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Open-source designs for turbidity and depth sensors are becoming increasingly capable and available, but the knowledge required to construct them limits their use compared to expensive, commercial sensors. Here, we present an open-source optical backscatter and water pressure sensor that can be ordered almost fully assembled, requires no coding to deploy, and costs approximately $50 USD. We share three examples of these sensors’ ability to facilitate new research. First, we observed complex changes in spatial and temporal patterns of suspended sediment transport in the Arctic Sagavanirktok River using a network of sensors. Second, we measured turbidity during the freeze-up period in the Tanana River, a period of high risk to sensors. Last, we built and deployed sensors with middle-school students to monitor turbidity under full ice cover on the Tanana River. The success of open-source sensors in these examples shows a marked increase in scale and accessibility of river science.

Himalayan lakes represent critical water resources, culturally important wa- terbodies, and potential hazards. Some of these experience dramatic water- level changes, responding to seasonal monsoon rains and post-monsoonal drain- ing. We present in situ observations of extreme (>12 meter) seasonal water- level changes in a 50-m deep lateral-moraine-dammed lake (lacking surface outflow), during a 16-month period, equivalent to a 5x106 m3 volume change annually. Data were used to construct a hydraulic model, which was used to estimate hydraulic conductivities of the moraine soils damming the lake. A higher moraine-dammed lake exhibits similar fluctuations. These lake levels lie above the nearby glacier surface. Several other lakes in the region exhibit minor seasonal water-level fluctuations, but each discharge through a surface outflow and lie below the glacier surface elevation. These results indicate that adjacent lakes may exhibit very different filling/draining dynamics and have different fates in the next few decades, based on presence/absence of surface outflows and elevation relative to retreating glaciers.

Estuaries worldwide have evolved over the past few centuries under development activities like dredging and shoreline reclamation, which commonly lead to increased channel depths and reduced intertidal areas. The Coos Estuary offers a useful example of how these changes, common to diverse global estuaries, have altered tidal and salt dynamics, with implications for estuarine habitats. In the past 150 years, the primary navigation channel has been deepened from ~ 6.7 to 11 m, generating a 12% decrease in estuary area and 21% increase in volume. To evaluate the present and future impacts on the Coos and similar estuaries, a hydrodynamic model was implemented using a detailed bathymetric dataset compiled from multiple data sources including agency charts, water-penetrating lidar, and single-beam-sonar small-vessel surveys. The model was then re-run using grids constructed from 1865 survey data and a future proposed dredging plan. Changes in the hypsometry from 1865 to present have driven a 33% increase in tidal amplitude, an 18% increase in salinity intrusion length, a doubling of the subtidal salt flux, and an increase in ebb dominance of currents. A proposed channel-depth increase from 11 to 14 m is predicted to generate a negligible change in tidal range and a small increase in the salinity intrusion length. These results highlight the utility of curating high-resolution bathymetric datasets for coastal management applications through modeling. The historical and modern models quantify how local bathymetric modifications can significantly alter tidal and salinity regimes and provide context for estuarine response to global climate-change drivers.

Optical backscatter sensors (OBSs) are commonly used to measure the turbidity, or light obscuration, of water in fresh and marine environments and various industrial applications. These turbidity measurements are commonly calibrated to yield total suspended solids (TSS) or suspended sediment concentration (SSC) measurements for water quality, sediment transport, and diverse other research and environmental management applications. Commercial sensors generally cost >$1000-3000. Here we leveraged simple, low-cost microprocessors, electronics, and housing components to design and construct open-source OBSs for

The 2011–2014 removal of two dams from the Elwha River, WA, delivered ~ 19 Mt of sediment to the marine environment, creating an opportunity to study the sensitivity of a coastal ecosystem to large-scale sediment input. Macroalgae, the primary habitat-forming species in the nearshore, disappeared from the region. It was hypothesized that this mortality event was caused by a reduction in benthic light availability due to increased turbidity. To investigate this connection, nearshore processes and benthic light availability were monitored at 7 locations along the 10-m isobath in 2016 and 2017. The primary driver of light attenuation was suspended sediment, with measured chlorophyll-a and CDOM concentrations contributing

Observations of ocean surface waves at three sites along the northern coast of Alaska show a strong coupling with seasonal sea ice patterns. In the winter, ice cover is complete, and waves are abse...

