The problem of underpowered rivers

This study has hypothesized that for many rivers the trade-off between flow accumulation and the decrease in slope along channel length means that stream power increases downstream and, moreover, that given the low slope angles in headwater and low-order streams, they would have insufficient stream power to erode let alone transport sediment. The study considered the stream power profile, the particle travel distances and the application of the Hjulström curve based on the velocity profile of nine, large UK catchments. The study showed that:

1. Some rivers never showed a maximum in their longitudinal stream power profile, implying that some rivers never develop a deposition zone before they discharge at the tidal limit.
2. Particle travel distances during a bankfull discharge event showed that for some rivers 91% of the upper main channel would not be cleared of sediment. Furthermore, while some rivers could transport a 2 mm particle their entire length in one bankfull event, for another river it would take 89 such events.
3. The Hjulström curve shows that for three of the study rivers the upper 20 km of the river was not capable of eroding a 2 μm particle.
4. The study has shown that for all rivers studied, erosion is focused downstream and deposition upstream. Many UK rivers have a dead zone where, on time scales of the order of centuries, no erosion or transport occurs and erosion only occurs in the lower courses of the channel where discharge rather than slope dominates – we propose these as underpowered rivers.

© 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

Transient streambed hydraulic conductivity in channel and bar environments,Loup River,Nebraska

Streambed hydraulic conductivity (K) and vertical K (K _v) are key controls on groundwater and surface water exchange and biogeochemical fluxes through the hyporheic zone, but drivers of transient hydraulic properties in different fluvial environments are poorly understood. This study combines hydrogeology, geophysics, and sedimentology to reveal mechanisms of K and K _v transience in the upper 0.5 m of a sandy streambed during low discharge. Hydraulic tests (44 slug tests, 130 falling-head permeameter tests) and 130 grain-size analyses were repeated three times over 8 weeks on a 1,200 m² grid spanning: (a) a channel with continuously flowing water and mobile bed load; (b) an adjacent mid-channel bar that was stationary and infrequently submerged. Aerial photographs and ground-penetrating radar show scour and complete reworking of fluvial sediments in the channel. Bar sediments below the water table remained immobile, but infrequent flows of moderate discharge reworked the uppermost few centimetres of the bar top. Despite differences in sediment mobility and stream flow characteristics across environments, K and K _v exhibited order-of-magnitude differences in spatial heterogeneity and temporal variability in both the channel and bar. Mean K and K _v values in the channel were comparatively stable over time. In the immobile bar, mean K declined 20% and K _v declined 26% after increased discharge temporarily inundated the bar. Grain-size distributions were steady across both environments over time, but repeat geophysical surveys of the bar show a decrease in electrical conductivity, likely from porosity reduction. These findings suggest that sediment dynamics and stream flow characteristics in different streambed environments are important drivers of K transience during low discharge conditions. Specifically, pore clogging can be an important mechanism of transience over short durations (weeks to months) in immobile sediments subject to infrequent flows and minor reworking.     

Considering river structure and stability in the light of evolution: feedbacks between riparian vegetation and hydrogeomorphology

River ecological functioning can be conceptualized according to a four‐dimensional framework, based on the responses of aquatic and riparian communities to hydrogeomorphic constraints along the longitudinal, transverse, vertical and temporal dimensions of rivers. Contemporary riparian vegetation responds to river dynamics at ecological timescales, but riparian vegetation, in one form or another, has existed on Earth since at least the Middle Ordovician (c. 450 Ma) and has been a significant controlling factor on river geomorphology since the Late Silurian (c. 420 Ma). On such evolutionary timescales, plant adaptations to the fluvial environment and the subsequent effects of these adaptations on fluvial sediment and landform dynamics resulted in the emergence, from the Silurian to the Carboniferous, of a variety of contrasted fluvial biogeomorphic types where water flow, morphodynamics and vegetation interacted to different degrees. Here we identify several of these types and describe the consequences for biogeomorphic structure and stability (i.e. resistance and resilience), along the four river dimensions, of feedbacks between riparian plants and hydrogeomorphic processes on contrasting ecological and evolutionary timescales. Copyright © 2014 John Wiley & Sons, Ltd.     

