Alluvial Storage Research Articles

Sediment storage, sea level, and sediment delivery to the ocean by coastal plain rivers

Coastal and marine sedimentary archives are sometimes used as indicators of changes in continental sediment production and fluvial sediment transport, but rivers crossing coastal plains may not be efficient conveyors of sediment to the coast. Where this is the case, changes in continental sediment dynamics are not evident at the river mouth. Stream power is typically low and accommodation space high in coastal plain river reaches, resulting in extensive alluvial storage upstream of estuaries and correspondingly low sediment loads at the river mouth. In some cases there is a net loss of sediment in lower coastal plain reaches, so that sediment input from upstream exceeds yield at the river mouth. The lowermost sediment sampling stations on many rivers are too far upstream of the coast to represent lower coastal plain sediment fluxes, and thus tend to overestimate sediment yields. Sediment which does reach the river mouth is often trapped in estuaries and deltas. Assessment of sediment flux from coastal plain rivers is also confounded by the deceptively simple question of the location of the mouth of the river. On low-gradient coastal plains and shelves, the location of the river mouth may have varied by hundreds of kilometers due to sea-level change. The mouth may also differ substantially according to whether it is defined based on channel morphology, network morphology, hydrographic or hydrochemical criteria, elevation of the channel relative to sea level, or the locus of deposition. Further, while direct continent-to-ocean flux may be very low at current sea-level stands, sediment stored in estuaries and lower coastal plain alluvium (including deltas) may eventually become part of the marine sedimentary package. The role of accommodation space in coastal plain alluvial sediment storage has been emphasized in previous work, but low transport capacity controlled largely by slope is also a crucial factor, as we illustrate with examples from Texas.

Residence time of alluvium in an aggrading fluvial system

AbstractRelationships between the surface area and age of alluvial deposits were used to estimate the residence time of alluvium in the 2205 km2 Waipaoa River basin, New Zealand. The contemporary Waipaoa River is an efficient transporter of sediment to the continental shelf, but the basin has been characterized by rapid channel and valley aggradation in the historic period, and by extensive mid‐ to late Holocene alluvial storage in the lower reaches. The area‐weighted mean age of alluvial deposits in the lower part of the river basin is ∼4400 yr. These deposits comprise terrace remnants isolated by downcutting, and Holocene to Recent sediments that are potentially remobilizable by the modern river. Even though the amount of storage is small relative to downstream transport, the majority of the potentially remobilizable alluvium is likely to remain in storage for >100 yr, and its half‐life (time for 50 per cent removal) is >2000 yr. Within the confines of the flfloodplain, the apparent ‘loss’ of older deposits is due primarily to burial, but losses of the most recent deposits are due almost entirely to remobilization (30–40 per cent), with the remainder preserved in the alluvial record for at least 104 yr. Most of this sediment is likely to remain in storage until there is a shift to a degradational state. Copyright © 2006 John Wiley & Sons, Ltd.

Dam-to-delta sediment inputs and storage in the lower trinity river, Texas

Livingston Dam on the Trinity River in SE Texas, USA disrupts the transport of sediment to the lower Trinity River and the Trinity Bay/Galveston Bay estuary. However, a sediment budget of the lower basin shows that the effects of this disruption are undetectable in the lower river. Sediment trapped in Lake Livingston is partly offset by channel erosion downstream of the dam and by inputs from the lower basin. Most importantly, however, the lower coastal plain reaches of the Trinity are characterized by extensive alluvial storage and are a bottleneck that buffers the bay from effects of upstream changes in sediment flux. Storage is so extensive that the upper Trinity basin and the lowermost river reaches were essentially decoupled (in the sense that very little upper-basin sediment reached the lower river) long before the dam was constructed. Whereas sediment storage in Lake Livingston is extensive, alluvial storage on the Trinity flood plain is even more extensive. Dam-related sediment starvation effects are noted for about 52 km downstream, and the sediment budget suggests that a majority of the sediment in this reach is likely derived from channel scour and bank erosion. The capacious alluvial storage in the lower Trinity not only limits flux to the bay, but the large amount of remobilizable alluvium also allows the system to adjust to localized sediment shortages, as illustrated in the dam-to-Romayor reach. Internal adjustments within the lower Trinity River valley thus buffer the bay from changes in sediment supply upstream.

