## 3 Methods and Data
### Field Data
#### 3.1.3 Water Level Data  
Water levels were continuously measured along the study reach at 15-min intervals using pressure transducers (Solinst Gold Leveloggers) mounted to concrete slabs that were sunk to the riverbed at the beginning of the field work period and left to collect data throughout the ice-free period (Figure 4). The pressure transducers of these devices have an accuracy of \(\pm\)0.005 m. Seven Leveloggers were positioned throughout the study area, three of which were located at the meander bend under investigation, one upstream, one in the middle of the bend in the deepest part of the meander and one downstream of the bend (Figure 5). The loggers were positioned far enough from the bank to be in deep enough water to remain submerged during summer low flow conditions.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Date**} & \multirow{2}{*}{**GNSS Points**} & \multicolumn{2}{c}{**ADCP**} & \multirow{2}{*}{**3D Points**} & \multirow{2}{*}{**Interpolated**} & \multirow{2}{*}{**3D Point Density**} \\  & & & & & **3D Points** & **in Meander (\(\mathbf{pts}\)\(\mathbf{m}^{-3}\))** \\ \hline May 6 & 82,823 & 7,444 & 70,556 & 2,587 & 15.2 \\ May 17 & 92,047 & 8,006 & 64,944 & 3,030 & 11.9 \\ May 18 & 497,716 & 17,293 & 155,584 & 3,243 & 2.2 \\ May 19 & 110,001 & 6,217 & 49,112 & 4,928 & 5.5 \\ May 20 & 96,532 & 8,552 & 69,397 & 4,983 & 7.7 \\ May 21 & 59,871 & 4,422 & 36,884 & 4,456 & 4.6 \\ May 23 & 87,028 & 6,368 & 49,744 & 5,703 & 4.8 \\ May 24 & 51,064 & 4,138 & 36,708 & 4,836 & 4.2 \\ \hline \hline \end{tabular}
\end{table}
Table 1: The number of points measured for each day at the meander bend AOI. GNSS points is the number of accepted points after baseline calculation (RMSE <0.05 m). ADCP points are raw measured point samples (ensembles). 3D points are the number of points in the point cloud for each day, and interpolated points are the number of points in the regular matrix interpolated from the original point cloud. 3D point density denotes the number of points per \(\mathrm{m}^{3}\) in the meander bend AOI.  
Figure 4: (**A**) Hydrograph of the ice-free period for 2013 of the Pulmanki River; (**B**) The rising stage of the spring flood is in meters above sea level (m.a.s.l). ADCP-flow measurement times are indicated with red circles.  
The water levels were measured at the beginning and the end of the study period using VRS-GNSS topography point measurements. The topography point method involves measuring a series of consecutive points and averaging the resulting coordinates in order to increase accuracy by averaging small positional errors that may result from slight movements of the GNSS receiver. This results in a measurement with better than 2-cm precision [31], which is suitable, given that the gradient of the river reach translates to a height difference of 0.2 m from the upstream to the downstream end of the reach. RTK-GNSS has previously been successfully used for water surface reconstructions [32; 33; 34].  
The Leveloggers log pressure and temperature data. A Barologger was located near the middle of the study reach to collect atmospheric pressure data for post-processing the pressure data of the Leveloggers into water depths. Figure 4 shows the hydrograph measured by the water level loggers at the meander bend AOI for the ice-free period of 2013.