Fluvial geomorphology of Middle Sakarya River (Between İnhisar-Osmaneli)

The Sakarya River, which is the longest river in the Northwest Anatolia, drains water of the Middle Sakarya Plateaus Region in its middle section. This section, which has a plateau appearance by descending towards the Sakarya valley of the Sündiken Mountains in the west, is tectonically located between two strike-slip fault zones known as the North Anatolian Fault Zone from the north and the Eskişehir-İnönü Fault Zone from the south and which are among the most active fault zones in Turkey. The study area covers the westernmost part of the geomorphological unit in the Middle Sakarya Plateau. This part of the Sakarya River, where the east-west extension of the Sakarya River changes towards the north-south direction, contains extremely important geomorphological units (plateau surfaces, gorges, valley slopes, terrace systems, and stream bed) in terms of revealing the tectonic and geomorphological evolution of the region. In this study, the river terrace systems and gorges, which are noteworthy among the main geomorphological units of the region and formed especially between İnhisar and Osmaneli, were examined in detail. The goal of the study is to reveal the processes that are effective in the formation of the terrace systems and straits in the context of regional/local tectonic movements in the Quaternary period and sea level changes in the Black Sea due to global climate changes. Besides traditional methods such as comprehensive field observations and sedimentologic and stratigraphic interpretation in geomorphology studies, modern methods such as morphometric analysis, Geographic Information Systems, and Remote Sensing techniques were applied. In addition, using the "Optical Stimulated Luminescence (OSL)" dating method, a dating study was carried out for the first time in the region and the terrace systems were placed in a chronostratigraphic framework. In the study area, the main morphological units are the gorges (Hamitabat, Satankaya, Darca, and Pasalar Gorge) and the terrace systems formed under their control. The terrace systems of the Sakarya River, followed in several levels from the valley floor, are four (+10 m, +18 m, +34 m and +52 m) in Yakacık and three (+12 m, +24 m, and +52 m) in Hamitabat. 36 m), two in Gemiciköy (+10 m and +19 m), two in Küçükyenice (+9 m and +21 m), and four in Selimiye (+9 m, +24 m, +37 m and + 50 m) level was found. Three different ages, 158.03 ± 12.93 ka, 150.97 ± 8.49 ka, and 55.07 ± 4.57 ka, were obtained as a result of three OSL dates to reveal the geochronological features of these terrace systems. The long-term rise rate of terrace levels was determined as 0.22 ± 0.03 mm/year near Hamitabat, 0.18 ± 0.03 mm/year near Gemiciköy and 0.14 ± 0.03 mm/year around Küçükyenice. Accordingly, the constant regional velocity of the terraces, which have different locations and altitudes from each other, has been determined as 0.18 ± 0.03 mm/year, starting from the Middle-Late Pleistocene. Consequently, the OSL dating results reveal the presence of river split in the study area, despite the regional uplift that has continued for at least the last 158,000 years. Accordingly, the deposits in the terrace systems point to the accumulation and excavation processes of approximately 50-160 ka. Based on Marine Isotope Stages (MIS), the younger ones correspond to Late Pleistocene MIS3c (interglacial), while older ones correspond to MIS6b (interstadial) late Middle Pleistocene. As a result, the Sakarya River accumulated in its bed during the relatively warm and humid interglacial and interstadial periods, while excavation of its bed during the cold and/or cold-hot transition periods formed the terraces. The datings indicate "Penultimate Glacial Maximum" and "Last Glacial Maximum" for excavation of stream bed. In addition, various morphometric analyses were applied in order to quantitatively evaluate the geomorphological units in the study area and to explain their morphometric features. The results revealed that tectonic activity in the region has important effects on the drainage system. In conclusion, the terrace systems have formed under the control of the gorges in the middle section of the Sakarya River valley from the Middle-Late Pleistocene to the present have provided important geomorphological and sedimentological data in terms of both revealing the regional/local tectonic effects and clarifying the geomorphological formation of the region. This study revealed that the terrace systems in the Middle Sakarya River valley formed within a complex system controlled mostly by regional/local tectonic and partly by global climate changes.