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This study assessed historical morphology changes of the Msimbazi River in Dar es Salaam City, Tanzania, following heavy rainfall events using historical Google earth images. The erosion and sedimentation processes that cause morphological changes of a river are also associated with flooding within the river catchments. In this study, the bank-line movement, erosion and sedimentation areas and changes in river section width were analyzed under three-time periods: 2005-2012, 2012-2018, and 2018-2020. Data shows that, the timing for floods coincides with the historical records for heavy rains. It was found that, area in the upper reaches of the river such as Kinyerezi suffer from river bank erosion most, as indicated by the large increase in river width. This has resulted into washing away of infrastructure including houses and loss of land. The river channel width at Kinyerezi has widened a lot since 2005, with some sections having widths of more than 90 m. On the lower reaches for example around Kigogo and Jangwani areas, deposition of the soil materials prevails, resulting into raised river bed and reduced river widths. As the river section an hence carrying capacity is reduced, inundation of river banks occurs. This is among the causes of river bank flooding reported at these areas whenever there is a heavy storm event. It is therefore recommended that interventions to solve the flooding events in Dar es Salaam city should also include preventing catchment and river bank erosion on the upstream areas. [ABSTRACT FROM AUTHOR]

For streams draining urban catchments, sediment transport capacity is the key driver of physical impacts including bed sediment removal and channel incision. The main unanswered question is the relative role of flow alteration compared to sediment supply in influencing sediment transport capacity. With this objective, we computed sand and gravel bed sediment transport capacity using the Wilcock and Kenworthy two-fraction bedload transport relation for nine streams in catchments covering a gradient of urbanisation. Computations were done for typical natural bed surface material, based on conditions in the least urban study streams. We compared transport capacity distributions and cumulative transport capacity over one-year between the streams. Transport capacity was up to three orders of magnitude higher in urban streams than in forested-catchment streams. This was driven overwhelmingly by the urbanisation-induced alterations to the flow regime, with only minor feedback from channel form changes. Transport capacity was two to three orders of magnitude greater than measured bedload transport in all but the least urban streams. This excess bedload transport capacity mobilises and removes bed sediment, produces channel incision and enlargement and reduces channel complexity. Rebalancing transport capacity with sediment supply therefore requires significant flow mitigation towards pre-urban conditions. Other responses, which may theoretically help to regain this balance – channel widening, grade control, increasing roughness, sediment augmentation – are either inappropriate or only feasible following flow mitigation measures. [ABSTRACT FROM AUTHOR]

Abstract Megaclast-dominated fluvial systems have been largely ignored by researchers. Yet megaclasts exert an important control on channel geomorphology, flow behaviour and sediment transport, and such systems are probably far more common than this neglect would suggest. Despite the explicit identification of megaclasts as the products of cataclysmic flow, observation of modern fluvial systems confirms that megaclasts can be entrained by flows that lie within the normal flow regime. The pattern of fining along the megaclast-dominated Sabeto River of western Viti Levu, Fiji reveals a strong negative exponential relationship between particle size and distance downstream, and a rate of downstream fining comparable with that of other gravel-bed rivers. The downstream fining pattern is straightforward and step-like discontinuities in the fining gradient are absent. This is suggestive of a sedimentary system only minimally disrupted by the supply of sediment from tributaries or the reworking of material from sources such as hillslopes and alluvial terraces. There is evidence, however, of anomalously coarse sub-populations at two sites. These coarser components must pre-date the other deposits at the sites. We speculate that they represent the products of earlier and higher-magnitude events. The bed sediments may thus be composite features, made up of individual components each transported by a specific event of particular magnitude. Our work confirms the relationship between the rate of downstream fining and particle size. This may arise because the distance over which individual particles are transported is likely to be size dependent. It is also possible that abrasion may be more effective in the case of larger particles than smaller ones, with the result that the coarser fraction of the river's bed sediment fines downstream more rapidly than the finer fraction. There is a long-standing debate about the relative roles of abrasion and sorting in downstream fining. Our data reveal little evidence for a downstream increase in the roundness of the river's gravels, implying that abrasion is likely to play a minor role in the fining process in this system. Highlights • Megaclast-dominated rivers occur widely, but have been largely ignored by researchers. • Megaclasts may be entrained by normal flows and not solely by catastrophic events. • Megaclast downstream-fining rates are comparable with those of other gravel-bed rivers. • There is a close relationship between bed-sediment size and rate of downstream fining. • Bed-sediment assemblages consist of discrete components, each transported by individual events. [ABSTRACT FROM AUTHOR]

