Strath Terraces Research Articles

Morphotectonic records of neotectonic activity in the vicinity of North Almora Thrust Zone, Central Kumaun Himalaya

The landform development and valley floor morphology of active regions is significantly controlled by tectonic processes. In the present study the upper catchments of Kosi and Gagas river valleys have been investigated with special emphasis on aggradational landforms to explain the spatial and temporal variability of aggradation/incision in response to tectonic activity during the late Quaternary and Holocene in Central Kumaun Himalaya. Major tectonic elements such as the North Almora thrust (NAT), Rasiyari fault (RF), and Gagas fault (GF) are considered to be controlling the development of landforms in the region. The valleys have preserved debris flow terraces, bedrock strath terraces, and fluviolacustrine terraces that provide signatures of tectonic activity. Morphostratigraphy of the terraces reveals that the oldest landforms are preserved in the hanging wall block of the NAT, RF, and along the GF. Reconstructions based on morphostratigraphy through the application of optical chronology suggests multiple phases of fluvial aggradation in the upper catchment of the Kosi and Gagas rivers that were initiated around 34ka. The youngest phase of aggradation is dated to be around early to mid-Holocene (7–1ka). Based on terrace morphology, four major phases of enhanced uplift have been estimated during 34, 15.8, 7, and 3ka. The older uplift event dated to be around 34ka is represented uplift along GF, which accommodated onset of valley-fill aggradation in Gagas valley. The 15.8ka event represents uplift along RF, which led to the generation of debris flow from adjacent hillslopes. The 7ka event represents uplift of a hanging wall block of the NAT and deposition of debris flow terraces. However, the youngest 3ka event represents enhanced uplift, which is responsible for the incision of the older valley fill sediments and bedrock. Following this, a phase of accelerated incision/erosion owing to an increase in uplift rate occurred, as evident from the strath terraces between 7 and 1ka.

Comparative evaluation of age and weathering condition of the Sirvan river terraces in western Iran

Comparative evaluation of age and weathering condition of alluvial deposits which associate to some environmental settings during their formation is a matter of interest in various fields of studies. The aim of this study was to compare age and weathering of two strath terraces of the Sirvan river in western Iran at its three cut-banks (1, 2 and 3) accessible for sampling. AMS 14C dating revealed 15090 ± 50 and 18510 ± 60 yrs BP for terrace T1 at the cut-banks 1 and 2, respectively, and 10870 ± 40, 11300 ± 40 and 14610 ± 40 yrs BP for terrace T2 at the cut-banks 1, 2, and 3, respectively. Evaluation of weathering using some coordination systems, where crystallinity ratio of iron oxides was considered as x-axis and indexes of either Parker or Product were set as y-axis showed more intensive weathering among older deposits for each of terraces T1 and T2. However, weathering condition of the two terraces were incomparable mostly due to deeper deposits and finer particle size distribution of terrace T1 than terrace T2 and also colder climatic condition of terrace T1 before formation of terrace T2. It seems all of these differences have discouraged weathering processes in terrace T1 through reducing of weathering factors influences. Color parameters of a* and b* in CIE color standard explained the intensity of reddish and yellowish of crystalline iron oxides of the river deposits, which functioned to discuss the order of emergence of the river terraces in the studied area.

Late glacial and Holocene history of the Penobscot River in the Penobscot Lowland, Maine

When the Laurentide ice sheet retreated rapidly (~150 m/a) across the Penobscot Lowland between ~16 and ~15 ka, the area was isostatically depressed and became inundated by the sea. Silt and clay were deposited, but no significant moraines or deltas were formed. The Penobscot River was reborn at ~14 ka when ice retreated onto land in the upper reaches of the river’s East Branch. As isostatic rebound exceeded sea level rise from melting ice, the river extended itself southward. Between ~13.4 and 12.8 ka, it established a course across marine clay and underlying glacial till in the Lowland. Its gradient was low as differential rebound had not begun. Discharge, however, was higher and the river transported and deposited outwash gravel. During the cold, dry Younger Dryas, ~11 ka, eolian sand began to accumulate in dunes in the Lowland. Some of this sand, along with fluvial sediment from the headwaters, was redistributed into terraces along gentler stretches of the river and into a paleodelta in Penobscot Bay. Eolian activity continued to ~8 ka and aggradation in terraces until ~6 ka. The climate became wetter and warmer after ~6 ka, the dunes were stabilized by vegetation, the river began to downcut, and braiding became less intense. Pauses in the downcutting are reflected in discontinuous strath terraces. In due course, the river re-encountered the old outwash gravels, marine clay, glacial till, and, in a few places, bedrock. Its profile is now stepped, with gentle, gravel-bedded reaches between bedrock ribs that form rapids.

