## 3 Methods and Data
### Field Data
#### 3.1.1 Remote-Controlled ADCP  
We used a Sontek RiverSurveyor M9 ADCP mounted to a custom-built remote-controlled mini-boat (RCFlow) to survey a dense pattern of flow measurements at the meander bend AOI. The boat is 1.43-m long and 0.425-m wide with the sensor emerging from the hull without its edge protruding at a water depth of 0.06 m. The hull is designed to minimize drag and turbulence, so that the boat can move against a fairly strong current and cause minimal interference to the measurements. RCFlow makes it possible to survey even shallow reaches in sub-meter depths that cannot be accessed by larger boats. The boat is propelled by an electric motor powered by either NiCad or LiPo battery sets providing 20,000 mA/h. This allows for uninterrupted measurements exceeding 2 h, depending on flow conditions and the load required. To measure the long reaches of the study area, we mounted a Sontek S5 ADCP to a Hydroboard floating platform attached with a pole to a motorboat (Figure 3). This allowed us to cover a greater distance and better cope with the current than the battery-operated RC boat. We aimed for boat speeds not exceeding 1 \(\mathrm{ms}^{-1}\). Both ADCPs had the SmartPulseHD feature enabled. Using this measurement mode, the sensor simultaneously emits coherent and incoherent pulses, potentially at multiple frequencies, and a processing algorithm selects the best return signal for the given water depth and velocity. The Sontek M9 uses 3.0-MHz, 1.0-MHz and 0.5-MHz and the S5 uses 3.0-MHz and 1.0-MHz pulses. The ADCP sample measurements were carried out in discharge measurement mode using the ADCPs' differential GPS (DGPS) for tracking. Bottom tracking was not an option, because the sandy river bed is mobile during the spring flood. Measurements were collected in a zig-zag pattern as very long transects and later exported as raw data in MATLAB format. The ADCPs measure one sample (also referred to as an ensemble) each second, and each sample is divided vertically into separate cells, also referred to as bins. The number of cells depends on water depth, and the cell height is dynamically adjusted depending on depth and speed. Measurement cells may be as small as 2 cm in shallow (<1.5 m), slow moving  
Figure 3: ADCP-GNSS set-up on a remote-controlled boat (**A**) and on the Hydroboard tethered to a zodiac (**B**). A Trimble R8 GNSS receiver is mounted on the GPS pole of the ADCP exactly above the sensor, and the ADCP’s GPS antenna is mounted on the deck of the vessel. (<0.4 \(\mathrm{ms^{-1}}\)) water or as large as 2 m in depths over 20 m. In our case, depths did not exceed 4 m, so the maximum cell size was 0.2 m.  
By design, ADCPs average the flow of increasingly large areas into one measurement cell, with increasing distance from the sensor. That is, the measurement sample is in the shape of a pyramid consisting of different layers [9, 29]. Each layer in the sample pyramid is one measurement cell. The true horizontal size of the measurement cell depends on the angle of the receivers on the sensor. Both Sontek ADCPs have the receivers arranged in a Janus configuration at a 25\({}^{\circ}\) angle. This means that, typically, the first cell measured after the blanking distance [30] (the space too close to the sensor to measure, generally 25 cm) is 0.03 \(\mathrm{m^{2}}\); at a 1-m depth, the cell is 0.45 \(\mathrm{m^{2}}\), and at 2 m depth, the cell is 1.77 \(\mathrm{m^{2}}\) in area, while both cells may be the same thickness, say 0.1 m. Conceptually, however, the measurement sample is treated as a column of stacked cells, each of the same size, the width of which depends on the distance between consecutive samples, which depends on the boat speed. The measured value is often considered as a point located at the center of the cell, vertically and horizontally [9] centered under the instrument.