMapOther factors may well have influencedthe creation of these steps such asdifferences in geology (Conwy Falls cutsthrough a volcanic ash flow tuffsurrounded upstream <strong>and</strong> downstreamby siltstone <strong>and</strong> mudstone) <strong>and</strong> changesin base level caused by absolute sealevel(Eustatic) changes or rebound ofplates after glacial melt (Isostaticrebound).Both knick points are in a state ofrecession, with a series of waterfalls<strong>and</strong> rapids slowly moving upstream.Conwy Falls – second knick point on the Afon Conwy

Rivers: Afon ConwyFSC Rhyd-y-creuau Field CentreYsbyty IfanYsbyty Ifan is the first settlement ofany significance moving from sourceto mouth on the Afon Conwy. By thispoint the river has been joined by anumber of tributaries making it muchlarger.Ysbyty Ifan is normally the third siteat which students sample on theAfon Conwy.The river at this location is of streamorder 4 (GR 842 488).The bridge over the Conwy at the village of Ysbyty IfanUntil 1189, the village was known as Dôl Gynwal. Then, it came to the attention of theKnights of St John, who set up a hospice. The church marks the spot where the old hospicestood.Upstream of Ysbyty Ifan the river channelbecomes significantly steeper for a shortsection before flattening out back up ontoMigneint Moor.This is the Conwy’s first ‘knick point’, likelyto have been caused by the meeting oftwo large glaciers in the Devensian glacialevent, gaining erosive energy <strong>and</strong>subsequently carving a step in thel<strong>and</strong>scape. This step or ‘knick point’ hasbeen receding ever since <strong>and</strong> today youcan see a small gorge as you drive to orfrom the Migneint Moors.National Truest properties are protected by reinforced banksThe renewal of a rivers potential energy <strong>and</strong> vertical erosion is called rejuvenation.Other points of interest at Ysbyty Ifan are the small floodplain, a river bar forming in thelow energy environment behind the bridge <strong>and</strong> the first signs of river defences includingreinforced banks.

Rivers: Afon ConwyFSC Rhyd-y-creuau Field CentreThe Conwy Falls is the largest series of rapids <strong>and</strong>waterfalls on the river course fallingapproximately 40m in height over a distance ofapproximately 800m (GR 808 535)The area is classified as a Site of Special ScientificInterest (SSSI) for its rare flora <strong>and</strong> fauna.Formation of the gorge <strong>and</strong> knickpoint has beensuggested to originate from the large quantities ofice meeting at this point. Valley gl aciers from theLledr <strong>and</strong> Llugwy valley joined the Conwy valleyglacier around Betws-y-Coed causing aggressiveglacial erosion deepening the valley.Hanging valleys were created as a result higherupstream along the Lledr <strong>and</strong> the Conwy.Conwy Falls with a steep-sided gorge.Created as the knickpoint retreated over timeSince the the last glacial event, rejuvenation has been taking place with the rivers trying toregrade themselves. So far, the Conwy has cut back around a kilometre leaving a deepgorge downstream towards Betws-y-Coed. The original valley floor is at the height of theviewing platform <strong>and</strong> the café.Rhyd-y-creuauBy the time the Afon Conwy passesthe Field Studies Centre it has beenjoined by two major tributaries, theAfon Machno <strong>and</strong> the Afon Lledr<strong>and</strong> will soon be joined by the AfonLlugwy, it’s third major tributary. Allthree of these channels join theConwy within approximately 4km ofeach other. The Conwy only has44m to descend between Rhyd-y-Creuauthe sea; a distance of approximately 30 kilometres.The Afon Conwy passing Rhyd-y-creuau<strong>and</strong>River features typical of lower courses are evident here including channel braiding <strong>and</strong>developed pool-riffle sequences.

Rivers: Afon ConwyFSC Rhyd-y-creuau Field CentreLlanrwstThe town of Llanrwst is the largestsettlement so far on the Afon Conwy.At this point the riveris flowing alongthe bottom of the large Conwy Valleyglacial trough where a large floodplainhas developed after the last glacialmaximum.Pont Fawr bridge in LlanrwstThe Conwy has a very ‘flashy’response, meaning it responds veryquickly to rainfall events with a highpeak discharge.Being so close to the river means thesettlement is vulnerable to flooding.Flood defences are very much in forcehere as the river often overflows itsbanks.Since 2004, approximately £7millionhas been spent trying to protectproperties in the area. Defencesinclude an artificial flood culvert, anew system of levees <strong>and</strong> mountableflood gates.Pont Fawr in flood - LlanrwstFlood risk is elevated in Llanrwst as the river istidal until half a kilometre downstream. Thismeans that damming of river water can occur athigh tides.A fine example of an oxbow lake can be seenImmediately downstream from Llanrwst. The lakewas formed when the me<strong>and</strong>er became sosinuous that it was cut off at some point,probably during a flood event.Oxbow lake near Llanrwst

Rivers: Afon ConwyFSC Rhyd-y-creuau Field CentreConwy EstuaryThis site is the mouth of theConwy, 55km from the source.The river here is an estuary withmudflats <strong>and</strong> salt marsh.The estuary is a kilometre widein places. The sediment hasbeen broken into smaller pieces<strong>and</strong> is now predominantly silts<strong>and</strong> fine estuarine muds.The town of Conwy overlooking the Conwy estuaryThe river itself takes its name fromthe castled town it passes on itsfinal stretch before it flows into theIrish sea.Looking upstream from the mouth of the Afon ConwyThe estuary area is an importanthabitat for wading birds with anRSBP reserve established in 1991 inconjunction with the worlds firstimmersed tube tunnel taking themain North Wales A55 directlyunder the estuary.The reserve area would have beenreclaimed <strong>and</strong> grazed had it not been forthe RSPB <strong>and</strong> the local council. Now it isa thriving wildlife haven.Summary:Length – 55 kmSource – Llyn Conwy, 450m aslMouth – Conwy estuaryMajor tribuataries – Afon Machno, AfonLledr, Afon LlugwyThe Conwy estuary seen from the Great Orme