Climatic and tectonic controls on strath terraces along the upper Weihe River in central China

The Weihe River in central China is the largest tributary of the Yellow River and contains a well-developed strath terrace system. A new chronology for the past 1.11 Ma for a spectacular flight of strath terraces along the upper Weihe River near Longxi is defined based on field investigations of loess–paleosol sequences and magnetostratigraphy. All the strath terraces are strikingly similar, having several meters of paleosols that have developed directly on top of fluvial deposits located on the terrace treads. This suggests that the abandonment of each strath terrace by river incision occurred during the transition from glacial to interglacial climates. The average fluvial incision rates during 1.11–0.71 Ma and since 0.13 Ma are 0.35 and 0.32 m/ka, respectively. These incision rates are considerably higher than the average incision rate of 0.16 m/km for the intervening period between 0.71 and 0.13 Ma. Over all our results suggest that cyclic Quaternary climate change has been the main driving factor for strath terrace formation with enhanced episodic uplift.

Extracting palaeoflood data from coarse‐grained Pleistocene river terrace archives: an example from SE Spain

AbstractField‐based palaeoflood event reconstruction has the potential to contribute to the development of our understanding of long‐term landscape evolution. However, the reconstruction of past flow event histories (magnitude and frequency) over long‐term (Quaternary) timescales is fraught with difficulties. Here we make a preliminary exploration of some of the practicalities of flood reconstruction from fluvial terrace archives using commonly available sedimentological and geomorphological observations from a field perspective. We utilize Manning and palaeostage indicators to reconstruct historic events that can be used as benchmarks for a lesser used competence based approach, which is applied to coarse‐grained strath terrace deposits. We evaluate the results against gauged records for extreme and catastrophic events that affected the same region in 1973 and 2012. The findings suggest that the competence approach is most effectively applied to terrace deposits if the channel geometry is taken into account when sampling both in cross‐section and in longitudinal section and calibrated against the sedimentology for palaeo‐flow depth. Problems can arise where constrictive channel geometries allow boulder jams to develop, acting as sediment traps for the coarsest material and leading to downstream ‘boulder starvation’. Useful sites to target for palaeoflood reconstruction, therefore, would be upstream of such constrictive reaches where the coarsest transportable bedload has been effectively trapped. Sites to avoid would be downflow, where the deposited material would poorly represent palaeoflood competence. Underestimation from maximum boulder preservation and limited section exposure issues would appear to outweigh possible overestimation concerns related to fluid density and unsteady flow characteristics such as instantaneous acceleration forces. Flood data derived from river terrace deposits suggests that basal terrace geometries and coarse boulder lags common to many terrace sequences are likely the result of extreme flow events which are subsequently filled by lesser magnitude flood events, in this environmental setting. Copyright © 2016 John Wiley & Sons, Ltd.

The role of pre-existing topography in the evolution of post-glacial fluvial landforms in the middle Satluj valley, north-western Himalaya, India

