River widening: an approach to restoring riparian habitats and plant species

‘River widenings’ are commonly used in river restoration to allow channel movement within a spatially limited area. Restoration seeks to restore fluvial processes and to re‐establish a more natural riparian community. This study investigates the performance of five river widenings in Switzerland, focusing on the re‐establishment of riparian (semi‐)terrestrial habitats and species, and highlights some factors that seem to influence their performance. The restoration projects are compared with pre‐restoration conditions and near‐natural conditions, which are assumed to represent the worst‐ and best‐case conditions along a gradient of naturalness. Fuzzy ordination of vegetation data and calculation of landscape metrics based on habitat maps revealed marked differences between the degree of naturalness achieved by each individual restoration project. However, in general river widenings were found to increase the in‐stream habitat heterogeneity and enhanced the establishment of pioneer habitats and riparian plants. Analyses of species pools based on a hierarchic list of indicator species and correspondence analysis showed that the ability of river widenings to host typical riparian species and to increase local plant diversity strongly depends on the distance to near‐natural stretches. Species dispersal and establishment might be hampered by decisions taken outside the scope of the restoration project. Therefore we conclude that action on the catchment scale is needed to maximize the benefits of local management. Copyright © 2005 John Wiley & Sons, Ltd.     

Beaver assisted river valley formation

We examined how beaver dams affect key ecosystem processes, including pattern and process of sediment deposition, the composition and spatial pattern of vegetation, and nutrient loading and processing. We provide new evidence for the formation of heterogeneous beaver meadows on riverine system floodplains and terraces where dynamic flows are capable of breaching in‐channel beaver dams. Our data show a 1.7‐m high beaver dam triggered overbank flooding that drowned vegetation in areas deeply flooded, deposited nutrient‐rich sediment in a spatially heterogeneous pattern on the floodplain and terrace, and scoured soils in other areas. The site quickly de‐watered following the dam breach by high stream flows, protecting the deposited sediment from future re‐mobilization by overbank floods. Bare sediment either exposed by scouring or deposited by the beaver flood was quickly colonized by a spatially heterogeneous plant community, forming a beaver meadow. Many willow and some aspen seedlings established in the more heavily disturbed areas, suggesting the site may succeed to a willow carr plant community suitable for future beaver re‐occupation. We expand existing theory beyond the beaver pond to include terraces within valleys. This more fully explains how beavers can help drive the formation of alluvial valleys and their complex vegetation patterns as was first postulated by Ruedemann and Schoonmaker in 1938. Copyright © 2010 John Wiley & Sons, Ltd.     

Habitat Restoration in the Context of Watershed Prioritization: The Ecological Performance of Urban Stream Restoration Projects in Portland,Oregon

In Portland (Oregon, USA), restoration actions have been undertaken at the watershed scale (e.g. revegetation and stormwater management) to improve water quality and, where water quality and quantity are adequate at the reach scale, to increase habitat heterogeneity. Habitat enhancement in urban streams can be important for threatened species, but challenging, because of altered catchment hydrology and urban encroachment on floodplains and channel banks. To evaluate reach‐scale restoration projects in the Tryon Creek watershed, we sampled benthic macroinvertebrates and conducted habitat quality surveys pre‐project and over 4 years post‐project. Species sensitive to pollution and diversity of trophic groups increased after restoration. Taxonomic diversity increased after restoration but was still low compared with reference streams. We found no significant changes in trait proportions and functional diversity. Functional diversity, proportion of shredders and semivoltine invertebrates were significantly higher in reference streams than in the restored stream reaches. We hypothesized that inputs of coarse particulate organic matter and land use at watershed scale may explain the differences in biodiversity between restored and reference stream reaches. Habitat variables did not change from pre‐project to post‐project, so they could not explain community changes. This may have been partly attributable to insensitivity of the visual estimate methods used but likely also reflects the importance of watershed variables on aquatic biota—suggesting watershed actions may be more effective for the ecological recovery of streams. For future projects, we recommend multihabitat benthic sampling supported by studies of channel geomorphology to better understand stream response to restoration actions. Copyright © 2014 John Wiley & Sons, Ltd.     

