Diverse patterns of ascent,degassing, and eruption of rhyolite magma during the 1.8 ka Taupo eruption,New Zealand: Evidence from clast vesicularity

The 22 km³ (DRE) 1.8 ka Taupo eruption ejected chemically uniform rhyolite in a wide range of eruptive styles and intensities. The 7 eruptive units include the ‘type examples’ of phreatoplinian (units 3 and 4) and ultraplinian fall (unit 5) deposits, and low-aspect-ratio ignimbrite (unit 6). Contrasts in bulk vesicularity, vesicle (and microlite) number densities and the size distributions of bubbles (and crystals) in the Taupo ejecta can be linked to the influence of shallow conduit processes on volatile exsolution and gas escape, before and during eruption, rather than changes in pre-eruptive chemistry. Existing work has modeled the individual phases of this complex eruption but not fully explained the abrupt shifts in style/intensity that occur between phases. We link these rapid transitions to changes in vent position, which permitted contrasts in storage, conduit geometry, and magma ascent history.     

Discontinuous headwater stream networks with stable flowheads,Salmon River basin,Idaho

Headwater streams expand, contract, and disconnect in response to seasonal moisture conditions or those related to individual precipitation events. The fluctuation of the surface flow extent, or active drainage network, reflects catchment storage characteristics and has important impacts on stream ecology; however, the hydrological mechanisms that drive this phenomenon are still uncertain. Here, we present field surveys of the active drainage networks of four headwater streams in Central Idaho's Frank Church‐River of No Return Wilderness (7–21 km²) spanning the spring and summer months of 2014. We report the total length of the active drainage networks, which varied as a power law function with stream discharge with an average exponent of 0.11 ± 0.03 (range of 0.05–0.20). Generally, these active drainage networks were less responsive to changes in discharge than many streams in past studies. We observed that the locations where surface flow originates, or flowheads, were often stable, and an average of 64% of the change in active drainage network length was explained by downstream discontinuities. Analysis of geologic and geomorphic characteristics of individual watersheds and flowheads suggests that most flowheads below approximately 2200 m are supported by stable flowpaths controlled by bedrock structure. At higher elevations, small accumulation areas and saturation of shallow and conductive soil and colluvium after snowmelt result in more mobile flowhead locations. The dynamics of active drainage networks can help illuminate the spatiotemporal structure of flowpaths supporting surface flow. Copyright © 2016 John Wiley & Sons, Ltd.     

Identification of steps and pools from stream longitudinal profile data

Field research on step–pool channels has largely focused on the dimensions and sequence of steps and pools and how these features vary with slope, grain sizes and other governing variables. Measurements by different investigators are frequently compared, yet no means to identify steps and pools objectively have been used. Automated surveying instruments record the morphology of streams in unprecedented detail making it possible to objectively identify steps and pools, provided an appropriate classification procedure can be developed.To achieve objective identification of steps and pools from long profile survey data, we applied a series of scale-free geometric rules that include minimum step length (2.25% of bankfull width (W_b)), minimum pool length (10% of W_b), minimum residual depth (0.23% of W_b), minimum drop height (3.3% of W_b), and minimum step slope (10° greater than the mean slope). The rules perform as well as the mean response of 11 step–pool researchers who were asked to classify four long profiles, and the results correspond well with the channel morphologies identified during the field surveys from which the long profiles were generated. The method outperforms four other techniques that have been proposed. Sensitivity analysis shows that the method is most sensitive to the choice of minimum pool length and minimum drop height.Detailed bed scans of a step–pool channel created in a flume reveal that a single long profile with a fixed sampling interval poorly characterizes the steps and pools; five or more long profiles spread across the channel are required if a fixed sampling interval is used and the data suggest that survey points should be located more frequently than the diameter of the step-forming material. A single long profile collected by a surveyor who chooses breaks in slope and representative survey points was found to adequately characterize the mean bed profile.     

A roughness-corrected index of relative bed stability for regional stream surveys

