Stream corridor and upland sources of fluvial sediment and phosphorus from a mixed urban-agricultural tributary to the Great Lakes

Like many impaired Great Lakes tributaries, Apple Creek, Wisconsin (119 km²) has Total Maximum Daily Load (TMDL) targets for reducing suspended sediment and total phosphorus by 51.2 % and 64.2 %, respectively. From August 2017 - October 2018, a stream sediment budget and fingerprinting integrated study was conducted to quantify upland and stream corridor sources of suspended sediment and sediment-bound phosphorus. Phosphorus concentrations varied among source groups and fluvial sediments, with higher concentrations among suspended sediment and cropland soils. Eroding streambanks identified in the stream corridor sediment budget accounted for 100 % of the TMDL Soil and Water Assessment Tool (SWAT) suspended sediment load but only 20 % of the total phosphorus load. Fine-grained streambed sediment equated to approximately-three years of modeled suspended sediment load but only one third of total phosphorus load. The two primary sources of fine-grained streambed sediment were streambanks and cropland, with relative streambank contributions increasing with downstream direction and watershed area. The relative proportion of suspended sediment varied by season and streamflow; however, cropland and streambank erosion accounted for 54 % and 23 % of the suspended sediment when weighted by of the proportion for representative streamflow. Urban land was a source in the upper watershed, but the signature was sequestered by a mid-watershed detention basin. Contributions from construction sites were higher in the fall 2018, likely corresponding to increased activity following a wet spring. These integrated techniques helped describe sources, transport, and sinks of fluvial sediment and phosphorus throughout the watershed at a range of spatial and temporal scales.     

Mechanically reshaping stream banks alters fish community composition

Mechanically reshaping stream banks is a common practice to mitigate bank erosion in streams that have been extensively channelised and lowered for land drainage. A common perception regarding this activity is that fish populations will be largely unaffected, at least in the short term, because the low‐flow wetted channel remains undisturbed. However, the response of fish populations to this practice has rarely been quantitatively evaluated. Using a Before‐After‐Control‐Impact design, we assessed fish community responses to a catchment‐scale bank reshaping event in a fourth‐order low‐gradient stream that drains an intensive agricultural landscape. Quantitative electric fishing and fish habitat data were collected 2 months before and annually for 3 years after the reshaping event. After reshaping, deposited fine sediment levels increased in impact reaches, and there was a significant reduction in anguillid eel biomass (by 49%). In contrast, densities of obligate benthic gobiid bully species increased significantly in impact reaches—potentially due to reduced predation pressure from eels. Three years after bank reshaping, fish community structure had largely returned to its preimpact state in the reshaped areas. Our results suggest that, even in highly modified stream channels, further bank modification can reduce instream habitat quality and displace eels for at least 1 year. Managers should endeavour to use bank erosion control measures that conserve bank‐edge cover, especially in streams with populations of anguillid eels, because these fish are declining globally.     

Assessing the effectiveness of enhancement activities in urban streams: I. Habitat responses

