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K.M. Cohen [email protected] Dept

Rhine reoccupation of the IJssel valley by divide dissection north of

K.M. Cohen [email protected] Dept. Physical Geography, Fac. Geosciences, University PObox 80.115 3508 TC Utrecht, ABSTRACT Cohen, K.M., E. Stouthamer, W.Z. Hoek, H.J.A. Berendsen & H.F.J. Kempen (2009) Zand in banen. 3rd fully revised edition. : Provincie . 130 pp. + CD-ROM + Digital Maps. In Dutch, with a summary in English. The Gelderse IJssel is the youngest natural of the Rhine delta. When it came into existence between 300 and 700 AD, it annexed a valley that had not previously been part of the Holocene delta plain before. Understanding the drainage configuration of the valley and the position of the divide, which separated ‘Rhine tributaries’ from ‘drainage going north’ prior to the formation of the IJssel, is key when reconstructing the timing and mechanics of the annexation process. The initiation of the IJssel is of archeological- historical interest, and the features from the initial stage - preserved some meters above deposits from later stages - offer a great opportunity to quantify a major flood of the Rhine. Kampen During 5 years of field campaign covering the full valley length and width, we mapped and dated landscape features related to the IJssel diversion and the area’s many dissected older elements. This allowed us to position the paleodivide between Zutphen and , and to explain how it formed. The outcomes of this research falsify former paleodrainage reconstructions that assume the divide lay further south and that showed the and Oude IJssel draining northward, instead of heading west towards the Rhine. DIVIDE CONTROL ON REOCCUPATION

The channel deposits of the Gelderse IJssel show a two-staged division. The oldest stage (‘initial stage’) saw multiple shallow channels and occupied a relatively wide belt within the valley. The younger stage (‘mature stage’) occupies a relatively narrow channel belt of greater thickness, and showed all the phenomena of a meandering river. The river was embanked in the 14th century, continued to for two more centuries (still ‘mature stage’), and then lost most of its due to discharge redistribution events at the upstream delta apex. Drainage and (straightened map) BEFORE DURING AFTER divide area Early-Holocene inherited drainage situation Avulsion splay formation [350-700 AD] Gelderse IJssel channel belt maturisation GLD | OV Deventer † ~768 AD OOST VELUWE WOLDBERG V V V IJsselmeer 205 210 215 205fortified 866 / 882 AD 210 215 205 210 215 ZUIDOOST V V V V () D D VELUWE V V V Cs V V V V V V V V V V VELUWEZOOM V V V V V V V V V V V V V V V V V V N Dorth V V V V V V V V V V V V V V V V V V V <1311 AD V V V V V V V V V V V V V V V KampenV V V V Arnhem V V V V V V V V V V Dt Hunnepe Dortherbeek V V V V V V V V V V V V V V V V V V Zutphen V V V V V V V V V V V V Olst V V Wijhe V V V V V V Deventer ZwolleV V V V Wilp † Wp IJssel V V V V V V V V V V Doesburg V V V V V V V V V ~965 AD Aa V V V V V V V V V V V V V V V V V V V V V V V V activeFormer c. 40.000-20.000 valley of river yr Berkelago 10 km Oude IJssel Berkel Dortherbeek Ov. Zwarte Water KMC ‘09 ther e after coversand bur GLD | OV Pingo-remnant cluster: G LIEMERS SALLAND IJSSEL DELTA paleo-seepage zone low high Veluwe periglacial fans uncovered covered 7.5 Former drainage, present former Higest coversand ridges ?? predating the IJssel channel of the river IJssel valley rim topography Berkel periglacial fan uncovered covered last active 12.500-11.700 yr ago sourced from periglacial alluvial fans ied 7.5 Annotated geological section along the axis of the IJssel valley seepage zone (FeO) Holocene V V E Lower reaches of combined Valley edge Valley edge Coversand ridges Incised lower reaches of combined † Voorsterbeek < 893 AD Voorsterbeek Oude IJssel + Berkel local rivers Oude IJssel 40-25 ka Rijn Voorst-Gorssel-Wilp Salland +Veluwe-source local rivers kunstmatige bocht- afsnijding 1357 AD 14 slope and fan IJssel delta Pingo-remnant cluster: paleo-seepage zone Zutphen, since ~300 AD Berkel , water mill Berkel fan Veluwe fan complex complex Woldberg since 1000 yr 7.5 fortified 866 / 882 AD Arnhem river Berkel 1225-1250 AD 12 Doesburg seepage zone (FeO) Z 7.5 Holocene Berkel Nederrijn IJssel Zutphen +10 Z W Warnsveld † 10 < 1121 AD Deventer (m) Olst 8 V V V V V V V V V V V V V valley Wijhe Rijndelta V V V V V V V V V V Oude IJssel 460 465 470 Withmundi 6 [since 5 ka] [25-15 ka] Zwolle 460 465 470 794 † 797 AD 460 465 470 [40-25 ka] [40-15 ka] Kampen [13-9 ka] IJssel 4 top IJssel natural ? ka] 60-25 IJs embankment Wychmoith 2 ine [ sel v top sand IJssel pointbars † < 1500 AD Rh alle V nd’ y R v v v v Eerbeek <1096 AD rla hi v v v v ntfe ne V V V IJsselmeer ‘Gelderse o [8 V V V V V V V v v v v NAP d-M 0- Poort’ on 4 V V V V V V V v v v v 794 AD ‘R 0 v v v v v v v v NAP Rhine k V V V V V V V national RD grid (km) Netherlands’ a] [40-15 ka] V V V V V V V V V V [25-15 ka] V V V V V V V V -2 ‘Zutphen Member’ v v v v ‘’ Rhine [130-80 ka] V V V valley periglacial outwash fan Brook valleys Splays and initial multi-channel avulsion belt formation Channel belt Active channelsand moats Ov. -4 V V V V V V Vecht V V V V coversand dune ridge Brook channels, traced Crevasse channels Present river IJssel Silted-up splay Residual channels -6 V V V V [?60-40 ka] Rhine-filled glacial tongue basin: ‘Twello Mb.’ V V V V Deventer coversand relative lows Brook channels projected Early Medieval fortified settlements ‘Zutphen’ and ‘Deventer’ Isolated breach splays Medieval cities, first dikes End of Saalian [145-130 ka], Termination II V V V V [80-?60 ka] 7.5 Minimal elevation(m O.D.) breached by crevasse channels -8 = historic church move 890 880 880 890 900 910 920 930 940 950 960 970 980 990 1000 RWS km langs geul Dortherb. All ages based on oldest (known) historic toponym mentioning; ‘<’ = ‘existed from before’; † = church / chapel founding