Abstract In tectonically active orogen like the Himalaya while computing the bedrock incision rate it is assumed that the underlying incised bedrock is genetically related to the overlying alluvium. This assumption may not be applicable universally, considering that the trans-Himalayan rivers were in existence before Miocene or earlier. Therefore, the sediments preserved at various locations represent the youngest phase of aggradation on the pre-existing valley topography – a cumulative expression of multiple cycles of erosion and aggradation. Considering that the fluvial terraces are extensively used to estimate the incision/uplift rates, it is therefore pertinent to ensure that the underlying bedrock topography is indeed genetically related to the overlying fluvial sediments and is not a relic of the pre-existing topography. The present study is aimed at understanding the relationship between the valley and channel-fill sediments and the underlying bedrock topography in the evolution of fluvial landforms. Based on the detailed field mapping of the relict channel/fossil valley-fill sequences, geomorphometry and limited optical ages the study suggests that the bedrock topography is cumulative expression of multiple cycles of erosion while the post glacial valley-fill sediments (dated to 14 ka and 8 ka) represent the most recent aggradation phase. The discreet linear strath terraces and epigenetic gorges represent a combination of climate and tectonically induced mid-Holocene river adjustment particularly in the areas proximal to local thrusts. Further, the study suggests that the securely dated strath terraces may provide the average tectonically induced incision/uplift rates whereas the epigenetic gorges largely represent climatically controlled focused and local uplift/incision in the study area. A preliminary estimate based on the strath terraces indicate that the terrain is being incised/uplifted at ∼0.6–1.3 mm/yr whereas as expected the epigenetic gorges provide a relatively higher incision rate (3.8 mm/yr to 12.5 mm/yr). Finally, the study cautions against using the conventional age-depth relationship of valley-fills and the underlying incised bedrock to ascertain the incision/uplift rates. Instead it suggests that the morphology of the underlying bedrock must be taken in to account in order to avoid erroneous estimate of incision/uplift rates (crustal deformation) in a tectonically active orogen.

Reconstruction of Late Quaternary climate and seismicity using fluvial landforms in Pindar River valley, Central Himalaya, Uttarakhand, India

Fluvial landforms in the Pindar River valley are investigated to understand the role of temporal variability in the Indian Summer Monsoon (ISM) and the spatial changes in crustal deformation. Employing the conventional geomorphological and sedimentological concepts, supported by optical dating, three major phases of valley-fill aggradation separated by phases on non-deposition are discerned. An oldest aggradation phase is dated to 33.5 ka during relatively strengthened ISM corresponding to the later part of pluvial Marine Isotopic Stage-3 (MIS-3). Following this, alluvial fan sedimentation impersistently continued during the declining phase of the ISM (until around 21 ka). A second major aggradation phase (17.5 ka–13 ka) occurred during the post-LGM/lateglacial strengthening of the ISM in which contribution from tributary valleys overwhelmed the valley-fill sedimentation. A youngest aggradation phase dated to the middle to late-Holocene (8 ka–3.6 ka), represents the transitional climate during which sporadic high sediment fluxes both from the upper catchment and tributary streams led to the development of fossil valleys (abandonment of old channels).The spatial variability in the incision/uplift rate, based on river strath terraces reveals that the terrain is undergoing differential crustal deformation. Two zones of relatively high crustal deformation are identified. These are located in the vicinity of the Main Central Thrust (MCT) and is attributed to the activity along the transverse fault and the in the inner Lesser Himalaya for which the thrust propagated fold associated with regional north-south compression is implicated. The study suggests that fluvial landforms (valley-fill and strath terraces) in the Pinder river valley are genetically related to the multi-millennial scale changes in the ISM and spatial and temporal changes in the crustal deformation associated with regional compression.

Late Quaternary tectonic landforms and fluvial aggradation in the Saryu River valley: Central Kumaun Himalaya

The present study has been carried out with special emphasis on the aggradational landforms to explain the spatial and temporal variability in phases of aggradation/incision in response to tectonic activity during the late Quaternary in the Saryu River valley in central Kumaun Himalaya. The valley has preserved cut-and-fill terraces with thick alluvial cover, debris flow terraces, and bedrock strath terraces that provide signatures of tectonic activity and climate. Morphostratigraphy of the terraces reveals that the oldest landforms preserved south of the Main Central Thrust, the fluvial modified debris flow terraces, were developed between 30 and 45ka. The major phase of valley fill is dated between 14 and 22ka. The youngest phase of aggradation is dated at early and mid-Holocene (9–3ka). Following this, several phases of accelerated incision/erosion owing to an increase in uplift rate occurred, as evident from the strath terraces. Seven major phases of bedrock incision/uplift have been estimated during 44ka (3.34mm/year), 35ka (1.84mm/year), 15ka (0.91mm/year), 14ka (0.83mm/year), 9ka (1.75mm/year), 7ka (5.38mm/year), and around 3ka (4.4mm/year) from the strath terraces near major thrusts. We postulate that between 9 and 3ka the terrain witnessed relatively enhanced surface uplift (2–5mm/year).

