Avulsions, channel evolution and floodplain sedimentation rates of the anastomosing upper Columbia River, British Columbia, Canada

Ages of channels of the anastomosing upper Columbia River, south‐eastern British Columbia, Canada, were investigated in a cross‐valley transect by ¹⁴C dating of subsurface floodplain organic material from beneath levees. The avulsion history within the transect was deduced from these data, and morphological stages in channel development were recognized. Additionally, floodplain sedimentation rates were established. The new data demonstrate that the upper Columbia River is a long‐lived, dynamic anastomosing system. Results show that anastomosis at the study location has persisted since at least 2700 cal. years BP, with avulsions occurring frequently. At least nine channels have formed in the studied cross‐valley transect within the past 3000 years. Channel lifetimes from formation to abandonment appear to be highly variable, ranging from approximately 800 to 3000 years. Log jams provoking avulsions and/or silting up of old channels are proposed as reasons for this variability. Long‐term average floodplain sedimentation rates appear to be significantly lower than previously proposed by Smith (1983, Int. Assoc. Sedimentol. Spec. Publ., 6, 155–168). A long‐term (4550 years) average of 1·75 mm year^?1 (after compaction) was based on ¹⁴C dates, while a short‐term sedimentation rate of 0·8 mm was determined for a single, relatively small, seasonal flood in 1994 using sediment traps. However, short‐term sedimentation rates vary considerably over the floodplain, with levees aggrading up to four times faster than floodbasins. Channels of the upper Columbia River anastomosed reach follow a consistent pattern in their development, with each stage being characterized by different morphology and processes. Channel evolution comprises the following succession: (1) avulsion stage, in which a crevasse splay channel deepens by scour and levee sedimentation; (2) widening and deepening stage, in which bank slumping and bed scouring dominates; (3) infilling stage, in which either channel narrowing (bank accretion) or channel shallowing (bed accretion) takes place; and (4) abandonment stage, in which the residual (remnant) channel is filled exclusively by silt, clay and organic material. Vertical stacking (super‐ imposition) of active channels on recent channel‐fill sand bodies is a notable feature of the upper Columbia River, which suggests that reoccupation of residual channels is a common process.     

<Emphasis Type="Italic">Phragmites australis</Emphasis> invasion and expansion in tidal wetlands: Interactions among salinity,sulfide, and hydrology

Through their physiological effects on ion, oxygen, and carbon balance, respectively, salinity, sulfide, and prolonged flooding combine to constrain the invasion and spread ofPhragmites in tidal wetlands. Initial sites of vigorous invasion by seed germination and growth from rhizome fragments appear limited to sections of marsh where salinity is <10‰, sulfide concentrations are less than 0.1 mM, and flooding frequency is less than 10%. In polyhaline tidal wetlands the invasion sites include the upland fringe and some high marsh creek banks. The zones of potential invasion tend to be larger in marshes occupying lower-salinity portions of estuaries and in marshes that have been altered hydrologically. Owing to clonal integration and a positive feedback loop of growth-induced modification of edaphic soil conditions, however, a greater total area of wetland is susceptible toPhragmites expansion away from sites of establishment. Mature clones have been reported growing in different marshes with salinity up to 45‰, sulfide concentration up to 1.75 mM, and flooding frequency up to 100%. ForPhragmites establishment and expansion in tidal marshes, windows of opportunity open with microtopographic enhancement of subsurface drainage patterns, marsh-wide depression of flooding and salinity regimes, and variation in sea level driven by global warming and lunar nodal cycles. To avoidPhragmites monocultures, tidal wetland creation, restoration, and management must be considered within the context of these different scales of plant-environment interaction.     

Neoformation of Ni phyllosilicate upon Ni uptake on montmorillonite: A kinetics study by powder and polarized extended X-ray absorption fine structure spectroscopy