Effects of stream enhancement on habitat conditions in five spring‐fed urban streams in Christchurch, New Zealand, were investigated. Stream enhancement consisted of riparian planting at three sites, and riparian planting and channel modifications at two sites, where a concrete dish channel and a timber‐lined channel were removed, and natural banks reinstated. Sites were surveyed prior to enhancement activities and 5 years after, and changes in riparian conditions (composition, horizontal and vertical cover), instream conditions (bank modifications, inorganic and organic material on the streambed), and hydraulic conditions (wetted perimeter, cross‐sectional area, depths and velocities) quantified. Enhanced sites generally had higher marginal vegetation cover, as well as increased overhanging riparian vegetation, reflecting planting of Carex sedges close to the water. Bed sediments changed at some sites, with the greatest change being replacement of a concrete channel with gravel and cobble substrate. Bryophyte cover declined at this site, reflecting loss of stable habitat where these plants grew. Bed sediments changed less at other sites, and cover of fine sediments increased in some enhanced sites, presumably from sediment runoff from nearby residential development. Filamentous algal cover decreased at one stream where shade increased, but increased in another stream where the removal of timber‐lined banks and creation of a large pond decreased shade. Stream enhancement increased variability in velocity at three of the five sites, but overall changes to stream hydraulics were small. Although enhancement activities altered the physical conditions of the streams, major changes occurred only to riparian vegetation and bank conditions. Lack of other major changes to instream physical conditions most likely reflected the limited range of channel morphology alterations undertaken. Moreover, the flat topography of Christchurch and naturally low stream discharge further constrained changes to instream physical conditions from enhancement activities. Sediment inputs from continuing urban development also negated the effects of adding coarse substrates. These over‐arching factors may constrain the success of future stream enhancement projects within Christchurch. Copyright © 2005 John Wiley & Sons, Ltd.     

Fingerprinting the contribution of quarrying to fine‐grained bed sediment in a mountainous catchment,Iran

The contribution of quarrying in the context of multiple catchment sources of fine‐grained sediment has rarely been investigated. This study assessed the relative importance of quarrying as a sediment source alongside rangeland surface soils and channel banks in a mountainous catchment in northern Tehran, Iran, using fingerprinting. Eight geochemical tracers were measured on 24 potential sediment source samples and four fine‐grained sediment samples. Statistical analysis to select three different composite fingerprints for discriminating the potential sediment sources comprised: (a) the Kruskal–Wallis H test (KW‐H), (b) a combination of KW‐H and discriminant function analysis (DFA), and (c) a combination of KW‐H and principal components and classification analysis (PCCA). A Bayesian unmixing model was used to apportion sediment source contributions using the three composite fingerprints. Using the KW‐H composite signature, the respective relative contributions (with uncertainty ranges) from channel banks, rangeland surface soils, and quarrying were estimated as 28.4% (10.9–46.8), 15.1% (6.6–22.7), and 56.6% (38.3–74.2), compared with 35.4% (11.9–60.1), 13.4% (4.1–22.2), and 51.3% (26.5–74.3) using a composite signature selected using a combination of KW‐H and DFA, or 20.7% (3.9–41.7), 17.2% (4.4–29.9), and 61.4% (44–78.8) using a fingerprint selected using KW‐H and PCCA. The different composite signatures therefore all consistently suggested that quarrying is the dominant source of the fine‐grained sediment samples. Potential mitigation measures targeting this land use include closure to permit revegetation to reduce exposure of bare surfaces to sediment mobilization. Limitations and uncertainties associated with this preliminary investigation are briefly discussed.     

Determination of bank erodibility for natural and anthropogenic bank materials using a model of lateral migration and observed erosion along the Willamette River,Oregon, USA

Many large rivers flow through a variety of geologic materials. Within the span of several kilometres, bends may alternately flow against recently reworked sediments, older, more indurated sediments or highly resistant materials. As sediment size, cementation, and other properties strongly influence the erodibility of river banks, erosion rates and channel planform are likely to vary significantly along the length of large rivers. In order to assess the role of bank materials on bank erosion rates, we develop a method for detecting relative differences in erodibility between bank materials along large floodplains. By coupling historic patterns of channel change with a simple model of bank erodibility we are able to track relative changes in bank erodibility among time intervals and bank materials. We apply our analysis to the upper Willamette River, in northwestern Oregon for three time periods: 1850–1895, 1895–1932 and 1972–1995 and compute relative differences in bank erodibility for Holocene alluvium, partially cemented Pleistocene gravels, and revetments constructed in the 20th century. Although the Willamette is fundamentally an anastomosing river, we apply the model to single‐thread portions of the channel that evolved through lateral migration. Our simple model of bank erodibility reveals that for all three‐time periods, banks composed of Holocene alluvium are at least 2–5 times more erodible than banks composed of Pleistocene gravels. Revetment installed in the 20th century is highly resistant to erosion and is at least 10 times less erodible than Pleistocene gravels. Copyright © 2006 John Wiley & Sons, Ltd.     