Abstract: Bed load sediment transport is an inherently challenging process to measure within a river, which is further complicated by the typically transient nature of the hydrograph. Here we use laboratory experiments to explore how sediment flux under transient—unsteady and intermittent—flow differ from those under steady flow. For a narrow unimodal sediment distribution, we calculated fluid stress and measured sediment flux for a range of hydrograph durations, magnitudes, shapes, and sequences. Within a hydrograph, we find considerable variability in sediment flux for a given stress above the threshold for motion. However, cumulative bed load flux resulting from a flood scales linearly with the integrated excess transport capacity (flow impulse). This scaling indicates that, to first order, flow magnitude, duration, shape, and sequence are only relevant to bed load flux in terms of their contribution to the total flow impulse, in agreement with prior field results. The flood impulse represents a quantitative parameter through which the effects of transient flow on coarse sediment transport may be parsed. [ABSTRACT FROM AUTHOR]

Understanding Earth surface responses in terms of sediment dynamics to climatic variability and tectonics forcing is hindered by limited ability of current models to simulate long-term evolution of sediment transfer and associated morphological changes. This paper presents pyBadlands, an open-source python-based framework which computes over geological time (1) sediment transport from landmasses to coasts, (2) reworking of marine sediments by longshore currents and (3) development of coral reef systems. pyBadlands is cross-platform, distributed under the GPLv3 license and available on GitHub (). Here, we describe the underlying physical assumptions behind the simulated processes and the main options already available in the numerical framework. Along with the source code, a list of hands-on examples is provided that illustrates the model capabilities. In addition, pre and post-processing classes have been built and are accessible as a companion toolbox which comprises a series of workflows to efficiently build, quantify and explore simulation input and output files. While the framework has been primarily designed for research, its simplicity of use and portability makes it a great tool for teaching purposes. [ABSTRACT FROM AUTHOR]

Understanding the spatial and temporal variability of nitrogen load and characteristics of retention along a river network is important for land management and water resources protection. This study employs a geomorphology-based non-point source pollution model (GBNP) to simulate the hillslope hydrological processes and transport of sediment and pollutants in the upper Xin'anjiang (XAJ) catchment. Based on the simulation, the spatial and temporal distribution of total nitrogen (TN) load from hillslopes and retention characteristics along the river network are analyzed. The results indicate that annual TN load ranges from 0.54 ton/km2 to 1.88 ton/km2 and is relatively higher during spring and summer. Average TN load positively correlates with irrigated cropland area (r =0.820) and negatively correlates with forest (r = -0.43). Seasonal TN retention ratios in the river network range from 0% to 81%, and streams of order 1 in the Horton-Strahler system have the highest retention ratio and are followed by orders 2, 3, and 4, which are mainly determined by the river hydraulic properties. Results of scenarios analysis demonstrate that TN retention ratios in the river network increase with TN load from hillslopes, but reach a maximum value rapidly, which indicates the limitation of the self-purification capacity of rivers. [ABSTRACT FROM AUTHOR]

Preliminary analysis of long-term hydrological data shows that sediment dynamics of Russian Arctic rivers is largely affected by hysteresis effects in relation of water discharge to suspended sediment concentration (SSC). The role of large-scale geomorphic and geocryological conditions in sediment transport is still relatively understudied. This research aims to assess the links between hydrogeomorphology and regularities of sediment formation and movement in Russian Arctic rivers. A dataset containing information on measured water discharges and SSC for 27 gauges on 16 medium and large rivers of the Russian Arctic was used in this research. Clockwise hysteresis is typical during spring events, while in summer counterclockwise and 'figure eight' curves are widely observed. The results show that the form of the rating curve can be attributed to the dominant sediment source, dominant channel pattern and, to certain extent, to the cross-section position within the river basin. Seasonality in hysteresis effects reflects the role frozen ground dynamic plays in sediment flux formation. Thus, reaction of permafrost landscapes on the widely observed climate shift should lead to significant changes in sediment transfer systems. [ABSTRACT FROM AUTHOR]