0 50 100 km downstream distance Preserved channels of the initial stage are ‘avulsion-splay crevasse channels’. They reworked lows Late Holocene in coversand ridges of the divide, and had splays downstream of these locations. As time progressed 130 80 40 25 15 11.7 ka 11.7 5 2 1 0 ka 0 = 2000 AD Local eolian and fluvial deposition Local deposition and repeated Rhine floods forced themselves to the divide breach, the number of active channels Eolian dunes (Wierden & Delwijnen Members) Level maintained by Early Holocene local Late glacial local drainage trunk systems drainage (top of brook channel deposits) dropped, and the width and incision-depth of the remaining channels increased. This caused the V V V Legend Veluwe periglacial alluvial fans and fan toes (Nieuwkoop Formation) V V V V Gelderse IJssel to mature into a single channel meandering system. In the divide region the mature Berkel periglacial local river deposits Rhine delta deposion Overijsselsche Vecht periglacial local river deposits Boxtel Formation Floodplain and -basin deposits, clay meander belt occupies a slightly incised architectural position: the top of IJssel sand bars is Rhine deposition IJssel embankment level (situation 1995) Overbank deposits ( Member) Level of top of IJssel natural encountered decimeters to a meter lower than equivalent initial stage deposits. To the south and Channel deposits: sands and gravelly sands, with lateral and vertical dissective architecture indicated. Level of top of subrecent pointbar sands the north of the divide, patches of the brook valley deposits occur preserved buried between Echteld Formation Kreftenheye Formation Hippo and Mammoth skull not to scale. along Nederrijn and IJssel Rhine branches crevasse channels. They are dated and used to constrain the begin of avulsion splay formation. GEOLOGY OF DIVIDE FORMATION DATING THE GELDERSE IJSSEL

