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L'Hevsel à Amida-Diyarbakır

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Martine Assénat

Geomorphological Setting and Tectonic Context of the Tigris Valley in the Diyarbakır Region

Sabri Karadoğan et Ahmet Yıldırım

Résumé

Tigris River has had a significant impact on the historical and human geographical structure in the Middle East and southeastern Turkey as well as on the formation and evolution of landforms. The course and derange shows crustal deformation features and late tectonic activities. Young tectonics and other natural conditions have affected the change of the associated river channels. Tectonic movements have accelerated the impact side and deep etching. Tigris River and its arms in the north, due to this process in accordance with orogenic lines (East-West), have complied with the obvious subsequent depression. This has led to the further south -basically epigenetic- buried straits in some places. Karacadağ volcanism is the main phenomenon affecting the river flow around the city. Near the city of Diyarbakır, the Tigris valley reveals unique geomorphological shapes. In particular, the expansion of the city south of the valley is important for the formation of Hevsel gardens.

Texte intégral

Our on-going researches on the geomorphology of the River Tigris valley and associated landforms, as well as on the palaeoenvironments associated with past climates, past land use (e.g. in the Hevsel gardens) and past fluvial dynamics, have received the logistic and financial support of the Diyarbakır Townhall and its UNESCO project, the Dicle University, the French Foreign Affairs Ministry (AMIDA project), and the French CNRS (PaléoMex/ArchéoMed project).

Introduction

1Together with the River Euphrates, the River Tigris is one of the most important rivers of southeastern Turkey. Its total length in Turkey is 1900 km, and its recharge basin of 57,600 km². The stream forming its upper course is named Maden. The initial stream springs from the southern foothills of the Hazarbaba Mountain (2230 m a.s.l.) which forms the southern watershed of the Hazar Lake closed drainage basin. When crossing the southeastern Taurus range through deeply incised gorges, the stream changes its name to Dibni. South of these gorges, the river takes its final name of Tigris. Downstream Diyarbakır city the river turns eastward. Its valley widens to reach a >1 km width. Important tributaries running from the north on its left bank (mainly the rivers Batman, Garzan and Botan) contribute to increasing its water discharge. East of Batman, the river flows into very deep and narrow gorges until it reaches the Cizre plain where it forms the Syrian frontier a few kilometers before crossing the Irak frontier (Yıldırım 2004, Fig. 1).

Figure 1 - Location map of the Upper Tigris basin

Figure 1 - Location map of the Upper Tigris basin

2The regime of the Tigris River is of a pluvio-nival type. All through the Taurus range, the upper course of the river is fed by yearlong discharge produced by high precipitation (snow and rainfall) as well as by spring snow melting. The regime of all rivers flowing in the highlands down to lower areas reflects the topographic contrasts, which influences the precipitation seasonal distribution. Thus, while high discharges occur in winter and spring, decreasing significantly in summer until the first rainfall that occurs in the highlands (Özgen 2007); the Tigris regime becomes increasingly that of an exogenous river when in the Cizre plain and southward in Irak. Also, due to high topographic and climatic contrasts, as well as to the fact that its left bank tributaries downstream Diyarbakır are also widely fed by the Taurus range, the River Tigris carries a heavy alluvium load that varies throughout the year. When water discharge decreases at the end of spring, alluvium are thus abandoned in the bed where islands appear and meander branches are abandoned. The same phenomenon also occurs when in exceptionally dry years.

3Together with these climate and topography-triggered flow and drainage characteristics, the geological structure, and especially its tectonic component, also plays an important role in shaping the geomorphology of the region.

4Of course, starting with the Paleolithic but also especially with the earliest Neolithic of the so-called “Fertile Crescent” until modern times, the course and geomorphological dynamics of the River Tigris played an important role with regard to the distribution and development of the human settlements.

I. Geomorphological Setting and Tectonic Context of the Tigris Valley Network in the Diyarbakır Basin

5The geomorphic evolution of the upper basin of the River Tigris in southeastern Turkey reflects the interplay of surface processes and the development of topographic relief and lithospheric deformation since the closing of the Southern Neo-Tethyan Ocean. The study of the Tigris system is a proxy for understanding the deep crustal processes associated with the active Alpine-Himalayan mountain belt. The drainage morphology is linked to the uplift of the eastern Taurus Mountains and deformation of the Arabian foreland system following continent-continent collision. At the junction of the Arabian Plate, the river may be viewed as an antecedent stream system that developed since the Miocene (Fig. 2). In the reach near Diyarbakır, valley development (and possibly incision) has been influenced by recent effusive volcanism (Kathleen 2010).

Figure 2 - The main tectonic structures and geomorphic features of the plate tectonics model in Anatolia

Figure 2 - The main tectonic structures and geomorphic features of the plate tectonics model in Anatolia

Revised from Keskin 2003

6The basin, which looks monotonous at a glance, is a sedimentation and subsidence basin forming a wide variety of landforms with neo-tectonic, fluvial and implicit karstic processes (Karadoğan 2013).