Complex relationships between stream functions and processes make evaluation of stream modification projects difficult. Informed by vague objectives and minimal monitoring data, post-construction project evaluations can often be a subjective attribution of success or failure. This article provides a simple framework to rapidly describe the degree of damage in stream modification projects performed in constrained settings. Based on widely accepted evaluations of physical habitat quality and stream stability, the damage states framework describes a continuum of damage in multiple categories that relate natural stream functions to the often desired state of static equilibrium. Given that channel form is closely related to stream function, it follows that changes to the channel form result in changes in function. The damage states focus on damage to flow hydraulics, sediment transport and channel equilibrium, hydraulic, and geomorphic parameters that describe basic stream functioning and support higher level functions in the modified channel. The damage states can be used in decision making as a systematic method to determine the need for repair and design adjustments. [ABSTRACT FROM AUTHOR]

Abstract: We present an analysis of daily water discharge and suspended sediment concentration measurements for the three main drainage basins in Nepal, on the basis of recent published papers. We first show how precipitation-discharge data can be used to highlight the impact of groundwater storage on the annual hydrological cycle of Himalayan rivers. Then, we show how the concentration of suspended sediment in rivers varies at the year scale depending on the river discharge cycle, as well as how the release of groundwater impacts the concentration of materials in rivers. Finally, we propose a new conceptual model for the mobilization and transportation of material within the monsoonal discharge cycle in the central Himalayas. [Copyright &y& Elsevier]

Abstract: This contribution presents a model approach to simulate the erosion, entrainment, deposition, and burial of the surface fine-grained laminae (SFGL) in low-gradient streams. We hypothesized that SFGL transport acts as a geomorphic control of the erosion and transport of sediment sources in a bedrock-controlled, low-gradient stream with fluvial deposits located in central Kentucky, USA. Results indicate that the SFGL development and temporary storage on the streambed surface plays a key role in maintaining an equilibrium streambed. The SFGL provides an easily erodible sediment source that satisfies the transport capacity of the stream during low to moderate flow events and thus protects deeper streambed sediment and streambank sediments from erosion. Modeling results showed that the simulation period was characterized by low to moderate hydrologic events which flush the SFGL from the stream and high flow events which deposit fine sediments into the SFGL. During the former, transported sediments were dominated by sediments of SFGL origin, i.e., 89% of sediments. During the latter, 33% of sediment input to the stream is deposited to the SFGL, which shows its importance as a temporary sink. Our model results support a novel concept that the ephemeral SFGL layers can satisfy sediment transport capacity of flow at low to moderate flows and thus protect other sources from erosion. While perhaps the bedrock, low-gradient stream studied provides an end-member to SFGL control, future research might investigate the importance of SFGL control in other stream systems. [Copyright &y& Elsevier]

Abstract: Networks fed by subsurface flow are a natural, but dynamically simple, system in which to consider general problems of network growth. Here we present four examples in which this dynamic simplicity can be used to develop a quantitative understanding of a natural kilometer-scale network of streams. In these four sections, we investigate the relation between the position of a spring in the network and the groundwater flux into it, the flow of water through streams and the stream shape, and the influence of solar radiation on the rate of sediment transport around the streams. In each case, a quantitative comparison is made between theory and observation. [Copyright &y& Elsevier]

Summary: This study evaluated the potential impacts of sub-bankfull flows produced by stormwater control measures (SCMs) on stream geomorphic stability. In part, design standards for SCMs include peak flow attenuation to maintain pre-development flow conditions to those of undeveloped watersheds or return urbanized, developed watersheds back to the pre-developed state. Most SCMs target lower frequency storms, usually the 2-and/or 10-year discharge events, but leave peak flows resulting from higher frequency storms uncontrolled. SCMs are possibly subjecting streams to longer and more frequent periods of erosion, increasing stream channel instability. The d 65 substrate size, pattern, profile, and dimension of 33 reference stream cross-sections in Piedmont North Carolina were modeled using the continuous simulations program, SWMM, to develop (1) a unit critical discharge metric in L/s/ha, Qc =0.0035(d 65)1.5048, (2) allowable annual erosional hour standard, Log(AAEH)=−1.26Log(d 65)+1.21, and (3) allowable volume of eroded bedload standard, Log(AV)=−0.64(Qc )−1.52, for watersheds containing SCMs discharging into surface waters. The unit critical discharge represents a threshold that, once exceeded, incipient motion of the d 65 particle can occur. These standards represented benchmarks of stable, naturally eroding reference streams. Ninety-four percent of the unit critical discharges were less than the 2-year 24-h event, indicating the necessity of controlling higher frequency sub-bankfull flows. The standards were applied to an urbanized watershed (one sub-catchment containing a structural SCM and two sub-catchments without) in Raleigh, North Carolina. The unit critical discharge metric appeared to adequately represent urbanized stream geomorphic processes for the sub-catchment undergoing urbanization (4.5% difference) but not for the mature urbanized sub-catchments (47.5% and 68.8% difference). Depending on the long-term management goal of the unstable stream, this metric is not applicable for all urbanized watersheds due to the discrepancy between the field and calculated unit critical discharges. Standards developed from urbanized reference streams could possibly better represent SCMs in urbanized watersheds. All three sub-catchments failed to meet the erosional standards demonstrating the ability of the standards to predict unstable geomorphic processes in streams. The addition of a detention SCM (wet pond), in the urbanized sub-catchment extended the duration of erosive flows from 37 to 87h/ha/yr, but reduced the estimated volume of eroded bedload from 1.81 to 0.99m3/m/ha/yr when compared to uncontrolled urbanization (no wet pond). Alterations to the design of the wet pond, increased volume size and change in orifice diameter, were explored to see if erosional standards could be better met. This study demonstrated the effect of current SCM design standards on stream stability and why geomorphic processes of stream channels should be incorporated in SCM design standards. [ABSTRACT FROM AUTHOR]

