Increase In Sediment Discharge Research Articles

Late Quaternary climatic control on erosion and weathering in the eastern Tibetan Plateau and the Mekong Basin

High-resolution siliciclastic grain size and bulk mineralogy combined with clay mineralogy, rubidium, strontium, and neodymium isotopes of Core MD01-2393 collected off the Mekong River estuary in the southwestern South China Sea reveals a monsoon-controlled chemical weathering and physical erosion history during the last 190,000 yr in the eastern Tibetan Plateau and the Mekong Basin. The ranges of isotopic composition are limited throughout sedimentary records: 87Sr/ 86Sr = 0.7206–0.7240 and ε Nd(0) = −11.1 to −12.1. These values match well to those of Mekong River sediments and they are considered to reflect this source region. Smectites/(illite + chlorite) and smectites/kaolinite ratios are used as indices of chemical weathering rates, whereas the bulk kaolinite/quartz ratio is used as an index of physical erosion rates in the eastern Tibetan Plateau and the Mekong Basin. Furthermore, the 2.5–6.5 μm/15–55 μm siliciclastic grain size population ratio represents the intensity of sediment discharge of the Mekong River and, in turn, the East Asian summer monsoon intensity. Strengthened chemical weathering corresponds to increased sediment discharge and weakened physical erosion during interglacial periods. In contrast, weakened chemical weathering associated with reduced sediment discharge and intensified physical erosion during glacial periods. Such strong glacial–interglacial correlations between chemical weathering/erosion and sediment discharge imply the monsoon-controlled weathering and erosion.

Kerguelen Plateau Quaternary–late Pliocene palaeoenvironments: from diatom, silicoflagellate and sedimentological data

Gravity cores from the South Kerguelen Plateau contain sections representing two Quaternary intervals: (1) <0.62 Ma and (2) 1.6–1.25 Ma; three Pliocene intervals: (1) 2.2–1.6 Ma, (2) 3.1/3.0–2.64 Ma, (3) 3.2–3.1/3.0 Ma, and at least three disconformities: (1) 0.62 Ma until possibly the Last Glacial Maximum, (2) 2.2–1.6 Ma, and (3) 3.1–2.2 Ma. The disconformities probably formed during intense glacial intervals when the velocity of the Antarctic Circumpolar Current increased. Increased current velocity winnowed away finer biogenic sediment, to create ice rafted debris lags that protected the underlying sediment. Sediment deposition occurred during the less cold climatic intervals. In these deposits, three diatom assemblages were identified by comparison with extant diatoms. The diatom data indicate fluctuating Quaternary open pack-ice and open water conditions. The open pack-ice assemblage differs from modern sea-ice assemblages, and may reflect differences in the sea-ice setting during past colder intervals at latitudes north of the Antarctic Divergence. A silicoflagellate ratio was used to reconstruct Pliocene summer sea-surface temperatures. Two intervals with surface water temperatures warmer than today were identified, during which conditions were 3.0–4.5°C warmer (3.1–2.64 Ma), and 4.5°C warmer (3.2–3.1 Ma) as far south as 62°S. This would have occurred if either the Antarctic Polar Front Zone was ∼1200 km further south, or if the temperature gradient across the associated oceanic fronts would have been significantly lower. The Pliocene intervals contain a large amount of ice rafted debris that may reflect an environment heavily influenced by icebergs due to warmer glacial conditions in Antarctica, with increased sediment discharge and iceberg calving.

Post-collisional sediment budget history of the Alps: tectonic versus climatic control

Based on the sediment budget of the Eastern, Swiss and Western Alps since the Oligocene the regional tectonic evolution has been identified as the dominant factor. It is superimposed on the influence of both regional and global climate change and global sea-level change. During late Pliocene and Pleistocene times, climate became the dominant factor because of cyclic glaciations. The early post-collisional history of the Alps is characterized by a doubling of sediment discharge rates around the Rupelian/Chattian boundary. This increase is attributed to isostatic re-adjustment either after large-scale thermal reorganization of the lithosphere related to slab break-off, or to crustal thickening as continental crust became subducted. From Middle Oligocene to Late Miocene times, the overall trend of sediment discharge rates in the entire Alps was modified only during relatively short-lived phases. These are characterized by an increase in Aquitanian (ca. 23–21 Ma) and late Burdigalian times (ca. 18–16.4 Ma), and a decrease in early to middle Burdigalian (21–19 Ma) and Langhian to Serravalian times (16.4–12 Ma). An important, still ongoing period of uplift, reflected by rapidly increasing sediment discharge rates, started in latest Miocene times in the Swiss and Western Alps and affected the Eastern Alps some 2 million years later. The reason for this uplift is not clear, but deep-seated lithospheric processes appear to be likely.