Alluvial storage and the long‐term stability of sediment yields

AbstractSeveral recent studies have shown general consistency of fluvial denudation rates over long time periods, or historical and contemporary sediment yields of the same general magnitude as sediment yields or accumulation rates over geologic time. This consistency of fluvial sediment export from some drainage basins, despite substantial climate, hydrological, ecological, base level, and other environmental changes, suggests that long‐term sediment yields may be controlled by factors that are independent of and overwhelm environmental changes (e.g. tectonics), or that the fluvial sediment system is at some level dynamically stable. The latter is explored via a model based on the notion that all debris produced by weathering within a drainage basin over any time period is either retained as part of the regolith, transported out of the basin as solid or dissolved sediment yield, or stored as alluvium within the fluvial system. This system is dynamically stable if alluvium is always potentially available for transport; e.g. to be converted to yield, and if regolith development exerts a negative feedback on weathering rates. This supports the argument that the long‐term consistency of sediment yields (where it exists) may be attributable to the storage and remobilization of alluvium, which buffers the system against environmental change. Environmental changes are manifested primarily in reorganizations within the fluvial sediment system, such as variations between net increases and decreases in alluvial storage, and changes in the spatial locus of deposition. These ideas are illustrated and tested using data from the lower Trinity River in southeast Texas.

Erosion of an Ancient Mountain Range, The Great Smoky Mountains, North Carolina and Tennessee

Analysis of 10Be and 26Al in bedrock (n=10), colluvium (n=5 including grain size splits), and alluvial sediments (n=59 including grain size splits), coupled with field observations and GIS analysis, suggest that erosion rates in the Great Smoky Mountains are controlled by subsurface bedrock erosion and diffusive slope processes. The results indicate rapid alluvial transport, minimal alluvial storage, and suggest that most of the cosmogenic nuclide inventory in sediments is accumulated while they are eroding from bedrock and traveling down hill slopes. Spatially homogeneous erosion rates of 25 - 30 mm Ky−1 are calculated throughout the Great Smoky Mountains using measured concentrations of cosmogenic 10Be and 26Al in quartz separated from alluvial sediment. 10Be and 26Al concentrations in sediments collected from headwater tributaries that have no upstream samples (n=18) are consistent with an average erosion rate of 28 ± 8 mm Ky−1, similar to that of the outlet rivers (n=16, 24 ± 6 mm Ky−1), which carry most of the sediment out of the mountain range. Grain-size-specific analysis of 6 alluvial sediment samples shows higher nuclide concentrations in smaller grain sizes than in larger ones. The difference in concentrations arises from the large elevation distribution of the source of the smaller grains compared with the narrow and relatively low source elevation of the large grains. Large sandstone clasts disaggregate into sand-size grains rapidly during weathering and downslope transport; thus, only clasts from the lower parts of slopes reach the streams. 26Al/10Be ratios do not suggest significant burial periods for our samples. However, alluvial samples have lower 26Al/10Be ratios than bedrock and colluvial samples, a trend consistent with a longer integrated cosmic ray exposure history that includes periods of burial during down-slope transport. The results confirm some of the basic ideas embedded in Davis’ geographic cycle model, such as the reduction of relief through slope processes, and of Hack’s dynamic equilibrium model such as the similarity of erosion rates across different lithologies. Comparing cosmogenic nuclide data with other measured and calculated erosion rates for the Appalachians, we conclude that rates of erosion, integrated over varying time periods from decades to a hundred million years are similar, the result of equilibrium between erosion and isostatic uplift in the southern Appalachian Mountains.

Correlation of reservoir sandstones using quantitative heavy mineral analysis

Heavy mineral analysis is one of a group of provenance-based methods that complement traditional biostratigraphic correlation of clastic reservoirs. A variety of processes give rise to stratigraphic changes in sediment composition, including source area uplift, unroofing, changes in climatic conditions, extent of alluvial storage on the floodplain and the interplay between different depositional systems. Heavy mineral analysis is a reliable and proven technique for the correlation of clastic successions because prolonged and extensive research has provided detailed understanding of the effects of processes that alter the original provenance signal during the sedimentary cycle, such as hydrodynamics and diagenesis. The technique has been successfully applied to a wide range of clastic reservoirs, from fluvial to deep marine and from Devonian to Tertiary, using a combination of different types of parameters (provenance-sensitive mineral ratios, mineral chemistry and grain morphology). The application of heavy mineral analysis as a non-biostratigraphic correlation tool has two limitations. The first is that valid correlations cannot be made in sequences with uniform provenance and sediment transport history, but this is a problem inherent with all provenance-based methods. The other is that the technique can be applied only to coarse clastic lithologies and is not suitable for fine-grained sediments or carbonates.

