## 4 Results and Discussion
### Electrical Resistivity Tomography 2D Survey  
As a general rule, the penetration depth is approximately 15-25% of the longest array length for any four-electrode array [42]. Resistivity results of this study present higher values owing to the sort and material type as [PERSON] et al. [43] and [PERSON] et al. [44] have alluded in their study. The ERT4 profile is the largest profile of the whole study that reaches around 6 m of investigation depth. This profile identifies the internal structure of the gravel-bed channel. BH4 was made as to coincide with the ERT4 profile approximately in the central part of the profile, as expected [6, 7]. The depth at which the marl substrate is encountered is 1.1 m. Additionally, profile ERT4 intersects with profiles ERT1, ERT2, and ERT3 at 11 m, maintaining a uniform distribution of the position of the marl substrate (Figure 3). At the top, an alluvial channel-fill is identified with high values of resistivity above \(\approx\)600 \(\Omega\).m, while at the bottom, the resistivity progressively decreases to low values (\(<\)300 \(\Omega\).m), indicating the presence of the marl substrate. The white dashed line marks the border between them, see Figure 3.  
The ERT4 profile provides a general overview of the channel. The profiles ERT1-ERT3 were limited by the channel width, reaching a depth of investigation of only \(\approx\)3 m. Figure 4 shows the resulting 2D geoelectrical sections of ERT1, ERT2, and ERT3 profiles. After the eighth iteration, the ERT1 profile yielded a section with a root mean square (RMS) of 9.47% and 1.05 of L2. In the case of the ERT2 profile, the RMS is 9.51% and L2 = 1.03, and finally, the ERT3 profile presents RMS = 10.21% and L2 = 1.09. Figure 3: Perpendicular ERT profile ERT4. The other three profiles intersect vertical with arrows to mark the position of ERT1-ERT3. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c} \hline \hline  & \multicolumn{10}{c}{**Zingg Shape Classes**} & \multicolumn{10}{c}{**Shape–Sphericity**} \\ \cline{2-13}  & \multicolumn{2}{c}{**Depth**} & \multicolumn{2}{c}{**Discoid or Oblate**} & \multicolumn{2}{c}{**Equid.–Spheroid**} & \multicolumn{3}{c}{**Blade**} & \multicolumn{3}{c}{**Rod**} & \multicolumn{3}{c}{**([PERSON] \& [PERSON], 1958)**} \\ \cline{3-13}  & \multicolumn{2}{c}{**(m)**} & \multicolumn{1}{c}{**\%**} & \multicolumn{1}{c}{**I/L**} & \multicolumn{1}{c}{**S/L**} & \multicolumn{1}{c}{**\%**} & \multicolumn{1}{c}{**I/L**} & \multicolumn{1}{c}{**S/L**} & \multicolumn{1}{c}{**\%**} & \multicolumn{1}{c}{**I/L**} & \multicolumn{1}{c}{**S/L**} & \multicolumn{1}{c}{**\%**} & \multicolumn{1}{c}{**I/L**} & \multicolumn{1}{c}{**S/L**} & \multicolumn{1}{c}{**S/L**} & \multicolumn{1}{c}{**DR1**} & \multicolumn{1}{c}{\(\psi_{\text{p}}\)} \\ \hline BH1 & 0.0–1.1 & 57.1 & 0.81 & 0.55 & 0.0 & - & - & 14.3 & 0.45 & 0.60 & 28.6 & 0.58 & 0.75 & 0.42 & 0.51 & 0.63 \\  & 1.1–3.0 & & & & & & & & & Loamy substrate & & & & & & & \\ \hline BH2  
ERT3 was in the downstream direction. This distribution allowed us to identify the bar geometry, which is wider in ERT1 but narrower in ERT3. The bar presents a triangular form, indicating that the bar decreases in the downstream direction. Four mechanical boreholes were drilled on ERT3 (Figure 4). Figure 4 presents a horizontal layer with a relatively constant thickness of about \(\approx\)1 m which is associated with a high resistivity (>2000 \(\Omega\).m). The results from boreholes support ERT results and link this layer to the pattern located in the active channel composed of medium and coarse gravels. The third pattern is related to BH2. In this pattern exists a top layer composed of coarser materials (gravel), corresponding to resistivities > 5000 \(\Omega\).m (ERT3) and a second layer of sand layer 1 m thick. The sand layer does not appear in the profile ERT 3 although it appears in profile ERT2 with resistivities > 3000 \(\Omega\).m.