STREAM RESTORATION AND CRIBWALL PERFORMANCE: A CASE STUDY OF CRIBWALL MONITORING IN SOUTHERN ONTARIO

Stream restoration focusing on adaptable natural and inert material use has been implemented through soil bioengineering designs aimed at the stabilization of urbanized streams. Within each design application materials such as large wood, sediment fill and vegetation must be suited to diverse settings. This paper discusses the application of cribwalls as soil bioengineering designs found in two Southern Ontario watersheds and the criteria that influence their performance. Field measurements of cribwall cuttings, sediment sampling, erosion pin monitoring, and computer‐generated stream power analysis are used to compare design performance at several sites. It is determined that the technical specifications of the design and site characteristics such as stream power distribution, sediment, and channel planform are equally involved in long‐term streambank stability. The results indicate that cribwalls with dense cutting growth perform well on streambanks that offer a greater amount of soil cohesion, nutrients, and infiltration in the mid and upper sections of the bank. In streams with moderate channel slopes and stream power distribution that is above the watershed mean, streams with well‐developed floodplains, sinuous channel planforms, and low bank height ratios perform better than those that are confined, straightened, and have greater bank height ratios. Throughout the comparison of several cribwall sites, the implication of this work is to demonstrate how to assess the fitness of similar soil bioengineering designs for application to diverse stream settings and to further validate their significance in stream restoration as designs that are multifunctional. Copyright © 2013 John Wiley & Sons, Ltd.     

The storage and provenance of fine sediment on the channel bed of two contrasting lowland permeable catchments,UK

Fine sediment (<63 µm) storage in river channels frequently represents a significant term in catchment sediment budgets and plays an important role in diffuse pollution problems. A combination of a sediment remobilization technique and the fingerprinting approach was used to examine the storage and provenance of fine sediment on the channel bed of two contrasting lowland permeable catchments in the UK. In the upper Tern (∼231 km²) study catchment, estimates of mean fine sediment storage on the channel bed ranged between 860–5500 g m⁻², with an overall average of 2391 g m⁻², compared to 470–2290 g m⁻² and 1065 g m⁻² in the Pang (∼166 km²) and 770–1760 g m⁻² and 1255 g m⁻² in the Lambourn (∼234 km²) sub‐catchments. Mean total fine sediment storage on the bed of the main channel was equivalent to 37% (upper Tern), 38% (Pang) and 21% (Lambourn) of the mean annual suspended sediment loads measured at the catchment outlets. Over the study period, the total gain (1427 t) and loss (1877 t) to fine sediment storage on the channel bed in the upper Tern catchment were equivalent to 82% and 108% of the mean annual suspended sediment load, respectively, compared to 149% (740 t) and 136% (678 t) in the Pang sub‐catchment, and 39% (422 t) and 49% (528 t) in the Lambourn sub‐catchment. The source of the fine sediment stored on the channel bed within each study area varied. In the upper Tern catchment, the weighted mean relative contributions from individual source types were estimated to be 35 ± 5% (pasture), 51 ± 5% (cultivated) and 14 ± 3% (channel banks and subsurface sources). The corresponding estimates were 49 ± 8%, 33 ± 5% and 18 ± 5% for the Pang sub‐catchment, compared to 19 ± 6%, 64 ± 5% and 17 ± 5% for the Lambourn sub‐catchment. These sediment source estimates have important implications for the design and implementation of targeted sediment control policies within the study areas. Copyright © 2007 John Wiley & Sons, Ltd.     