Post-Eocene evolution of the North Alpine Foreland Basin and its response to Alpine tectonics

The post-flysch (Oligocene–Miocene) palaeogeographic evolution of the entire North Alpine Foreland Basin (NAFB) between Savoy (France) and Lower Austria is presented in eight sketch maps. The compilation considers the palinspastic evolution of the Alps. It includes intramontane deposits, which represent a continuous marginal facies of the NAFB during Rupelian to Early Burdigalian times. The facies distribution in the NAFB was driven by two major types of processes, which are related to the tectonic evolution of the Alpine orogen. The first type, representing tectonic processes at the thrust front of the Alps, directly influenced the facies distribution of the narrowing NAFB. The second type represents an indirect impact of Alpine uplift and tectonics to the NAFB, transformed by varying sediment discharge. A strong increase in sediment discharge due to uplift of the Alps is the major reason for the generally regressive coarsening- and shallowing-upward cycle from the Lower Marine to the Lower Freshwater Molasse (UMM, USM) between 33 and 21 Ma. The development of the “Burdigalian Seaway” at ∼20 Ma was coeval with a reduction of thrust advance rates in the western and central part of the NAFB. Shallow marine conditions (Upper Marine Molasse, OMM) prevailed for ∼3 million years. In the Eastern Alps, reduction of relief due to lateral (east–west-oriented) extension caused a strong reduction in sediment discharge. Closure of the “Burdigalian Seaway” around 17 Ma occurred during a phase of tectonic reorganisation in the Alpine orogen and is coeval with a short-term increase in sediment discharge. Between 17 and 12 Ma, the NAFB was constantly overfilled (Upper Freshwater Molasse, OSM), despite strongly decreasing sediment discharge. Termination of sedimentation in the (unfolded) NAFB occurred diachronously in an undramatic process. It started in the western NAFB in the course of uplift of the Swiss Jura Mountains after 11 Ma and reached Lower Austria around 6–5 Ma. Strong uplift of the Alps and the NAFB started at around 6 Ma in the Swiss and Western Alps and at 4–3 Ma in the Eastern Alps. The uplift was followed by reworking and erosion of more than 2 km of Molasse sediments in the western NAFB.

Holocene delta evolution and sediment discharge of the Mekong River, southern Vietnam

Evolutionary changes, delta progradation, and sediment discharge of the Mekong River Delta, southern Vietnam, during the late Holocene are presented based on detailed analyses of samples from six boreholes on the lower delta plain. Sedimentological and chronostratigraphic analyses indicate clearly that the last 3 kyr were characterized by delta progradation under increasing wave influence, southeastward sediment dispersal, decreasing progradation rates, beach-ridge formation, and steepening of the face of the delta front. Estimated sediment discharge of the Mekong River for the last 3 kyr, based on sediment-volume analysis, was 144±36 million t yr −1 on average, or almost the same as the present level. The constant rate of delta front migration and stable sediment discharge during the last 3 kyr indicate that a dramatic increase in sediment discharge owing to human activities, as has been suggested for the Yellow River watershed, did not occur. Although Southeast Asian rivers have been considered candidates for such dramatic increases in discharge during the last 2 kyr, the Mekong River example, although it is a typical, large river of this region, does not support this hypothesis. Therefore, estimates of the millennial-scale global pristine sediment flux to the oceans must be revised.

Progradation of the Changjiang River delta since the mid-Holocene

Subaqueous deltaic deposits with approximately 30 radiocarbon ages show that the Changjiang River delta was strongly affected by tides and that the delta progradation rate after 2 kaBP was almost double the rate before 2 kaBP. This change in the progradation rate correlates well with the active extension of the subaerial delta plain shown by previous work. Widespread human activities, such as farming, deforestation, and dike construction, probably resulted in an increase in sediment discharge to the river-mouth area.