Late Pleistocene–Holocene uplift driven terrace formation and climate-tectonic interplay from a seismically active intraplate setting: An example from Kachchh, Western India

Fluvial terrace formation is often regulated by external forcings like climate, tectonic and eustatic changes. These terraces, particularly in a dryland environment, preserves the discrete signatures of these external forcings, thus enabling us to reconstruct the fluvial response to the late Quaternary palaeoenvironmental changes and factors governing them. The present study focuses on reconstructing the aggradation/incision phases in the Lotia River which is located in the eastern segment of the Northern Hill Range (NHR) of the Kachchh Peninsula. The Lotia river drains through Mesozoic rocks before cutting across the Kachchh Mainland Fault (KMF) and finally debouch in the Banni Plains. Reconstruction based on tectonic geomorphology, sedimentology, sediment geochemistry, mineral magnetic, and OSL chronology suggests the fluvial response to monsoon variability archived during the last 15ka. The time frame was also marked by incision enhanced by uplift along the KMF, which led to strath terrace formation. The accommodation space thus created was filled by an aggradational event between 14.8ka and 10.6ka. Sedimentological and geochemical parameters have also suggested that the time period between 12.5ka and 11.5ka showed a decline in the monsoon strength, which coincides with ‘Younger Dryas’. It has been observed that the sediments spanning between 10.6ka and 7.8ka are absent from the archive, which is most likely the manifestation of the early Holocene optimum that led to severe erosional processes. The period between 7.8ka and 3.3ka is marked as another aggradational phase with fluctuating climatic conditions. At 3.3ka, the region has experienced an incision of 4m, which led to the formation of Holocene terrace T1, most likely due to tectonic uplift. During the last 3.3ka, another pulsative uplift has occurred, which led to the formation of unpaired Holocene terrace T2, along with tilting of the Lotia basin. Based on the OSL chronology of bedrock strath terrace, we reported a minimum uplift rate of 1.04mm/a for the eastern KMF during the Late Pleistocene–Holocene period, hinting seismically active nature of the KMF during this period. The terrace formation in the eastern Northern Hill Range is mostly regulated by tectonic uplifts along the KMF.

Time-dependent evolution of rock slopes by a multi-modelling approach

This paper presents a multi-modelling approach that incorporates contributions from morpho-evolutionary modelling, detailed engineering-geological modelling and time-dependent stress-strain numerical modelling to analyse the rheological evolution of a river valley slope over approximately 102kyr. The slope is located in a transient, tectonically active landscape in southwestern Tyrrhenian Calabria (Italy), where gravitational processes drive failures in rock slopes. Constraints on the valley profile development were provided by a morpho-evolutionary model based on the correlation of marine and river strath terraces. Rock mass classes were identified through geomechanical parameters that were derived from engineering-geological surveys and outputs of a multi-sensor slope monitoring system. The rock mass classes were associated to lithotechnical units to obtain a high-resolution engineering-geological model along a cross section of the valley. Time-dependent stress-strain numerical modelling reproduced the main morpho-evolutionary stages of the valley slopes. The findings demonstrate that a complex combination of eustatism, uplift and Mass Rock Creep (MRC) deformations can lead to first-time failures of rock slopes when unstable conditions are encountered up to the generation of stress-controlled shear zones. The multi-modelling approach enabled us to determine that such complex combinations may have been sufficient for the first-time failure of the S. Giovanni slope at approximately 140ka (MIS 7), even without invoking any trigger. Conversely, further reactivations of the landslide must be related to triggers such as earthquakes, rainfall and anthropogenic activities. This failure involved a portion of the slope where a plasticity zone resulted from mass rock creep that evolved with a maximum strain rate of 40% per thousand years, after the formation of a river strath terrace. This study demonstrates that the multi-modelling approach presented herein is a useful tool for estimating the progressive development of slope failures because it can highlight time-dependent continuous deformations as the major processes that drive rocky slopes to failure. This type of approach can be devoted to the best selection of risk mitigation strategies with respect to both human life and anthropic infrastructure.

