## 2 Materials and Methods
### Indirect Measurement of Suspended Load
The probe measures optical reflections of infrared light according to ISO 7027, while the handheld turbidity meter measures 90\({}^{\circ}\) dispersion according to EPA 180.1. The analyzed sample is also different, the turbidity probe is in situ, while the handheld turbidity meter analyzes a disturbed sample several times. The multiparameter probe is fixed and built as a remote registration station. The station was installed on the sand-bed alluvial reach of the Danube River in the \(\sim\)1479.6 rkm section of the river on a floating pontoon next to the left bank (Figure 3) in January 2020. The support structure, consisting of a protective tube and an instrument box, is mounted on the downstream edge of the pontoon. The placement on the float ensures that the measuring point is always at a depth of \(\sim\)1.5 m. The subsurface placement provides greater protection from driftwood. The probe is located in a protective basket at the bottom of the protective tube attached to the pontoon. The station consists of a probe, a data logger, additional electronics, and the supporting structure. The probe measures the parameters every hour and sends them to the data logger located in the instrument box, which transmits the data to the LDVWD, where the measured data can be seen with minimal delay. Thanks to the low power consumption of the devices, the installed 50 W solar panel and the 22 Ah buffer battery supply the system with power without problems. The turbidity measured by the probe is checked by comparative measurements regularly, but at almost every flood wave. During the regular inspection, a sample is taken next to the probe, and then the turbidity of the sample taken is measured also with a hand-held turbidity meter, as described in Section 2.2. Before carrying out further calculations, the adequacy of the values recorded by the probe was checked. According to the user information, the probe must be calibrated before the first use and at regular intervals. The calibration standard series consists of seven samples (0.1-4000 NTU), of which at least five are required for the Danube River (0.1-1000 NTU). The probe must therefore be calibrated to improve its measurement accuracy. In order to monitor and improve the measurement accuracy of the probe without the calibration series, water samples are regularly taken from the probe and the turbidity of the sample taken is measured with a handheld turbidimeter. The turbidity of a sample is measured three times and a sample is taken twice from next to the probe. The correct functioning of the chlorophyll sensor has to be checked with campaign-like measurements as well. During the occasional sampling, the correct registration of chlorophyll content and water temperature is checked, and additional chemical parameters (pH, specific conductivity, dissolved oxygen content, and saturation) are also measured. These parameters are measured and checked for purposes not relevant to this study. In the used method, the turbidities registered by the installed probe were corrected based on the control measurements made with the handheld turbidity meter. Based on the reliable relationship between the turbidity and the sediment concentration, the corrected turbidity was converted into a near-bank suspended sediment concentration. Using the relationships established based on the simultaneous suspended sediment measurements and near-bank turbidity measurements, the sediment yield along the section was calculated by using a relationship between it and the near-bank sediment concentration (near-bank SSC in Figure 4). A flowchart of the method used is shown in Figure 4.  
Figure 3: Automatic turbidity monitoring station (Danube, 1479.6 rkm, left bank, authors’ photo).