Recurrent shoaling and channel dredging, Middle and Upper Mississippi River, USA

A 34-year record of dredging in a 484-km reach of the Middle and Upper Mississippi River documents the spatial and temporal patterns of bed aggradation in an intensively engineered river. Between 1964 and 1997, 183  million m³ was removed from the study reach, with 112.6 km of the channel undergoing dredging of some kind, 12.1 km requiring ≥5 dredgings during this period, 2.6 km requiring ≥10 dredgings, and one site requiring 29 dredgings in 34 years. Forty-three sites were identified where dredging volume was exceptionally high and/or were frequently repeated. These sites occur in five settings: (1) where flow is divided through chutes or side channels; (2) at the mouths of largely unregulated tributary streams; (3) at thalweg crossings in meander bends; (4) in long, straight reaches of the channel; and (5) near problematic engineered structures, the principal one in the study area being the outlet of the Chain of Rocks Canal. In the pooled reach of the Upper Mississippi, impoundment has had a major influence on shoaling, with 80% of dredged volume required in the upstream halves of the pools. Although past engineering modifications have reduced sediment aggradation in the Mississippi channel, structural solutions to continuing shoaling can adversely impact the river's ecology, flood response, or hydrological function, and many structural solutions appear to be approaching their effective limits. The exception to this may be at highly localized shoaling locations, such as the Chain of Rocks Canal outlet, where flaws in the original design may still be remedied by structural means Targeted structural solutions may be effective at solving limited local shoaling problems, but we suggest that more sweeping new engineering modifications to reduce or eliminate dredging requirements are likely to be ineffective and/or outweighed by their adverse effects on the river system.     

The effects of deforestation on slope and channel evolution in the tectonically active Darjeeling Himalaya

The main indicators of Quaternary tectonic uplift are the young mountain slopes of the Darjeeling Himalaya, rising straight above the Ganga–Brahmaputra foredeep, fragments of uplifted river terraces and fresh fault scarps. Evidence for the continuation of the uplift includes downcutting of the Tista and other straight rivers in the bedrock, continuing aggradation in the plains and overriding of the metamorphic rocks on the alluvia. Owing to deforestation and extensive land use, the earlier natural tendency of a dominance of channel incision over slope degradation has changed to prevailing aggradation, even in steep valley reaches, caused by intensive slope mass movements and the overloading of the mountain creeks. Aggradation progresses upstream along the rivers dissecting the mountain front.     

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

The 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.     

Evolution of the Yellow River delta,China: Impacts of channel avulsion and progradation

The evolution of the Yellow River delta is characterized by heavy sediment load,rapid seaward migration,frequent avulsions,and intense anthropogenic disturbances.Evolution of the delta channel following avulsions is very complex and has not yet been thoroughly understood.In the research presented by this paper,we conducted comprehensive analyses of the changes in the water stages,slopes,longitudinal profiles,and the erosion and deposition in the Yellow River delta channels during a time period of over five decades.Results showed that,following each avulsion,channels migrated seaward at decaying rates and the slopes at the downstream of the avulsion point decreased exponentially with time and completed its major adjustment within about four to five years.A generalized geometric model was proposed to describe the changes in the longitudinal profiles of the delta channels.A calculation method to determine the characteristic water stages at the delta was proposed based on the geometric model and the delayed response model for the morphological responses of fluvial rivers to perturbations.Water stages corresponding to a discharge of 3000 m3/s at Lijin and Xihekou during 1954 through 2012 were calculated by using the proposed method.The proposed method may be used to predict the evolution of the delta channels in response to artificial avulsions at the Yellow River delta in the future.     

Medium-term effects of two landsliding episodes on channel storage of sediment

Two landsliding episodes between late 1973 and early 1975 delivered about 60000 m³ of sediment to six small deeply incised streams draining a 2·7 km² area. About 4700 m³ of logs in the landslide debris formed major log jams in five streams, which impounded large volumes of landslide-derived sediment. Five years after the landsliding, 42 per cent (25000 m³) of sediment was still in storage behind 35 log jams ranging from 1·4–8·2 m high. The landsliding episodes have produced multi-stepped stream profiles, aggradation of channel reaches up to 150 m long to mean depths between 1·2 and 4·1 m, reductions in gradient, fining of bed material size, and related changes in bedforms and channel width:depth ratios that seem likely to persist for at least several decades. Sediment presently stored behind log jams is equivalent to between 50 and 220 years normal supply of sediment from hillslopes to stream channels. Long-delayed, large magnitude impacts on higher-order channels may occur if sudden failure of log jams is induced by a large storm at some future date.     