7With time, both climate changes and tectonic movements influenced the hydrographical and hydrological characteristics of the Tigris River, thus modifying geomorphological features and organization in the drainage basins and valleys in the whole basin.

  • 1 Epigeny occurs when a river is forced by rapid uplift to incise deeply a gorge in a hard substratum (...)

8Prior to the Miocene, a consequent watercourse (i.e. adapted to the N-S geological structural dip) was continuously flowing southwards even to today’s Mardin region. All over its course, it deposited a thick alluvial cover of which many remnants can be seen on the highlands south of today’s River Tigris elbow south of Diyarbakır and Bismil. This palæo-Tigris changed direction and dynamics towards the end of the Pliocene, when continental collisions occurred in relation to the closing of the Neo-Tethys. The collisions generated an orogenesis (i.e. formation of the Taurus range) accompanied by increased tectonic movements (faults and folds, uplifts and block breaking). These movements caused the palæo-Tigris first to stop flowing southwards all the way to the Mardin area, and to concentrate in the trough that it occupies now between Diyarbakır and Batman. Meanwhile, the regional uplift forced the flow to incise the trough, forming an increasingly wide and deep valley, which attracted today’s northern tributaries of this newly constrained river. At the beginning of the Quaternary, these events had led the River Tigris to flow and incise its valley within an E-W orientation, topographically disconnected from its previous N-S oriented course still visible south of today’s Diyarbakır elbow. In these hills and E-W oriented uplifted highlands that separate the River Tigris valley from the Mardin plateau, southward oriented small rivers now flow into epigenic gorges1. Here, the extreme rapidity of the uplift caused the new Quaternary rivers heading southward out of the Tigris basin a) to incise blindly the alluvial pre-Miocene cover (not hard), and in second phase b) to keep thus the memory of the pre-Miocene N-S oriented palæo-Tigris which still controls their direction and location (see the Eğil and Raman gorges). During the Quaternary, the River Tigris continued to flow in a wide valley concentrating the deposition of thick alluvium that formed with time a set of terraces between Batman and the south of Eğil. During the Quaternary, the volcanism that had given birth during the Miocene to the Karacadağ massif went on erupting and producing lava flows which invaded the region east and south of Diyarbakır. At a yet unknown period, these lava flows disputed the valley occupied by the River Tigris and one of its tributary which was still flowing N-S at the base of the Diyarbakır lava flow (Fig. 3). The dispute ended in the obstruction of the Tigris valley south of Diyarbakır where it forms today a sharp elbow. After the obstruction, a lake must have occurred at the place of today’s elbow. This lake would have formed because of the continuing input of the Tigris river water accumulating behind the lava dam. Afterwards although at some time yet unknown, the previous E-W directed Tigris upper course (flowing from today’s Batman area) must have been captured by a river flowing W-E. This connection would have determined today’s organization of the Tigris basin south of Diyarbakır. Between Diyarbakır/Bismil and Batman, the E-W geometry of the confluences between the main Tigris course and its northern tributaries keeps the memory of this direction change.

Figure 3 - Generalized geological and tectonic map of Southeast Regional Turkey

Figure 3 - Generalized geological and tectonic map of Southeast Regional Turkey

Karadoğan and Yıldırım 2010

II. Geomorphological characteristics and effects of the Tigris River around the city of Diyarbakır

9On a large scale, the geomorphological units of the Tigris valley and its surroundings can be grouped into plateaus, alluvial terraces, and flood plains.

10South of Diyarbakır, the River Tigris bed adopts various patterns: linear, meandering, braided, or incision of the bedrock. The latter case means that the bed responses to an active tectonic uplift, as in the part of the valley when the Tigris deeply incises a narrow valley within the Raman-Gercüş anticlinal. The three other cases are geomorphological and geohydrological indicators of local variations in the ratios between (i) water discharge characteristics (e.g., intensity of flood peaks, seasonal to inter-annual variability of discharge, etc.), (ii) amount and type of transported sediment load, and (iii) steepness of the bed slope. Of course, man-made modifications to the beds (e.g. concrete-transformed banks, quarries in the alluvial fill, dykes etc.) as well as dams in the upper course of the main river and its tributaries or water withdrawals from the active bed etc… may also modify the dynamics of the bed geometry.

11At places, some meanders are incised in the bedrock. Such features are also characteristic of a rapid uplift occurring while the meanders had already formed over a flat and low-sloped surface. Because of the rapidity of the uplift, the river keeps its meandering pattern in spite of the increasing energy needed to incise such a pattern in a hard bedrock. The result is the formation of a deep and steep gorge, and the absence of a floodplain at the bottom of the gorge.