Human‐induced sedimentation patterns of a channelized lowland river

Channelization of the severely polluted Odra and Vistula Rivers in Poland induced intensive accumulation of fine‐grained deposits rich in organic matter and heavy metals. These sediments have been identified in vertical profiles in a narrow zone along river banks both in groyne‐created basins and on the floodplain. Grain size, organic matter, zinc (Zn), lead (Pb), copper (Cu) content and cesium‐137 (¹³⁷Cs) was used for sediment dating and, stratigraphy and chemistry have been diagnostic features for these deposits, named industrial alluvium. In the most polluted river reaches stabilized by bank reinforcements and groynes, 2‐m‐thick slack water groyne deposits are composed of uniform strata of polluted silts with organic matter content over 10%, Zn content over 1000 mg/kg and average Cu and Pb over 100 mg/kg. The average rate of sediment accretion in groynes is higher than on the floodplain and reaches 5 cm/yr. Stratification which appears at higher levels in the groyne fields and on the levees reflects a change from in‐channel to overbank deposition and is typified by dark layers separated by bright, sandy, and less polluted strata. Stratified, 4‐m‐thick, sediment sequences have been found in groyne fields of incised river reaches. The average rate of sediment accretion in these reaches is of the order of 5 cm/yr. In stable and relatively less polluted river reaches, vertical‐accretion organic deposits are finely laminated and the average rate of deposition amounts to a few millimeters per year. Investigations indicate that groyne construction favors conditions for long‐term storage of sediments at channel banks. For this reason, groynes should be considered as structures that efficiently limit sudden release of sediment‐associated heavy metals stored in channels and in floodplains of the historically polluted rivers. Copyright © 2014 John Wiley & Sons, Ltd.     

The contribution of aeolian processes to fluvial sediment yield from a desert watershed in the Ordos Plateau,China

In arid zones, many active aeolian dunes terminate at ephemeral and perennial desert rivers. The desert rivers show very high rates of sediment transport that cause deleterious downstream effects on the river system and ecology. High sediment loading has been attributed to severe water erosion of sparsely covered watersheds during infrequent but heavy rainfall. Although aeolian erosion is known to lead to high rates of wind‐blown sand transport, direct confirmation of whether the aeolian processes accelerate or inhibit fluvial sediment loss is lacking. Here, we show that an aeolian‐fluvial cycling process is responsible for the high rate of suspended sediment transport in a Sudalaer ephemeral desert channel in the Ordos Plateau of China. Frequent aeolian processes, but low frequency (once every 3 years on average) flooding, occur in this region. Wind‐blown saltating grains appeared to be unable to cross the desert channel because of interruption of channel‐induced recirculating air flow, and therefore tended to settle in the channel during the windy seasons, leading to channel narrowing. During flooding, this narrowed channel was found to yield a threefold increase in suspended sediment loading and a 3.4‐fold increase in the weight percentage of the 0.08–0.2 mm sediment fraction on 18 July 2012. Loss of stored aeolian sand due to channel erosion accounted for about half of the total sediment yield in this watershed. These findings show that aeolian processes play an essential role in accelerating the sediment yield from a watershed characterized by aeolian‐fluvial interplay and also suggest that the drier the region and the greater the aeolian process, the more the aeolian process contributes to fluvial sediment yield. Copyright © 2013 John Wiley & Sons, Ltd.     

Silicate‐oxide mineral chemistry of mafic–ultramafic rocks as an indicator of the roots of an island arc: The Chilas Complex,Kohistan (Pakistan)

The Chilas Complex is a major lower crustal component of the Cretaceous Kohistan island arc and one of the largest exposed slices of arc magma chamber in the world. Covering more than 8000 km², it reaches a current tectonic width of around 40 km. It was emplaced at 85 Ma during rifting of the arc soon after the collision of the arc with the Karakoram plate. Over 85% of the Complex comprises homogeneous, olivine‐free gabbronorite and subordinate orthopyroxene–quartz diorite association (MGNA), which contains bodies of up to 30 km² of ultramafic–mafic–anorthositic association (UMAA) rocks. Primary cumulate textures, igneous layering, and sedimentary structures are well preserved in layered parts of the UMAA in spite of pervasive granulite facies metamorphism. Mineral analyses show that the UMAA is characterized by more magnesian and more aluminous pyroxene and more calcic plagioclase than those in the MGNA. High modal abundances of orthopyroxene, magnetite and ilmenite (in MGNA), general Mg–Fe–Al spatial variations, and an MFA plot of whole‐rock analyses suggest a calc‐alkaline origin for the Complex. Projection of the pyroxene compositions on the Wo–En–Fs face is akin to those of pyroxenes from island arcs gabbros. The presence of highly calcic plagioclase and hornblende in UMAA is indicative of hydrous parental arc magma. The complex may be a product of two‐stage partial melting of a rising mantle diaper. The MGNA rocks represent the earlier phase melting, whereas the UMAA magma resulted from the melting of the same source depleted by the extraction of the earlier melt phase. Some of the massive peridotites in the UMAA may either be cumulates or represent metasomatized and remobilized upper mantle. The Chilas Complex shows similarities with many other (supra)subduction‐related mafic–ultramafic complexes worldwide.     