Quantifying the hydrological effects of stream restoration in a montane meadow,northern California,USA

Stream restoration efforts, particularly within meadow systems, increasingly rely on ‘pond and plug’ type methods in which (a) alluvial materials are excavated from the floodplain, forming ponds; (b) excavated alluvial materials are used to plug incised channels and (c) smaller dimension channels are restored to the floodplain surface. A commonly stated objective of these efforts is to restore ecologically significant hydrological processes to degraded riparian systems. However, little research has been conducted to evaluate and quantify the restoration of these hydrological processes. Direct comparisons of pre‐ and post‐restoration hydrological observations are often misleading due to an inter‐annual climatic variability. To overcome this issue and accurately quantify the hydrological effects of restoration, we developed, calibrated and validated a hydrological model of a 230 ha mountain meadow along a 3.6 km restored reach of Bear Creek in the northeastern California. We then applied the model to simulate the pre‐ and post‐restoration scenarios by altering the floodplain topography and stream channel networks. Our results document three general hydrological responses to the meadow restoration effort: (1) increased groundwater levels and volume of subsurface storage; (2) increased frequency/duration of floodplain inundation and decreased magnitude of flood peaks and (3) decreased annual runoff and duration of baseflow. This study supports and quantifies the hypothesis that ‘pond and plug’ type stream restoration projects have the capacity to re‐establish hydrological processes necessary to sustain riparian systems. In addition, the results of this study can be used to improve quantitative objectives for ‘pond and plug’ type stream restoration activities in similar settings. Copyright © 2008 John Wiley & Sons, Ltd.     

Fish assemblage change following the structural restoration of a degraded stream

Decades of anthropogenic pressure have harmed riverscapes throughout North America by degrading habitats and water quality and can result in the extirpation of sensitive aquatic taxa. Local stream restoration projects have increased in frequency, but monitoring is still infrequent. In 2010, Kickapoo Creek in East Central Illinois was subjected to a stream restoration project that included implementation of artificial riffles, riprap, scouring keys, and riparian vegetation. We monitored the restoration efforts for 6 years after the restoration through annual sampling efforts at restored and reference sites to determine changes in habitat and fish assemblage using standard habitat sampling and electrofishing techniques. We observed distinct temporal and spatial shifts in physico‐chemical parameters along with changes in fish community structure. Although biotic integrity remained moderately low in reference assemblages, restored reaches showed 3‐year delay in response to restoration, with biotic integrity positively linked to additional instream habitat and altered channel morphology. Larger substrate sizes, submerged terrestrial vegetation, and newly formed scour pools along with reduced siltation were found in the restored sites, in contrast to the reference sites. These changes resulted in increased species diversity, reduced number of opportunistic species and consequently an overall increase in health of fish communities. We also observed recruitment of habitat specialists and increase in species with reproductive strategies that rely on complex substrates. The results of this study highlight some of the complex dynamics driving reach‐scale restoration projects. We demonstrate the usefulness of structural restoration as a management tool to increase biotic integrity through long‐term alteration of critical habitat. The delay in the response of species to the restoration efforts emphasizes the need for long‐term continuous temporal and spatial monitoring.     

Low potential for restraint of anadromous salmonid reproduction by beaver Castor fiber in the Numedalslågen River catchment,Norway

After a long absence, beaver Castor fiber are rapidly returning to Europe. Their dam‐building and tree‐felling behaviour may have consequences for salmon Salmo salar and sea trout Salmo trutta management. In 2003 we investigated the parallel use of stream sections by beaver, sea trout and salmon and determined the potential hindrance that beaver dam‐building presented for reproducing salmon and sea trout along 65 km of the Numedalslågen River and tributaries, a major Norwegian catchment. We also surveyed landowner attitude to having beaver on salmon and sea trout streams. Most salmon spawned in the river and most sea trout in 51 tributaries. Nine of these tributaries also hosted spawning salmon. 15 (29%) of the 51 tributaries with spawning sea trout and six (67%) of the nine with spawning salmon had intermittently been occupied by beaver. Though beaver preferred to colonize the same sections of stream used for spawning, only 15% of the stream length navigable by salmon and sea trout on the 51 tributaries had actually been used by beaver, and only three colonies were occupied autumn 2003 (1 colony/25.0 km). Five dams were functioning during autumn 2003 on the 51 tributaries (1 dam/14.3 km). These potentially hindered sea trout and salmon from reaching 18% and 3%, respectively of their potential spawning habitat, though all dams were low (≤0.5 m). Though the autumn density of occupied beaver colonies along the river (1 colony/2.5 km) was 10.0 times the density on the 51 tributaries, no dams were built on the river. Thus most salmon reproduction in the catchment was unhindered by beaver. Nine of 14 landowners were unequivocally positive about having beaver together with salmon and sea trout. We conclude that the presence of beaver on similar catchments will likely have only an insignificant negative impact on the reproduction of sea trout and salmon. Copyright © 2007 John Wiley & Sons, Ltd.     

