## 2 Materials and Methods
### Indirect Measurement of Suspended Load  
Sediment concentrations were as well determined indirectly, using in situ turbidity measurement. The turbidity of water expresses its reduced transparency, which is caused by particles in water, by scattering or absorbing light rays passing through water. The instruments used in daily use are based on the laws of nephelometry (nephelometers). Nephelometric measurement measures a 90\({}^{\circ}\) scattering of light in the visible or infrared range. Wedges measuring optical reflection measure light rays scattered in 140-165\({}^{\circ}\) in the infrared range. Such devices are also suitable for analyzing a sample of small volume (a few cm\({}^{3}\)). The dispersion of light depends on the size of the particles in the sample, their color, and the shape of the particles [22]. The advantage of this method is that the probe can be fixed in a suitable place and its operation can be automated, which greatly increases the temporal resolution of sediment data. The disadvantage of the method is that turbidity depends on the size, composition, color, and shape of the sediment. The disadvantage of  
Figure 2: Calculation of the suspended sediment load, based on [20,21]. (**a**) shows the suspended sediment concentration in a vertical, (**b**) shows the flow velocity in a vertical, (**c**) shows the suspended sediment transport in a vertical and its calculation, (**d**) shows the suspended sediment transport in a cross-section and its calculation. h is the water depth, \(s_{0}\) is the suspended sediment concentration, \(v\) is the flow velocity, \(q_{\text{si}}\) is the suspended sediment transport in a vertical, \(Q_{\text{s}}\) is the suspended sediment transport in a cross-section. installation is that the accuracy can be greatly reduced by biofilm formation on the probe, so it is necessary to ensure continuous cleaning. Being an indirect method, calibration is required to convert turbidity into sediment concentration. When using direct methods, data are generated only if the sampling team is physically present on site, takes the samples, and performs the necessary laboratory and processing work. The temporal resolution of data obtained by direct methods should not be arbitrarily low, as they depend on the available human resources and measurement conditions. Indirect methods are based on the use of a physical relationship, so each requires a calibration between the measured and the parameter being sought. An increased number of samplings is required until the calibration is established, after which it is sufficient to check the correctness of the calibration set up by sampling on a regular basis. However, the need for routine calibration is expected to diminish over time [8]. Indirect methods do not give information specific to the entire cross-section, only point information. A separate calibration is required for cross-sectional extension of point information. Another advantage of the indirect methods is that they can be remotely controlled, so they can provide data without on-site presence, at almost any temporal resolution. To continuously detect turbidity, an OTT Hydrolab HL7 multiparameter probe was used. The parameters measured by the probe can be selected at the time of purchase, and the manufacturer offers a total of 13 sensors for the probe. The probe operated in this specific research measures turbidity, \(\alpha\)-chlorophyll, and water temperature. The turbidity sensor can measure turbidity in the range of 0-3000 NTU. Measurement is carried out according to ISO 7027 using light with a wavelength of 880 nm (infrared). Since the measurement accuracy of optical turbidity meters is significantly impaired by the biofilm formed on the sensor, the probe is equipped with a central brush that keeps the instrument clean at all times. The operation of the turbidity probe and the handheld turbidity meter differ based on the standards used.