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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]

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: 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]

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]

The article examines the implications of the saltation-abrasion model for steady-state river longitudinal profile relief and concavity. It investigates the controls on river longitudinal profile concavity and relief using an experimentally calibrated model. It focuses on the case of topographic steady state, in which the rate of vertical rock uplift is equal to the rate of channel incision. The study also carried out a comprehensive sensitivity analysis and explores a variety of key potential linkages among the variables that together control river longitudinal profile form.

Relationships 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 floodplain, 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. [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]