Dr. Caamaño-Avendaño, Diego

Recent work in the undeveloped Rio Murta Basin, located in Chilean Patagonia, identified an evolutionary trend in the fluvial system as it progresses toward and away from dynamic equilibrium. A location-for-time-substitution model employed over the longitudinal extent of a 16 km study site assessed the performance of extremal hypotheses in identifying dynamic equilibrium conditions. Numerous extremal hypotheses were successful in identifying the spatial trend, but no means were available to discern differences between them. Thus, this study uses field measurements within the evolutionary trend to propose a new metric for evaluating extremal hypotheses. A thorough review and synthesis of the extremal approach are additionally presented. The new method compares theoretical predictions against field-measured values to determine which extremal hypothesis is most effective in identifying the condition of dynamic equilibrium in a gravel-bed river. Channel width and depth are identified as the dependent stream variables that uniquely differentiate most extremal hypotheses from one another. The results indicate that extremal hypotheses based on energy metrics of the flow are most successful, with the strongest support for minimum kinetic energy and minimum specific stream power.

Surface runoff, channel activation and sediment transport processes in desert environments have been convincingly shown to strongly depend on the duration and intensity of local and convective rainfall events. Among these environments, the Atacama, considered the driest desert on Earth, is situated in a remote and rugged area, where documentation of historical and recent hydro-sedimentary processes is rare. We characterize the hydrological processes in an endorheic watershed of the Atacama's Altiplano Desert, where the occurrence of flash floods was evaluated on event-based signatures of water and sediment in a small ephemeral playa. Twelve pits were dug in the playa, with five identified event sediment couplets, each corresponding to computed flood volumes that gave rise to transport and depositional events occurring between 1978 and 2019. Detailed topography allowed reconstruction of a 3D terrain model, from which we estimated a 11.3 t/yr/km2 local historic sediment yield. The timing of the older identified events did not match local rainfall records, and proved to be uncorrelated with occurrences of El Niño–Southern Oscillation (ENSO). This suggests a high spatial patchiness of rainfall events in terms of depth and intensity, and implies that small playa records are not necessarily always helpful in reconstructing the regional climatic history of the recent past. The sediment concentration and volume of the reconstructed hypercontracted events are not well corrected with the magnitude of the rainfall event, suggesting the important role played by variable sediment availability and connectivity at basin scale. This spatiotemporal variability plays a major role in understanding the present and historic hydro-sedimentary processes in the Atacama's altiplano.

Data provided by products 3B42 V7 (TRMM) and its successor 3IMERG (GPM) are compared with discrete rainfall information throughout the Chilean territory covering four macro hydroclimatic zones. Precipitation data was obtained from weather stations available on a daily basis from the 1930s to present. A total of 143 stations were chosen and rainfall estimates performed for years 2014 through 2018. Applying the same metrics we showed how GPM performances improve as the temporal aggregation increases. Several drawbacks were detected in the coastal areas, which were characterized by lower accuracy performances than internal areas. However, the 3IMERG product could be a strong source of data to study the impact that climatic disturbances have on the hydrologic cycle in the Central and South zones of Chile. Additionally, its offers a fundamental source of data for remote zones or areas where access is complicated to install weather stations.

The indeterminate channel problem arises from uncertainty in finding a closure relation for alluvial channels created by self-organizing erosional and depositional processes. Extremal hypotheses have been proposed as one potential approach to closing the system of governing equations for alluvial channels. Many different extremal hypotheses have been presented, but no substantive evidence has been developed to select which hypothesis may be most appropriate for natural alluvial river systems. This paper evaluates the ability of ten extremal hypotheses to identify dynamic equilibrium across a geomorphic gradient in the remote and undeveloped mid-latitude watershed of Rio Murta, Chile. This study (a) introduces extremal hypotheses, (b) describes the field site and geomorphic conditions, and (c) examines which extremal hypotheses are supported by the field data in identifying the evolutionary trend toward dynamic-equilibrium. The extremal hypotheses that identified dynamic equilibrium within the geomorphic gradient in the field are: (1) minimum kinetic energy, (2) minimum specific stream power, (3) maximum friction factor, and (4) maximum total friction factor, which collectively support minimizing kinetic energy of the system.