This progress report on the discipline of fluvial geomorphology reviews 134 papers, 112 of which were published in Earth Surface Processes and Landforms (ESPL) during the calendar years of 2008 and 2009. It continues where the last report for 2006 and 2007 (Stott, 2010) ended. Papers are again grouped by themes to cover 10 subdisciplines within the subject area. The themes were chosen by classifying all geomorphological articles published in ESPL for the same period, of which 38% were within the subject area of fluvial geomorphology. Themes (in order of number contributing to the total) were: fluvial sediment transport; soil erosion and control; modelling the fluvial environment; river management, restoration and effects of vegetation on fluvial systems; gully and hillslope sediment transfer; river regulation, channel change and human influences; advances in methodology in fluvial geomorphology; fluvial hydraulics; fluvial chronology; and bank erosion in fluvial systems. The 2006–2007 and 2008–2009 periods are compared and it was found that broadly the same themes retained their popularity (in terms of numbers of papers published) as in the previous report for 2006–2007. [ABSTRACT FROM AUTHOR]

Abstract: We explore the novel technique of using short-lived fallout radionuclides to date fluvial surfaces with the goal of determining lateral migration rates. Specifically, we use the fallout radionuclide 210Pb (half-life t 1/2 ~22.3years) to determine the lateral migration rates of meandering reaches on the Winooski River, VT, the upper Connecticut River, NH, and the Genesee River, NY. We find that, particularly near the channel, 210Pbex inventories are affected by the initial 210Pbex inventories of freshly deposited sediment (inheritance). Inheritance differs at each site, with lower inheritance occurring at sites with larger upstream drainage areas. After accounting for inheritance, 210Pbex inventories in surface sediments along a transect orthogonal to the channel yield migration rates of 0.7, 3.1, and 4.7m/y for the Winooski, Connecticut, and Genesee Rivers, respectively. These rates agree well with values derived independently from historical aerial photography. Variations in 210Pbex inventories along the transects also provide details of the channel migration histories not evident from the limited available aerial photographs, including evidence for both nearly constant linear and episodic channel migrations. Results suggest the broad applicability and added value of our approach for quantifying decadal-scale variations in fluvial surface ages. [Copyright &y& Elsevier]

Channel conveyance, channel size and water levels in the Athabasca River Delta were investigated in detail during three summer seasons. The Athabasca Delta is between 58°N and 59°N, in Alberta, Canada, and is formed immediately upstream of where the river enters Lake Athabasca. Ice-free flow rates in the river and in the various distributary channels were measured by acoustic Doppler profiler, and water surface elevations by differential GPS and pressure transducer recorders. Approximate values were measured for river bank heights, and a profile of elevation for the river bank for a distance of 80 km upstream of the lake was established. Computed bankfull flows for cross-sections starting at the delta apex were found to reduce significantly in the downstream direction. High values for bankfull conveyance at the head of the delta, equivalent to 50-year return period flood flows, were noted. For medium Athabasca River flows (about 800-1100 m3/s), a comparison was made of water surface elevations at the delta apex during the survey, and data from 1971. A significant increase of water surface height for the same flow rate, by as much as 0.8 m, is apparent, indicating rapid channel aggradation. [ABSTRACT FROM AUTHOR]