Human impacts on suspended sediment and turbidity in the River Murray,South Eastern Australia: Multiple lines of evidence

European settlement has led to increased loads of fine suspended sediment (SS) entering the River Murray, Australia's largest, and arguably, most important river. The River Murray's anthropogenic sediment history can be divided into four periods with varying source areas, sediment loads, and seasonal patterns. The Aboriginal period (before 1840) was characterized by clear water at summer low‐flows in the River Murray and its southern tributaries, with more sediment coming from the northern catchment than the southern, and the Darling River being turbid at all flows. There is little evidence that Aboriginal burning resulted in any measurable increase in SS. SS loads peaked in the 1870s and 1880s (the gold and gully period, 1850–1930) as valley floors were incised by gullies (mostly in northern tributaries), and gold sluicing flushed huge amounts of sludge into southern tributaries. Sedimentation in wetlands and on floodplains increased by 2–10 times in this period, and the biota in wetlands switched from clear water to turbid water communities. In the hiatus period (1930–1960) sediment supply from gullies and gold mining waned and low flow SS concentrations returned to low levels. Dam construction through the 1960s and 1970s (the regulation period, 1960 on) disconnected the River Murray from catchment derived sediment. Despite this, SS levels increased again: now largely derived from instream sources including bank erosion from long duration summer irrigation flows, the spread of bottom‐feeding carp (Cyprinus carpio), and wave erosion from boats. Erosion switched from winter to summer dominated. Significant investment in securing water for the environment in the Murray‐Darling Basin could be complemented by addressing in‐channel sediment sources in the River Murray itself to reduce turbidity. Overall, European era SS concentrations remain relatively low with small sediment delivery to the ocean (0.1 Mt per annum), despite high catchment erosion rates. This is due to poor sediment delivery efficiency through the low‐gradient landscape.     

黄河下游泥沙沉积汇对入海悬移质泥沙粒度的影响

黄河下游泥沙沉积汇在黄河流域系统的泥沙收支平衡(Sediment budget)中起着重要的作用。通过河床主槽中泥沙的淤积和冲刷,运动泥沙的组成发生变化;通过滩地上泥沙淤积和河岸坍塌,河漫滩上前期淤积的泥沙与洪水所挟带的泥沙发生交换,使悬移质泥沙的组成发生变化。泥沙冲淤量对不同粒径组入海泥沙百分比的影响是不同的。小于0.01mm细泥沙的百分比与下游河道淤积量呈正相关,0.025~0.05mm和大于0.05mm的较粗泥沙的百分比与下游河道淤积量呈负相关。入海泥沙平均粒径与深泓年摆幅之间也存在着负相关关系。自20世纪60年代,中期以来,特别是自80年代中期以来,由于黄河下游径流量显著减小,黄河下游河道发生萎缩,河宽减小,因而主流线的摆动幅度也显著减小。这使得原来十分强烈的河岸侵蚀和滩槽泥沙交换强度大为减弱,主槽洪水得到的来自河岸侵蚀的细粒泥沙越来越少,因而使入海泥沙粒径变粗。     