Evolution of the Late Pleistocene Aspe River (Western Pyrenees, France). Signature of climatic events and active tectonics

We make use of the cosmogenic nuclide 10Be exposure to date an alluvial terrace of the Aspe River in the foothills of the northwestern Pyrenees. Initially ascribed to the Rissian glaciation, our dating shows that the terrace was abandoned at 18±2kyr. In reference to the Late Pleistocene climatic chronology, two kinds of terraces can be distinguished: high-standing fill terraces probably deposited during glacial events and lower cut-in-fill and strath terraces cut during the postglacial river incision. A part of the terrace aggradations could have occurred during the Würmian glacial episodes. Hence, the dated terrace fits in with the prevailing view of incision during climate transitions. Our study also shows that elevation is not a good criterion of terrace correlation, which should be better carried out on the basis of absolute dating. In addition, this dating also suggests a potential Late Pleistocene fault reactivation of the Mail Arrouy thrust in this tectonically active area of the Western Pyrenees.

Lithologic controls on valley width and strath terrace formation

Valley width and the degree of bedrock river terrace development vary with lithology in the Willapa and Nehalem river basins, Pacific Northwest, USA. Here, we present field-based evidence for the mechanisms by which lithology controls floodplain width and bedrock terrace formation in erosion-resistant and easily friable lithologies. We mapped valley surfaces in both basins, dated straths using radiocarbon, compared valley width versus drainage area for basalt and sedimentary bedrock valleys, and constructed slope-area plots. In the friable sedimentary bedrock, valleys are 2 to 3 times wider, host flights of strath terraces, and have concavity values near 1; whereas the erosion-resistant basalt bedrock forms narrow valleys with poorly developed, localized, or no bedrock terraces and a channel steepness index half that of the friable bedrock and an average channel concavity of about 0.5. The oldest dated strath terrace on the Willapa River, T2, was active for nearly 10,000years, from 11,265 to 2862 calibrated years before present (cal YBP), whereas the youngest terrace, T1, is Anthropocene in age and recently abandoned. Incision rates derived from terrace ages average 0.32mmy−1 for T2 and 11.47mmy−1 for T1. Our results indicate bedrock weathering properties influence valley width through the creation of a dense fracture network in the friable bedrock that results in high rates of lateral erosion of exposed bedrock banks. Conversely, the erosion-resistant bedrock has concavity values more typical of detachment-limited streams, exhibits a sparse fracture network, and displays evidence for infrequent episodic block erosion and plucking. Lithology thereby plays a direct role on the rates of lateral erosion, influencing valley width and the potential for strath terrace planation and preservation.

Rates and mechanisms of bedrock incision and strath terrace formation in a forested catchment, Cascade Range, Washington

Measurements of channel bed and bank incision into bedrock coupled with mapping and radiocarbon dating of strath terraces in the West Fork Teanaway River, Washington, provide insight into rates and mechanisms of river incision and strath terrace formation in a forested landscape. The West Fork drains 102 km 2 of the tectonically quiescent southeastern North Cascade Range, and it is rapidly eroding its bed and creating strath terraces in its lower reach. Minimum vertical incision, measured annually relative to nails embedded in the streambed, was greater in the seasonally exposed, weathering-dominated, high-flow channel (mean = 10.9 mm yr −1 ) than in the perennially wet, abrasion-dominated, low-flow channel (3.8 mm yr −1 ), documenting unsteady lowering of the channel margin. Ages of radiocarbon-dated materials from alluvium on strath terraces, 0.1 m to 5.4 m above the water surface, suggest three episodes of strath abandonment at maximum ages of ca. A.D. 830, A.D. 1560, and A.D. 1890, and average incision rates of 1.3 mm yr −1 , 1.4 mm yr −1 , and 7.4 mm yr −1 for the oldest to youngest surfaces, respectively. Weathering-promoted vertical incision in the high-flow channel provides a mechanism for “top-down” rapid lateral strath planation in which scour of alluvium on incipient strath terraces incorporates the surface into the high-flow channel, allowing rapid removal of bedrock weathered during wetting and drying cycles. Relationships among channel width, channel confinement by bedrock terrace risers, modeled bankfull shear stress, and alluvial bed cover suggest that rapid channel widening could also internally limit vertical incision by slowing incision as shear stresses decline and more alluvium is retained on the bed. The timing of the most recent (ca. A.D. 1890) strath abandonment corresponds with historic anthropogenic removal of fluvial wood, suggesting that the relative abundance of fluvial wood may influence episodes of vertical bedrock incision by affecting the retention of alluvium on streambeds.