Authigenic Apatite in a Fluvial Sandstone Sequence: Evidence for Rare-Earth Element Mobility During Diagenesis and a Tool for Diagenetic Correlation

ABSTRACT Apatite grains in heavy-mineral separates of samples from the Statfjord Formation (Triassic-Lower Jurassic, North Sea), exhibit variably corroded detrital cores and euhedral diagenetic overgrowths. In the overlying marine deposits of the Nansen Formation, apatite grains show no evidence of either corrosion or overgrowths. Compositions of detrital apatite in both formations are indistinguishable, indicating that the source areas responsible for Statfjord and Nansen deposition included similar apatite-bearing lithologies. Apatite corrosion in the Statfjord Formation is considered to be a response to leaching by acidic groundwaters during sediment storage on the fluvial floodplain. Any variations in the degree of corrosion that may have been generated by differences in floodplain residence time have been homogenized by sediment reworking. Laser-ablation microprobe inductively coupled plasma mass spectrometry and wavelength-dispersive electron microprobe analysis indicate that the authigenic apatite contains high abundances (up to 3.5% total) of the rare earth elements (REE), implying a degree of REE mobilization during diagenesis. REE profiles (NASC normalized) for the authigenic apatites are convex-upwards with enrichment of the MREE. The REE are considered to have been derived from dissolution of other REE-bearing heavy minerals (e.g., garnet) during alluvial storage, with possible contributions from dissolved feldspars (which may account for slight positive Eu anomalies) and REEs adsorbed onto clay surfaces. REE mobilization was probably enhanced by fluoride and/or phosphate complexes, although organic complexing related to pedogenic processes is also a possibility. Through the Statfjord Formation, authigenic apatite displays subtle variations in major-element, minor-element, and trace-element chemistry, which may find application as a tool for diagenetic correlation.

Rates of Soil Erosion

In his report “Decreased rates of alluvial sediment storage in the Coon Creek Basin, Wisconsin, 1975–93” (20 Aug., p. [1244][1]), Stanley W. Trimble presents the significant findings that there is a relatively constant supply of sediments released to the Mississippi River from Coon Creek and

A noncalibrated rainfall‐runoff model for large, arid catchments

A distributed, field‐based rainfall‐runoff model was developed for the 1400‐km2 arid catchment of Nahal Zin, Israel. No calibration with measured flow data was performed. The model used rainfall radar input applied over a catchment that was spatially disaggregated into different terrain types according to hydrologically relevant surface characteristics. Hortonian overland flow generation on each type was parameterized independently using values of initial loss and temporal decay of infiltration determined from existing field experiments. Delimited by topography, this catchment wide pattern of rainfall excess was distributed over 850 tributary catchments (model elements). Runoff delivery from the model elements to the adjoining channel segments was timed by applying a mean response function determined in an environmentally similar experimental catchment. Inside the channel network the MVPMC3 method of the Muskingum‐Cunge technique was used for streamflow routing, accounting for channel dimensions and roughness. For each channel segment a constant infiltration rate was applied to account for transmission losses and discontinued when the wetting front reached the bottom of the available alluvial storage. Within two model tests, one separate for the routing component (October 1979) and one for the complete model (October 1991), observed hydrographs and reconstructed peak discharges were successfully simulated. The spatially distributed model output showed that during the October 1991 test, tributaries produced preceding peaks that wetted the channel alluvium before the main flood had arrived and transmission losses lost their significance downstream. Total maximum model uncertainty was estimated including the uncertainty ranges of each model parameter. In general, this study shows that field‐based data on generation and losses of runoff may be incorporated into a distributed hydrologic model to overcome calibration with the poor data records of arid high‐magnitude events.