Porosity, permeability, and fluid flow in the Yellowstone geothermal system, Wyoming

Cores from two of 13 U.S. Geological Survey research holes at Yellowstone National Park (Y-5 and Y-8) were evaluated to characterize lithology, texture, alteration, and the degree and nature of fracturing and veining. Porosity and matrix permeability measurements and petrographic examination of the cores were used to evaluate the effects of lithology and hydrothermal alteration on porosity and permeability. The intervals studied in these two core holes span the conductive zone and the upper portion of the convective geothermal reservoir. Variations in porosity and matrix permeability observed in the Y-5 and Y-8 cores are primarily controlled by lithology. Y-8 intersects three distinct lithologies: volcaniclastic sandstone, perlitic rhyolitic lava, and non-welded pumiceous ash-flow tuff. The sandstone typically has high permeability and porosity, and the tuff has very high porosity and moderate permeability, while the perlitic lava has very low porosity and is essentially impermeable. Hydrothermal self-sealing appears to have generated localized permeability barriers within the reservoir. Changes in pressure and temperature in Y-8 correspond to a zone of silicification in the volcaniclastic sandstone just above the contact with the perlitic rhyolite; this silicification has significantly reduced porosity and permeability. In rocks with inherently low matrix permeability (such as densely welded ash-flow tuff), fluid flow is controlled by the fracture network. The Y-5 core hole penetrates a thick intracaldera section of the 0.6-Ma Lava Creek ash-flow tuff. In this core, the degree of welding appears to be responsible for most of the variations in porosity, matrix permeability, and the frequency of fractures and veins. Fractures are most abundant within the more densely welded sections of the tuff. However, the most prominent zones of fracturing and mineralization are associated with hydrothermal breccias within densely welded portions of the tuff. These breccia zones represent transient conduits of high fluid flow that formed by the explosive release of overpressure in the underlying geothermal reservoir and that were subsequently sealed by supersaturated geothermal fluids. In addition to this fracture sealing, hydrothermal alteration at Yellowstone appears generally to reduce matrix permeability and focus flow along fractures, where multiple pulses of fluid flow and self-sealing have occurred.     

Channel avulsion and river metamorphosis: The case of the Thomson River,Victoria, Australia

Channel avulsion occurred on the Thomson River in Victoria, Australia, in 1952 along a 12 km length of the valley. A comparison of the old and new channels reveals considerable differences in channel characteristics. The old channel was perched above the floodplain on an alluvial ridge such that when bankfull capacity was exceeded, floodwaters concentrated on the lowest part of the floodplain some distance away. This is where the new channel formed. It is an incised channel with larger capacity and longer meander wavelength than the old channel and is also shorter and steeper. The new channel is subject to larger floodflows and a more variable flood regime than the old course because of the differences in the channel/floodplain relationship and channel capacity. The resulting concentration of stream power along the new course is responsible for the contrast in channel characteristics and for the more rapid meander migration. This example shows that river metamorphosis can occur without major environmental changes. Measures of channel geometry such as gradient, sinuosity, and meander wavelength therefore cannot be used in palaeohydrological work to infer climatic or other environmental changes without independent supporting evidence. Differences in channel geometry can arise simply from changes in the relationship between the channel and its loodplain.     

Contemporary and palaeo channel patterns and the late quaternary stratigraphy of Cooper Creek,Southwest Queensland,Australia

Braided and anastomosing channels make up two major coexistent networks in the mud-dominated fluvial system of Cooper Creek, Southwest Queensland. The floodplain is characterized by a system of mud braids operative when floods inundate the whole alluvial surface. Anastomosing channels are inset deeper into the floodplain, operate at modern flows, and transport a traction load of sand. Shallow stratigraphic data show that an underlying sand sheet is unrelated to surface channel patterns and was formed by a system of meandering streams. According to preliminary dates based on thermoluminescence, the change from a sand- to mud-dominated fluvial regime took place between 50 000 and 200 000 years B.P., and probably reflects increasing aridity.     