Since ~40,000 years ago - after the Rhine abandoned its IJssel valley An apparent gap exists in the dated age of the IJssel ‘upstream’ and ‘downstream’ course between 60,000 and 40,000 years ago - local periglacial of the divide area. This is due to the (i) the static presence of the divide until ca. depositional sedimentary systems buried the valley floor. Orphaned 550 AD and (ii) the centuries necessary for the IJssel breach to mature.

divide area Zutphen former Rhine tributaries built out low gradient alluvial fans from both Reaches upstream of the divide were gradually flooded by Rhine waters from 2500 sides of the valley. The fans were fed from catchments draining the Berkel yr ago onward. Rhine-flood slackwater deposits accumulated up the tributary valleys Veluwe ice-pushed ridge complex and the eastern Netherlands uplands, of the rivers Oude IJssel and Berkel, between 2000 and 1500 AD in the Zutphen respectively. vicinity. The reaches downstream of the divide record hydrological changes from 350 AD and 550 AD onwards. Rhine clay deposition began later, around 750 AD Through deposition the fans raised the local surface elevation. Around (recent 14C results; Cohen, in prep.) and expanded around 900 AD (Makaske et 20,000 years ago, the largest two fans grew so large that they met in al. NJG 2008; 14C dating). Historic sources and archeological evidence imply that the axis of the abandoned valley. At this time lobe avulsions of the the IJssel exists through the divide region since at least 600 AD (e.g. Fermin & Berkel fan redirected this river to a position south of the most extensive Groothedde, ZAP, Gem. Zutphen). Veluwe-sourced fan. As a result, the orphaned Berkel tributary once Combined sedimentary and dating evidence constrains the stages as follows: again became a Rhine tributary. Coversand ridges formed on top of 1) The ‘Initial Stage’ occurred between ca. 500 and 900 AD, with 550 AD as a best the abandoned part of the fans, sourced from the ephimeral streams guess age for an initial rare-magnitude flood triggering divide breaching to on the active parts. This caused further raising of the surface at a local commence, based on dendrochronology and paleohydrology from a former level. oak at Zwolle (Sass-Klaassen & Hanraets NJG 2006); 2) The ‘Mature Stage’ occurred between 900 and 1550 AD, based on increased clay deposition in the lower IJssel, increased in the IJssel delta In sequence, these periglacial developments (from ~40,000 to ~11,000 since that time, and historical information on measures mitigating ongoing years ago) created the saddle topography that became the drainage channel migration near the medieval cities of Zutphen, Deventer, and Doesburg. divide. The associated drainage network stayed active until late in the Holocene, until the Gelderse IJssel branch reoccupied the area as part of a deltaic avulsion-by-annexation event (in early medieval times). METHOD The reconstruction is based on systematic lithogenetic mapping, Doesburg coring to 5 meters below surface depth (and occassionally deeper). 10-20 boreholes / km2 were strategically placed based on high resolution digital Oude IJssel elevation data (AHN 1 5x5m; RWS-AGI, 2005). The geomorphological analysis and dating methods included: separation of in-situ Rhine deposits from locally reworked units on architectural and sedimentological grounds; palaeohydrological Arnhem indications from periglacial and interglacial times; OSL dating of Rhine and Gelderse IJssel local periglacial strata; 14C and pollen-biozone dating of Late Glacial, Early Holocene and later Holocene brook valleys buried below IJssel overbank deposits; separation of mature-stage meandering IJssel channel deposits from initial-stage crevasse deposits on architectural and sedimentological grounds; 14C, pollen-biozone and archeology-based dating of younger brook deposits and IJssel deposits, and application of established coversand stratigraphy and climate-hydrology-discharge- yield morpho-stratigraphical schemes for the time period for the Netherlands.

Thanks to Wim Hoek, Esther Stouthamer, Henk Berendsen, Henny Kempen, Freek Busschers, Michel Groothedde, Nico Willemse, Maarten Kleinhans Annelies van Hoesel, Rens Quak, Marieka Brouwer and many others. Thanks to ~100 BSc and 10 MSc students since 2005.

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