Abstract: Groundwater dominance has important effects on the hydrological and geomorphological characteristics of river systems. Low suspended sediment concentrations and high water clarity are expected because significant inputs of sediment-free spring water dilute the suspended sediment generated by storms. However, in many Mediterranean rivers, groundwater dominance is characterised by seasonal alternations of influent and effluent discharge involving significant variability on the sediment transport regimes. Such areas are often subject to soil and water conservation practices over the centuries that have reduced the sediment contribution from agricultural fields and favour subsurface flow to rivers. Moreover, urbanisation during the twentieth century has changed the catchment hydrology and altered basic river processes due to its ‘flashy’ regime. In this context, we monitored suspended sediment fluxes during a two-year period in the Na Borges River, a lowland agricultural catchment (319 km2) on the island of Mallorca (Balearic Islands). The suspended sediment concentration (SSC) was lower when the base flow index (i.e., relative proportion of baseflow compared to stormflow, BFI) was higher. Therefore, strong seasonal contrasts explain the high SSC coefficient of variation, which is clearly related to dilution effects associated with different groundwater and surface water seasonal interactions. A lack of correlation in the Q-SSC rating curves shows that factors other than discharge control sediment transport. As a result, at the event scale, multiple regressions illustrate that groundwater and surface water interactions are involved in the sedimentary response of flood events. In the winter, the stability of baseflow driven by groundwater contributions and agricultural and urban spills causes hydraulic variables (i.e., maximum discharge) to exert the most important control on events, whereas in the summer, it is necessary to accumulate important volumes of rainfall, creating a minimum of wet conditions in the catchment to activate hydrological pathways and deliver sediment to the drainage network. The BFI is also related to sediment delivery processes, as the loads are higher with lower BFI, corroborating the fact that most sediment movement is caused by stormflow and its related factors. Overall, suspended sediment yields were very low (i.e., <1 t km−2 yr−1) at all measuring sites. Such values are the consequence of the limited sediment delivery attributable to soil conservation practices, low surface runoff coefficients and specific geomorphic features of groundwater-dominated rivers, such as low drainage density, low gradient, steep valley walls and flat valley floors. [Copyright &y& Elsevier]

To evaluate anthropogenic sedimentation in United States (U.S.) Pacific Northwest coastal streams, we applied an index of relative bed stability ( LRBS*) to summer low flow survey data collected using the U.S. Environmental Protection Agency’s Environmental Monitoring and Assessment Program field methods in a probability sample of 101 wadeable stream reaches. LRBS* is the log of the ratio of bed surface geometric mean particle diameter ( Dgm) to critical diameter ( D*cbf) at bankfull flow, based on a modified Shield’s criterion for incipient motion. We used a formulation of LRBS* that explicitly accounts for reductions in bed shear stress that result from channel form roughness due to pools and wood. LRBS* ranged from −1.9 to +0.5 in streams within the lower quartile of human riparian and basin disturbance, and was substantially lower (−4.2 to −1.1) in streams within the upper quartile of human disturbance. Modeling results suggest that the expected range of LRBS* in streams without human disturbances in this region might be generally between −0.7 and +0.5 in either sedimentary or volcanic lithology. However, streams draining relatively soft, erodible sedimentary lithology showed greater reductions in LRBS* associated with disturbance than did those having harder, more resistant volcanic (basalt) lithology with similar levels of basin and riparian disturbance. At any given level of disturbance, smaller streams had lower LRBS* than those with larger drainages. In sedimentary lithology (sandstone and siltstone), high-gradient streams had higher LRBS* than did low-gradient streams of the same size and level of human disturbance. High gradient streams in volcanic lithology, in contrast, had lower LRBS* than low-gradient streams of similar size and disturbance. Correlations between Dgm and land disturbance were stronger than those observed between D*cbf and land disturbance. This pattern suggests that land use has augmented sediment supplies and increased streambed fine sediments in the most disturbed streams. However, we also show evidence that some of the apparent reductions in LRBS*, particularly in steep streams draining small volcanic drainages, may have resulted in part from anthropogenic increases in bed shear stress. The synoptic survey methods and designs we use appear adequate to evaluate regional patterns in bed stability and sedimentation and their general relationship to human disturbances. More precise field measurements of channel slope, cross-section geometry, and bed surface particle size would be required to use LRBS* in applications requiring a higher degree of accuracy and precision, such as site-specific assessments at individual streams. [ABSTRACT FROM AUTHOR]