Wet chemistry kinetics and powder and polarized extended X-ray absorption fine structure (EXAFS and P-EXAFS) spectroscopy were combined to investigate the mechanism of Ni uptake on montmorillonite, at pH 8, high ionic strength (0.2 M Ca(NO₃)₂), initial Ni concentration of 660 μM, and solid concentration of 5.3 g/L. Approximately 20% of Ni sorbed within the first 24 h; thereafter, the Ni uptake rate slowed, and 12% of the initial Ni concentration remained in solution after 206 d of reaction time. Powder EXAFS spectra collected on wet pastes at 1, 14, 90, and 206 d showed the presence of Ni-Ni pairs at ∼3.08 Å in an amount that gradually increased with time. Results were interpreted by the nucleation of a Ni phase having either an α-Ni-hydroxide- or a Ni-phyllosilicate-like local structure. The latter possibility was confirmed by recording P-EXAFS spectra of a highly textured, self-supporting montmorillonite film prepared in the same conditions as the wet samples and equilibrated for 14 d. The orientation distribution of the c*-axes of individual clay particles off the film plane, as measured by quantitative texture analysis, was 32.8° full width at half maximum, and this value was used to correct from texture effect the effective numbers of Ni and Si nearest neighbors determined by P-EXAFS. Ni atoms were found to be surrounded by 2.6 ± 0.5 Ni atoms at 3.08 Å in the in-plane direction and by 4.2 ± 0.5 Si atoms at 3.26 Å in the out-of-plane direction. These structural parameters, but also the orientation and angular dependence of the Ni and Si shells, strongly support the formation of a Ni phyllosilicate having its layers parallel to the montmorillonite layers. The neoformation of a phyllosilicate on metal uptake on montmorillonite, documented herein for the first time, has important geochemical implications because this dioctahedral smectite is overwhelmingly present in the environment. The resulting sequestration of sorbed trace metals in sparingly soluble phyllosilicate structure may durably decrease their migration and bioavailability at the Earth’s surface and near surface.     

Biomarkers in upper Holocene Eastern North Sea and Wadden Sea sediments

Total extracts of sediment cores from five different stations in the North Sea and Wadden Sea were analysed for their biomarker composition. Only sediments of the Skagerrak contained significant amounts of marine biomarkers (mainly alkenones), other sites contained predominantly terrestrial biomarkers. Bioturbation in the Skagerrak is, however, far too high to determine sea surface temperature (SST) changes within short time intervals. These results indicate that biomarkers contained in these sediments are not useful to reconstruct climate fluctuations during the upper Holocene. High amounts of α-, β- and ω-hydroxy fatty acids as well as small amounts of α, β-dihydroxy fatty acids were released from the insoluble organic material of the sediments from the Wadden Sea station, indicating a significant input of the eelgrass Zostera marina. This was confirmed by microscopic observations. This is the first time the α,β-dihydroxy fatty acids have been found in a sediment core and they have proven to be potential biomarkers for these seagrass species.     

Faulting history at the eastern termination of the High Atlas Fault (Western High Atlas,Morocco)

At its eastern termination, the High Atlas Fault in the Western High Atlas in Morocco, consists of a splay of three faults. In the interjacent fault blocks, Neo- and Paleoproterozoic basement, forming the northernmost extremity of the NW-African Craton, is cropping out. The Precambrian basement witnesses a long history of brittle deformation starting at the end of the Pan-African Orogeny. A subsequent episode of normal faulting can be related to the development of a Hercynian basin along the northern passive margin of the cratonic promontory. With regard to the main tectonic activity in the Western High Atlas, basically two models exist: one emphasising block tectonics reflecting Mesozoic rifting followed by Alpine uplift and inversion, the other emphasising Late Paleozoic dextral wrench tectonics. The analysis of the fault activity along the splay faults reveals a predominantly Alpine history, consisting of the Triassic development of the Atlas Rift along the axial zone of the orogen, followed by uplift and inversion. The Late Jurassic to Cenozoic fault activity took place in a sinistral transpressive regime and was partitioned over the three splay faults. Dextral strike-slip fault activity could not be demonstrated in the fault blocks nor along the splay faults. Therefore the faults were probably not involved in Late Paleozoic dextral wrench tectonics.     

Modelling the Effect of Tree Foliage on Sprayer Airflow in Orchards

The effect of tree foliage on sprayer airflow through pear trees in a fruit orchard was studied and modelled in detail. A new three-dimensional (3-D) computational fluid dynamics model that integrates the 3-D canopy architecture with a local closure model to simulate the effect of the stem and branches and leaves of trees separately on airflow was developed. The model was validated with field observations made in an experimental orchard (pcfruit, Sint-Truiden, Belgium) in spring and summer 2008 and was used to investigate the airflow from three air-assisted orchard sprayers (Condor V, Duoprop and AirJet quatt). Velocity magnitudes were measured before and behind leafless and fully-leafed pear canopies across the row while the operating sprayers are passing along the row, and were compared with the simulations. The simulation results predicted the measured values well with all the local relative errors within 20%. The effect of foliar density on airflow from the three air assisted sprayers was manifested by changing the magnitude and direction of the sprayers’ air velocity behind the canopy, especially at the denser regions of the canopy and by changing the pattern of velocity decay horizontally along the jet. The developed methodology will also allow a thorough investigation of atmospheric airflow in canopy structures.     