Dynamic process-response model of river channel development

Feedback mechanisms, which operate upstream through drawdown and backwater effects and downstream through sediment discharge are responsible for channel evolution. By combining these mechanisms with channel processes it euables a dynamic process-response model to be developed to simulate the initial evolution of straight gravel-bed channels. When erosion commences on a land surface, sediment entrained in the headwater reach by hydraulic action is selectively transported, deposited and reworked. This produces a damped oscillation between degradation and aggradation as the channel and valley respond to spatial and temporal variations in sediment calibre and hydraulic conditions. The initial cut and fill phases are responsible for valley incision and floodplain development while secondary and subsequent activity can produce river terraces. Eventually sediment entrainment in the headwaters declines as slopes are reduced. Subsequent channel evolution is relatively insignificant because it is dependent on local weathering activity producing material that can be transported on declining slopes. Therefore landforms produced during the initial phase of development, when local weathering was non-limiting, dominate the landscape.     

Vegetation recruitment in an enhanced floodplain: Ancillary benefits of salmonid habitat enhancement

Widespread loss and degradation of riverine habitats due to dams, diversions, levees, and human development have led to an increase in river habitat enhancement projects in recent decades. These projects typically focus on improving either terrestrial (e.g., riparian vegetation) or aquatic (e.g., fish spawning and rearing) habitats, and do not commonly address the relationship between the two systems. However, there is abundant evidence that fundamental linkages exist between terrestrial and aquatic ecosystems, and anthropogenic impacts such as urban expansion, agricultural activities, and river impoundment can synergistically degrade both systems. This study examines the effects of adult and juvenile salmonid habitat restoration on recruitment, density, and composition of riparian vegetation in an area heavily impacted by mining and flow regulation. For a year following in-channel coarse sediment placement and floodplain construction in an area previously covered with coarse mine tailings, we compared the abundance, richness and diversity of vegetation across four treatments: the newly constructed floodplain, isolated mine tailings, mine tailings near an access road, and a remnant riparian area that was less impacted by mining. Richness and diversity were higher in the floodplain than in any of the other treatments; we identified a total of 15 plant families in the floodplain treatments, as compared to three to five families in the other treatments. We observed significant differences in plant assemblage composition between treatments, with higher richness of primarily obligate or facultative wetland plant taxa in the floodplain treatment. This study demonstrates that restoring hydrological linkages between aquatic and terrestrial habitats, and redistribution of sediment size classes altered by mining, can create conditions that promote rapid wetland plant colonization, enhancing biodiversity and improving ecosystem function.     

Channel change and the significance of floodplain stratigraphy: 1990 flood event,lower river Tay,Scotland

During February 1990 a flood event on the River Tay with an estimated recurrence interval of 70 years in its lower reaches caused extensive flooding and geomorphological change. The most extensive area of flooding and dramatic geomorphological change, the erosion of two new channels across the floodplain, was in the vicinity of the village of Caputh, Perthshire. The location of the eroded channels relates to the position of former river courses, identifiable by depressions in the floodplain surface and floodplain stratigraphy. The lower floodplain surface elevation and occurrence of gravel and sand deposits along the line of the former river courses caused the area to be more vulnerable to erosion and controlled the morphology of the eroded channels.     

Floodplain storage of mine tailings in the belle fourche river system: A sediment budget approach

Arsenic-contaminated mine tailings that were discharged into Whitewood Creek at Lead, South Dakota, from 1876 to 1978, were deposited along the floodplains of Whitewood Creek and the Belle Fourche River. The resulting arsenic-contaminated floodplain deposit consists mostly of overbank sediments and filled abandoned meanders along White-wood Creek, and overbank and point-bar sediments along the Belle Fourche River. Arsenic concentrations of the contaminated sediments indicate the degree of dilution of mine tailings by uncontaminated alluvium. About 13 per cent of the 110 × 10⁶ Mg of mine tailings that were discharged at Lead were deposited along the Whitewood Creek floodplain. Deposition of mine tailings near the mouth of Whitewood Creek was augmented by an engineered structure. About 29 per cent of the mine tailings delivered by Whitewood Creek were deposited along the Belle Fourche River floodplain. About 60 per cent of that sediment is contained in overbank deposits. Deposition along a segment of the Belle Fourche River was augmented by rapid channel migration. The proportions of contaminated sediment stored along Whitewood Creek and the Belle Fourche River are consistent with sediment storage along the floodplains of perennial streams in other, similar sized watersheds.     