Offshore-onshore correlation of upper Pleistocene strata, New Jersey Coastal Plain to continental shelf and slope

High-resolution seismic reflection profiles (,1‐5 m resolution), including Geopulsee, Uniboome, minisparker, small air gun, and water gun sources, are used to trace the d 18 O stage 5 portion of the outcropping Cape May Formation across the shelf to the continental slope. The d 18 O stage 5/6 boundary identified at Ocean Drilling Project (ODP) Site 903 on the continental slope anchors the onshore-offshore seismic correlations. Above the d 18 O stage 5 sequence, there are distinguishable lowstand systems tracts (LST), transgressive systems tracts (TST) and highstand systems tracts (HST) that correlate with d 18 O stages 4 through 1. Atlantic Margin Coring Project (AMCOR) holes 6009, 6010, 6011, 6020, and 6021C provide age and paleoenvironmental indicators that agree with these correlations. The sub-arctic paleoenvironmental indicators in sequences of d 18 O stage 3 agree with the cooler temperatures and lower sea-level highstands of that time. Thicker d 18 O stage 3 and 4 sequences are preserved in the Paleo-Hudson River incised valley across the shelf. The expanded ice sheets during stage d 18 O 3 compared to d 18 O stages 1 and 5 probably increased sediment discharge in the Hudson River drainage system. q 2000 Elsevier Science B.V.

Water table control on rill initiation and implications for erosional response

On sloping sandy agricultural soil sites near Toronto, Canada, summer storms adequate to generate runoff occur frequently, but rill development occurs mainly in spring when snowmelt or rainfall-induced runoff occurs above frozen subsoil. This suggests that on low and moderate slopes on these soils rill initiation is controlled primarily by hydraulic impedance close to the surface, rather than critical hydraulic conditions in runoff. Laboratory flume experiments were carried out on 10 m slopes at 1.5°, 5° and 9° with loamy sand/clay composite soil sample to test this hypothesis. Runoff with hydraulic conditions adequate for rill initiation occurred rapidly in most tests, but on 1.5° and 5° slopes little knickpoint scour or sediment transport occurred before water table development. This coincided with reduced surface soil strength, knickpoint scour and marked increase in sediment discharge, particularly on 5° slopes where increase was 20- to 30-fold. Further increase in sediment discharge occurred when water tables reached the surface. On 9° slopes runoff occurred more quickly, with higher hydraulic values. Significant rill incision and sediment discharge occurred well before water table development, and ultimately reached much higher values than on lower slopes. Results show that soil erodibility can change dramatically over short time periods during storms due to soil moisture conditions, and that the presence of a hydraulic impedance close to the surface which causes a perched water table to develop can strongly influence rill incision and sediment transport. The influence is unlikely to be marked on soils which are very erodible regardless of moisture conditions, or on extremely resistant soils. It will also be limited on very gentle or steep sites, but can be a significant factor in rill development on intermediate slopes.

Suspended sediment discharge from snowmelt: Ikushunbetsu River, Hokkaido, Japan

Sediment discharge generated from snowmelt was observed in the Ikushunbetsu River, Hokkaido, Japan (drainage basin area: 60 km 2) in April–May 1992 and April 1993. Continuous records of water discharge Q and suspended sediment concentration C s indicated synchronous temporal variations at a station on the riverbank and a downstream increase in sediment discharge Q s( = Q· C s) between two stations along a river channel. The sediment suspended over 90% of water depth consisted of more than 95 wt.% of silt (3.91 < d ≤ 62.5 μm) and clay ( d ≤ 3.91 μm). Sources of the fine suspended sediment were identified by comparing grain size distributions of riverbed and side gorge slope sediment samples, being supplemented by mineralogical analysis. It is concluded that the sediment discharge is generated by the fluvial erosion of weathered soils which are accumulated on side gorge slopes from the collapse of the top soil, and the amount of sediment eroded per unit area per unit time is approximately proportional to the fluvial shear stress.