Alaskan Arctic coastlines are protected seasonally from ocean waves by the presence of coastal and shorefast sea ice. \ud This study presents field observations collected during the autumn 2019 freeze up near Icy Cape, a coastal headland in the Chukchi Sea of the Western Arctic. The evolution of the coupled air-ice-ocean-wave system during a four-day wave event was monitored using drifting wave buoys, a cross-shore mooring array, and ship-based measurements. The incident wave field with peak period of 2.5 s was attenuated by coastal pancake and frazil sea ice, reducing significant wave height by 40\% over less than 5 km of cross-shelf distance spanning water depths from 13 to 30 m. Spectral attenuation coefficients are evaluated with respect to wave and ice conditions and the proximity to the ice edge. Attenuation rates are found to be three times higher within 500 m of the ice edge, relative to values farther in the ice cover. Attenuation coefficients are in the range of $\langle 2.3,2.7\rangle$ \si{\meter^{-1}}, and follow a power-law dependence on frequency.

Alaskan Arctic coastlines are protected seasonally from ocean waves by the presence of coastal and shorefast sea ice. This study presents field observations collected during the autumn 2019 freeze up near Icy Cape, a coastal headland in the Chukchi Sea of the Western Arctic. The evolution of the coupled air-ice-ocean-wave system during a four-day wave event was monitored using drifting wave buoys, a cross-shore mooring array, and ship-based measurements. The incident wave field with peak period of 2.5 s was attenuated by coastal pancake and frazil sea ice, reducing significant wave height by 40\% over less than 5 km of cross-shelf distance spanning water depths from 13 to 30 m. Spectral attenuation coefficients are evaluated with respect to wave and ice conditions and the proximity to the ice edge. Attenuation rates are found to be three times higher within 500 m of the ice edge, relative to values farther in the ice cover. Attenuation coefficients are in the range of $\langle 2.3,2.7\rangle$ \si{\meter^{-1}}, and follow a power-law dependence on frequency.

Rapid decline in seasonal sea ice has been linked to increased surface wave activity and shoreline erosion in the coastal Arctic. This trend poses a risk to communities vulnerable to flooding and storm surges. Here we focus on quantifying the relationship between coastal erosion, increasing wave activity and the role of sea ice in protecting the coast. In November 2019, we observed a three day wave event in the Chukchi Sea along the coastal barrier system near Icy Cape, Alaska. The wave event was sampled using multiple drifting SWIFT (Surface Wave Instrument Float with Tracking) buoys, a cross-shore mooring array, and ship-based CTD casts. This provided datasets for different ice types in both Eulerian and Lagrangian reference frames. Pancake and frazil sea ice near the coast attenuated the incident wave field, such that the significant wave height reduced from 3 to 1.5 m over less than 5 kilometers. The wave data combined with in-situ ice observations and satellite imagery are used to calculate spectral attenuation of wave energy segregated by ice type. Furthermore, observed temperature, mean circulation and surface heat fluxes are used to address the evolution of sea ice throughout the event. Supported by the National Science Foundation and the Office of Naval Research.