Influence of rainfall and beaver dams on upstream movement of spawning Atlantic salmon in a restored brook in Nova scotia,Canada

In a restored, third‐order stream in northern Nova Scotia, Canada, we used redd counts over 12 years to examine the influence of beaver dams and the timing and intensity of autumn rains on spawning activity of Atlantic salmon. Most beaver dams in most years had no detectable effect on the distribution of spawning redds, but in 2004 the density of redds downstream from a three‐dam complex was significantly greater than that above, suggesting the dams were a barrier to many fish. A second complex of dams blocked salmon passage completely in 2003 and 2004 until they were notched to provide access upstream. The length of stream used by salmon for spawning was linearly correlated with total precipitation in the basin in October plus November (R² = 0.60), to a ceiling of 325 mm, above which the fish had access to the entire brook, if beaver dams were notched. Number of redds in the whole brook was strongly correlated (R² = 0.94) with the coefficient of variation (CV) of daily rainfall in October, but only for 7 of 11 years. This relationship disappeared when the impassable beaver dam complex failed in 2005, allowing salmon free access to 4 km of the upper brook. Variation in rainfall, and hence discharge, in this flashy brook evidently influences migration and spawning of Atlantic salmon in conjunction with channel blocking by beaver dams. Copyright © 2009 John Wiley & Sons, Ltd.     

Physical and Biological Responses to an Alternative Removal Strategy of a Moderate‐sized Dam in Washington,USA

Dam removal is an increasingly practised river restoration technique, and ecological responses vary with watershed, dam and reservoir properties, and removal strategies. Moderate‐sized dams, like Hemlock Dam (7.9 m tall and 56 m wide), are large enough that removal effects could be significant, but small enough that mitigation may be possible through a modified dam removal strategy. The removal of Hemlock Dam in Washington State, USA, was designed to limit channel erosion and improve fish passage and habitat by excavating stored fine sediment and reconstructing a channel in the former 6‐ha reservoir. Prior to dam removal, summer daily water temperatures downstream from the dam increased and remained warm long into the night. Afterwards, a more natural diel temperature regime was restored, although daily maximum temperatures remained high. A short‐lived turbidity pulse occurred soon after re‐watering of the channel, but was otherwise similar to background levels. Substrate shifted from sand to gravel–cobble in the former reservoir and from boulder to gravel–cobble downstream of the dam. Initially, macroinvertebrate assemblage richness and abundance was low in the project area, but within 2 years, post‐removal reaches upstream and downstream of the dam had diverse and abundant communities. The excavation of stored sediment and channel restoration as part of the dam removal strategy restored river continuity and improved benthic habitat while minimizing downstream sedimentation. This study provides a comparison of ecological effects with other dam removal strategies and can inform expectations of response time and magnitude. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.     

LONG‐TERM MONITORING AND ASSESSMENT OF A STREAM RESTORATION PROJECT IN CENTRAL NEW YORK

Monitoring, assessment and reporting of stream restoration projects have historically lagged far behind implementation. However, in recent years, rigorous post‐project assessments (PPAs) of modern stream restoration practices have steadily increased. This has helped to stimulate debate and inquiry regarding the effectiveness of restoration techniques and has provided critical feedback to practitioners and planners useful in restoration design and implementation. Nonetheless, few studies exist that track the performance of modern restoration projects over a protracted period. Instead, most are based on a brief snapshot taken during the initial post‐construction period, which may not always accurately characterize longer‐term project performance. Here, we re‐visit a stream restoration project implemented in 2005 on a third‐order stream in central New York. By repeating several of our quantitative and qualitative evaluation procedures from the original 2007 PPA we demonstrate that (i) despite several recent large flood events and the fact that the current channel geometry differs from the design/as‐built configuration, the project has made substantial progress towards the goals of channel stabilization and habitat enhancement; (ii) this more favourable, mid‐term outcome was not necessarily evident during or well‐predicted by our 2007 PPA; (iii) although continued deformation of in‐stream structures may be a harbinger of future channel instability, riparian vegetation is playing an increasingly important role in maintaining channel stability; and (iv) accurately predicting local scour depths proximal to in‐stream structures, performing a detailed sediment budget analysis, and prescribing adequate bank protection are critical to project success, especially during early stages of a project. Copyright © 2013 John Wiley & Sons, Ltd.     