Some methodological developments in the analysis of sediment transport processes using age distribution of floodplain deposits

Floodplain areas are primary storage sites for river sediment. In the Saru River, ages of floodplain surfaces were examined by tree ring analysis, and vertical and horizontal configurations of floodplain deposits were measured by field and aerial-photo surveying. A flood in 1992 provided a good opportunity to examine depositional and erosional processes of floodplain sediment before and after the flood. Predominant disturbances were observed in the unconstrained, wide reaches where floodplains develop. This event indicated that eroded areas of floodplain deposits in each age class linearly increased with sediment volume and that the proportion of the total area eroded decreased exponentially with increase in the age of sediment. We constructed basic equations expressing continuity of age distribution in order to analyze river sedimentation in a time series according to the results of the 1992 flood. The floodplain disturbance rates determined by this analysis showed similar temporal changes with sediment transport rates monitored at the Iwachishi Reservoir. A sharp increase in sediment discharge was seen after 1962 associated with the historical maximum rainfall in the temporal analysis of floodplain sediment. The time series approach presented here is useful for evaluating the speed of sediment waves and the cumulative impact of sedimentation in a river basin.

Acid sulphate soil-landscape relationships in the Pearl River Delta, southern China

Relationships between the development of acid sulphate soil and landscape characteristics were examined in a fluvial-dominated, estuarine-deltaic plain of the South China coast. It was found that the accumulation of pyrite (potential acid sulphate material) has been limited generally by rapid sedimentation rates and insufficient supply of sulphate ion. The occurrence of the pyritic sediments is, however, closely associated with a particular depositional pattern, i.e. “surrounding island deposition”. The acid sulphate soils in the area frequently occur within narrow zones surrounding present-day's rocky hills (previously bedrock islands). This provides a guide for predicting the occurrence of potential acid sulphate soils in the area. Apart from the naturally large input of fluvial sediment, human activities have also accelerated the delta progradation and limited the vertical accretion of the deltaic surface. These effects have had significant impacts on the thickness and occurrence depth of the pyritic layer. With a history of more than 1000 years of land reclamation, the intensive cultivation in the area has turned most of the potential acid sulphate soils into actual acid sulphate soils, however, the soil properties vary markedly with landuse types. The case of the Pearl River Delta is generally in agreement with Pons and van Breemen's (1982) conclusion in that the accumulation of pyrite is inversely related to the rate of sedimentation. Our results show that the existence of bedrock islands in the estuary basin provided zones of slow sedimentation and therefore enhanced pyrite accumulation. On the other hand, human intervention over the past 2000 years has acted to reduce pyrite accumulation both through landward catchment clearing that increased sediment discharge to the estuary and through empolderment and associated land management practices.

Hydraulic conditions for rill incision under simulated rainfall: A laboratory experiment

AbstractA series of controlled laboratory experiments were conducted in order to obtain precise data on the hydraulic and sediment transport conditions during rill formation. Tests were carried out using a crusting‐prone binary mixed soil in a 15 m long flume at an average slope of 0·087 under simulated rainfall. Rainfall intensities varied from 30–35 mm h−1 and developed about 70 per cent of the kinetic energy of natural rainfall of similar intensity.Runoff and sediment discharge measured at the downstream weir were strongly influenced by rill forming processes. Essentially, rill incision reduced runoff discharge as a result of increased percolation in rill channels but increased sediment discharge. Secondary entrainment processes, such as bank collapse, also increased sediment discharge at the weir. Knickpoint bifurcation and colluvial deposition, however, decreased sediment discharge. Rills always developed through the formation of a knickpoint. The critical condition for knickpoint initiation was the development of supercritical flow and waves which mould and incise the bed. Prior smoothing of the soil surface by entrainment and redistribution of sediment facilitated supercritical flow. Statistical analysis showed that hydraulic and sediment transport conditions differed significantly in rilled and unrilled flows. The relationship between sediment discharge, rill erosion, and flow hydraulics was found to be nonlinear, conforming to a standard power function in the form y = axb. Rills were also associated with significantly increased sediment transport capacities. However, rill initiation was not clearly defined by any specific hydraulic threshold. Instead, rilled and unrilled flows were separated by zones of transition within which both types of flow occur.