The Mekong River Delta is one of the largest in Asia. To understand its sediment distribution, thickness, mass budget, stratigraphic sequences and sediment-transport process, extensive geophysical and geochemical surveys were conducted on the inner portions of the adjacent continental shelf. Analyses of > 80 high-resolution Chirp-sonar profiles show the Mekong River has formed a classic sigmoidal cross-shelf clinoform in the proximal areas, up to 15 m thick, with topset, foreset and bottomset facies, but constrained to water depths of 250 > 300 km southwestward to the tip of the Ca Mau Peninsula, forming a distal mud depocenter up to 22 m thick, and extending into the Gulf of Thailand. A large erosional trough or channel (up to 8 m deeper than the surrounding seafloor and parallel to the shore) was found on the top of the clinoform, east of the Ca Mau Peninsula. Based on the thicknesses and distribution revealed by Chirp sonar profiles, the total estimated volume of the Mekong River subaqueous clinoform on the shelf is ~120 km 3 , which is equivalent to ~120–140 × 10 9 t of sediment using an average sediment dry-bulk density of 1.0–1.2 g/cm 3 . Assuming the subaqueous deltaic deposit has formed within ~1000 yr, the calculated millennial-timescale average sediment discharge to the shelf could be 120–140 × 10 6 t per year. Spatially, the proximal subaqueous delta has accumulated ~45 × 10 9 t (~33%) of sediment; the distal part around the Ca Mau Peninsula has received ~55 × 10 9 t (~42%) of sediment; and the remaining ~35 × 10 9 t (~25%) has accumulated within the central transition area, although the coastline and shoreface in this area are presently eroding. The spatially averaged 1000-yr-scale accumulate rate is up to 2 cm/yr. Compared to other tide-dominated fluvial dispersal systems, the Mekong River system has a relatively young (≤1000 yr) subaqueous delta, a shallow rollover at 4–6 m water depth, gentle foreset gradients (0.03–0.57°), and a short cross-shelf dimension of 15–20 km within 20-m water depth. Like the Amazon, Po, and Yangtze rivers, the Mekong River has developed a pervasive along-shelf deposit, which in this case extends > 250 > 300km to the southwest as a result of the superimposed tidal processes, wave-induced resuspension, and a strong low-flow season coastal current.

Thirty-two kasten cores, collected from the proximal Mekong continental shelf, have been analyzed for their excess 210Pb distributions in an effort to establish rates of sediment accumulation over the past 100 years. The length of the cores varied from 0.5 to 3 m, and stations sampled topset, foreset, and bottomset beds (water depths 7–21 m). Apparent excess 210Pb sediment accumulation rates ranged from > 10 cm/y (no down-core decrease of excess activity over 300 cm core length) near the Song Hau river mouth, to 1–3 cm/y in topset and foreset beds within 20–50 km of the river mouth, to rates as low as 0.4 cm/y in cores from bottomset beds. The 210Pb sediment accumulation rates yield an overall sediment burial rate of 6.1 × 1013 g/y for the proximal deltaic deposits, which corresponds to 43% of the total modern Mekong sediment burial on the southern Vietnam shelf (1.4 × 1014 g/y; based on our 210Pb and seismic data and 210Pb data from the literature). This shelf burial rate is in reasonable agreement with current long-term estimates of Mekong River sediment discharge (1.3–1.6 × 1014 g/y) from the literature. The inventory of excess 210Pb in the proximal Mekong deltaic deposits indicates that the shoreward flow of offshore water (entrained during river/ocean mixing) is approximately twice the flow of the Mekong freshwater discharge. Organic-carbon 14C ages were measured on 10 cores from the proximal Mekong delta and compared to 210Pb sediment accumulation rates in the same core. The 210Pb accumulation rates in all 10 cores were considered to be more robust and accurate than the 14C geochronologies, primarily because of down-core variations in the source of organic carbon deposited on the seafloor (old terrestrial carbon versus younger marine carbon). Variations in the source of organic carbon accumulating in the seabed were resolved by measuring the δ13C value of the seabed organic carbon.

Fluvial sediment is the major source for the formation and development of the Mekong Delta. This paper aims to analyse the dynamics of suspended sediment and to investigate the roles of different processes in order to explore flux pattern changes. We applied modelling on two scales, comprising a large-scale model (the whole delta) to consider the upstream characteristics, particularly the Tonle Sap Lake's flood regulation, and a smaller-scale model (tidal rivers and shelf) to understand the sediment processes on the subaqueous delta. A comprehensive comparison to in-situ measurements and remote sensing data demonstrated that the model is capable of qualitatively simulating sediment dynamics on the subaqueous delta. It estimates that the Mekong River supplied an amount of 41.5 mil tons from April 2014 to April 2015. A substantial amount of sediment delivered by the Mekong River is deposited in front of the river mouths in the high flow season and resuspended in the low flow season. A sensitivity analysis shows that waves, baroclinic effects and bed composition strongly influence suspended sediment distribution and transport on the shelf. Waves in particular play an essential role in sediment resuspension. The development of this model is an important step towards an operational model for scientific and engineering applications, since the model is capable of predicting tidal propagation and discharge distribution through the main branches, and in predicting the seasonal SSC and erosion/deposition patterns on the shelf, while it is forced by readily available inputs: discharge at Kratie (Cambodia), GFS winds, ERA40 reanalysis waves, and TPXO 8v1 HR tidal forcing.