SEDIMENT DYNAMICS IN THE RESTORED REACH OF THE KISSIMMEE RIVER BASIN,FLORIDA: A VAST SUBTROPICAL RIPARIAN WETLAND

Historically, the Kissimmee River Basin consisted of a broad nearly annually inundated riparian wetland similar in character to tropical Southern Hemisphere large rivers. The river was channelized in the 1960s and 1970s, draining the wetland. The river is currently being restored with over 10 000 hectares of wetlands being reconnected to 70 river km of naturalized channel. We monitored riparian wetland sediment dynamics between 2007 and 2010 at 87 sites in the restored reach and 14 sites in an unrestored reference reach. Discharge and sediment transport were measured at the downstream end of the restored reach. There were three flooding events during the study, two as annual flood events and a third as a greater than a 5‐year flood event. Restoration has returned periodic flood flow to the riparian wetland and provides a mean sedimentation rate of 11.3 mm per year over the study period in the restored reach compared with 1.7 mm per year in an unrestored channelized reach. Sedimentation from the two annual floods was within the normal range for alluvial Coastal Plain rivers. Sediment deposits consisted of over 20% organics, similar to eastern blackwater rivers. The Kissimmee River is unique in North America for its hybrid alluvial/blackwater nature. Fluvial suspended‐sediment measurements for the three flood events indicate that a majority of the sediment (70%) was sand, which is important for natural levee construction. Of the total suspended sediment load for the three flood events, 3%–16% was organic and important in floodplain deposition. Sediment yield is similar to low‐gradient rivers draining to the Chesapeake Bay and alluvial rivers of the southeastern USA. Continued monitoring should determine whether observed sediment transport and floodplain deposition rates are normal for this river and determine the relationship between historic vegetation community restoration, hydroperiod restoration, and sedimentation. Published in 2011 by John Wiley & Sons, Ltd.     

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 Impacts of Natural Flood Management Approaches on In‐Channel Sediment Quality

Natural Flood Management (NFM) techniques aim to reduce downstream flooding by storing and slowing the flow of stormwater to river channels. These techniques include a range of measures, including setback stormwater outfalls and the physical restoration of channels and floodplains, to improve the natural functioning of catchments. An additional benefit of NFM measures is the potential reduction in sediment and pollutant delivery to the channel. Urban development releases a variety of heavy metal and nutrient pollutants that enter rivers through stormwater outfalls with adverse effects on the aquatic ecosystem. In this study, the influence of channel modification and quality of the river habitat on the sediment quality surrounding stormwater outfalls was assessed. Sediment samples were taken at several outfalls within the Johnson Creek catchment, Oregon, USA, and analysed for a variety of urban pollutants. The level of river habitat quality and modification at each site were assessed using a semi‐quantitative scoring methodology. Significant increases in pollutant levels were observed at outfalls, with a greater and more variable increase at direct compared to setback outfalls. Removal efficiency of certain pollutants was found to be significantly correlated to the level of habitat quality or modification (for Fe, Ba, Sn, Mg, P, K) indicating that more natural reaches had greater potential for pollutant removal. The findings highlight the multiple benefits associated with NFM and river restoration approaches in relation to sediment quality and pollutant content. © 2016 The Authors River Research and Applications Published by John Wiley & Sons, Ltd.     