Topographic variation within fluvial systems is essential for providing a mosaic of physical habitats and supporting the dynamic hydraulic, geochemical, and biological processes that determine both aquatic and riparian ecosystem function. In highly-modified rivers through both urban and rural settings, the physical heterogeneity of alluvial channels has been diminished by anthropogenic activities. As riparian areas are increasingly under pressure from agricultural and urban development, identifying the geomorphic controls on physical heterogeneity through these environments is critical. In this study, we use the bed coefficient of variation (CV) extracted from a high-resolution bathymetric LiDAR survey as a dimensionless metric for topographic variation and physical heterogeneity over 100 km of the Boise River corridor that spans an urban-rural gradient. Our CV results for both the streambed and channel demonstrate that the average topographic variation of reaches in urban areas is 22–25% lower than reaches located in rural areas along the same river. While these results initially support the application of the urban stream syndrome hypothesis, CV values had similar magnitudes in both urban and rural reaches suggesting there is a dominant control on topographic variation that was not directly related to urban land use. Analysis of CV values relative to normalized levee width indicates that the causative driver of morphologic simplification in the channel was lateral constraints from levees. In the Boise River, topographic variation increased linearly with normalized levee widths that ranged between 50% and >300% of the average channel width. Further, topographic variation was maximized in reaches where flow expansion during high discharge inundated between 1 and 2 times the average channel width (approximately 65–70% of the available floodplain). Our simple and objective watershed-scale approach leverages highresolution topography data to identify reaches of high physical heterogeneity for river conservation, as well as help guide environmental flow releases in managed rivers.

Hydrodynamics are a major environmental factor on intertidal rocky shores. Morphological responses to this factor are expected to strongly influence spatial distribution of species across environmental gradients. We here analysed the shell phenotypic variability of the limpet Fissurella latimarginata using geometric morphometric analysis. The limpets were obtained from a sheltered intertidal coastal area and a wave-exposed environment. To determine whether the shell shape variation of the intertidal molluscs is linked to their resistance to differential intertidal wave exposure, mesocosm studies were developed in a hydraulic flume to explore the effects of hydrodynamic forces on this limpet species. A unidirectional current was used to test the impacts of step-by-step increased current flow velocities for each limpet. The phenotypic variability observed in the populations of F. latimarginata was associated with habitats characterized by contrasting wave exposure. Limpets from exposed environments showed a flattened, round to laterally wider and posteriorly narrower shell shape, larger foot and higher full limpet height, and were dislodged at higher velocities. A more laterally compressed and peaked shape was found in limpets from sheltered areas and these showed a lower resistance to wave action by dislodging at lower velocities.

Restoration of an alluvial wet-meadow system was conducted in the late 1990s to reestablish hydraulic interactions between the river, floodplain, and groundwater to support aquatic–riparian ecosystem function. A single-discharge approach sized the bankfull channel dimensions to the effective discharge (Qe) and three degrees of channel widening relative to the Qe design were explored to identify which design attained dynamic equilibrium in the shortest time. The three experimental channel designs were implemented with bankfull widths of 96%, 157%, and 191%, respectively, of the Qe geometry. Response trajectories were documented for channel dimensions, sediment mobility, channel morphology, floodplain connectivity, and riparian vegetation for the three channel designs, and the efficacy of a single-discharge approach for restoration was examined. Analysis of 20 years of monitoring data and hydraulic modeling revealed that each design responded differently to the imposed initial channel conditions and evolved at substantially different rates. The design with bankfull dimensions most closely approximating Qe reached dynamic equilibrium within four years of restoration, whereas the moderately over-widened channel (57% larger) exhibited slower responses toward dynamic equilibrium for some metrics and did not fully attain the Qe design dimensions within the monitoring period. The extremely-over-widened channel (91% larger) mainly induced slow rates of bed deposition that are projected to take nearly 300 years for the bankfull dimensions to narrow to the Qe width. All reaches had low bed mobility (bankfull Shields stress < 0.03) 14 years after restoration, demonstrating the challenge of reducing the drivers of channel widening, while maintaining sufficient competence for bedload transport and a sustained supply of coarse bed material for salmonid habitat. Restoration that sizes the channel to Qe can provide rapid dynamic equilibrium, but is a first-order simplification of 1) channel dynamics and 2) the range of flows needed for restoring physical and biological processes in wet-meadow systems.

