Journal of Geophysical Research F: Earth Surface

Ice‐cored permafrost landscapes are highly sensitive to disturbance and have the potential to undergo dramatic geomorphic transformations in response to climate change. The acceleration of thermokarst activity in the lower Mackenzie and Peel River watersheds of northwestern Canada has led to the development of large permafrost thaw slumps and caused major impacts to fluvial systems. Individual “mega slumps” have thawed up to 10⁶ m³of ice‐rich permafrost. The widespread development of these large thaw slumps (up to 40 ha area with headwalls of up to 25 m height) and associated debris flows drive distinct patterns of stream sediment and solute flux that are evident across a range of watershed scales. Suspended sediment and solute concentrations in impacted streams were several orders of magnitude greater than in unaffected streams. In summer, slump impacted streams displayed diurnal fluctuations in water levels and solute and sediment flux driven entirely by ground‐ice thaw. Turbidity in these streams varied diurnally by up to an order of magnitude and followed the patterns of net radiation and ground‐ice ablation in mega slumps. These diurnal patterns were discernible at the 10³ km² catchment scale, and regional disturbance inventories indicate that hundreds of watersheds are already influenced by slumping. The broad scale impacts of accelerated slumping are indicated by a significant increase in solute concentrations in the Peel River (70,000 km²). These observations illustrate the nature and magnitude of hydrogeomorphic changes that can be expected as glaciogenic landscapes underlain by massive ice adjust to a rapidly changing climate.

Coastal zone management requires the ability to predict coastline response to storms and longer‐term seasonal to interannual variability in regional wave climate. Shoreline models typically rely on extensive historical observations to derive site‐specific calibration. To circumvent the challenge that suitable data sets are rarely available, this contribution utilizes twelve 5+ year shoreline data sets from around the world to develop a generalized model for shoreline response. The shared dependency of model coefficients on local wave and sediment characteristics is investigated, enabling the model to be recast in terms of these more readily measurable quantities. Study sites range from microtidal to macrotidal coastlines, spanning moderate‐ to high‐energy beaches. The equilibrium model adopted here includes time varying terms describing both the magnitude and direction of shoreline response as a result of onshore/offshore sediment transport between the surf zone and the beach face. The model contains two coefficients linked to wave‐driven processes: (1) the response factor (φ) that describes the “memory” of a beach to antecedent conditions and (2) the rate parameter (c) that describes the efficiency with which sand is transported between the beach face and surf zone. Across all study sites these coefficients are shown to depend in a predictable manner on the dimensionless fall velocity (Ω), that in turn is a simple function of local wave conditions and sediment grain size. When tested on an unseen data set, the new equilibrium model with generalized forms ofφandcexhibited high skill (Brier Skills Score, BSS = 0.85).

Many Karakoram glaciers periodically undergo surges during which large volumes of ice and debris are rapidly transported downglacier, usually at a rate of 1–2 orders of magnitude greater than during quiescence. Here we identify eight recent surges in the region and map their surface velocities using cross‐correlation feature tracking on optical satellite imagery. In total, we present 44 surface velocity data sets, which show that Karakoram surges are generally short‐lived, lasting between 3 and 5 years in most cases, and have rapid buildup and relaxation phases, often lasting less than a year. Peak velocities of up to 2 km a⁻¹ are reached during summer months, and the surges tend to diminish during winter months. Otherwise, they do not follow a clearly identifiable pattern. In two of the surges, the peak velocity travels down‐ice through time as a wave, which we interpret as a surge front. Three other surges are characterized by high velocities that occur simultaneously across the entire glacier surface, and acceleration and deceleration are close to monotonic. There is also no consistent seasonal control on surge initiation or termination. We suggest that the differing styles of surge can be partly accounted for by individual glacier configurations and that while some characteristics of Karakoram surges are akin to thermally controlled surges elsewhere (e.g., Svalbard), the dominant surge mechanism remains unclear. We thus propose that these surges represent a spectrum of flow instabilities and the processes controlling their evolution may vary on a glacier by glacier basis.

Observing the critical zone (CZ) below the top few meters of readily excavated soil is challenging yet crucial to understanding Earth surface processes. Near‐surface geophysical methods can overcome this challenge by imaging the CZ in three dimensions (3‐D) over hundreds of meters, thus revealing lateral heterogeneity in subsurface properties across scales relevant to understanding hillslope erosion, weathering, and biogeochemical cycling. We imaged the CZ under a soil‐mantled ridge developed in granitic terrain of the Laramie Range, Wyoming, using data from five boreholes and a 3‐D volume (970 by 600 by 80 m) of seismic velocities generated by ordinary kriging of 25 two‐dimensional seismic refraction transects. The observed CZ structure under the ridge broadly matches predictions of two recently proposed hypotheses: the uppermost surface of weathered bedrock is consistent with subsurface weathering driven by bedrock drainage and subsurface topography defining the top of unweathered protolith is consistent with fracturing predicted from topographic and regional stresses. In contrast, differences in slope aspect along the ridge are too subtle to explain observed variations in regolith structure. Our observations suggest that multiple processes, each of which may dominate at different depths, work in concert to regulate deep CZ structure.

Here we examine 31 glaciers in the West Kunlun Shan of the northwestern Tibetan Plateau and identify 9 as surge type. The method is based on satellite synthetic aperture radar and Landsat optical images, the former going back to 1992, the latter to 1972. To identify surge‐type glaciers, we consider temporal changes in velocity, changes in glacier terminus position, propagation of a surge bulge, presence of looped and/or contoured medial moraines, and extensive crevassing. Other than the nine surge‐type glaciers, we identify two that have likely surged, and six that may be surge type. But no glacier surges more than once during the observation period, meaning that the recurrence interval exceeds 42 years. In addition, we examine the evolution of the surface velocities at two surging glaciers with the unprecedented temporal resolution of down to 11 days over ∼7 years. The results show clear seasonal modulations by as much as ∼200% in early winter against those in early summer. This seasonal modulation in surface velocity suggests the presence of surface meltwater that reroutes through the englacial and subglacial drainage systems. Thus, our findings suggest that the hydrological processes originating in the surface meltwater play an important role in maintaining the yearlong active surging phase.

