Hydromorphologically-driven variability of thermal and oxygen conditions at the block ramp hydraulic structure: The Porębianka River, Polish Carpathians.

Growing anthropopressure in mountain streams aimed at limiting erosion and flood protection often caused adverse effects on the natural environment. In recent years, great attention has been paid to the restoration and conservation of natural habitats in mountain streams using environmentally friendly solutions such as the Block Ramp (BR) Hydraulic Structures. In this study we investigated the factors responsible for spatial variability in thermal and oxygen conditions at the single BR structure in the growing season, and the relation between water temperature and dissolved oxygen (DO) concentration. This has been done by measurements of hydraulic characteristics along with physicochemical properties of water, such as water temperature and DO concentration, at two different discharges. The redundancy analysis has been applied in order to describe the relationships among hydraulic parameters and physicochemical variables, and extract potential sources of water temperature and DO variability within the BR hydraulic structure. Results have shown that DO and water temperature distributions within the BR hydraulic structure depend on discharge conditions and are associated with the submergence of the block ramp. The highest heterogeneity in hydraulic, DO and water temperature conditions occurs at low flow and is associated with the presence of crevices between protruding cobbles at the block ramp. The lowest variability, in turn, occurs at high discharge, when the block ramp is completely submerged. The results indicated that thermal and oxygen conditions within the BR hydraulic structure are independent of hydraulic parameters at low flow. Moreover, the relation between DO concentration and water temperature is positive at low flow indicating potential impact of biological processes. On the contrary, at high discharge both, the DO concentrations and water temperature within the BR structure, depend on bed shear velocity and maximum Reynolds number.