Processes controlling the composition of heavy mineral assemblages in sandstones

Sandstone compositions result from a complex interplay between provenance and factors that operate during the sedimentation cycle. Accurate identification and discrimination of provenance depends on isolating provenance-sensitive features, and avoiding parameters that are influenced by other factors. Heavy mineral analysis offers a high-resolution approach to determination of sandstone provenance, because of the diversity of mineral species found in sandstones and because the factors affecting assemblages have been comprehensively evaluated. This paper presents the current understanding of the effects of processes operative during the sedimentation cycle. The original provenance signal may be overprinted by weathering at source prior to incorporation in the transport system; by mechanical breakdown during transport; by weathering during periods of alluvial storage on the floodplain; by hydraulic processes during transport and final deposition; by diagenesis during deep burial; and by weathering at outcrop. The most influential of these processes are hydraulics, which fractionates the relative abundance of minerals with different hydraulic behaviour, and burial diagenesis, which reduces mineral diversity through progressive dissolution of unstable mineral species. There is also evidence that weathering during alluvial storage plays a significant role. Two alternative, complementary approaches are recommended to identify provenance from heavy mineral data. The relative abundances of minerals with similar hydraulic and diagenetic behaviour are largely unaffected by processes operative during the sedimentation cycle, and utilize information gained from the entire heavy mineral suite. Determination of such ratios can be augmented by acquisition of varietal data, concentrating on the varieties shown by mineral types within the assemblage. A number of different varietal techniques are recommended, including optical differentiation of types based on colour, habit and internal structure, single-grain geochemical analysis, and single-grain geochronology.

Spatial patterns of historical overbank sedimentation and floodplain evolution, Blue river, Wisconsin

Increased soil erosion and overbank flooding associated with land use changes following European settlement have buried pre-settlement floodplain surfaces in many Driftless Area watersheds with as much as several meters of alluvium. This paper examines the processes influencing spatial variations in the storage of post-settlement vertical accretion alluvium and the impact on floodplain evolution in the Blue River watershed, Wisconsin. The cross-sectional area of historical vertical accretion sediments stored on floodplains was measured at 41 cross-valley transects in the 208 km 2 watershed. The storage of historical overbank sediments is influenced by valley width, stream power, and the development of meander belts. Post-settlement alluvial storage increases with valley width and decreases with cross-sectional stream power. In a multivariate regression model, valley width and cross-sectional stream power explain 67% of the variance in the amount of post-settlement alluvium. The development of meander belts, enlarged channels confined between alluvial terraces, by accelerated lateral channel migration influences historical floodplain storage by decreasing the influence of valley width and modifying stream power characteristics for flows, greater than bankfull. Meander belts are located in headwater and mid-basin valley reaches where high stream power produces rapid lateral channel migration, whereas low gradient channels in downstream valleys lack the power necessary to migrate laterally and develop meander belts. Meander belts influence the spatial pattern of historical sedimentation by: (1) containing large magnitude floods and preventing overbank flow, (2) increasing sediment-transport capacities, (3) initiating the erosion of previously deposited historical sediments, (4) reducing the attenuation of flood peaks, and (5) increasing the amount of sediment routed downstream to wide, low gradient valley bottoms lacking meander belts. Continued high rates of lateral channel migration will eventually be damped as widening of the meander belt decreases flow depths and stream power.

Chemical and Physical Weathering of Fluvial Sands in an Arctic Environment: Sands of the Sagavanirktok River, North Slope, Alaska

ABSTRACT Sands of the Sagavanirktok River drainage basin, on the North Slope of Alaska, are composed primarily of calcite, quartz, and sedimentary and metasedimentary lithic fragments. The relative proportions of these components vary systematically through the drainage basin. Sands sampled in the river headwaters, in the Brooks Range, average 58% calcite, 35% lithic fragments, and 7% quartz. With increased distance from river headwaters, calcite is progressively removed from the sediments. Rivers sampled in the Arctic Foothills physiographic province average 29% calcite, 53% lithic fragments, and 18% quartz. Further transport results in enrichment of quartz relative to lithic fragments. Sands from the Arctic Coastal Plain average 9% calcite, 38% lithic fragments, and 53% quartz; the northernm st sample collected, from near the head of the Sagavanirktok delta, is 70% quartz, 5% calcite, and 25% lithic fragments. We attribute these increases in compositional maturity to chemical and physical weathering of sediments during temporary storage in alluvial sequences such as alluvial fans, river braid plains, point bars, and alluvial terraces. As river channels migrate and reincorporate such stored alluvium into bed load, there is a net increase in the proportion of sediment that has been stored--and subjected to weathering during alluvial storage. Climate change, tributary dilution, and anthropogenic perturbation of the river system are insufficient to account for these compositional changes. This process is similar to that demonstrated previously in tropical river systems. The magnitude of the compositional variation among the Sagavanirktok River sands suggests that weathering during alluvial storage is a dominant process in shaping sediment composition in Arctic settings as well. Chemical weathering in the arid, Arctic setting of the North Slope of Alaska is probably enhanced by high organic acid activity associated with the stagnant water perched on permafrost of the tundra. Physical weathering in the form of grain disaggregation through freeze-thaw cycles, crushing between cobbles, and dragging by ice probably plays an important role as well.