Quantitative regional assessments of streambed sedimentation and its likely causes are hampered because field investigations typically lack the requisite sample size, measurements, or precision for sound geomorphic and statistical interpretation. We adapted an index of relative bed stability (RBS) for data calculated from a national stream survey field protocol to enable general evaluation of bed stability and anthropogenic sedimentation in synoptic ecological surveys. RBS is the ratio of bed surface geometric mean particle diameter (D_gm) divided by estimated critical diameter (D_cbf) at bankfull flow, based on a modified Shield's criterion for incipient motion. Application of RBS to adequately depict bed stability in complex natural streams, however, has been limited because typical calculations of RBS do not explicitly account for reductions in bed shear stress that result from channel form roughness. We modified the index (RBS) to incorporate the reduction in bed shear stress available for sediment transport that results from the hydraulic resistance of large wood and longitudinal irregularities in channel dimensions (“form roughness”). Based on dimensional analysis, we derived an adjustment to bankfull shear stress by multiplying the bankfull hydraulic radius (R_bf) by the one-third power of the ratio of particle-derived resistance to total hydraulic resistance (C_p/C_t)^1/3, where both resistances are empirically based calculations. We computed C_p using a Keulegan equation relating resistance to relative submergence of bed particles. We then derived an empirical equation to predict reach-scale hydraulic resistance C_t from thalweg mean depth, thalweg mean residual depth, and large wood volume based on field dye transit studies, in which total hydraulic resistance C_t was measured over a wide range of natural stream channel complexity, including manipulation of large wood volumes. We tested our estimates of C_t and RBS by applying them to data from a summer low flow probability sample of 104 wadeable stream reaches in the Coastal Ecoregion of Oregon and Washington, USA. Stream discharges calculated using these C_t estimates compared favorably with velocity–area measurements of discharge during summer low flow, and with the range of 1 to 2-year recurrence floods (scaled by drainage area) at U.S.Geological Survey gauged sites in the same region. Log [RBS] ranged from − 4.2 to + 0.98 in the survey region. D_gm ranged from silt to boulders, while estimated bankfull critical diameter, D_cbf, ranged from very fine gravel to large boulders. The median value of D_cbf (adjusted for form roughness influences) averaged 40% (inter quartile range 28 to 59%) of the unadjusted estimate D_cbf. Log[RBS] was consistently negatively related to human disturbances likely to produce excess sediment inputs or hydrologic alteration. Log [RBS] ranged from − 1.9 to + 0.5 in the streams within the lower quartile of human disturbance in their basin and riparian areas and was substantially lower (− 4.2 to − 1.1) in streams within the upper quartile of human disturbance. The synoptic survey methods and designs we used appear adequate to evaluate regional patterns in bed stability and sedimentation and their general relationship to human disturbances. Although the RBS concept also shows promise for evaluating sediment and bed stability in individual streams, our approach is relatively coarse, so site-specific assessments using these rapid field methods might prudently be confined to identifying severe cases of sedimentation or channel alteration. Greater confidence to discern subtle differences in site-specific assessments could be gained by calculating RBS using more precise field measurements of channel slope, bed particle size and bankfull dimensions, and by refining our adjustments for energy loss from channel form roughness.     

Low amplitude anisotropic wave trains in cosmic rays

In the present study the occurrence of an unusual class of low amplitude anisotropic wave trains in the cosmic ray neutron intensity, which is distinctly different from the average diurnal variation as well as from other recognized types of low amplitude anisotropic wave trains are noted and the directional distribution in the interplanetary space determined. The major objective of this paper is to study the first three harmonics of low amplitude anisotropic wave trains of cosmic ray intensity over the period 1981–1994 for Deep River neutron monitoring station. The significant characteristic of these events is that the low amplitude wave trains shows a maximum intensity of diurnal component in a direction earlier than 18:00 h/co-rotational direction. It is noticed that these events are not caused either by the high-speed solar wind streams or by the sources on the Sun responsible for producing these streams such as polar coronal holes. However the possibility of occurrence of these events during high-speed solar wind streams cannot be denied. The occurrence of low amplitude events is dominant for positive polarity of B_z. The disturbance storm time index i.e. D_st, remains consistently negative only for majority of the low amplitude wave train events, which is never been reported earlier. The amplitude as well as direction of first two harmonics seems to remain unaffected with the variation in the D_st and A_p-index. However, the amplitude as well as direction of third harmonic found to deviates with the increase of D_st and A_p-index. The corotating streams produce significant deviations in cosmic ray intensity as well as in solar wind speed during low amplitude anisotropic wave train events.     

Stream macroinvertebrate communities change with grassland afforestation in central Argentina

Lotic ecosystems are highly affected by land use changes such as afforestation of natural areas for management or commercial purposes. The aim of this study was to analyze the effect of pine plantations on benthic invertebrate communities in mountain grassland streams. Additionally, we assessed if the hydrological period modifies the effect of afforestation on stream invertebrates. Three headwater streams draining grasslands (reference streams) and three draining plantations of Pinus elliottii were selected in a mountain watershed of Córdoba province (Argentina). Hydrologic and physicochemical variables were registered and benthic invertebrate samples were collected in each stream at two different hydrological periods. Total invertebrate abundance, richness and diversity were reduced in afforested streams as well as the number of indicator taxa. In addition, invertebrate functional structure (i.e. taxonomic richness and total and relative abundance of functional feeding groups, FFG) showed differences between streams with different riparian vegetation and between hydrological periods. Total abundance of all FFGs was lower in afforested streams and scrapers’ relative abundance was higher in grassland streams at the low water period. In addition, in most FFGs richness was diminished in afforested streams. Changes in light intensity, hydrology and coarse organic matter inputs produced by afforestation alter fluvial habitats and consequently the composition and trophic structure of invertebrate communities in grassland streams of Córdoba mountains.     