ADJUSTMENT OF PRAIRIE POTHOLE STREAMS TO LAND‐USE,DRAINAGE AND CLIMATE CHANGES AND CONSEQUENCES FOR TURBIDITY IMPAIRMENT

Changes in land use and drainage have contributed to channel adjustment in small‐order to medium‐order streams in the prairie pothole region of south‐west Minnesota. Although conversion from prairie to agriculture occurred a century ago, recent decades have seen increased subsurface tile drainage, annual row crop coverage and channel modifications, particularly at road crossings such that channel adjustment is ongoing. Channel evolution in Elm and Center Creeks, two fourth‐order streams in the Blue Earth River basin, was studied to understand relationships between changes in channel morphology and suspended sediment concentrations. The construction of drainage ditches and expanded subsurface tiling has connected isolated basins to stream channels, effectively increasing drainage areas of Elm and Center Creeks by 15–20%. Sinuosity has been reduced by grading and drainage of first‐order sloughs, channel straightening at road crossings and natural cut‐offs and agricultural ditching that have shortened Elm Creek by 15% between 1938 and 2003. Stream cross‐sectional area was enlarged in response to the land‐use and drainage changes. In the headwaters, public ditches are wider than historic channels and entrenched by berms. Unchannelized headwater and upper mainstem portions of Elm Creek are also highly entrenched (up to 1.07 meters below the pre‐channelization bed elevation with a bank height ratio > 1.5) but have not widened substantially. In contrast, the lower main channel has widened by an average of 68%. These channel adjustments contribute to the suspended sediment load and violations of Minnesota's turbidity and Index of Biotic Integrity standards. The watershed has a low sediment delivery ratio because it is a flat, poorly connected landscape and likely delivers less sediment to the Minnesota River than steeper rivers downstream, such as the Blue Earth River. Entrenchment and increased sediment transport capacity in the lower reaches of the river have lead to increased sediment delivery to the downstream Blue Earth and Minnesota rivers. Understanding geomorphic changes will be important for addressing water‐quality impairments in the region. Copyright © 2011 John Wiley & Sons, Ltd.     

Putting down roots: Afforestation and bank cohesion of Icelandic Rivers

Riparian vegetation is widely recognized as a critical component of functioning fluvial systems. Human pressures on woody vegetation including riparian areas have had lasting effects, especially at high latitude. In Iceland, prior to human settlement, native downy birch woodlands covered approximately 15%–40% of the land area compared to 1%–2% today. Afforestation efforts include planting seedlings, protecting native forest remnants, and acquiring land areas as national forests. The planted and protected nature of vegetation along rivers within forests provides a unique opportunity to evaluate the various taxa within riparian zones and the channel stabilizing characteristics of the vegetation used in afforestation. We investigated bank properties, sediment textures, and root characteristics within riparian zones along four rivers in forests in Iceland. Bank sediment textures are dominantly sandy loam overlying coarser textures. Undercut banks are common because of erosion of the less cohesive subsurface layer. Quantitative root data indicate that the woody taxa have greater root densities, rooting depths, and more complex root structures than forbs or graminoids. The native downy birch has the highest root densities, with <1 mm roots most abundant. Modeling of added bank cohesion indicates that willow provides up to six times and birch up to four times more added cohesion to the coarse sediment textures comprising stream banks compared to no vegetation. We conclude that planting and protecting the native birch and willow helps to reduce bank erosion, especially where long-term grazing exclusion can be maintained.     

Monitoring and assessment of a river restoration project in central New York

A widespread lack of post‐project appraisals (PPAs) not only hinders progress in the field of river restoration but also limits the application of adaptive management – a powerful heuristic tool particularly well suited to dynamic fluvial environments. In an effort to contribute to the limited body of scientific literature pertaining to PPAs, we evaluated a stream restoration project completed in the fall of 2005 in central New York. Using a variety of evaluation approaches, we documented both successes (e.g. enhanced in‐stream habitat) and short‐comings (e.g. channel avulsions). Overall, we concluded that the project was marginally successful in achieving its stated goals and that future prospects remain uncertain based on current trajectory. Lessons learned from this monitoring study include: (i) protect vulnerable banks and floodplains until vegetation is established, e.g. via integrated bio‐ and geo‐technical methods, (ii) perform scour depth analyses and excavate scour pools associated with hydraulic structures to design depth to prevent clogging of the channel during post‐construction floods, (iii) orient bank vanes such that cross‐stream flows are not deflected towards the bank, (iv) cross‐validate restoration designs via multiple methods, including process‐based sediment transport relations, especially in unstable gravel‐bed rivers, (v) anticipate and fund for fixing natural channel design (NCD) projects for 3–5 years after completion to account for uncertainties and (vi) identify measurable, goal‐specific success criteria that account for watershed scale stressors and site constraints prior to construction to facilitate successful project design and ensure effective outcomes appraisal. Copyright © 2010 John Wiley & Sons, Ltd.     

