Thomas Dunne

Ongoing and planned hydroelectric dam construction in the Andes will alter water and sediment delivery to downstream rivers, with likely substantial impacts on river and floodplain morphology, hydrology, and habitats. These disturbances will be greatest along laterally dynamic rivers with strong current channel-floodplain coupling, such as Andean foreland rivers. Using a hydro-morphodynamic river model, we show that these floodplains are rapidly transformed through widespread vertical and lateral erosion with persisting long-term effects: Erosion creates secondary floodplains entrenched several meters below pre-dam levels within a century. This degradation of the channel-floodplain complex changes floodplain inundation patterns and frequency and disconnects riparian water bodies from the active river. Such decoupling will be most severe in rivers characterized by high sediment loads and lateral migration rates before dam construction. Our simulations reveal that predictive approaches that neglect geomorphic changes, including lateral river mobility, may be of limited utility in assessing dam impacts on the construction and functioning of wetland habitats.

Quantifying sequences of events and materials involved in the growth and dispersal of post‐wildfire debris flows across entire mountain catchments and piedmont fans is rarely possible. However, understanding these processes facilitates assessing future flow magnitudes and recurrence risk. This study analyzed the evolution of debris flows generated by a rainstorm following near‐complete burning of vegetation in six mountain watersheds. The flows transported large volumes of boulders to downstream fans, devastating Montecito, California. With rainfall‐runoff modeling, lidar, photogrammetry, and field surveys, we quantified the hydrological and sedimentological components of the debris flows as they evolved from hillslope runoff to boulder‐rich fan deposits and ocean discharge. Runoff from burned soils drove larger amounts of rill erosion and slurry generation on shale hillslopes than on sandstones. Hillslope slurry mobilized ravel deposits and fine sediments stored in the channel network, mainly on shales. Channels draining sandstones mainly supplied the flows' boulder loads. One‐quarter of the mountain‐shed sediment escaped to the ocean, while all boulders settled on the fans. Flows confined to primary channels remained erosive across the fans except where channel gradients and dimensions decreased in response to fault‐related topography and where bridges trapped boulders, intensifying in‐channel and overbank deposition. Although the results derive from a single event, they illustrate how a sequence of processes and landscape conditions determine debris‐flow evolution across catchments and fans. Identifying debris‐flow components highlights useful measurement and modeling methods to improve prediction while highlighting current limits on understanding critical processes and transient antecedent conditions.

Quantifying sequences of events and materials involved in the growth and dispersal of post-wildfire debris flows across entire mountain catchments and piedmont fans is rarely possible. Information on these growth and dispersal processes clarifies debris-flow magnitude and risks of recurrence. We analyzed the evolution of debris flows generated by a rainstorm following near-complete burning of vegetation on six mountain watersheds. The flows transported large volumes of boulders to downstream fans, devastating Montecito, California. With rainfall-runoff modeling, lidar, photogrammetry, and field survey, we quantified the hydrological and sedimentological components of the debris flows as they evolved from hillslope runoff to boulder-rich fan deposits and ocean discharge. Runoff from burned soils drove larger amounts of rill erosion and slurry generation on shale hillslopes than on sandstones. The hillslope slurry mobilized ravel deposits and fine sediments stored in the channel network, mainly on shales. Channels draining sandstones mainly supplied the flows' boulder loads. Of the volumes leaving the mountains, one-quarter escaped to the ocean, but all boulders settled on the fans. Flows confined to primary channels remained erosive across the fans except where channel gradients and dimensions decreased in response to fault-related topography and where bridges trapped boulders, intensifying in-channel and overbank deposition. Although the results derive from a single event, they illustrate how a sequence of processes and landscape conditions determined debris-flow evolution over catchments and fans. Identifying debris-flow components highlights useful measurement and modeling methods to improve prediction as well as current limits on understanding critical processes and transient antecedent conditions.