Compound deltas, composed of a subaerial delta plain and subaqueous clinoform, are common termini of large rivers. The transition between clinoform topset and foreset, or subaqueous rollover point, is located at 25–40-m water depth for many large tide-dominated deltas; this depth is controlled by removal of sediment from the topset by waves, currents, and gravity flows. However, the Mekong Delta, which has been classified as a mixed-energy system, has a relatively shallow subaqueous rollover at 4–6-m depth. This study evaluates dynamical measurements and seabed cores collected in Sep 2014 and Mar 2015 to understand processes of sediment transfer across the subaqueous delta, and evaluate possible linkages to geometry. During the southwest rainy monsoon (Sep 2014), high river discharge, landward return flow under the river plume, and regional circulation patterns facilitated limited sediment flux to the topset and foreset, and promoted alongshore flux to the northeast. Net observed sediment fluxes in Sep 2014 were landward, however, consistent with hypotheses about seasonal storage on the topset. During the northeast rainy monsoon, low river discharge and wind-driven currents facilitated intense landward and southwestward fluxes of sediment. In both seasons, bed shear velocities frequently exceeded the 0.01–0.02 m/s threshold of motion for sand, even in the absence of strong wave energy. Most sediment transport occurred at water depths 4 cm/yr at

Underwater melting How fast does warm ocean water melt glaciers that terminate in the sea? That question is central to understanding how fast ice sheets may lose mass, and thus how fast sea level will rise, in response to global warming, but there are few data about the process. Sutherland et al. used repeat multibeam sonar surveys to observe an Alaskan subsurface tidewater glacier face to create a time series of its melting and calving patterns. They observed melt rates up to a hundred times larger than those predicted by theory, observations that compel us to reevaluate predictions of such ice loss. Science , this issue p. 369

Sediment supplied by small mountainous rivers (SMRs) represents a major fraction of the global ocean sediment budget. Studies from the past two decades have shown that much of this sediment is dispersed by episodic wind and wave energy along storm-dominated coasts. In tidally dominated environments, however, different transport styles and deposits may result from persistent tidal dispersal. This study investigates episodic sediment releases generated by dam removal from a SMR in Washington State, in order to evaluate the mechanics of tidally dominated sediment dispersal in an energetic marine environment. The results indicate that asymmetric tidal currents with peak magnitudes of ∼50 to >80 cm/s produce daily sediment export in the direction of the dominant tidal phase (i.e., the semi-diurnal phase with faster currents and longer duration), resulting in dispersal of fluvially derived fine sediment to distal sinks. These effects are observed throughout all seasons in the presence or absence of wave events. During the first two years of dam removal, more than 8 million tonnes of sediment were discharged to the coast. The net result was little to no change in grain size at 10–60 m water depth across >70% of the seabed offshore of the river mouth. Over the remaining ∼2 to 3 km2 of the subaqueous delta, several cm of mud and sand accumulated in a sheltered coastal embayment adjacent to the river mouth. These results demonstrate that SMR discharge events may form patchy, isolated deposits—or even no deposits—along coastlines with strong tidal currents, in contrast to the mid-shelf mud belts formed on storm-dominated shelves. Over longer timescales, knowledge of the erosional capacity of local and regional tidal currents may be key to interpreting the terrestrial event record preserved in (or possibly excluded from) marine SMR deposits.