Coarse bed load transport in an alluvial gravel bed stream,Dupuyer Creek,Montana

Coarse bed load was sampled in a gravel/cobble bed stream during two major floods in the snowmelt runoff season. The channel is characterized by high rates of bank erosion and, therefore, high rates of sediment supply and bed load flux. Peak discharge reached four times bank‐full, and bed load was sampled at flows 0·7–1·7 times bank‐full. A large aperture bed load sampler (1 m by 0·45 m) captured the largest particles in motion, and specifically targeted the coarse bed load size distribution by using a relatively large mesh (32 mm or D₂₅ of streambed surface size distribution). Bed load flux was highly variable, with a peak value of 0·85 kg/s/m for the coarse fraction above 38 mm. Bed load size distribution and maximum particle size was related to flow strength. Entrainment was size selective for particles D₇₀ and larger (88–155 mm), while particles in the range D₃₀–D₇₀ (35–88 mm) ceased to move at essentially the same flow. Bed load flux was size selective in that coarse fractions of the streambed surface were under‐represented in or absent from the bed load. Painted tracer particles revealed that the streambed surface in the riffles could remain stable even during high rates of bed load transport. These observations suggest that a large proportion of bed load sediments was sourced from outside the riffles. Repeat surveys confirmed major scour and fill in pools (up to 0·75 m), and bank erosion (>2 m), which together contributed large volumes of sediment to the bed load. Copyright © 2007 John Wiley & Sons, Ltd.     

Thermal‐infrared remote sensing of surface water–groundwater exchanges in a restored anastomosing channel (Upper Rhine River,France)

Ecohydrological processes are a key element to consider in functional river restorations. In the framework of a LIFE+ European restoration program, we have investigated the potential for airborne thermal‐infrared remote sensing to map surface water–groundwater exchanges and to identify their driving factors. We focused our attention on anastomosing channels on an artificial island of the Upper Rhine River (Rohrschollen), where a new channel was excavated from the floodplain to reconnect an older channel in its upstream part. These hydraulic engineering works led to an increased inflow from the Rhine Canal. Here, we propose an original data treatment chain to (a) georeference the thermal‐infrared images in geographic information system based on visible images, (b) detect and correct data errors, and (c) identify and locate thermal anomalies attributed to groundwater inputs and hyporheic upwellings. Our results, which have been compared to morpho‐sedimentary data, show that groundwater upwelling in the new channel is controlled by riffle–pool sequences and bars. This channel is characterized by large bedload transport and morphodynamic activity, forming riffles and bars. In the old channel, where riffle–pool sequences no longer exist, due to impacts of engineering works and insufficient morphodynamic effects of the restoration, thermal anomalies appeared to be less pronounced. Groundwater inputs seem to be controlled by former gravel bars outcropping on the banks, as well as by local thinning of the low‐permeability clogging layer on the channel bed.     

Suspended sediment,nutrients, and chlorophyll in tropical floodplain lakes with different patterns of hydrological connectivity

The ecological functioning of floodplain lakes is largely influenced by the interaction with the river mainstem. In this study, seasonal variation in water chemistry and the relationship with the river conditions were compared between floodplain lakes that differ in the level of connection to the Usumacinta River, the largest river of Mesoamerica. Samples for suspended solids, nutrients, and chlorophyll a were collected through the year in lakes permanently connected to the river and in lakes that only received water from the Usumacinta for a short period during peak flow. Floodplain lakes showed higher total suspended solids than the river during the dry season while during the rainy season greater differences were observed between the river and the lakes, probably explained by higher concentrations in the river and greater sedimentation in the lakes. Greater organic matter content in the suspended solids was observed in the floodplain lakes, particularly in the more isolated lakes, likely related to high algal biomass. Nitrate concentrations were always higher in the river than in the lakes and lower nitrate concentrations occurred at the isolated lakes, suggesting that processes that remove nitrate occur through the year and are a common feature of floodplain lakes. Phosphorus in the connected lakes was higher than in the river only during the dry season, while in the isolated lakes concentrations were always greater than in the river. Chlorophyll a concentrations were higher in the connected lakes than in the river only during the dry season, while the more isolated lakes exhibited higher values through the year, showing signs of eutrophication. Suspended organic matter, nitrate, and chlorophyll showed larger differences between lake and river sites in the more isolated lakes, probably related to greater water residence time and its influence on primary production. Less connected lakes are more vulnerable to flow alteration because the brief period of connection to the river can be compromised and the effects of eutrophication exacerbated.     