Differential uplift and incision of the Yakima River terraces, central Washington State

AbstractThe fault‐related Yakima folds deform Miocene basalts and younger deposits of the Columbia Plateau in central Washington State. Geodesy implies ~2 mm/yr of NNE directed shortening across the folds, but until now the distribution and rates of Quaternary deformation among individual structures has been unclear. South of Ellensburg, Washington, the Yakima River cuts a ~600 m deep canyon across several Yakima folds, preserving gravel‐mantled strath terraces that record progressive bedrock incision and related rock uplift. Here we integrate cosmogenic isochron burial dating of the strath terrace gravels with lidar analysis and field mapping to quantify rates of Quaternary differential incision and rock uplift across two folds transected by the Yakima River: Manastash and Umtanum Ridge. Isochron burial ages from in situ produced 26Al and 10Be at seven sites across the folds date episodes of strath terrace formation over the past ~2.9 Ma. Average bedrock incision rates across the Manastash (~88 m/Myr) and Umtanum Ridge (~46 m/Myr) anticlines are roughly 4 to 8 times higher than rates in the intervening syncline (~14 m/Myr) and outside the canyon (~10 m/Myr). These contrasting rates demonstrate differential bedrock incision driven by ongoing Quaternary rock uplift across the folds at rates corresponding to ~0.13 and ~0.06 mm/yr shortening across postulated master faults dipping 30 ± 10°S beneath the Manastash and Umtanum Ridge anticlines, respectively. The reported Quaternary shortening across the anticlines accounts for ~10% of the ~2 mm/yr geodetic budget, suggesting that other Yakima structures actively accommodate the remaining contemporary deformation.

Rates of river incision and scarp retreat in eastern and central Grand Canyon over the past half million years: Evidence for passage of a transient knickzone: COMMENT

Abbott et al. (2015) and Crow et al. (2014), two Grand Canyon incision studies, come to very different conclusions despite apparent methodological similarities. Crow et al. (2014) used U-Th, Ar-Ar, and cosmogenic burial dating of material associated with Colorado River (CR) strath terrace sequences at 6 sites throughout Grand Canyon and concluded that average bedrock incision rates have been temporally steady at each site over at least the last 650 ka, but vary spatially from 100 to 160 m/Ma due to differential mantle-driven uplift. Abbott et al. (2015) used our unpublished Ar-Ar dating on a dike in western Grand Canyon, near river mile 159, plus U-Th dating of travertine-cement in sidestream alluvium, near Hermit Rapid, to suggest that incision rates were 1–4 km/Ma from 500 to 400 ka and <200 m/Ma after 400 ka, a difference they ascribe to a migrating knickpoint. The conclusions of these studies are contradictory.

Active segmentation assessment of the tectonically active South Wagad Fault in Kachchh, Western Peninsular India

ABSTRACT In present study the results of integrated studies of landform development and geomorphic indices of drainage networks across the South Wagad Fault (SWF) zone have been presented. The SWF is a structure that is morphogenic in nature, creating a 40-m-high E–W trending escarpment that extends ~ 80 km at the southern end of the Wagad uplift. The SWF got reactivated during the Bhuj earthquake of 2001, continued aftershock activity of M ≥ 4 levels were recorded in the epicentral zone, north of the Kachchh Mainland Fault and west of the SWF. Strath terraces along the river valleys and the truncation of alluvial fans along the mountain front, incision, and offset of rivers suggests an active nature of the SWF. To assess the relative tectonic activity, morphometric indices such as stream-gradient (SL) index, hypsometric integral (HI), basin asymmetry (AF), mountain front sinuosity (Smf) index, sinuosity index (SI), and basin shape (BS) have been analyzed. The landform development along the river valley in the SWF zone suggests reactivation of the SWF between 12 and 3 ka. The southern front of Wagad is found to be uplifting at the rate of 2.8 mm/y during the last 12 ka as estimated through OSL dates. The relative index of active tectonics (RIAT) by the study of different geomorphic parameters and field checks indicates that most of the Wagad highland is active. The compressional strain of 0.05 μstrain/y inferred from the GPS study in the SWF area attests to the activeness.