We used lidar differencing and field observations to map volumes, and interpret the origins of, sediment mobilized from mountain canyons by large post‐wildfire debris flows near Montecito, CA, USA in 2018. The debris flows progressively entrained and partially redeposited 550,000 m³ of previously stored channel sediments throughout the canyon networks. The observations that scour depths and volumes were highest where the largest volumes of bouldery colluvium and debris‐flow deposits had accumulated, and that scour persisted beyond the mountain front, indicates that debris‐flow volumes in this extreme event were ultimately controlled by the coarse sediment reservoir available for scour. Because the volumes of available stored sediment result from the stochastic interaction of colluvial mass wasting, the magnitude and frequency of previous debris flows, and the accommodation space provided by valley morphology, the study reinforces the importance of estimating stored sediment volumes when developing debris‐flow hazard assessments.

Landscapes after wildfire commonly experience accelerated hillslope erosion, which often contributes to the mobilization and volume of debris flows. However, quantitative studies of the erosion and its relationship to rainfall, runoff, and landscape characteristics have been limited to a narrow range of physiographic conditions. We estimated the volume and delivery rate of slurry (a water‐sediment mixture) supplied to stream channels during a post‐wildfire rainstorm that generated large debris flows in six catchments above Montecito, CA, in 2018. We mapped the distribution of rills and measured their cross‐sectional geometries to quantify the influences of runoff, lithology, and hillslope characteristics on the sediment volumes released by rill erosion, and we scaled the results up to the 19.5 km² of burned hillslopes in the source catchments. We computed the likely rate of surface runoff during the rainstorm and developed an empirical model for the evolution of a representative hillslope‐spanning rill to illustrate the magnitude and speed of the erosion process. Rilling was the dominant form of erosion across the hillslopes of the source catchments, and the rapid evacuation and mixing of water and sediment during rill formation supplied a slurry with high solids concentrations to stream channels. Colluvium on shale formations was more continuous, finer‐grained, and probably less permeable than colluvium on sandstones, and these differences affected the extent and dimensions of rills. As a result, shale hillslopes were the dominant source of slurry to the debris flows and supplied over twice as much slurry per unit burn area as sandstones.

Overland flow self-organized into rills and eroded colluvium from steep hillslopes after a wildfire in the Santa Ynez Mountains. During 12-15 minutes of runoff, rill erosion generated a slurry that scoured bouldery alluvium from mountain canyons to form large debris flows that severely impacted the community of Montecito, CA. The timing, volume, and peak discharge of the debris flows, and their capacity to scour boulders from canyons, depended on the mechanism and generation rate of the granular fluid (slurry) responsible for the sediment transport. Field surveys and aerial imagery revealed dense networks of rills on bare, burned hillslopes, and levees of fine-grained sediment lining the rill margins indicated that flows were highly viscous and the water and sediment were already intimately mixed before entering stream channels. We mapped networks of rills and measured their cross-sectional geometry to quantify the influences of lithology, hillslope length, gradient, and planform on the sediment volumes released by the rills and their volumetric contribution to the canyon-scale debris flows in six watersheds. We interpreted the mechanism of slurry generation by rill erosion, as far as field evidence will allow, through the lens of field and laboratory experiments conducted by others under comparable conditions. We used the geometry of rills, and the duration of intense rainfall and modelled surface runoff, to develop an empirical model of the rate of slurry generation by rill erosion and to suggest a mechanism by which the necessary intensive mixing occurred.

