Effects of stream channel morphology, transport processes and effective discharge on salmonid spawning habitats

Research to understand the drivers of river form and processes has focussed on alluvial sand and gravel-bed channels. However, boulder-bed rivers are also an abundant channel type, particularly in previously glaciated and mountainous regions. Understanding boulder distribution in rivers is important because of their effects on &#160;channel hydraulics and sediment transport processes. Boulder-bed channels in low-relief, previously glaciated landscapes may be considered semi-alluvial since the boulders likely were not deposited by fluvial processes (unlike in e.g., step-pool mountain channels). However, the relative importance of glacial legacy sediment and fluvial processes as drivers of boulder-bed river morphology is poorly understood. This is especially true in northern Sweden where channel clearance for timber floating has resulted in the removal of boulders from most rivers. Restoration of these rivers involves the replacement of boulders but is challenged by a lack of geomorphological understanding.This study aimed to quantify the morphological characteristics of northern Swedish boulder-bed streams and determine the association between fluvial and glacial legacy controls on these channels. We undertook a large-scale field campaign surveying 20 rivers (drainage area: 15 - 112 km2) that have not been cleared for timber floating. At each reach, we measured channel morphology using a total station over approximately 100 m river length, surveying the channel planform, thalweg and 5 cross sections. In addition, we measured the location, diameter and protrusion of every boulder (> D84) within each reach. We also conducted a survey of the size and density of boulders on the floodplain to compare to in-channel boulder distributions. We coupled this field campaign with analysis of digital elevation models, surficial geology, glacial landform maps, and hydrological data to investigate potential landscape controls on reach-scale geomorphology. Associations between drainage area, channel slope, width and D84&#160;as well as longitudinal clustering of boulders into fluvial bed-forms would indicate fluvial rather than legacy glacial drivers.Preliminary results show high variability in the morphology of reference sites, from low-gradient reaches with high floodplain connectivity to steep and narrow channels (Slope ranged 1.1 - 8.8%). D84&#160;ranged from 0.4 m to 2.1 m with some sites having as many as 500 large boulders (> 1 m diameter) in a 100 m reach. D84&#160;was not associated with channel slope and boulders were not clustered longitudinally in most reaches. This suggests that boulder spacing is the result of glacial legacy controls. These results are important for understanding geomorphic processes in boulder-bed channels and how channel form relates to reach- and landscape-scale controls. The relative importance of fluvial versus glacial legacy controls on boulder-bed channel morphology is also important to help restoration practitioners more accurately identify reference states of boulder-bed channels in previously glaciated landscapes.

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Modelling braided river morphodynamics using a particle travel length framework

10.5194/esurf-2018-17 ◽

2018 ◽

Cited By ~ 1

Author(s):

Alan Kasprak ◽

James Brasington ◽

Konrad Hafen ◽

Richard D. Williams ◽

Joseph M. Wheaton

Keyword(s):

Path Length ◽

Numerical Models ◽

Hybrid Approach ◽

Length Distribution ◽

Transport Processes ◽

Model Verification ◽

Event Scale ◽

Effective Discharge ◽

Volumetric Assessment ◽

Decadal Scale

Abstract. Numerical models that predict channel evolution are an essential tool for investigating processes that occur over timescales which render field observation intractable. The current generation of morphodynamic models, however, either oversimplify the relevant physical processes, or in the case of more physically-complete CFD based codes, have computational overheads that restrict severely the space-time scope of their application. Here we present a new, open-source, hybrid approach that seeks to reconcile these modelling philosophies. This framework combines steady-state, two-dimensional CFD hydraulics with a rule-based sediment transport algorithm to predict particle mobility and transport paths which are used to route sediment and evolve the bed topography. Data from two contrasting natural braided rivers (Rees, New Zealand and Feshie, United Kingdom) were used for multi-scalar model verification incorporating reach-scale quantitative morphological change budgets and volumetric assessment of different braiding mechanisms. The model was able to simulate eight of ten empirically observed braiding mechanisms from the parameterized bed erosion, transport, and deposition. Representation of bank erosion and bar edge trimming necessitated the inclusion of a lateral channel migration algorithm. Comparisons between simulations based on steady effective discharge versus event hydrographs represented as a series of steady states were found to only marginally increase the predicted volumetric change, with greater deposition offsetting erosion. A decadal-scale simulation indicates that accurate prediction of event-scale scour depth and subsequent deposition present a methodological challenge because the predicted pattern of deposition may never catch up to erosion if a simple path-length distribution is employed, thus resulting in channel over-scouring. It may thus be necessary to augment path length distributions to preferentially deposit material in certain geomorphic units. We anticipate that the model presented here will be used as a modular framework to explore the effect of different process representations, and as a learning tool designed to reveal the relative importance of geomorphic transport processes in rivers at multiple timescales.

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Ecological impacts on aquatic macroinvertebrates following upland stream invasion by a ponded pasture grass (Glyceria maxima) in southern Australia

Marine and Freshwater Research ◽

10.1071/mf04043 ◽

2004 ◽

Vol 55 (7) ◽

pp. 709 ◽

Cited By ~ 26

Author(s):

Amber Clarke ◽

P. S. Lake ◽

Dennis J. O'Dowd

Keyword(s):

Ecosystem Engineer ◽

High Capacity ◽

Ecosystem Processes ◽

Channel Morphology ◽

Ecological Impacts ◽

Freshwater Wetlands ◽

Stream Channel ◽

Pasture Grass ◽

Functional Feeding Group ◽

Sediment Matrix

Glyceria maxima (Reed sweet-grass), an emergent aquatic grass native to Eurasia, has naturalised in North America, New Zealand and southern Australia. Introduced as a ponded pasture species, it threatens native biodiversity and ecosystem processes in freshwater wetlands and waterways. We compared paired sections, either invaded or not invaded by G. maxima, of three upland streams in Gippsland, Victoria, Australia to infer its impacts on macroinvertebrate abundance, morphospecies density, and morphospecies and functional feeding group (FFG) composition. Its potential effects in altering sedimentation were explored by comparing stream channel morphology in paired stream sections at one site. Invasion by G. maxima appears to drive changes in macroinvertebrate morphospecies composition and FFG composition, reducing a diverse array of macroinvertebrates to a more uniform fauna. The estimated volume of plant/sediment matrix in a 50 m invaded stream section was around 1100 m3, nearly 15-fold greater than in an adjacent uninvaded stream section. Glyceria maxima is an autogenic ecosystem engineer, with the ability to convert sections of fast-flowing aerobic streams into partially anaerobic swamps. By generating a root-mat swamp with a high capacity to process nutrients, G. maxima may facilitate its own growth and spread, as well as that of secondary invaders.

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The impacts of ski slope development on stream channel morphology in the White River National Forest, Colorado, USA

Geomorphology ◽

10.1016/j.geomorph.2008.07.003 ◽

2009 ◽

Vol 103 (3) ◽

pp. 375-388 ◽

Cited By ~ 18

Author(s):

Gabrielle C.L. David ◽

Brian P. Bledsoe ◽

David M. Merritt ◽

Ellen Wohl

Keyword(s):

Channel Morphology ◽

National Forest ◽

Stream Channel ◽

White River

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