مطالعه نقش فرایندهای رودخانه‌ای و برافراشتگی ساحلی بر ایجاد بوم‌سازگان‌های جنگلی خلیجی - عمانی در جلگه غربی مکران

Background and Objectives: Different phenomena in different regions of natural geography acquire their characteristics through the dynamics of the environment and by receiving energy from various external and internal sources of the earth. Therefore, if for some reasons, changes are made in the quality of the environmental dynamics, different characteristics will be obtained in the environment (Pirestani & Shafaghati, 2009). Dams have acted as geomorphic factors since their construction (Bayati Khatibi, 2007), reducing water flow and food concentration. They leave adverse effects on the plant and animal communities downstream (Naderi & Tadari, 2004). Among other factors that cause changes in the coastal environment are tectonic movements (Aghanabati, 2013). Such phenomena as the uplift of coast of Oman Sea, from Jask to Gwatar and the continuation of subduction under the Makran zone are the reason that these movements are still active in this zone (Aghanabati, 2013). It seems that at least a part of the Sahara Sindhi forest habitat of Hormozgan Province, in the western plains of Makran, has been established on the abandoned riverbeds. This research examines the effects of river processes and uplifting of Oman Sea coasts on creating a geomorphological environment suitable for establishment of Sahara Sindhi forests as an ecosystem in arid and semi-arid regions.
Materials and Methods: The coasts of Oman Sea start as a belt from east of the Persian Gulf (Minab fault) and extend to the north of coast of Indian Ocean in Pakistan. These lowlands are mainly based on the eroded structure of Makran lithology, which was deposited by large flood rivers such as Jegin, Gabrik, Sedich, and Kehir, originating from inner Makran. Wherever Makran folded structure has shown relatively more resistance against erosion factors, the width of the plains has been minimized and the remnants of the Makran folds have divided the Oman plain into six parts in the form of a mountain tongue. Based on the location of the first and third capes, which are respectively located in Bunji (west of Koh Mubarak), Bahal (east of Jask), and Guhert (100 km east of the latter), the study area is divided into three main sites. Considering the differences in the stretch of the plain, characteristics of the rivers, and extent of the Sahara Sindhi forests, the first site in the east of Jask and the third site on the edge of the Strait of Hormuz are the scopes of this research.
The research data includes spatial distribution of geomorphological forms, river dynamics, and extent of Sahara Sindhi forests in the coastal plain. Topographic maps, fieldwork tools, and computer software such as ArcGIS and Google Earth were used as research tools. Initially, morphodynamics of the rivers was studied. Geomorphological forms were analyzed by interpretation of satellite images, Google Earth software, and repeated fieldwork, and the morphodynamic changes of rivers affected by coastal uplift were investigated by referring to the study records. The map of Sahara Sindhi forests was also prepared by visual interpretation of satellite images and field works. Then, by combining and analyzing the results, the role of tectonic and river hydrodynamics on the establishment and expansion of Persian Sahara Sindhi forests on deltas of Jegin, Gabrik, and Sedich was studied.
Results: Sahara Sindhi forests have a significant extent on the deltas of Sedich, Gabrik, and Jegin rivers in the eastern part of Jask county. In the west, along the coast of the Strait of Hormuz, apart from scattered trees, there is no dense coverage of Sahara Sindhi forests on the deltas of Guz, Torkend, Zarani, and Hivi rivers. The extensive old riverbeds resulting from the various changes in the courses of Sedich, Gabrik, and Jegin rivers occupy a wide area around these rivers. These basins have similar erosion conditions, and river discharge can be used as an indicator of their sediment yield. The Jegin River has the highest discharge, followed by the Sedich and Gabrik rivers. The Gaz and Zarani rivers (both on Site 3) have less discharge compared to the rivers on Site 1.
Conclusion: Results showed that the extent of Sahara Sindhi forests is consistent with the abandoned riverbeds. Since the sinuosity coefficients of the rivers in the western parts are higher than those in the eastern part of the plain, the extent of the abandoned riverbeds is not solely related to the sinuosity of the rivers. It seems that factors other than sedimentological characteristics and slope of the plain have been involved in changing the river courses, leading to the extensive abandoned riverbeds on the eastern deltas (Sedich, Jegin, and Gabrik). Among these factors, tectonics and differences in coastal uplift due to the movement of Oman plate beneath the Makran plate can be mentioned. Due to the difference in coastal uplift, the eastern rivers change their course and create extensive abandoned riverbeds upstream of the plain when they reach a certain sinuosity threshold, which is lower than that of the western rivers. These abandoned riverbeds have gradually provided the conditions for the establishment and expansion of the Sahara Sindhi forests over time.