Sediment is listed as one of the leading causes of water-quality impairments in surface waters of the United States (U.S.). A water body becomes listed by a State, Territory or Tribe if its designated use is not being attained (i.e., impaired). In many cases, the prescribed designated use is aquatic health or habitat, indicating that total maximum daily loads (TMDL) targets for sediment should be functionally related to this use. TMDL targets for sediment transport have been developed for many ecoregions over the past several years using suspended-sediment yield as a metric. Target values were based on data from “reference” streams or reaches, defined as those exhibiting geomorphic characteristics of equilibrium. This approach has proved useful to some states developing TMDLs for suspended sediment, although one cannot conclude that if a stream exceeds the target range, the aquatic ecosystem will be adversely impacted. To address this problem, historical flow-transport and sediment-transport data from hundreds of sites in the Southeastern U.S. were re-examined to develop parameters (metrics) such as frequency and duration of sediment concentrations. Sites determined as geomorphically stable from field evaluations and from analysis of gauging-station records were sorted by ecoregion. Mean-daily flow data obtained from the U.S. Geological Survey were applied to sediment-transport rating relations to determine suspended-sediment load for each day of record. The frequency and duration that a given concentration was equaled or exceeded were then calculated to produce a frequency distribution for each site. “Reference” distributions were created using the stable sites in each ecoregion by averaging all of the distributions at specified exceedance intervals. As with the “reference” suspended-sediment yields, there is a broad range of frequency and duration distributions that reflects the hydrologic and sediment-transport regimes of the ecoregions. Ecoregions such as the Mississippi Valley Loess Plains (#74) maintain high suspended sediment concentrations for extended periods, whereas coastal plain ecoregions (#63 and 75) show much lower concentrations. [ABSTRACT FROM AUTHOR]

A landscape's sediment grain size distribution is the product of, and an important influence on, earth surface processes and landscape evolution. Grains can be large enough that the motion of a single grain, infrequently mobile in size-selective transport systems, constitutes or triggers significant geomorphic change. We define these grains as boulders. Boulders affect landscape evolution; their dynamics and effects on landscape form have been the focus of substantial recent community effort. We review progress on five key questions related to how boulders influence the evolution of unglaciated, eroding landscapes: 1) What factors control boulder production on eroding hillslopes and the subsequent downslope evolution of the boulder size distribution? 2) How do boulders influence hillslope processes and long-term hillslope evolution? 3) How do boulders influence fluvial processes and river channel shape? 4) How do boulder-mantled channels and hillslopes interact to set the long-term form and evolution of boulder-influenced landscapes? 5) How do boulders contribute to geomorphic hazards, and how might improved understanding of boulder dynamics be used for geohazard mitigation? Boulders are produced on eroding hillslopes by landsliding, rockfall, and/or exhumation through the critical zone. On hillslopes dominated by local sediment transport, boulders affect hillslope soil production and transport processes such that the downslope boulder size distribution sets the form of steady-state hillslopes. Hillslopes dominated by nonlocal sediment transport are less likely to exhibit boulder controls on hillslope morphology as boulders are rapidly transported to the hillslope toe. Downslope transport delivers boulders to eroding rivers where the boulders act as large roughness elements that change flow hydraulics and the efficiency of erosion and sediment transport. Over longer timescales, river channels adjust their geometry to accommodate the boulders supplied from adjacent hillslopes such that rivers can erode at the baselevel fall rate given their boulder size distribution. The delivery of boulders from hillslopes to channels, paired with the channel response to boulder delivery, drives channel-hillslope feedbacks that affect the transient evolution and steady-state form of boulder-influenced landscapes. At the event scale, boulder dynamics in eroding landscapes represent a component of geomorphic hazards that can be mitigated with an improved understanding of the rates and processes associated with boulder production and mobility. Opportunities for future work primarily entail field-focused data collection across gradients in landscape boundary conditions (tectonics, climate, and lithology) with the goal of understanding boulder dynamics as one component of landscape self-organization. • We synthesize work to date exploring how boulders affect the evolution of nonglaciated, eroding hillslopes and channels. • Boulder dynamics are one facet of landscape self-organization, influencingtransient adjustment and steady-state form. • The effects of boulders on landscape shape can be quantified by combining field data with simplified theoretical models. • The key research need is for more field data on boulder dynamics spanning gradients in boundary conditions. [ABSTRACT FROM AUTHOR]