Phản hồi giữa động lực học của thảm thực vật, quá trình hình thành đất và sự phát triển địa hình ảnh hưởng đến "vùng quan trọng" — bộ lọc sống của chu kỳ thủy văn, địa hóa, và chu trình đá/trầm tích của Trái đất. Đánh giá tầm quan trọng của những phản hồi này, đặc biệt rõ nét trong các hệ thống hạn chế nước, vẫn là một thách thức cơ bản xuyên ngành. Các "đảo trời" ở miền nam Arizona cung cấp một thí nghiệm tự nhiên được xác định rõ ràng liên quan đến những phản hồi này bởi vì lượng mưa trung bình hàng năm thay đổi đáng kể theo hệ số năm trên khoảng cách khoảng 10 km ở những khu vực có kiểu đá (đá granit) và lịch sử kiến tạo tương tự. Tại đây, chúng tôi tổng hợp dữ liệu phân bố không gian có độ phân giải cao về Chuyển đổi Năng lượng và Khối lượng Hiệu quả (EEMT: năng lượng có sẵn để điều khiển phong hoá đá gốc), sinh khối trên mặt đất, độ dày đất, độ dốc địa hình theo quy mô sườn đồi, và mật độ thoát nước trong hai dãy núi như vậy (Santa Catalina: SCM; Pinaleño: PM). Tồn tại sự tương quan mạnh giữa các biến thảm thực vật - đất - địa hình, biến đổi phi tuyến theo độ cao, vì vậy những phần thấp, khô, ấm của các dãy núi này được đặc trưng bởi sinh khối trên mặt đất tương đối thấp, đất mỏng, chất hữu cơ trong đất tối thiểu, dốc đứng, và mật độ thoát nước cao; ngược lại, ở độ cao cao hơn, mát mẻ, ẩm ướt hơn, có sinh khối cao hơn một cách hệ thống, đất dày hơn giàu chất hữu cơ, dốc hơn nhẹ nhàng, và mật độ thoát nước thấp hơn. Để kiểm tra xem các phản hồi eco-pedo-địa hình có điều khiển mô hình này hay không, chúng tôi đã phát triển một mô hình tiến hóa cảnh quan kết hợp quá trình hình thành đất và phát triển địa hình trên quy mô thời gian địa chất, với tốc độ phụ thuộc rõ rệt vào mật độ thảm thực vật. Mô hình tự tổ chức thành các trạng thái tương tự như đã quan sát ở SCM và PM. Kết quả của chúng tôi nhấn mạnh tầm quan trọng tiềm năng của các phản hồi eco-pedo-địa hình, trung gian bởi độ dày đất, trong các hệ thống hạn chế nước.

The transition of flow between laterally confined channels and the unchannelized delta front controls the morphodynamic evolution of river deltas but has rarely been measured at the field scale. We quantify flow patterns and bathymetry that define the evolution of the subaqueous delta front on the Wax Lake Delta, a rapidly prograding delta in coastal Louisiana. A significant portion of flow (∼59%) departs the channel network over lateral channel margins as opposed to the downstream channel tips. Bathymetric surveys and remotely sensed estimates of flow direction allow spatial changes in flow velocity to be quantified and patterns of erosion and deposition to be estimated. Shallowing along channel margins produces spatial acceleration and erosion. Lateral spreading, deceleration, and deposition occur within three to eight channel widths outside of the channel margins. In interdistributary bays, the shape of each flow path is constrained by “nourishment boundaries” that separate the outflows from neighboring channels. Deposit elevation decreases with a basinward slope of 2.4 × 10⁻⁴ with distance from a channel margin along any flow path, regardless of the channel or location that flow departed the network. Bathymetric depressions called “interdistributary troughs” form along nourishment boundaries where flow paths are the longest and deposit elevation is correspondingly low. We conclude that the deposit morphology exerts a strong control on bathymetric evolution and that interaction between neighboring channels and even neighboring deltas can influence delta front morphology.

Alluvial rivers are shaped by interactions of flow and sediment transport. Their lower reaches to the world's oceans are highly dynamic, often presenting engineering and management challenges. Here we analyzed over 6,000 single‐beam cross‐sectional measurements surveyed in 1992, 2004, and 2013 in the last 500‐km reach of the highly engineered Mississippi River, also known as the lowermost Mississippi River or LmMR, starting from the river's Gulf outlet to its avulsion into the Atchafalaya River. We applied inverse distance weighted interpolation to downscale the survey records into 10 × 10 m digital elevation models. We assessed riverbed deformation from bank to bank and quantified changes in riverbed sediment volume. The goal of our study is to test the hypothesis that the lower reach of a large alluvial river can function as a conduit for sediment transport under the current engineering focus of navigation safety and flood control. Our analysis shows that in the past two decades, nearly 70% of the riverine sand was trapped within the LmMR, and continuous riverbed aggradation occurred below the Mississippi‐Atchafalaya diversion, presenting favorable backwater conditions for avulsion. Backwater effects have mainly controlled riverbed deformation in the LmMR, while flow reduction may have also contributed to the channel aggradation in the uppermost and lowermost reaches. The study reveals the considerable complexity of geomorphic responses of a large alluvial river to human interventions, strongly suggesting that future river engineering and management of the world's alluvial rivers should focus on strategies and solutions that will improve sediment transport.