The Use of Stream Power as an Indicator of Channel Sensitivity to Erosion and Deposition Processes

Stream power is a measure of the main driving forces acting in a channel and determines a river's capacity to transport sediment and perform geomorphic work. Recent digital elevation models allow the calculation of channel gradient and consequently stream power at unprecedented spatial resolution, opening promising and novel opportunities to investigate river geomorphic processes and forms. The present paper investigates the suitability of map‐derived information on total and specific stream power (SSP) to identify dominant processes within the channel (i.e. erosion, transport or deposition). SSP has been already used to identify a threshold for channel stability. This paper tests this knowledge and investigates whether or not attributes of stream power profiles are statistically correlated with distinctive field morphological forms. Two gravel bed single‐thread English rivers are used as case studies, the Lune and the Wye. Available deposition and erosion features surveyed in the field from 124 different locations are used to classify channel reaches as erosion, transport or deposition dominated. Meaningful patterns emerge between the stream power attributes and the field‐based channel classification. An SSP threshold, which erosion is triggered, compares favourably with the ones in the literature. Information about upstream stream power profiles helps to determine the dominant processes. The joint configuration of local and upstream stream power information uniquely classifies reaches into four classes of different sensitivity to erosion and deposition. Copyright © 2013 John Wiley & Sons, Ltd.     

Contrasting stream stability characteristics in adjacent urban watersheds: Santa Clara Valley,California

A comparative study of two adjacent stream channels in the Santa Clara Valley region of California provided an opportunity to study the relative effects of multi‐faceted watershed‐urbanization impacts on channel evolution and stability. Berryessa Creek (15.5 km²) and Upper Penitencia Creek (61.3 km²) have similar intrinsic watershed characteristics; however, urbanization processes have imposed distinctly different evolutionary trends in each watershed. The influences of drainage network manipulation, hydrologic routing and engineering infrastructure has resulted in Upper Penitencia Creek remaining relatively stable throughout the course of urbanization, while Berryessa Creek has experienced system‐wide channel instability problems. This study enumerates the many anthropogenic impacts and provides insight into basin alterations that can have either positive or negative feedbacks in maintaining or degrading channel stability throughout the course of urbanization. Results show that infrastructure that disrupts the bed material sediment continuity (such as large drop structures or sedimentation ponds) generate long‐term downstream channel instabilities leading to channel degradation and continued maintenance. Off‐line flow diversions (in this study percolation ponds) that do not disrupt bed material transport can emulate pre‐urbanization conditions offsetting channel degradation resulting from changes in hydrology. This study also demonstrates the degradational responses of a stream due to losses in riparian vegetation from water table lowering transforming a perennial stream into an ephemeral stream resulting in increased bank instability. The importance of maintaining floodplains for flood access and channel stability has also been identified and compared to conditions of channel encroachment to facilitate maintenance, which have further exacerbated downstream channel degradation, long‐term channel maintenance and dredging. Copyright © 2009 John Wiley & Sons, Ltd.     

Hydraulic Interaction between Rock Cross Vane Stream Restoration Structures and a Bridge Crossing

The performance of a stream restoration project that incorporates a bridge crossing is evaluated within a 3‐year monitoring period. A goal of the project was to alleviate and prevent future sediment aggradation within the waterway of a low‐clearance bridge crossing. The stream restoration project included two rock cross vanes and stepped riprap and vegetation bank stabilization. Monitoring of the project involved the collection of channel survey data, pebble counts, and general observations of instream structure condition and sediment movement. The evaluated performance of the restoration structures is related to the general hydrologic conditions, the historical changes in the watershed and channel, and the hydraulic conditions created by the low‐clearance bridge crossing. Backwater effects created by the bridge crossing are found to be a substantial cause of the failure of the stream restoration project to meet its goals. The low‐clearance bridge hydraulics are preventing a rock cross vane located upstream of the bridge from creating a scour hole in the centre of the channel; instead, aggradation is occurring in this portion of the channel. However, degradation is occurring downstream of the bridge causing the failure of the second rock cross vane and of the riprap and vegetation bank. Although the hydraulic conditions may stem from the initial design of the bridge crossing, any restoration structure should be designed according to the current site hydraulics. In addition to providing insight into the design and construction of stream restoration structures, the results have implications for the design and management of bridge crossings. Copyright © 2014 John Wiley & Sons, Ltd.     

