Lagrangian drift experiment on a large lowland river during a spring flood

A real-time Lagrangian drift experiment was performed along the Middle Daugava River in South-east Latvia at the peak of the spring flood on March 28, 2007. The drift was performed downstream from Daugavpils City by a manned research platform constructed from a small maritime life raft and an inflatable boat. All measurements and water sampling procedures were performed in the middle of the river at hourly intervals. Main physical and chemical properties of the drifting water mass were measured at the 0.5 m depth by a Hach Hydrolab DS5 multiprobe. At each sampling position, subsurface samples of water, phytoplankton and zooplankton were also taken. The correlation analysis and the Principal Component Analysis were applied for the obtained data series. The drift continued uninterruptedly for 12 hours, and about 51 km long distance was travelled. The range of variation of most parameters exceeded their respective measurement accuracy levels. During the drift, the water temperature, pH and conductivity gradually increased, the turbidity and transparency remained almost constant, whereas the abundance, total biomass and diversity of the phyto-and zooplankton communities changed unpredictably. Possible impact of weather conditions, water mass turbulence and stagnant floodwaters are discussed in detail. The experiment highlighted the role of turbulence at different scales and validated hypotheses about the longitudinal transformation of the water mass properties and composition during its downstream drift during the spring flood. It also demonstrated the usefulness of the applied methodology for the in situ observations of the drifting water masses and highlighted possible ways for further improvements of such techniques in river system research.