Integrating remotely acquired and field data to assess effects of setback levees on riparian and aquatic habitats in glacial‐melt water rivers

Setback levees, in which levees are reconstructed at a greater distance from a river channel, are a promising restoration technique particularly for alluvial rivers with broad floodplains where river‐floodplain connectivity is essential to ecological processes. Documenting the ecological outcomes of restoration activities is essential for assessing the comparative benefits of different restoration approaches and for justifying new restoration projects. Remote sensing of aquatic habitats offers one approach for comprehensive, objective documentation of river and floodplain habitats, but is difficult in glacial rivers because of high suspended‐sediment concentrations, braiding and a lack of large, well‐differentiated channel forms such as riffles and pools. Remote imagery and field surveys were used to assess the effects of recent and planned setback levees along the Puyallup River and, more generally, the application of multispectral imagery for classifying aquatic and riparian habitats in glacial‐melt water rivers. Airborne images were acquired with a horizontal ground resolution of 0.5 m in three spectral bands (0.545–0.555, 0.665–0.675 and 0.790–0.810 µm) spanning from green to near infrared (NIR) wavelengths. Field surveys identified river and floodplain habitat features and provided the basis for a comparative hydraulic analysis. Broad categories of aquatic habitat (smooth and rough water surface), exposed sediment (sand and boulder) and vegetated surfaces (herbaceous and deciduous shrub/forest) were classified accurately using the airborne images. Other categories [e.g. conifers, boulder, large woody debris (LWD)] and subdivisions of broad categories (e.g. riffles and runs) were not successfully classified either because these features did not form large patches that could be identified on the imagery or their spectral reflectances were not distinct from those of other habitat types. Airborne imagery was critical for assessing fine‐scale aquatic habitat heterogeneity including shallow, low‐velocity regions that were not feasible or practical to map in the field in many cases due to their widespread distribution, small size and poorly defined boundaries with other habitat types. At the reach‐scale, the setback levee affected the amount and distribution of riparian and aquatic habitats: (1) the area of all habitats was greater where levees had been set back and with relatively more vegetated floodplain habitat and relatively less exposed sediment and aquatic habitat, (2) where levees confine the river, less low‐velocity aquatic habitat is present over a range of flows with a higher degree of bed instability during high flows. As river restoration proceeds in the Pacific Northwest and elsewhere, remotely acquired imagery will be important for documenting its effects on the amount and distribution of aquatic and floodplain habitats, complimenting field data as a quantitative basis for evaluating project efficacy. Copyright © 2008 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.     

Effects of Passive and Structural Stream Restoration Approaches on Transient Storage and Nitrate Uptake

Understanding nitrogen dynamics in headwater streams is important for ascertaining how they influence downstream nutrient loads and identifying strategies for reducing loading through stream restoration. We compare nitrate uptake associated with two restoration approaches in headwater streams, Sheep Creek and Nunn Creek, of northern Colorado, USA. Segments of Sheep Creek were exclosed (fenced off) from open rangeland cattle grazing in the 1950s, allowing riparian corridors of these segments to naturally revegetate (passive approach), while other segments have been continually grazed. In 2003, restoration structures including cross vanes, J‐hook vanes, rootwads, log vanes, and bank riprap (structural approach) were installed along portions of Nunn Creek for trout habitat enhancement and local bank stabilization. We performed detailed physical characterizations and multiple nutrient injections of Br^? and NO₃^? to estimate transient storage and nitrate uptake in four reaches along Sheep Creek (two reaches exclosed from grazing and two grazed reaches) and two reaches along Nunn Creek (one with restoration structures and one without structures). Parameters of transient storage and nitrate uptake were estimated with the one‐dimensional transport with inflow and storage model run through universal inverse modelling code for optimization. Responses of transient storage and nitrate uptake to restoration techniques depended upon the type and extent of restoration implemented, as well the context and physical setting of each study reach. For example, in the higher‐gradient pair of Sheep Creek reaches, the restored reach showed greater nitrate uptake, while in the lower‐gradient pair of reaches, the non‐restored reach had greater uptake. At Nunn Creek, the reach with instream wood but without restoration structures exhibited more transient storage and nitrate uptake when compared with the reach with extensive J‐hook vane structures. Copyright © 2016 John Wiley & Sons, Ltd.     

