Stream power represents the rate of energy expenditure along a stream reach and can be calculated using topographic data acquired via structure-from-motion (SfM) photogrammetry and terrestrial laser scanning (TLS). This study sought to quantitatively relate morphological adjustments in the Azohía Rambla, a gravel-bed ephemeral stream in southeastern Spain, to stream power (ω), critical power (ω_c), and energy gradients (∂ω/∂s), along different reference channel reaches of 200 to 300 m in length. High-resolution digital terrain models (HRDTMs), combined with ortophotographs and point clouds from 2018, 2019, and 2020, and ground-based surveys, were used to estimate the spatial variability of morphological sediment budgets and to assess channel bed mobility during the study period at different spatial scales: reference channel reaches (RCRs), pilot bed survey areas (PBSAs), and representative geomorphic units (RGUs). The optimized complementary role of the SfM technique and terrestrial laser scanning allowed the generation of accurate and reliable HRDTMs, upon which a 1-D hydrodynamic model was calibrated and sediment budgets calculated. The resulting high-resolution maps allowed a spatially explicit analysis of stream power and transport efficiency in relation to volumes of erosion and deposition in the RCR and PBSA. In addition, net incision or downcutting and vertical sedimentary accretion were monitored for each flood event in relation to bedforms and hydraulic variables. Sediment sources and sinks and bed armoring processes showed different trends according to the critical energy and stream power gradient, which were verified from field observations. During flows exceeding bankfull discharges (between 18 and 24 m³ s⁻¹ according to channel reach), significant variations in ∂ω/∂s values and ω/ω_c ratios (e.g., −15 < ∂ω/∂s < 15 Wm⁻³; ω/ω_c > 2 for a peak discharge of 31 m³ s⁻¹) were associated with a large amount of bedload mobilized upstream and vertical accretion along the middle reach (average rise height of 0.20 to 0.35 m for the same event). By contrast, more moderate peak flows (≤10 m³ s⁻¹) only produced minor changes resulting in surface washing, selective transport, and local bed scouring.

Mapping the streambed morphology is crucial for understanding fluvial forms and processes, for advancing both our knowledge and best management practice of riverine systems. It is often done by wading streams but recently topobathymetric Light Detection and Ranging (LiDAR) and imagery captured from airborne platforms are becoming promising and effective surveying methodologies. The recent use of remotely piloted aerial systems (RPAS) combined with structure-from-motion (SfM) photogrammetric processing provides a novel, high-resolution approach to modelling fluvial morphology. Nevertheless, a complicating factor of such data acquisition in fluvial settings is linked to water bodies, whose presence introduces errors and distortions because of light reflection and refraction at the air–water interface. Although proof-of-concept research has shown it is possible to reduce the effects of refraction in certain settings (e.g., clear waters, unbroken surfaces), corresponding validation methods remain limited to point-based assessments of error, these being typically limited to accessible parts of the channel. Here, we provide the first high-resolution, spatially continuous validation of a stream reach bathymetry surveyed with SfM and corrected with an advanced refraction method. This method required only the camera co-ordinates and is available as open source, coded in C++. We used RPAS imagery from a regulated reach of the Palancia River, Spain, where a diversion structure fully dewatered the reach and let us obtain the entire streambed topography, which we used as spatially continuous control topography to compare the bathymetry surveyed from imagery during normal flow conditions. We compared this method with the small angle refraction correction (SARC), which has less data requirements, and analysed the error distribution across habitat types (i.e. pools and riffles). These results showed that our approach had smaller errors than SARC in both habitat types, especially in the riffle. By analysing the relationship between error and channel roughness obtained from our dry-bed model, we found that the greatest errors arise in the peripheral zones of the water surface and in its areas where streambed roughness generates more turbulence. Quantitative validation confirmed the reliability of our method as a relatively low-cost tool for the modelling and management of geomorphology and habitat within small – to medium-sized streams.

This work proposes a methodological approach applied to ephemeral gravel-bed streams to verify the change in the magnitude and frequency of hydrological events affecting the morphological dynamics and sediment budget in this type of channel. For the case study, the Azohía Rambla, located in southeastern Spain, was chosen, emphasizing the research on two reference riverbed sections (RCRs): an upper one, with a predominance of erosion, and a middle one, where processes of incision, transport, and deposition were involved. First, this approach focuses on relationships between peak discharges and sediment budgets during the period 2018–2022. For this purpose, water level measurements from pressure sensors, a One-Dimensional Hydrodynamic model, and findings from comparative analyses of high-resolution differential digital elevation models (HRDEM of Difference-HRDoD) based on SfM-MVS and LiDAR datasets were used. In a second phase, the GeoWEPP model was applied to the period 1996–2022 in order to simulate runoff and sediment yield at the event scale for the watersheds draining into both RCRs. During the calibration phase, a sensitivity analysis was carried out to detect the most influential parameters in the model and confirm its capacity to simulate peak flow and sediment delivery in the area described above. Values of NS (Nash–Sutcliffe efficiency) and PBIAS (percent bias) equal to 0.86 and 7.81%, respectively, were found in the calibration period, while these indices were 0.81 and −4.1% in the validation period. Finally, different event class patterns (ECPs) were established for the monitoring period (2018–2022), according to flow stage and morphological channel adjustments (overtopping, bankfull and sub-bankfull, and half-sub-bankfull), and then retrospectively extrapolated to stages of the prior simulated period (1996–2018) from their typical sequences (PECPs). The results revealed a significant increase in the number of events and PECPs leading to lower bed incision rates and higher vertical accretion, which denotes a progressive increase in bed armoring and bank erosion processes.

This work proposes a methodological approach applied to ephemeral gravel-bed streams to verify the change in the magnitude and frequency of hydrological events affecting the morphological dynamics and sediment budget in this type of channel. For the case study the Azohía Rambla, located in southeastern Spain, was chosen, emphasizing the research on two reference riverbed sections (RCRs): an upper one, with a predominance of erosion, and a middle one, where processes of incision, transport, and deposition converge. First, this approach focuses on relationships between peak discharges and sediment budgets during the period 2018 to 2022. For this purpose, water level measurements from pressure sensors, a One-Dimensional Hydrodynamic model, and findings from comparative analyses of high-resolution differential digital elevation models (HRDEM of Difference - HRDoD) based on SfM-MVS and LiDAR datasets were used. In a second phase the GeoWEPP model was applied to the period 1996-2022 in order to simulate runoff and sediment yield at the event scale for the watersheds draining into both RCRs. During the calibration phase a sensitivity analysis was carried out to detect the most influential parameters in the model and to confirm its capacity to simulate peak flow and sediment delivery in the area described above. Values of NS (Nash-Sutcliffe efficiency) and PBIAS (percent bias) equal to 0.86 and 7.81%, respectively, were found in the calibration period, while these indices were 0.81 and -4.1% in the validation period. Finally, different event class patterns (ECPs) were established for the monitoring period (2018-2022), according to flow stage and morphological channel adjustments (overtopping, bankfull and sub-bankfull, and half-sub-bankfull), and then retrospectively extrapolated to stages of the prior simulated period (1996-2018) from their typical sequences (PECPs). The results revealed a significant increase in the number of events and PECPs leading to lower bed incision rates and higher vertical accretion, which denotes a progressive increase in bed armoring and bank erosion processes.