Forest fire,bank strength and channel instability: the ‘unusual’ response of Fishtrap Creek,British Columbia

In August 2003, the McLure forest fire burned 62% of the drainage basin of Fishtrap Creek. Streamflow has been measured there since the early 1970s, and suspended sediment concentration and channel morphology have been monitored since the fire. Although the short post‐fire period (four years) limits our ability to draw firm conclusions about streamflow changes, there has been no obvious increase in peak flows since the fire. However, the total runoff during the freshet period does appear to have increased and the onset of snowmelt appears to occur about two weeks earlier than it did prior to the fire. Suspended sediment records from Fishtrap Creek and from an unburnt reference stream nearby are similar, suggesting that the burnt areas have remained relatively stable and that the sediment supply to Fishtrap Creek has not been dramatically altered. In contrast, the stream channel morphology has changed, widening by over 100% of the original width in some places and transforming from a laterally stable plane‐bed morphology to a laterally active riffle‐pool morphology. The timing and magnitude of the observed morphologic changes are consistent with the predicted decline in bank strength due to root decay, implying that the observed changes are associated with an internal instability associated with changes to the stream boundaries, rather than with the more typically reported externally driven instabilities caused by changes in streamflow or sediment supply. This delayed response in the absence of large changes in streamflow or sediment supply, while ‘unusual’ in that it has not been documented in the previous literature, may be a common mode of response, particularly in wat'ersheds with nival flow regimes. Copyright © 2010 John Wiley & Sons, Ltd.     

Aeolian modification of moraine soils,Whiskey Basin,Wyoming, USA

Soils along catenas of Pinedale (15–20 ka) and Bull Lake (100–130 ka) age moraines at Whiskey Basin in the Wind River Range, Wyoming, USA, were sampled to assess the effects of aeolian processes on soil development here. Aeolian processes appear to have in?uenced soils by both depositing sediments and eroding topsoils. Pedogenic silt (often used as an indicator of wind deposition) accumulated in the Bull Lake soils moderately correlate with pedogenic clay accumulated, suggesting that ?ne sediments may have been deposited and incorporated into soil formation here. Following removal of previous topsoil by wind during Pinedale glaciation, Bull Lake B horizons have developed into contemporary A horizons. These data further link aeolian processes to soil development on piedmont moraines throughout the Wind River Range. Copyright © 2004 John Wiley & Sons, Ltd.     

Hydrodynamics of floodplain wetlands in a chalk catchment: The River Lambourn, UK

Variations in floodplain channel water levels and valley floor groundwater levels (measured in piezometers and boreholes) are examined at selected points along the course of the River Lambourn, a chalk river in southern England. A local alluvial gravel aquifer in the valley bottom is associated with numerous small wetlands that extend over much of the river's perennial profile. Variations in hydraulic gradient between local borehole levels and/or floodplain channel water levels are described for three sites in the seasonal section of the channel at Bockhampton, East Garston and West Shefford. The results indicate that observed groundwater levels are closely associated with flows from discrete springs at the margins of the channel and floodplain. However, as the floodplain widens and the alluvial gravel aquifer increases in size, the gravel aquifer accounts for a substantial down-valley component of groundwater flow with a diffuse vertical water flux. In the lower catchment, the exchange of flows between the gravel aquifer and the river enables some attenuation of floodplain water-table variability, providing a stable hydrological regime for valley-bottom wetlands. Catchment controls upon the local, valley-bottom, wetland regime are demonstrated with the application of a simple groundwater model developed using MODFLOW. The model is used to simulate groundwater discharge to the river in the upper and lower catchment, in addition to the water level regime at selected points in the valley bottom in the lower catchment. The results demonstrate the importance of taking catchment-scale water flow into account when managing isolated wetlands in a permeable catchment.     

Role of floodplain sediments in reducing the nitrate concentration of subsurface run-off: A case study in the Cotswolds,UK

Discharge of groundwater from a limestone aquifer through floodplain sediments is associated with a large decrease in the nitrate concentration of the water. Results are presented to show that only a small amount of this reduction is caused by dilution of groundwater by water already present within the floodplain sediments; most of the effect is an active reduction process, most probably biological denitrification. The nitrate reduction process appears to operate independently of surface vegetation type and tends to be focused in specific regions of the floodplain where sediments are anaerobic and carbon-rich. The results suggest that active denitrification can operate throughout the winter, when nitrate concentrations in groundwater are at their highest and that the process remains effective even during periods of maximum run-off. The results show that undrained floodplains can be used as buffer zones to protect surface waters from groundwater polluted with agriculturally derived nitrate.