SHRIMP constraints on sediment provenance and transport history in the Mesozoic Statfjord Formation, North Sea

Detrital zircons in sandstones of the Statfjord Formation (late Triassic-early Jurassic) in the northern North Sea have been dated with the sensitive high resolution ion microprobe (SHRIMP). These sandstones lack detailed biostratigraphic control, but display distinct variations in heavy mineral assemblages which facilitate a three-fold subdivision of the sequence. The lower and middle zones (heavy mineral Zones A and B) have similar characteristics, both displaying a marked upward decline in garnet abundance which has proved difficult to attribute unambiguously to change in either provenance or sedimentation history. The base of the highest zone (heavy mineral Zone C) is marked by changes in several heavy mineral indicators attributable to a shift in ultimate provenance compared with the underlying zones. The detrital zircon ages are a direct fingerprint of the sediment source terrains, and so resolve ambiguities in interpreting provenance from heavy mineral abundances alone. The age spectra indicate that both low-garnet and high-garnet parts of Zone A have the same ultimate provenance. This strongly suggests that garnet removal was not a function of changing provenance, but took place during the sedimentation process, probably by weathering during periods of prolonged alluvial storage. The source of Zone A is not represented in any presently exposed landmass: the required combination of Caledonian granitoids intruding or in immediate proximity to high-grade Archaean-sourced metasediments is likely to have been either to the west, in the northern part of the Shetland Platform, or to the north, in what is now the Norwegian Sea. Zircons in Zone C have a distinctly different age spectrum consistent with a source in the Western Gneiss Region of southern Scandinavia, where there is the required combination of Proterozoic protoliths and Caledonian reworking.

The Fate of Eroded Soil: Sediment Sinks and Sediment Budgets of Agrarian Landscapes in Southern Minnesota, 1851–1988

Abstract Human-induced soil erosion has produced real-world laboratories for studying the fates of eroded soil particles in watersheds all over the world. This article investigates the spatial distribution of historical sediment and the sediment budgets of three of these laboratories in medium-size watersheds of southern Minnesota. Sediment storage is measured in various geomorphic sites or sinks including colluvium, stream-order floodplains, and reservoirs. Two of these watersheds exhibit erosion histories and quantities and patterns of historical alluvium that are comparable with watersheds in Wisconsin's Driftless Area. In these watersheds, the patterns of historical sediment storage are two-tiered, with 47 to 61 percent of all sediment stored in the uplands (in low-order floodplains and colluvium) and 38 to 52 percent stored in the main lower floodplains. In one stream, backwater effects from the Mississippi River cause significantly greater alluviation in the lower floodplain; in another stream, the main floodplain is narrower and alluvial storage is thinner. In both, floodplain width is the major influence on alluvial storage. A third and smaller watershed in central Minnesota reports lower erosion and sedimentation rates and a different pattern of storage; 73 percent of the sediment is stored in colluvium and 27 percent in the lower main floodplain. Based on estimates of soil erosion, historical sediment yields and sediment budgets are estimated for each watershed. Historical sediment yields of about 13 to 36.5 percent are comparable to other estimates for these areas, which means that 63.5 to 87 percent or more of all historically eroded soil still resides within the basins and within 4 to 25 km of original points of detachment. Moreover, in the 137 years of European settlement, 38 to 73 percent of all eroded sediment has travelled no more than 4 km.