Replacement of culvert styles has minimal impact on benthic macroinvertebrates in forested,mountainous streams of Northern California

Culvert styles are being replaced on many road-stream crossings to provide long-term (>2 years) benefits, but these projects may result in short-term (0–2 years) biological impacts. We quantified the short-term effects of replacing steel-pipe culverts with open-arch structures on the benthic-macroinvertebrate communities of 6 streams in the Klamath National Forest of Northern California USA. Physical habitat showed notable site-specific effects in channel form and sedimentation, but no significant change among sites. In contrast, we observed small though significant impacts of the culvert style replacement on benthic macroinvertebrates among sites, including a statistically significant reduction in both taxa richness (p = 0.012) and abundance of intolerant taxa (p = 0.004). Moreover, there was also modest evidence of slightly elevated variability in the benthic-macroinvertebrate communities downstream following the replacement of culvert style. The long-term benefits of culvert style replacement that have been observed in other studies may outweigh the minor, short-term biological impacts observed in these streams.     

The effects of kinetic sorting on sediment mobility on steep slopes

In poorly mobile static armour, sorting is usually considered the result of hiding/exposure effects. We called this effect ‘static sorting’ in opposition to very efficient grain‐to‐grain mechanisms produced by a mobile mixture, called ‘kinetic sorting’. We hypothesized that kinetic sorting can be an important contributor to the morphodynamics of mountain streams and attempted to demonstrate this with new flume experiments. Two long runs were produced with natural poorly sorted sediments, and with transport stages of the coarse fraction (defined by the ratio between the shear stress and the critical shear stress for transport), smaller and higher than 1, respectively. Both runs produced an efficient transfer downstream of the injected material, but with a major difference: the first run (no kinetic sorting) produced permanent armour figuring clusters, akin to what has already been observed in similar experiments; the second run (with kinetic sorting) also produced bed armouring, but this armour was periodically totally destroyed, leading to substantial bed erosion. This phenomenon was explained by kinetic sorting, the effects of which are to produce an efficient downward migration of fine materials and bed surface armouring. The consequence is that fine materials are hidden to the flow during aggradation, allowing the slope to attain values much steeper than would have been expected at equilibrium for the mixture. However, whereas the surface armouring tends to stabilize the bed, construction of a layer of fine sediments at the subsurface also contributes to making it very unstable. These two contradictory effects explain the complex bed behaviours and the existence of very large bedload and slope fluctuations. Copyright © 2014 John Wiley & Sons, Ltd.     

Meteor streams associated with Jupiter family comets

Meteor showers have been observed for a considerable time, and the cause, meteoroids from a meteoroid stream ablating in the Earth's atmosphere, has also been understood for centuries. The connection between meteoroid streams and comets was also established 150 years ago. Since that time our ability both to understand the physics and to numerically model the situation has steadily increased. We will review the current state of knowledge. However, just as there are differences between the behaviour of long period comets, Halley family comets and Jupiter family comets, so also differences exist between the associated meteoroid streams. Streams associated with Jupiter family comets show much more variety in their behaviour, driven by the gravitational perturbations from Jupiter. The more interesting showers associated with Jupiter family comets will be discussed individually.     

Plankton of lake Ototoa,a sand‐dune lake in Northern New Zealand

The seasonal cycles of abundance of the phytoplankton and zooplankton in Lake Ototoa, Northland, at 36° 31 S, 174° 14 E, are described. Concentrations of chlorophyll a were low (range 0.04–4.61 mg.m^‐3; mean 0.97 mg.m^‐3), and highest values were in winter. Phytoplankton densities were also low; an oligo‐trophic diatom‐desmid assemblage associated with Botryococcus, Dinobryon, and Svhaerocyslis was found.

The zooplankton was dominated by the calanoid copepod Calamoecia lucasi whose numbers remained fairly constant throughout the year, and the only other copepod found was the cyclopoid Mesocyclops leuckarti which was present in very low numbers. Bosmina meridionalis was the only limnetic cladoceran and was most abundant during autumn and spring. A number of rotifer species were also common.