## 1 Introduction  
Ephemeral streams are watercourses of arid and semi-arid environments with unstable morphology and high temporal variability of runoff. Sudden, extreme discharge events, that are isolated in time and alternate with long dry periods. These types of streams are particularly sensitive to short-term climatic changes, and human impacts may alter their degree of response, sometimes leading to large morphological adjustments during flash floods [1, 2, 3]. As a result, the ephemeral channels show a changing geometry, highly conditioned by differences in slope and textural variations in the bed materials and banks. Often along their upper reaches and on alluvial fans, these channels have a steep slope which promotes a fast hydraulic regime. Under such conditions, and considering the abundant sediment stored within the channel, important transport rates contribute to most of the morphological changes in the channel. This dynamic is especially complex in gravel-bed ephemeral channels, subject to sporadic and torrential transport. A product of this is the mixture of sand, gravel, and pebbles laid in layers of irregular thicknesses and geometries. The bed material also shows high spatial variability in texture between bedforms, between channel reaches, and between the surface and the subsurface. An uncertainty in bedload estimates for this type of streams is largely driven by the inability to characterise arrangement, orientation, and resultant forces of fluvial sediment in river beds. Water working of grains leads to textural differences between areas of the bed through particle sorting, packing, imbrication, mortaring, and the degree of bed armouring.  
This study intends to solve the uncertainty related to the spatial variability of the thickness. Therefore, non-destructive electrical resistivity tomography (ERT) imaging in 2D and 3D is used to visualise, quantify, and assess the internal geometry of cross-sections of an ephemeral gravel-bed stream in combination with datasets extracted from borehole samples. The chosen pilot study area is the upper reach of the Rambla de la Azohia, a gravel-bed channel subjected to great recent geomorphological activity.  
In general, the electrical resistivity tomography method provides indirect information that need to be validated by direct measurements (borehole samples), this combination offers satisfactory results. Some studies utilised this synergetic combination for identifying specific elements [4; 5], determining deposition material paths [6] or even determining the stratigraphy of fluvial channels [7]. Electrical resistivity tomography (ERT) is a non-invasive method of rapid application, which is usually used to generate models of distributions of electrical properties of the subsurface, from which the geological structure and hydrogeological variations can be inferred [7; 8; 9]. Additionally, ERT can be employed to map spatial variations of qualities within a mineral deposit in 3D [10; 11]. Several authors conducted ERT surveys to study loose materials, assess mine tailings, and so forth [12; 13; 14; 15].  
ERT can provide spatial mapping models of the subsurface at the site scale, in comparison with intrusive sampling methods, the information from which is only valid for discrete locations. Various studies have focused on the detail achieving high-resolution results with low electrode spacing [16; 17; 18; 19]. This method is sensitive to variations in lithological composition with depth and can be used to distinguish between different types of materials, e.g., gravels and pebbles (characterised by high resistivity), or silts and clay (characterised by low resistivity) [20; 21].  
In the absence of direct porosity data, the objective of this research is to: (i) assess the grain shape, particle size distribution, and sorting in heterogeneous alluvial deposits; (ii) assess the vertical and lateral behaviour by creating a 3D model based on electrical resistivity tomography; and (iii) set a methodology to evaluate this type of ephemeral channels.