Marsh surface deposition, suspended sediment concentrations, tidal hydrodynamics, and marsh surface characteristics were measured within a tidal creek system of a west-central Florida, microtidal, open marine Juncus roemerianus marsh to examine the depositional relationship between a creek system and the surrounding marsh during nonstorm conditions. Results suggest that short-term marsh deposition is highly episodic with deposition showing an inverse relationship with marsh surface inundation. Deposition was greatest at marsh sites adjacent to secondary creek segments over all examined timescales. Variations in hydroperiod, time-velocity asymmetry, vegetation density, and sediment availability could not explain the observed differences in short-term deposition. [ABSTRACT FROM AUTHOR]

Abstract: A flume experiment on entrainment of woody debris is carried out. Woody debris is modeled using smooth, cylindrical dowels, in touch with the flume bed. The water depth and velocity are evaluated that initiate the motion of the partially submerged dowels. On the basis of the experimental finding, a theoretical model of log entrainment is developed, providing in dimensionless form the equilibrium equation for incipient motion. The experiment shows that the equilibrium equation must keep into account the modification of the local water profile, affecting the force balance at incipient motion. This issue has been apparently neglected in the wood entrainment models so far developed. The entrainment model is less sensitive to the choice of the apparent drag coefficient. The capability of the model in predicting the critical log diameter for initiation of motion is discussed and compared with that from the recent entrainment model from Braudrick and Grant, 2000. The comparison shows interesting results and provide evidence of the needs for further studies on wood entrainment in rivers. [Copyright &y& Elsevier]

A port complex was implanted in the coast region of Suape, PE-Brasil, in 1979–80, which altered the geomorphological and hydrodynamic conditions of the area, mainly in the estuary of the Ipojuca River. Water samples were collected from the surface and the deep layer to determine the salinity, pH, dissolved oxygen, transparency, and temperature from three fixed stations in the estuary of the Ipojuca River during the summer and winter before (1978) and after (1986–87) the port's construction, in order to evaluate the effects of the port-induced changes. The data were subjected to a principal component analysis and to individual t tests. The first principal component, the axis of maximum information, alone explained 50% of the total variance, being associated with the generalized increase of the chemical parameters after the port's construction, mainly the salinity. Despite the fact that the results of the t tests presented significant differences only for the dissolved oxygen, pH, and temperature, the increased salinity produced the greatest ecological effects in the estuary. The water flow of the Ipojuca River was diverted toward the ocean, decreasing the river-flow velocity at the mouth. This decrease in flow caused a sedimentation in the area, a decrease in the circulation and depth, and a transformation of the mouth of the river into a coastal lagoon with an increase in evaporation and salinity. A delay of more than 2 hours was observed in the dynamic tide in relation to the normal 6-hour cycles. Despite an increase in the concentration of dissolved oxygen and transparency values, effects that theoretically improve environmental conditions, a morphological transformation occurred that caused drastic changes in the estuary's circulation and in the tidal currents. [ABSTRACT FROM AUTHOR]

The primary geomorphic process active in the development of karst stream systems is generally regarded as bedrock dissolution. However, physical erosional processes may also be an important geomorphic agent in karst development. The objectives of this study were to determine the sediment transport threshold in two fluviokarst streams. The source of the sediment was internal to the karst basins. The approach used was to calculate basal and critical shear stresses from streams in two karst systems to determine if, and how frequently, storm-induced flows are capable of mobilizing stream sediment. The fluviokarst systems investigated as a part of this research are capable of transporting 50–85% of their stream substrates during bankfull discharge conditions. Based on the discharge and precipitation frequency, stream flows capable of entraining d50 and d85 particles occur at intervals of 2.4 and 11.7 months (0.98 yr), respectively. Thus, the sediment transport threshold in fluviokarst streams is exceeded by relatively common discharge events. [Copyright &y& Elsevier]

