STRUCTURAL CONTROL OF DRAINAGE IN THE UPPER PART OF THE LUDA YANA RIVER BASIN, BULGARIA

Drainage pattern is influenced by the underlying surface and tectonic features of the territory. Lithology and tectonics are major controls of stream network and drainage. This study is motivated by the need to investigate the influence of lithological and tectonic factors on the formation and development of drainage networks, with the goal of advancing structural-geomorphological research in Bulgaria and exploring its applicability in risk assessment. The Panagyurska and Strelchanska Luda Yana drainage basins are the objects of study, and the analysis focuses on the drainage network density. The water permeability and erodibility of various rock types and lithological formations were also studied. Fault density and drainage pattern were also determined for the study area. Results confirmed that unconsolidated sediments contribute to higher drainage density. Sedimentary rocks tend to result in relatively high drainage density values, while volcano-sedimentary rocks exhibit both high and low drainage density, depending on specific lithological and topographic factors. Intrusive igneous rocks are associated with a relatively high drainage density, greater than that of migmatized rocks. The density of 1st order streams in volcano-sedimentary rocks further supports the conclusion that, in the development of the drainage network, both lithological properties and topographic factors are significant. The Panagyurska Luda Yana basin exhibits a higher fault density, while the Strelchanska Luda Yana basin shows a lower fault density. In both basins, the rectangular drainage pattern prevails in the middle course, while the annular pattern is prevalent in the upper course, with parallel patterns occurring predominantly in areas of sedimentary and volcano-sedimentary rocks. The findings provide a basis for further research, particularly into the weathering resistance of rocks, and contribute to the development of hydrogeological and hydro-climatic analyses of the studied area. The results also have practical value for regional and local authorities in developing strategies to prevent risk events and implement effective measures to mitigate their impact.