Soil seed banks of degraded riparian zones in southeastern Australia and their potential contribution to the restoration of understorey vegetation

Although soil seed banks are understood to be integral to the vegetation dynamics and restoration of many ecosystems, little is known of their role in riparian zones. In this study, we investigated soil seed banks of riparian zones of contrasting condition in an agricultural landscape and evaluated their potential to influence riparian restoration. We examined the composition and structure of germinable soil seed banks along lateral gradients from stream channels in both cleared and wooded riparian zones of three lowland creeks within the Goulburn Broken catchment in temperate southeastern Australia. Environmental correlates of soil seed bank characteristics and similarity to extant vegetation were also examined. We found an abundant and species‐rich soil seed bank mostly comprising propagules of perennial rushes and sedges and annual and perennial grasses with many species of annual forbs. While the majority of identifiable germinants and species were native, exotic species were common at all locations. Soil seed bank composition was relatively homogeneous among streams and along lateral gradients from the channel. Riparian condition (i.e. cleared or wooded), however, had a strong influence on soil seed bank composition and structure with cleared reaches containing more species, more germinable annual grasses and higher total numbers of germinable seeds. Soil seed bank composition was correlated with site openness suggesting that extant vegetation structure plays an important role in soil seed bank dynamics. Recruitment from the in situ soil seed bank will help restore only some components of the riparian plant community and may hinder restoration by introducing undesirable species. Copyright © 2008 John Wiley & Sons, Ltd.     

The Disconnected Sediment Conveyor Belt: Patterns of Longitudinal and Lateral Erosion and Deposition During a Catastrophic Flood in the Lockyer Valley,South East Queensland,Australia

The sediment (dis)connectivity concept is the water‐mediated transfer of sediment between different compartments of a catchment sediment cascade involving four possible dimensions or linkages (longitudinal, lateral, vertical and temporal). Quantifying the strength of these linkages within and between compartments provides a means to understand the internal sediment flux dynamics of a catchment. The aims of this paper are to examine (1) the dynamics of longitudinal and lateral (dis)connectivity by quantifying patterns of erosion and deposition that occurred during a catastrophic flood, and (2) how the patterns of connectivity can be changed through management actions that better utilise floodplain sediment storages. Multi‐temporal LiDAR and air photos are used to quantify volumetric change with respect to geomorphic settings and units. The results show that over the length of the trunk stream, the high‐magnitude event was net depositional with high longitudinal sediment disconnectivity. At the reach scale, an alternating pattern of high and low longitudinal connectivity associated with contraction and expansion zones was evident. The efficiency of sediment transfer from the uppermost compartment to the most downstream compartment decreased exponentially, while the strength of lateral connectivity increased for each expansion reach. Modelling results show that increasing channel boundary roughness along expansion reaches with riparian revegetation can increase the frequency of lateral connectivity and floodplain sediment storage, thereby decreasing reach‐to‐reach connectivity and reducing end‐of‐catchment sediment delivery. This contrasts with the current trend of building levees along the bank tops of expansion reaches, which decrease lateral connectivity and increase reach‐to‐reach connectivity. Copyright © 2015 John Wiley & Sons, Ltd.     

LARGE‐WOOD LOADING FROM NATURAL AND ENGINEERED PROCESSES AT THE WATERSHED SCALE

Large wood, both live and dead, is essential for producing complex habitat in many streams, especially in forested watersheds that support salmonid populations. The addition of engineered wood structures is a common approach taken in many streams where past watershed management practices have resulted in reduced wood loading. We examined six 300‐m stream reaches in the Lagunitas Creek watershed, Northern California, to determine (i) the distribution of large wood in the bankfull channel and 10‐year floodplain, (ii) the influence of large wood on the size and distribution of pools and (iii) whether streams with engineered wood structures had greater diversity of pool habitat to support salmonid populations. We found that the amount of large wood in the bankfull channel and the amount available for recruitment from the 10‐year floodplain were highly variable among and within reaches examined and largely dependent on the local geomorphic setting. Stream reaches with engineered wood structures had elevated pool frequencies relative to reaches without these structures, suggesting a higher capacity to support salmonids during critical life stages. Among large wood pieces that had a strong influence on pool formation, 23% had an attached root wad and 66% were part of a cluster. All of the study reaches we examined had lower volumes of large wood in their bankfull channels than similar stream types with natural wood‐loading levels, suggesting that increased additions of large wood could provide ecosystem benefits over time. These principles can be understood and transferred effectively to other watersheds using a framework of wood‐loading process domains. Copyright © 2012 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.