Fan-delta and braid-delta systems in Pennsylvanian Sandia Formation, Taos Trough, northern New Mexico: Depositional and tectonic implications

Fan-delta and braid-delta deposits are distinguished in the Pennsylvanian Sandia Formation, Taos Trough, New Mexico, according to guidelines recently defined by McPherson and others. Subaqueous sedimentation styles and depositional sequences also are used for distinguishing fan-delta and braid-delta deposits in the Taos Trough. Fan deltas were deposited in close proximity to the active western boundary fault of the Taos Trough and occur as coarse-grained aggradational sequences. Alluvial processes in fan-delta complexes were characterized by unrestricted sheetflood and ephemeral streamflow deposition. Subaqueously deposited, fan-delta conglomerates and sandstones arranged in lenticular units encased by basinal mudstones were emplaced by a variety of high-density and dilute gravity-flow processes. Braid-delta complexes were characterized by channelized alluvial deposition in which discharge was variable, but perennial. Perennial discharge in braid deltas was in response to expansion of the drainage area in the Taos Trough hinterland through time. This resulted in increased sediment discharge into the trough and enabled basinward extension of the alluvial plain by braid-delta progradation. This study concludes that fan deltas are associated with steep basin margins in which overall subsidence rates exceed sedimentation rates. Braid deltas are related to low-gradient basin margins along which sedimentation rates are greater than subsidence rates. Distribution of fan-delta and braid-delta complexes in the Sandia Formation is dictated by tectonic load-induced subsidence in the Taos Trough and may be used as a guide for resolving evolutionary history of other types of active sedimentary basins.

Low‐Temperature Effects on Flow in Sand‐Bed Streams

Laboratory experiments were performed in a 40‐ft long, 2‐ft wide, 1‐ft deep (12‐m×0.6‐m×0.3‐m) tilting flume to elucidate the effects of temperature variations on the flow and sediment‐transport characteristics of streams with beds of fine sand (D50=0.11mm in these experiments). Nearly constant depth was maintained with three different constant discharges, and temperatures were varied from approximately 30° C to near 0° C. In the two series of experiments at higher velocities and Froude numbers, temperature reductions produced major increases in sediment discharge and uniformity of concentration distributions and smaller, but still significant, increases in friction factor and bed‐form height. An analysis of the experimental results within the framework of a theoretical sediment‐transport model revealed that the increased sediment discharge with decreasing water temperature in flows carrying significant amounts of suspended sediment are caused by increases in both bed‐layer sediment concentration and the uniformity of the concentration profiles. The sediment discharges of low‐velocity (F=≃0.30) flows, which were just above the threshold of sediment motion, were not significantly affected by water temperature. It was concluded that temperature affects the near‐bed concentration only if the bed‐layer Reynolds number (based on bed‐layer thickness) is greater than about 20.

The impact, control, and mitigation of off-road vehicle activity in Hungry Valley, California

Hungry Valley, and arid basin situated within the Transverse Ranges of Southern California, has been utilized by off-road vehicles (ORVs) since 1971. Loss of vegetation, severe soil erosion and gullying, alluvial fan formation, and increased sediment discharge are the direct results of ORV activity in the valley. These hydrologic-geologic effects have been documented and monitored utilizing sequential aerial photographs, ground surveys, and sediment transport measurements. Despite the increased usage of ORVs on public lands, and the well-documented short-term physical environmental impacts, little data exists on erosion or sediment transport rates in disturbed vs. adjacent undisturbed areas, the ability of natural vegetation to re-establish itself, or the feasibility of site rehabilitation techniques or methods.

Dissolved, suspended and bed load movement patterns in Two O'Clock Creek, Rocky Mountains, Canada, summer, 1969

During the summer of 1969, 12850 tons of material were removed as suspended sediment load, 440 tons as dissolved load and 65 tons as bed load from Two O'Clock Creek Basin in the Canadian Rockies. This is equivalent to a surface lowering of the basin by 0.0195 inches per year, a figure which agrees very well with rates of denudation reported by researchers for other mountain areas. During the peak snow melt generated flood in early June, 87 per cent of the total sediment load was exported. Most of the remainder was transported out of the basin by a secondary high flow resulting from rainfall and snow melt in early July. The single most intense rainstorm of the season on August 5 and 6 resulted in a minor increase in stream flow but no increase in sediment discharge.