Fluvial sediment budgets in the North Carolina Piedmont

Sediment budget for four large (> 1000 km 2) drainage basins in the Piedmont physiographic province of North Carolina were estimated from data compiled during erosion and sedimentation surveys. Budgets for the Upper Tar, Upper Neuse, Haw and Deep River basins showed broadly similar trends in allocation of eroded sediment among yield and storage. Sediment yield as a percentage of mean annual gross erosion within the basins averaged 10%. This was less than the rate of alluvial storage, which averaged 14% of annual gross erosion. About 76% of the mean annual erosion was stored as colluvium on hillslopes. There are relatively small differences in sediment delivery and storage ratios among the study basins. This suggests that the relative order of magnitude of the allocation of eroded sediment for southern Piedmont rivers is colluvial storage, alluvial storage and yield. Of the sediment that does reach the streams, more than half is stored as alluvium on an average-annual basis. Aggrading channels and floodplains, siltation of benthic habitats and sediment pollution problems are thus likely to persist unless dramatic reductions in upland erosion or in sediment delivery to streams are realized. With respect to longer-term basin evolution, results point to the need for an improved understanding of the temporal scales of colluvial and alluvial sediment storage.

Tectonic versus chemical‐weathering controls on the composition of fluvial sands in tropical environments

ABSTRACTThe compositions of fluvial sands collected in mountainous regions along the volcanic front in Central America and along the Andean front in Venezuela and Colombia correlate with the tectonic setting of their source terrains. The sands derived from the Central American volcanic arc are compositionally distinct from those derived from the Andean terrains, and minor variations in tectonic style can also be discriminated. Subtle differences in tectonic setting and source rock composition are recognizable only when modification by chemical weathering is slight; they are strongest in sands from low‐order streams draining mountainous areas with modest alluvial‐plain development. Sands that have been exposed to weathering for extended periods during alluvial storage are progressively enriched in chemically stable phases. With increased weathering, sands from continental terrains converge toward a supermature quartz arenite end member, and the imprint left by source rock composition and tectonic setting is obscured. In intraoceanic terrains, a similar but much less pronounced enrichment in stable phases relative to less‐stable phases is observed, similarly overprinting the tectonic signal. Although detailed tectonic information is easily lost, sands derived from intraoceanic terrains are distinguishable from those derived from continental terrains even after long exposure to intense chemical‐weathering conditions.

Chemical Weathering of Fluvial Sediments During Alluvial Storage: The Macuapanim Island Point Bar, Solimoes River, Brazil

ABSTRACT Modification of sediment composition by chemical weathering depends both on the overall intensity of weathering and on its duration. At Macuapanim Island, Solimboes River (Amazon main channel), Brazil, a number of alluvial deposits have been exposed to intense tropical weathering for differing lengths of time. In the very fine sand fraction, the detrital component shows a steady and marked increase in compositional maturity with increasing sediment age: immature sands of arkosic to litharenitic composition show a progressive loss of lithic fragments, feldspars, and accessory minerals, with a concomitant enrichment in quartz. This process produces first-cycle sands whose detrital component is of quartz arenite composition. Among the salts, quartz is enriched over time relative to more nstable phases, particularly plagioclase. This enrichment is probably largely caused by the destruction of plagioclase through chemical weathering, but silt-size quartz may also be produced through the breakdown of sedimentary and metasedimentary lithic fragments in the sand fraction. In the clay fraction, an increase in the relative abundance of smectite accompanies the breakdown of volcanic lithic fragments in the sand fraction, and probably reflects the degradation of volcanic glass. The clay fractions of the oldest samples examined contain less smectite, somewhat more kaolinite, and markedly more vermiculite as compared to younger samples, but an extensively-leached, cation-poor clay fraction analogous to the quartz-dominated silts and sands is never attained. The compositional mat rity of the clay fraction is everywhere lower than that of associated silts and sands, perhaps because of a continual influx into the clay fraction of immature cation-rich clays derived from the physical breakdown of lithic fragments in coarser size fractions. This effect is enhanced by the translocation of fines, whereby younger, less-weathered fine material is juxtaposed with older, more mature coarse material in the lower levels of the alluvial stratigraphy. Reworking of floodplains during channel migration results in the reincorporation of older material stored on the floodplains into the sediment load. If sediments have been intensely weathered during storage on the floodplain, then the result will be a net dilution and replacement of younger, relatively immature sands transported by the river by older, more mature material from the floodplain. The relative proportion of the older sands in the bedload will increase downstream, resulting in a progressive downstream increase in the compositional maturity of the bedload of the river.