Oblique aggradation: a novel explanation for sinuosity of low‐energy streams in peat‐filled valley systems

Low‐energy streams in peatlands often have a high sinuosity. However, it is unknown how this sinuous planform formed, since lateral migration of the channel is hindered by relatively erosion‐resistant banks. We present a conceptual model of Holocene morphodynamic evolution of a stream in a peat‐filled valley, based on a palaeohydrological reconstruction. Coring, ground‐penetrating radar (GPR) data, and ¹⁴C and OSL dating were used for the reconstruction. We found that the stream planform is partly inherited from the Late‐Glacial topography, reflecting stream morphology prior to peat growth in the valley. Most importantly, we show that aggrading streams in a peat‐filled valley combine vertical aggradation with lateral displacement caused by attraction to the sandy valley sides, which are more erodible than the co‐evally aggrading valley‐fill. Owing to this oblique aggradation in combination with floodplain widening, the stream becomes stretched out as channel reaches may alternately aggrade along opposed valley sides, resulting in increased sinuosity over time. Hence, highly sinuous planforms can form in peat‐filled valleys without the traditional morphodynamics of alluvial bed lateral migration. Improved understanding of the evolution of streams provides inspiration for stream restoration. Copyright © 2016 John Wiley & Sons, Ltd.     

Evaluation of aDcp processing options for secondary flow identification at river junctions

Secondary circulation in river confluences results in a spatial and temporal variation of fluid motion and a relatively high level of morphodynamic change. Acoustic Doppler current profiler (aDcp) vessel-mounted flow measurements are now commonly used to quantify such circulation in shallow water fluvial environments. It is well established that such quantification using vessel-mounted aDcps requires repeated survey of the same cross-section. However, less attention has been given to how to process these data. Most aDcp data processing techniques make the assumption of homogeneity between the measured radial components of velocity. As acoustic beams diverge with distance from the aDcp probe, the volume of the flow that must be assumed to be homogeneous between the beams increases. In the presence of secondary circulation cells, and where there are strong rates of shear in the flow, the homogeneity assumption may not apply, especially deeper in the water column and close to the bed. To reduce dependence on this assumption, we apply a newly-established method to aDcp data obtained for two medium-sized (~60–80 m wide) gravel-bed river confluences and compare the results with those from more conventional data processing approaches. The comparison confirms that in the presence of strong shear our method produces different results to more conventional approaches. In the absence of a third set of fully independent data, we cannot demonstrate conclusively which method is best, but our method involves less averaging and so in the presence of strong shear is likely to be more reliable. We conclude that it is wise to apply both our method and more conventional methods to identify where data analysis might be impacted upon by strong shear and where inferences of secondary circulation may need to be made more cautiously. © 2019 John Wiley & Sons, Ltd.     

Soil moisture effects on seasonal temperature and precipitation forecast scores in Europe

The Second Global Land Atmosphere Coupling Experiment (GLACE2) is designed to explore the improvement of forecast skill of summertime temperature and precipitation up to 8?weeks ahead by using realistic soil moisture initialization. For the European continent, we show in this study that for temperature the skill does indeed increase up to 6 weeks, but areas with (statistically significant) lower skill also exist at longer lead times. The skill improvement is smaller than shown earlier for the US, partly because of a lower potential predictability of the European climate at seasonal time scales. Selection of extreme soil moisture conditions or a subset of models with similar initial soil moisture conditions does improve the forecast skill, and sporadic positive effects are also demonstrated for precipitation. Using realistic initial soil moisture data increases the interannual variability of temperature compared to the control simulations in the South-Central European area at longer lead times. This leads to better temperature forecasts in a remote area in Western Europe. However, the covered range of forecast dates (1986–1995) is too short to isolate a clear physical mechanism for this remote correlation.