Climate-dependent fluvial architecture and processes on a suborbital timescale in areas of rapid tectonic uplift: An example from the NE Tibetan Plateau

The substantial tectonic uplift (1000–2500m in a few million years) of the Northeastern Tibetan Plateau (NETP), together with the major climatic changes during the Quaternary, provides an opportunity to study the impact of tectonic and climatic changes on the morphological development and sedimentary architecture of fluvial deposits. The effects of these processes are revealed by a terrace staircase, together with the stratigraphy of each individual terrace, in the confluence zone of the Huang Shui and Yellow Rivers in the NETP, during the late Quaternary. On the basis of morphological mapping and OSL-dating, at least seven strath terraces were identified that formed during the last glacial cycle, which are preserved at locations where tectonic uplift was sufficient to separate them altitudinally from one another. The terraces are composed of stacked fluvial gravels, sands and alluvial loams. The principal result is that we demonstrate that the terraces were formed in response to climatic cycles on a suborbital timescale. For each terrace, the lower coarse-grained sediments (gravel and sand) were deposited during cold periods (such as the LGM, MIS3b, MIS4 and MIS5d) associated with a strong Asian winter monsoon. The aggradation during cold periods was associated with floodplain widening. The river incised slightly during the transitions from cold to warm phases, resulting in the transformation of the previous river plain into a terrace. The coarse grained cold phase deposits are covered by inter-bedded, horizontally-laminated silt and sand (representing flood sediments that often contain reworked soil material), during the (cold to warm) transitional phases. The floodplain accumulation on the terrace continued during the subsequent warm period. The warm periods (such as MIS3a, MIS3c, and MIS5a) of the climatic cycles are associated with a strong Asian summer monsoon. Pronounced incision took place at the subsequent warm–cold transitions. After this warm–cold transition, aeolian loess accumulated on the abandoned terrace without any further fluvial reworking. Our results demonstrate that critical thresholds for fluvial response can be crossed at climatic changes on a suborbital timescale given conditions of accelerated tectonic uplift in the NETP. In addition, based on the OSL ages of different units in the sedimentary sequences of the terraces, we conclude that the durations of terrace aggradations and floodplain widening lasted much longer than the periods of fluvial downcutting.

Floodplain persistence and dynamic-equilibrium conditions in a canyon environment

Canyon river systems are laterally constrained by steep walls, strath terraces, and bedrock intrusions; however, semialluvial reaches are nested within these environments as discontinuous floodplains along the river margins. These semialluvial floodplains provide an example of dynamic-equilibrium set within high-energy fluvial systems, marking areas where the river is free to alter its boundary conditions. Most research has focused on hydraulic conditions necessary for floodplain formation and persistence in unconfined systems, whereas little is known about canyon streams. This paper focuses on (1) characterizing dynamic-equilibrium, (2) describing the controls on floodplain formation and distribution, and (3) evaluating the performance of extremal hypotheses to identify dynamic-equilibrium and floodplain persistence in high-energy, semiconfined canyon environments.These objectives were addressed with field and numerical data derived from a one-dimensional hydraulic model for bankfull and 100-year return interval flood events, supported by closely spaced cross sections for the lower 38-km canyon reach of the Deadwood River (Idaho). Under bankfull conditions, critical energy thresholds for equilibrium floodplain persistence at this study site present the upper limits of: slope=0.018, shear stress=175N/m2, and specific stream power=400W/m2. Channel and floodplains near equilibrium, quantified with a near-zero sediment transport divergence (Exner equation), were successfully identified by the minimum unit stream power extremal hypothesis and to a lesser degree by the other extremal hypotheses that minimize energy expenditure (minimum specific stream power, minimum total stream power, and minimum Froude number), provided backwater environments and major tributaries could be identified. Extremal results were compared to hydraulic geometry relations to evaluate how closely equilibrium floodplains approached values for unconfined alluvial river systems.