Sediment transfer and transformation of an alluvial valley floor: The river South Tyne, Northumbria, U.K.

AbstractThe amounts of alluvial storage and the mechanisms responsible for the dispersal of sediment associated with historic metal mining are examined for a 115 year timespan on a 22 km reach of the River South Tyne, northern England. Analysis of lateral and vertical channel change over this period, the extent of actively reworked gravels, and identification of metals in alluvial units, show five ‘sedimentation zones’ separated by more stable reaches. Aggradation in the late nineteenth century was followed by a period of storage and local reworking, and then incision. Some reaches show short‐term storage of sediment injected from tributaries which may be dispersed by floods. There is some evidence of transfer of sediment bodies downvalley within sedimentation zones, and of a sediment ‘wave’ movement between zones in lower parts of the South Tyne investigated. Present‐day incision, the variation in the area of actively reworked gravel spreads, and the widespread dispersal of fine‐grained toxic metal mining wastes all have practical implications. The intricate response of the channel system to historic changes in sediment supply is not readily described by either a complex response model or a simple translatory wave.

Hydrology of Alluvial Stream Channels in Southern Coastal Plain Watersheds

ABSTRACT THIRTEEN years of recorded rainfall, streamflow, and groundwater elevation data were used to analyze the hydrologic characteristics of the alluvial stream channels for three subwatersheds in the Coastal Plain of the southeastern U.S. Drainage areas of these subwatersheds range from 16.7 km2 to 50 km2. Digitized soil maps and cross-sectional transects were used to determine alluvial areal extent and alluvial depth, respectively. These data were used to compute the total alluvial volume in each sub watershed. Total alluvial depths and total alluvial volumes with groundwater elevation data were then used to determine the alluvial antecedent conditions and available alluvial storage for three subwatersheds. Results indicate that the available alluvial storage varies with both time of year and antecedent rainfall conditions. Lower available storage was observed for late winter and early spring due to higher rainfall and lowered evapotranspiration. Gradually increasing values were observed during summer and autumn due to lower rainfall and higher evapotranspiration. Results also indicate the effect of alluvial antecendent conditions on instantaneous peak discharge responses. For comparable storms, instantaneous peak discharge rates were higher by an order of magnitude for wet conditions than for dry conditions.

Effect of Weathering on Abrasion of Granitic Gravel, Colorado River (Texas)

Precambnan outcrops in the Llano Uplift of central Texas furnish the Colorado River with a distinctive granitic load which changes systematically as it moves downstream. In 160 miles between Austin and the Columbus-Eagle Lake area, coarse granitic gravel diminishes in size by about 50 percent and changes from a dommantly granite-gneissaplite assemblage to a dominantly pegmatite-graphic granite assemblage. Coarse quartz and chert gravel diminish in size by about 30 percent and 20 percent, respectively. In this reach, the Colorado flows on a flood plain whose alluvial width and depth stay fairly constant, a situation unfavorable for significant down-valley sorting (Mackin, 1963). Field observations generally lead to the same conclusion. Abrasion and its relationship to weathering was studied by running fresh and weathered specimens of Colorado River granitic gravel in a Kuenen-type abrasion tank at the University of Texas. Degree of weathering was slight to moderate, with biotite and feldspar the primary and secondary targets. Abrasion of fresh granitic gravel during 160 miles of travel produced only a 10 percent reduction in size. Abrasion of weathered gravel produced a reduction of more than 50 percent and was clearly related to lithology; the biotite-bearing rocks (granite, gneiss, and some aplite) were the least durable, the biotite-free rocks (pegmatite and graphic granite) most durable. Abrasion of quartz and chert produced a reduction in size of less than 10 percent. Changes in size and lithology of coarse granitic gravel along the lower Colorado River are best explained by abrasion of particles tha t weather slightly during periods of temporary alluvial storage. Reduction in size of quartz and chert are not satisfactorily explained for the reach between Austin and Columbus-Eagle Lake, but downstream from Eagle Lake, sorting appears to be in control.