A LIDAR‐DERIVED EVALUATION OF WATERSHED‐SCALE LARGE WOODY DEBRIS SOURCES AND RECRUITMENT MECHANISMS: COASTAL MAINE,USA

In‐channel large woody debris (LWD) promotes quality aquatic habitat through sediment sorting, pool scouring and in‐stream nutrient retention and transport. LWD recruitment occurs by numerous ecological and geomorphic mechanisms including channel migration, mass wasting and natural tree fall, yet LWD sourcing on the watershed scale remains poorly constrained. We developed a rapid and spatially extensive method for using light detection and ranging data to do the following: (i) estimate tree height and recruitable tree abundance throughout a watershed; (ii) determine the likelihood for the stream to recruit channel‐spanning trees at reach scales and assess whether mass wasting or channel migration is a dominant recruitment mechanism; and (iii) understand the contemporary and future distribution of LWD at a watershed scale. We utilized this method on the 78‐km‐long Narraguagus River in coastal Maine and found that potential channel‐spanning LWD composes approximately 6% of the valley area over the course of the river and is concentrated in spatially discrete reaches along the stream, with 5 km of the river valley accounting for 50% of the total potential LWD found in the system. We also determined that 83% of all potential LWD is located on valley sides, as opposed to 17% on floodplain and terrace surfaces. Approximately 3% of channel‐spanning vegetation along the river is located within one channel width of the stream. By examining topographic and morphologic variables (valley width, channel sinuosity, valley‐side slope) over the length of the stream, we evaluated the dominant recruitment processes along the river and often found a spatial disconnect between the location of potential channel‐spanning LWD and recruitment mechanisms, which likely explains the low levels of LWD currently found in the system. This rapid method for identification of LWD sources is extendable to other basins and may prove valuable in locating future restoration projects aimed at increasing habitat quality through wood additions. Copyright © 2011 John Wiley & Sons, Ltd.     

Sedimentation dynamics within a large shallow lake and its role in sediment transport in a continental-scale watershed

A comprehensive understanding of the sedimentation dynamics within Lake Winnipeg (surface area: 23,750 km²) and its role in sediment transport in the downstream river system was achieved by determining the properties of lake bottom sediment and patterns of sediment accumulation rates and by constructing a conceptual total (i.e., organic and inorganic) sediment budget. Net deposition was the governing process in the South and North Basins, whereas transportation dominated in the Narrows. The largest fluvial source of sediments to the lake, the Red River, supplies 35% of the total sediment load. Although accumulation rates in profundal zones progressively decreased northward from this source at the south end of the lake, high accumulation rates with low inventories of fallout radionuclides in the northern margin of the North Basin indicate a second sediment source, which was determined to be erosion of north shore banks, which accounts for up to 50% of the total sediment load to the lake. The nearshore-offshore gradient in bottom sediment properties in the North Basin confirmed that the signature of this source can reach at least 20 km southward into the lake. However, the properties of bottom sediments, sedimentation dynamics, and sediment budget suggested that some of the materials eroded from the north shore are exported without interaction with the lake bottom and this local sediment source is the dominant source for the downstream river system. It was concluded that Lake Winnipeg effectively disconnects the downstream Nelson River from sediment transport processes in its upstream watershed (953,250 km²).     