Two dams on the Elwha River, Washington State, USA trapped over 20 million m3 of mud, sand, and gravel since 1927, reducing downstream sediment fluxes and contributing to erosion of the river's coastal delta. The removal of the Elwha and Glines Canyon dams, initiated in September 2011, induced massive increases in river sediment supply and provided an unprecedented opportunity to examine the geomorphic response of a coastal delta to these increases. Detailed measurements of beach topography and nearshore bathymetry show that ~ 2.5 million m3 of sediment was deposited during the first two years of dam removal, which is ~ 100 times greater than deposition rates measured prior to dam removal. The majority of the deposit was located in the intertidal and shallow subtidal region immediately offshore of the river mouth and was composed of sand and gravel. Additional areas of deposition include a secondary sandy deposit to the east of the river mouth and a muddy deposit west of the mouth. A comparison with fluvial sediment fluxes suggests that ~ 70% of the sand and gravel and ~ 6% of the mud supplied by the river was found in the survey area (within about 2 km of the mouth). A hydrodynamic and sediment transport model, validated with in-situ measurements, shows that tidal currents interacting with the larger relict submarine delta help disperse fine sediment large distances east and west of the river mouth. The model also suggests that waves and currents erode the primary deposit located near the river mouth and transport sandy sediment eastward to form the secondary deposit. Though most of the substrate of the larger relict submarine delta was unchanged during the first two years of dam removal, portions of the seafloor close to the river mouth became finer, modifying habitats for biological communities. These results show that river restoration, like natural changes in river sediment supply, can result in rapid and substantial coastal geomorphological responses.

The coastal marine ecosystem near the Elwha River was altered by a massive sediment influx-over 10 million tonnes-during the staged three-year removal of two hydropower dams. We used time series of bathymetry, substrate grain size, remotely sensed turbidity, scuba dive surveys, and towed video observations collected before and during dam removal to assess responses of the nearshore subtidal community (3 m to 17 m depth). Biological changes were primarily driven by sediment deposition and elevated suspended sediment concentrations. Macroalgae, predominantly kelp and foliose red algae, were abundant before dam removal with combined cover levels greater than 50%. Where persistent sediment deposits formed, macroalgae decreased greatly or were eliminated. In areas lacking deposition, macroalgae cover decreased inversely to suspended sediment concentration, suggesting impacts from light reduction or scour. Densities of most invertebrate and fish taxa decreased in areas with persistent sediment deposition; however, bivalve densities increased where mud deposited over sand, and flatfish and Pacific sand lance densities increased where sand deposited over gravel. In areas without sediment deposition, most invertebrate and fish taxa were unaffected by increased suspended sediment or the loss of algae cover associated with it; however, densities of tubeworms and flatfish, and primary cover of sessile invertebrates increased suggesting benefits of increased particulate matter or relaxed competition with macroalgae for space. As dam removal neared completion, we saw evidence of macroalgal recovery that likely owed to water column clearing, indicating that long-term recovery from dam removal effects may be starting. Our results are relevant to future dam removal projects in coastal areas and more generally to understanding effects of increased sedimentation on nearshore subtidal benthic communities.

Fjords along the western Antarctic Peninsula contain rich marine ecosystems and tidewater glaciers potentially sensitive to ocean change, but much is unknown about the physical oceanography of these fjords. This paper presents a comprehensive description of the physical environment of Andvord Bay, a glacial fjord located in the Gerlache Strait on the northern end of the Peninsula. Measurements were collected as part of the FjordEco program between November 2015 and March 2017, including during three research cruises and from a number of fixed installations (sub-surface moorings, time lapse cameras, and automatic weather stations). Andvord Bay is located just north of a sill in the Gerlache Strait. This sill inhibits the direct influence of warm Upper Circumpolar Deep Water from the shelf, and deep waters in the fjord are therefore relatively cold. As a result, glaciers in Andvord Bay are not subject to the substantial ocean-driven retreat observed farther south on the Peninsula. Instead, mass flux from the glaciers occurs mainly through calving. The influence of meltwater in Andvord Bay is small compared to glacial fjords in the Arctic and in the South Shetland Islands, although summer freshening is evident in the fjord near-surface layer, likely as a result of glacial inputs. The combination of surface freshening and the absence of strong mean wind forcing in the fjord results in salinity stratification extending well into the euphotic zone, providing advantageous conditions for phytoplankton blooms. Andvord Bay is generally dynamically quiet compared to the ambient ocean, with the exception of during local katabatic wind events. Exchange with the Gerlache Strait is weak, although the deep waters of the fjord are replenished on an annual scale through slow lateral exchange with the outside. In addition, deep convection may play a role in ventilating subsurface waters in winter.