The River Saar is a heavily impounded river with an average discharge of 80 m 3 s À1 . The German reach of the River Saar, i.e. the lower 90 km, was gradually impounded from 1977-2000, resulting in a doubled average water depth (today 4.2 m). In parallel to river development, water pollution was decreased strongly, relieving the critical oxygen budget in the river. Nowadays, oxygen concentrations may still fall below 4 mg O 2 l À1 during low-flow periods in summer, when thermal stratification and depth gradients of oxygen occur. In August 2005, high-resolution measurements of temperature, conductivity, turbulence, as well as oxygen-levels were carried out over 48 hours. These data were used to develop and validate a 'quasi-two-dimensional', depthresolving modelling approach with the deterministic water quality model QSim. This model includes the mathematical description of the influence of flow velocity and solar radiation on thermal stratification, on which the exchange between water layers depends. Three data sets of continuous measurements were compared with the model outputs. Thermal stratification shows diurnal rhythms and longitudinal variability depending on solar radiation, water depth, and flow velocity. In the course of the day, measurements and model outputs showed best agreement during the strongest stratification in the evening. The modelled effects of thermal stratification on oxygen-budget rates were quantified and showed that the reduction of atmospheric re-aeration is partly compensated by an increase in algal oxygen production. For the River Saar, the high temporal and spatial variability of oxygen concentrations documented here is of major ecological significance.
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