A remote proglacial stream in Chilean Patagonia was examined at two temporal scales to evaluate the downstream spatial progression of morphodynamics in response to chronic climatic forcing. Historic aerial imagery indicates alluvial channel response to a reduction in glacigenic sediment delivery that is driving reach-scale alterations to the channel planform and affecting the extent and character of geomorphic reaches at centurial timescales. At the decadal timescale, fluvial morphodynamics show a downstream spatial convergence toward dynamic equilibrium. The attainment of dynamic equilibrium is not considered herein, but the trend toward such a condition is discussed. Metrics of flood magnitude, hydraulic energy thresholds, inter-annual energy expenditure, variability of channel dimensions, and continuity of sediment transport capacity illustrate how alluvial systems respond to chronic climatic forcing and deglaciation subject to the constraints of valley conditions. A conceptual proglacial alluvial model is proposed in order to characterize expected fluvial changes and to evaluate the downstream spatial progression. This model is based on a combination of previous observations of alluvial evolution and a location-for-time-substitution approach validated by an intensive 3 year field data collection program.

This study identifies and characterizes hydromorphological changes along the Rapel River downstream of the first large dam built in Chile (1968). A hydromorphological analysis is carried out to assess changes on the hydrological flow regime, bed sediments, and fluvial morphology along a 19 km river reach. Results classify current global hydrological quality as “Moderate” (according to the Indicator for Hydrological Alteration in RIverS, IAHRIS), however specific indicators within this classification scheme identified quality as “Poor”. The morphological quality decreased from “Very Good” to “Good” (assessed by the Morphological Quality Index, MQI). Changes in the planform were particularly intense during the post dam period when intensive lateral mobility occurred, with the corresponding loss of secondary river branches, and with generation of straighter and regular river sections with presence of an armor layer observed along the entire river reach. Between 1991 and 2015 channel stabilization with less lateral mobility was observed, which thought to be associated with the river new equilibrium trend. River width, sinuosity and braiding index changed at different rates along the studied river reach. Our investigation demonstrates that the Rapel River experienced changes differently than those described in the literature given its lower gradient and hydraulic interaction with the Pacific Ocean.

A cascading rainfall–landslide–tsunami event occurred on June 29th, 2022, in Todos los Santos Lake, located in southern Chile, affecting the tourist town of Petrohué. The event took place after several days of heavy rain during an extratropical cyclone. Important data were collected during a field survey, including hillslope 3D scans, lake–river bathymetry, orthomosaic photos, and an assessment of damage to public infrastructure. The analysis showed that the landslide had an estimated length, width, and depth of 175 m, 40 m, and 1.5 m, respectively, which resulted in a total volume of 10,500 m3. The underwater runout distance of the landslide was estimated at 40 m, with a final water depth of 12 m. The initial tsunami wave was observed to be ~1 m, and since the distance from the landslide to the town was ~500 m, an arrival time of ~1 min was observed. Despite the small tsunami amplitudes, the pedestrian bridge of the floating pontoon collapsed due to the flow current and vertical oscillations. The results of the numerical simulation of the tsunami supported the observed data. They showed that the impact of the tsunami was only in the near field and was influenced by the bathymetry, such that refraction and edge waves were observed. The landslide occurred in an area where previous debris flows took place in 2013 and 2015. The main finding of the present research is that the occurrences of this and previous landslides were controlled by the presence of the Liquiñe–Ofqui fault zone, which generates broad areas of structural damage, with mechanical and chemical weathering significantly reducing rock strength. These observations provide a warning regarding the susceptibility of similar regions to other trigger events such as earthquakes and rainfall. This recent landslide highlights the need for a more comprehensive hazard assessment, for which probabilistic analysis could be focused on large active strike-slip fault systems. It also highlights the importance of community awareness, particularly in areas where tourism and real estate speculation have significantly increased urban development.