Figure 4 - Geomorphological map of the River Tigris valley near Diyarbakır City

Figure 4 - Geomorphological map of the River Tigris valley near Diyarbakır City

12The Tigris river valley surrounding the city of Diyarbakır exhibits the geomorphological features pertaining to floodplains: secondary branch alluvial fans, concave bends and deposits, crevasse deposits, point bar deposits, sand islands, back swamp deposits and levees (Fig. 4). The lateral profile of the Tigris River valley at Diyarbakır around the city is asymmetrical; with a right bank widely expanding through alluvial terraces stepped at the foot of a plateau fixed by a hard lava flow. The lateral displacement of the river southwards on the right bank, allowed the development and preservation of the Hevsel Gardens where fruits and vegetables are traditionally produced. These gardens are watered (and irrigated through water control) by abundant springs outflowing from the underground network circulating below the basalt lava flow covering Pliocene coarse alluvium incising Miocene lake clay. In contrast, the left bank is regressing through normal erosion by the river meander concavity, and by the back-flowing mass movements in the clayey slope (lacustrine and terrestrial Pliocene substratum). This local widening of the valley as well as the lateral displacement of the bed southwards may indicate a subsidence in the substratum below the meander. The circular shape of the widening may point to a possible karstic origin –collapse) to the ground movement.

Conclusion

13The River Tigris which collects the waters of the Diyarbakır area, plays a fundamental role in the environmental dynamics of several ecosystems and landscapes, as well as in the environmental context of human life in the region.

14The drainage system of the river reflects the development of tectonics, erosion and sedimentation conditions of the basin. North of Diyarbakır, the river basin drainage system is adapted to the tectonic structure and orogenic lines (Taurus folds and blocks). Near the city of Diyarbakır, the river valley widens and a large meander develops around a series of alluvial terraces over which developed the Hevsel Gardens. These gardens, in turn, played an important role in fulfilling the needs of the Diyarbakır city population in fruits and vegetables throughout history. While the valley at Diyarbakır exhibits specific fluvial and floodplain morphology, there is also the possible impact of underground gypsum karst processes. These matters and other features yet unexplained or yet not clearly evidenced (e.g. a major change in the Tigris flow direction in relation to lava damming at the eastern foot of the Karacadağ volcano) deserve more scientific research in the very near future.

Bibliographie

Karadoğan, S., Yıldırım, A., (2010). Güneydoğu Anadolu’nun Jeo-Tektoniği ve Depremselliği, Çantay Yayınevi, İstanbul.

Karadoğan S., (2013). “Doğal Peyzaj Unsurları ve Yerleşmeye Etkileri Açısından Diyarbakır Kenti ve Yakın Çevresi [Diyarbakır City and Its Immediate Surrounding in Terms of Natural Landscape Elements And Their Effects On Settlement]” ICOMOS ICOFORT 2013: ‘Diyarbakır Kalesi Ve Tarihi Şehri’ toplantısı, 11-12 Nisan 2013, Diyarbakır. URL: http://www.dicle.edu.tr/a/skaradogan/diyarbakir/arshive/abstract_SKARADOGAN.pdf.

Kathleen, N., (2010). “Landscape development within a young collision zone: implications for post-Tethyan evolution of the Upper Tigris River system in southeastern Turkey”, International Geology Review, 52(4), p. 404-422. URL: http://dx.doi.org/10.1080/00206810902951072.

Keskin, M., (2003). “Magma generation by slab steepening and breakoff beneath a subduction-accretion complex: An alternative model for collision-related volcanism in Eastern Anatolia, Turkey”, Geophysical Research Letters, 30(24), 8046. URL: http://dx.doi.org/10.1029/2003GL018019.

Özgen, N., (2007). “Bismil İlçesinin Coğrafyası”, Ankara Üniversitesi Sosyal Bilimler Enstitüsü, unpublished doctoral thesis, Ankara.

Yıldırım, A., (2004). “Hasankeyf – Gercüş ve Yakın Çevresinin Fiziki Coğrafyası”, Ankara Üniversitesi Sosyal Bilimler Enstitüsü, unpublished doctoral thesis, Ankara.

Notes

1 Epigeny occurs when a river is forced by rapid uplift to incise deeply a gorge in a hard substratum instead of turning around it.

Table des illustrations

Titre Figure 1 - Location map of the Upper Tigris basin
URL http://books.openedition.org/ifeagd/docannexe/image/1233/img-1.jpg
Fichier image/jpeg, 260k
Titre Figure 2 - The main tectonic structures and geomorphic features of the plate tectonics model in Anatolia
Crédits Revised from Keskin 2003
URL http://books.openedition.org/ifeagd/docannexe/image/1233/img-2.jpg
Fichier image/jpeg, 168k
Titre Figure 3 - Generalized geological and tectonic map of Southeast Regional Turkey
Crédits Karadoğan and Yıldırım 2010
URL http://books.openedition.org/ifeagd/docannexe/image/1233/img-3.png
Fichier image/png, 540k
Titre Figure 4 - Geomorphological map of the River Tigris valley near Diyarbakır City
URL http://books.openedition.org/ifeagd/docannexe/image/1233/img-4.png
Fichier image/png, 3,5M

Auteurs

Dicle University, Ziya Gökalp Faculty of Education, Department of Geography, Diyarbakır
skaradogan@gmail.com

Dicle University, Ziya Gökalp Faculty of Education, Department of Geography, Diyarbakır
ayildirim@dicle.edu.tr

© Institut français d’études anatoliennes, 2015

Conditions d’utilisation : http://www.openedition.org/6540

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