The extent and intensity of impacts of multiple new dams in the Amazon basin on specific biological groups are potentially large, but still uncertain and need to be better understood. It is known that river disruption and regulation by dams may affect sediment supplies, river channel migration, floodplain dynamics, and, as a major adverse consequence, are likely to decrease or even suppress ecological connectivity among populations of aquatic organisms and organisms dependent upon seasonally flooded environments. This article complements our previous results by assessing the relationships between dams, our Dam Environmental Vulnerability Index (DEVI), and the biotic environments threatened by the effects of dams. Because of the cartographic representation of DEVI, it is a useful tool to compare the potential hydrophysical impacts of proposed dams in the Amazon basin with the spatial distribution of biological diversity. As the impact of Amazonian dams on the biota of both rivers and periodically flooded riparian environments is severe, DEVIs from different Amazonian tributary basins are contrasted with patterns of diversity and distribution of fish, flooded forest trees and bird species. There is a consistent relationship between higher DEVI values and the patterns of higher species richness and endemism in all three biological groups. An assessment of vulnerability at the scale of tributary basins, the assessment of biodiversity patterns related to DEVI, and the analysis of teleconnections at basin scale, demonstrate that recent construction of dams is affecting the biota of the Amazon basin. The evidence presented here predicts that, if currently planned dams are built without considering the balance between energy production and environmental conservation, their cumulative effects will increase drastically and represent a major threat to Amazonian biodiversity.

Large‐scale river restoration programs have emerged recently as a tool for improving spawning habitat for native salmonids in highly altered river ecosystems. Few studies have quantified the extent to which restored habitat is utilized by salmonids, which habitat features influence redd site selection, or the persistence of restored habitat over time. We investigated fall‐run Chinook salmon spawning site utilization and measured and modeled corresponding habitat characteristics in two restored reaches: a reach of channel and floodplain enhancement completed in 2013 and a reconfigured channel and floodplain constructed in 2002. Redd surveys demonstrated that both restoration projects supported a high density of salmon redds, 3 and 14 years following restoration. Salmon redds were constructed in coarse gravel substrates located in areas of high sediment mobility, as determined by measurements of gravel friction angles and a grain entrainment model. Salmon redds were located near transitions between pool‐riffle bedforms in regions of high predicted hyporheic flows. Habitat quality (quantified as a function of stream hydraulics) and hyporheic flow were both strong predictors of redd occurrence, though the relative roles of these variables differed between sites. Our findings indicate that physical controls on redd site selection in restored channels were similar to those reported for natural channels elsewhere. Our results further highlight that in addition to traditional habitat criteria (e.g., water depth, velocity, and substrate size), quantifying sediment texture and mobility, as well as intragravel flow, provides a more complete understanding of the ecological benefits provided by river restoration projects.

Repeated measurement of tephra erosion near Mount St. Helens over a 30‐year period at steel stakes, installed on 10 hillslopes in the months following the 1980 eruption, provides a unique long‐term record of changing processes, controls and rates of erosion. Intensive monitoring in the first three post‐eruption years showed erosion declined rapidly as processes shifted from sheetwash and rilling to rainsplash. To test predictions about changes to long‐term rates and processes made based on the 3‐year record, we remeasured sites in 1992, 2000 and 2010. Average annual erosion from 1983 to 1992 averaged 3.1 mm year⁻¹and ranged from 1.4 to 5.9 mm year⁻¹, with the highest rate on moderately steep slopes. Stakes in rills in 1983 generally recorded deposition as the rills became rounded, filled and indistinct by 1992, indicating a continued shift in process dominance to rainsplash, frost action and bioturbation. Recovering plants, where present, also slowed erosion. However, in the second and third decades even unvegetated hillslopes ceased recording net measurable erosion; physical processes had stabilized surfaces from sheetwash and rill erosion in the first few years, and they appear to have later stabilized surfaces against rainsplash erosion in the following few decades. Comparison of erosion rates with suspended sediment flux indicates that within about 6 years post‐eruption, suspended sediment yield from tephra‐covered slopes was indistinguishable from that in forested basins. Thirty years after its deposition, on moderate and gentle hillslopes, most tephra remained; in well‐vegetated areas, plant litter accumulated and soil developed, and where the surface remained barren, bioturbation and rainsplash redistributed and mixed tephra. These findings extend our understanding from shorter‐term studies of the evolution of erosion processes on freshly created substrate, confirm earlier predictions about temporal changes to tephra erosion following eruptions, and provide insight into the conditions under which tephra layers are preserved. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.