Paleodischarge of the Mojave River, southwestern United States, investigated with single-pebble measurements of10Be

The paleohydrology of ephemeral stream systems is an important constraint on paleoclimatic conditions in arid environments, but remains difficult to measure quantitatively. For example, sedimentary records of the size and extent of pluvial lakes in the Mojave Desert (southwestern USA) have been used as a proxy for Quaternary climate variability. Although the delivery mechanisms of this additional water are still being debated, it is generally agreed that the discharge of the Mojave River, which supplied water for several Pleistocene pluvial lakes along its course, must have been significantly greater during lake highstands. We used the 10 Be concentrations of 10 individual quartzite pebbles sourced from the San Bernardino Mountains and collected from a ca. 25 ka strath terrace of the Mojave River near Barstow, California, to test whether pebble ages record the timing of large paleodischarge of the Mojave River. Our exposure ages indicate that periods of discharge large enough to transport pebble-sized sediment occurred at least 4 times over the past ∼240 k.y.; individual pebble ages cluster into 4 groups with exposure ages of 24.82 ± 4.36 ka (n = 3), 55.79 ± 3.67 ka (n = 2), 99.14 ± 12.07 ka (n = 4) and 239.9 ± 52.16 ka (n = 1). These inferred large discharge events occurred during both glacial and interglacial conditions. We demonstrate that bedload materials provide information about the frequency and duration of transport events in river systems. This approach could be further improved with additional measurements of one or more cosmogenic nuclides coupled with models of river discharge and pebble transport.

Terrace styles and timing of terrace formation in the Weser and Leine valleys, northern Germany: Response of a fluvial system to climate change and glaciation

In glaciated continental basins accommodation space is not only controlled by tectonics and sea-level but also by the position of ice-sheets, which may act as a regional base-level for fluvial systems. Although the Pleistocene terrace record of major river systems in northwestern Europe has been investigated by many authors, relatively little attention has been paid to base-level changes related to glacier advance–retreat cycles and how these regional changes in base-level interacted with river catchment processes. This study provides a synthesis of the stratigraphic architecture of Middle Pleistocene to Holocene fluvial terraces in the upper Weser and middle Leine valley in northern Germany and links it to glaciation, climate and base-level change. The depositional architecture of the fluvial terrace deposits has been reconstructed from outcrops and high-resolution shear wave seismic profiles. The chronology is based on luminescence ages, 230Th/U ages, 14C ages and Middle Palaeolithic archaeological assemblages.The drainage system of the study area developed during the Early Miocene. During the Pleistocene up to 170 m of fluvial incision took place. A major change in terrace style from strath terraces to cut-and-fill terraces occurred during the early Middle Pleistocene before Marine Isotope Stage MIS 12, which may correlate with climate deterioration and the onset of glaciation in northern central Europe. During this time a stable buffer zone was established within which channels avulsed and cut and filled freely without leaving these vertical confines. Climate was the dominant driver for river incision and aggradation, whereas the terrace style was controlled by base-level changes during ice-sheet growth and decay. A major effect of glacio-isostatic processes was the post-Elsterian re-direction of the River Weser and River Leine.The Middle Pleistocene fluvial terraces are vertically stacked, indicating a high aggradation to degradation ratio, corresponding with a regional base-level rise during glacier advance. At the beginning of the Late Pleistocene the terrace style changed from a vertical to a lateral stacking pattern, which is attributed to a decrease in accommodation space during glacier retreat. The formation of laterally attached terraces persisted into the Holocene.Major incision phases took place during MIS 5e, 5d, 5c, and probably early MIS 4, early MIS 3 and MIS 2 (Lateglacial). During MIS 5e and the Lateglacial the braided river systems changed into meandering rivers, indicated by preserved organic-rich flood-plain and point bar deposits. The Late Pleistocene braided river systems (MIS 5c to MIS 3) are characterized by a high sinuosity, which may be a direct effect of an increased downstream gradient after deglaciation when the channel lengthened and the river adjusted to the increased gradient by increasing sinuosity. These Middle Pleniglacial fluvial deposits are unconformably overlain by Lateglacial to Holocene meandering river deposits, which form laterally attached terraces, recording millennial-scale channel shifts. The lack of Late Pleniglacial deposits might be related to Late Weichselian forebulge formation.