Isolated Archosaur teeth from the green sandstone capping the Coralline Limestone (Bagh Group) of the Narmada valley: Evidence for the presence of pre‐Late to Late Maastrichtian abelisaurids in India

Recent field prospecting in the Cretaceous sequences of the lower Narmada valley has led to the discovery of three isolated archosaur teeth from the upper part of marine Cretaceous rocks of the Bagh Group. The specimens were recovered by surface prospecting from an oyster‐bearing green sandstone bed occurring at the top of the Coralline Limestone (Coniacian) from a site near Phutibawri village, Dhar District, Madhya Pradesh, India. Of the three teeth recovered from this horizon, two are identified with abelisaurid dinosaurs and the third one with an indeterminate crocodile. The abelisaurid teeth conform to the premaxillary and maxillary tooth morphology of Majungasaurus and Indosuchus. Earlier reports of abelisaurid dinosaurs from India are from the Upper Cretaceous (Maastrichtian) Lameta Group of Jabalpur, Pisdura (Central India) and Balasinor (Western India) and Upper Cretaceous (Late Maastrichtian) Kallamedu Formation (South India). As no associated age diagnostic fossils are found, the specimens described here are considered to represent pre‐Late to Late Maastrichtian age based on the known ages of the underlying and overlying formations. The new finds, therefore, document stratigraphically the oldest occurrence of abelisaurid dinosaurs known from the Indian subcontinent.     

区域地质调查中对沉积混杂岩的识别与工作方法

介绍了沉积混杂岩的识别方法 , 提出了造山带沉积混杂岩区区调研究的技术路线 , 建议沉积混杂岩的制图单位采用三级划分体制 :一级为沉积混杂岩带、二级为沉积混杂岩层、三级为沉积混杂岩体。以中甸地区为例示范了这些制图单位在地质图上的表示方法。     

Trees as riparian engineers: the Tagliamento river,Italy

Pristine river corridors were characterized by island and floodplain development driven by the inter‐play of flows, sediments and woody vegetation. Here we explore these relationships within topographically controlled settings within the upper, middle and lower reaches of a large, semi‐natural alpine to mediterranean river. These reaches have expanding or contracting valley floors within which we show that there are more or less predictable patterns of stream power and rates of vegetation growth, reflecting water availability during dry periods and also the availability of sand and finer sediment. We relate these to the pattern of island distribution that is repeated within the three reaches and is indicative of the engineering role of riparian trees. Islands are shown to develop within thresholds defined by stream power, rates of woody vegetation growth and rates of sedimentation, and to develop most quickly where riparian species include those capable of sprouting from driftwood. Copyright © 2006 John Wiley & Sons, Ltd.     

Effects of the number of stations and time resolution on derivation

For at least 30 years now it has been well known that the Dst index can be modelled using the solar wind as input. Since then, many attempts have been made to improve the predictability of Dst using different approaches. These attempts are useful, for instance, to understand which features of the solar wind–magnetosphere interactions are most important in producing magnetospheric activity and how the Dst index would improve the space weather forecast. The Dst index is by far the most reliable and simple indication that a magnetic storm is in progress. In this work, the effect of using more than four magnetic stations and shorter time intervals than the hourly averages used in Sugiura's procedures is evaluated. The discussion is based on the results presented by Burton in 1975 and Feldstein in 1984 considering 4 or 12 magnetic stations and time averages of 2.5 min for a magnetic disturbed period that occurred from February 7–28, 1967, including two geomagnetic storms. The analysis has shown that the global representation of a magnetic storm by the standard Dst (Sugiura) is well preserved either using 4, 6, 12 magnetic stations or using 1 h, 2.5 min 1 min averages. A brief review of the current understanding of Dst has been included to support the discussions. The analysis performed has shown that a more refined Dst index (time and number of stations>4) would be useful to investigate the intrinsic processes and the different current systems involved in the ring current development during magnetic storms; the standard Dst, as it is conceived, is quite adequate to monitor geomagnetic storms and identify their overall features; concerning the magnetic stations normally considered, the inclusion of higher magnetic latitude stations (>35) may underestimate the observed Dst.