三峡水库蓄水以来水库淤积和坝下冲刷研究

为了分析三峡工程对库区及坝下长江中游河势的影响,基于实测资料,较为系统地研究了三峡水库蓄水运用以来水库泥沙淤积和坝下游河床冲刷特性。研究表明,1991年以来长江干流各站径流量变化不大,输沙量明显减小;三峡水库蓄水运用后的2003～2011年入库沙量继续大幅减少,仅为原设计值的40%,水库年均淤积泥沙1.40亿t,也仅为论证阶段的40%左右,且绝大部分淤积在常年回水区和死库容内;受上游来沙减小和三峡水库蓄水拦沙影响,坝下游输沙量大幅减小,悬移质泥沙颗粒也明显变粗,长江中游原有的冲淤相对平衡状态被打破,河床发生沿程冲刷,2002年10月至2010年10月,宜昌至湖口河段总冲刷量为9.79亿m3,河床冲淤形态转变为“滩、槽均冲”,主要冲刷发生在宜昌至城陵矶河段。     

Lithological controls influence river form in contrasting sub-catchments: The Scamander and Avenue Rivers,Tasmania

Bedrock influence on river channel form is difficult to assess, with many catchments dominated by glacial erosion and with blanketing sediments from both pluvial and fluvial sources. The Scamander catchment in Tasmania lacks glacial history and features two bedrock-confined sub-catchments of similar area and maximum flow length, but one dominated by Mathinna Group sedimentary rocks, and the other with large areas in granite lithology. Lithology types, stream network parameters and near stream slope angles were analysed using geographical information systems, and results of stream cross sections, channel form and planforms were compared for each lithology. Results showed that granite features low gradient, shallow streams with channel slope cross sections of <10°. Granite channels are irregular and dominated by large boulders that create channel roughness and resist incision. Weathering products of coarse quartz sand provide anchorages for in-channel vegetation. By contrast, sedimentary Mathinna Group rocks feature steeply incised trough-like channels, with near-channel hillslope gradients mostly between 30° and 50°. Rectangular blocks disintegrate to gravel cobbles on the channel bed, providing poor anchorage for vegetation. Mathinna lithology influences steep, low roughness channels, and stable, incised recurved meanders formed by river interaction with vertical layers of resistant rock. The resistant bedrock lithology is demonstrated in this study to influence varied river planforms, near-channel slope gradients, channel sediments and cross sections.     

Sediment Loading During the 20th Century in Presque Isle Bay,Lake Erie,Pennsylvania

Mud-dominated sediments in Presque Isle Bay are contaminated with metals and hydrocarbons derived from developed watershed and atmospheric sources. Prior to this study, the quantities, rates, and spatial distribution of long-term sedimentation and erosion in the bay were largely unknown. As a result, the fate of contaminated bay-floor sediments and possible rates of natural recovery for this Area of Concern (AOC) could not be determined. To provide baseline data useful to state and federal agencies monitoring recovery of the bay, this paper identifies: (1) the quantities, rates and patterns of 20^th Century sedimentation and erosion, (2) the major sediment inputs and outputs for the bay, and (3) the implications of the sedimentary regime on possible future rates of bay recovery. Bathymetric and sedimentological data show that 20th Century net accumulation totaled approximately 3.94 × 10⁶ m³ which is equivalent to a dry sediment loading of 5.92 × 10⁹ kg (5.92 × 10⁶ t), or 6.29 kg/m²/yr (1.28 lb/ft²/yr) when averaged over the accretional 70% of the bay. This external loading represents approximately 50% of total accretion because externally derived sediments are augmented with resuspended sediments from shallow-water parts of the bay. The principal sediment inputs were littoral drift from ephemeral and permanent inlets (∼42%), artificial infilling along the shoreline (∼28%), streams (∼16%), bank/bluff erosion (∼12%), and biological production (∼2%). Dredging was the principal output. Based on long-term average sedimentation rates and patterns, recovery of the AOC through natural sediment capping will take at least several decades if source contaminants are removed.