In this paper, we develop and apply a framework for estimating the potential global-scale impact of reservoir construction on riverine sediment transport to the ocean. Using this framework, we discern a large, global-scale, and growing impact from anthropogenic impoundment. Our study links information on 633 of the world''s largest reservoirs (LRs) (≥0.5 km3 maximum storage capacity) to the geography of continental discharge and uses statistical inferences to assess the potential impact of the remaining >44,000 smaller reservoirs (SRs). Information on the LRs was linked to a digitized river network at 30′ (latitude×longitude) spatial resolution. A residence time change (ΔτR) for otherwise free-flowing river water is determined locally for each reservoir and used with a sediment retention function to predict the proportion of incident sediment flux trapped within each impoundment. The discharge-weighted mean ΔτR for individual impoundments distributed across the globe is 0.21 years for LRs and 0.011 years for SRs. More than 40% of global river discharge is intercepted locally by the LRs analyzed here, and a significant proportion (&ap;70%) of this discharge maintains a theoretical sediment trapping efficiency in excess of 50%. Half of all discharge entering LRs shows a local sediment trapping efficiency of 80% or more. Analysis of the recent history of river impoundment reveals that between 1950 and 1968, there was tripling from 5% to 15% in global LR sediment trapping, another doubling to 30% by 1985, and stabilization thereafter. Several large basins such as the Colorado and Nile show nearly complete trapping due to large reservoir construction and flow diversion. From the standpoint of sediment retention rates, the most heavily regulated drainage basins reside in Europe. North America, Africa, and Australia/Oceania are also strongly affected by LRs. Globally, greater than 50% of basin-scale sediment flux in regulated basins is potentially trapped in artificial impoundments, with a discharge-weighted sediment trapping due to LRs of 30%, and an additional contribution of 23% from SRs. If we consider both regulated and unregulated basins, the interception of global sediment flux by all registered reservoirs (n&ap;45,000) is conservatively placed at 4–5 Gt year−1 or 25–30% of the total. There is an additional but unknown impact due to still smaller unregistered impoundments (n&ap;800,000). Our results demonstrate that river impoundment should now be considered explicitly in global elemental flux studies, such as for water, sediment, carbon, and nutrients. From a global change perspective, the long-term impact of such hydraulic engineering works on the world''s coastal zone appears to be significant but has yet to be fully elucidated. [Copyright &y& Elsevier]

The suspended sediment load carried by rivers is highly variable in space and time. On a given river, the suspended sediment load is generally related to the discharge. However, at any given discharge on a specific river the suspended sediment load can vary by a couple of orders of magnitude. The temporal variations can be due to many factors including: varying water sources through a season (i.e., rain versus snowmelt), variability in the amount of easily mobilized sediment stored in the channel, changing channel morphology due to intermittent events or changing climate, and variations in erosional processes supplying sediment to the river. Many studies have looked at the causes of the temporal variability in suspended sediment load on a particular river over specific time periods and created models to simulate the variability. A review of the physical mechanisms responsible for the variability in suspended sediment load reveals the necessary components of a general suspended sediment model for ungauged (and gauged) basins. A generally applicable model is presented that accounts for the inter- and intra-annual variability of suspended sediment load of rivers. The strength of this new model is that the coefficients have strong trends between river basins that can be related to drainage basin parameters. The model accounts for basin wide characteristics through a mean exponent. A variable exponent captures the annual variability and is related to the size of the river basin. The inter-annual variability is accounted for by a variable coefficient, which has large values on small rivers. This is a result of small rivers having larger changes in the intra-annual flows and large rivers tending to have less variability. [Copyright &y& Elsevier]

The rivers of India reveal certain special characteristics because they undergo large seasonal fluctuations in flow and sediment load. The rivers are adjusted to an array of discharges, and most rivers exhibit morphologies that are related to high-magnitude floods. In the last 100 years primarily hydraulic engineers have contributed to the understanding of the fluvial forms and processes. Though this trend has continued even today, in recent decades some interesting fluvial research has also been carried out by earth scientists. Four large rivers, namely Brahmaputra, Kosi, Indus and Narmada, have received greater attention from fluvial geomorphologists. The major themes in Indian fluvial geomorphology include the hydrology of monsoonal rivers; forms and processes in alluvial channels; causes of avulsion, channel migration; and anomalous variations in channel patterns; dynamics of suspended sediment; and the geomorphic impacts of floods. Studies of bedrock channels are far less than similar studies in alluvial channels. Only a few rivers have been investigated in this respect. Studies indicate that the Himalayan rivers are different in many respects from those of the Indian Peninsula. The former occupy a highly dynamic environment with extreme variability in discharge and sediment load. Earthquakes and landslides also have a great impact on these rivers from time to time. Consequently, the rivers are characterized by frequent changes in shape, size, position and planform. In comparison, the adjustments in Peninsular rivers are less frequent and of a much smaller magnitude. An inescapable conclusion is that in the tropical monsoonal environment, large floods are important geomorphic agents that temporarily affect the forms and behavioural characteristics of some rivers, but leave a lasting effect on others. In magnitude-frequency terms, large floods are major formative events in many rivers of the Indian region in general and the Indian Peninsula in particular. [ABSTRACT FROM AUTHOR]