Rocko Brown - Academia.edu (original) (raw)

Papers by Rocko Brown

ABSTRACT The description of fluvial form has evolved from anecdotal descriptions to artistic rend... more ABSTRACT The description of fluvial form has evolved from anecdotal descriptions to artistic renderings to 2D plots of cross section or longitudinal profiles and more recently 3D digital models. Synthetic river valleys, artificial 3D topographic models of river topography, have a plethora of potential applications in fluvial geomorphology, and the earth sciences in general, as well as in computer science and ecology. Synthetic river channels have existed implicitly since approximately the 1970s and can be simulated from a variety of approaches spanning the artistic and numerical. An objective method of synthesizing 3D stream topography based on reach scale attributes would be valuable for sizing 3D flumes in the physical and numerical realms, as initial input topography for morphodynamic models, stream restoration design, historical reconstruction, and mechanistic testing of interactions of channel geometric elements. Quite simply - simulation of synthetic channel geometry of prescribed conditions can allow systematic evaluation of the dominant relationships between river flow and geometry. A new model, the control curve method, is presented that uses hierarchically scaled parametric curves in over-lapping 2D planes to create synthetic river valleys. The approach is able to simulate 3D stream geometry from paired 2D descriptions and can allow experimental insight into form-process relationships in addition to visualizing past measurements of channel form that are limited to two dimension descriptions. Results are presented that illustrate the models ability to simulate fluvial topography representative of real world rivers as well as how channel geometric elements can be adjusted. The testing of synthetic river valleys would open up a wealth of knowledge as to why some 3D attributes of river channels are more prevalent than others as well as bridging the gap between the 2D descriptions that have dominated fluvial geomorphology the past century and modern, more complete, 3D treatments.

ABSTRACT An emerging view in fluvial geomorphology is that rivers are systems with multiple scale... more ABSTRACT An emerging view in fluvial geomorphology is that rivers are systems with multiple scales of variability. This is especially true in mountain rivers that have spatially variable alluvial-bedrock boundaries as multiple scales of topography, from individual boulders to valley scale deposits, can steer flow paths affecting the erosion and deposition patterns of transported sediments. We hypothesize that depending on flow discharge and stage, different scales of channel topography can become dominant in routing sediment such that the resulting topography is a layered sequence of past flows. Here we evaluate gravel and cobble channel change associated with hydraulic unit to reach scale (e.g. 10-1-102 channel widths) changes in topographic and stage-dependent hydrodynamic variability in a mixed alluvial-bedrock river canyon. This study takes advantage of a unique opportunity where 4,535 metric tons of gravel ranging from 6-128 mm was augmented directly below a dam for spawning habitat rehabilitation in a 1,200 m long mountain river reach with no other sources of gravel sediment supply and an existing substrate of bedrock, large cobbles (>250mm), angular shot rock (>0.5m) and boulders (>1m). While the study site is a regulated river flows above 117 m3/s, just below the bankfull discharge, still overtop the dam so natural aspects of the hydrograph are still retained such that the reach still experiences large floods capable of considerable topographic change. We utilize kite-blimp aerial photography, kayak-based surveying, topographic change detection, and 2D modeling to understand how flow discharge can activate topographic features that ultimately control channel change following a controlled gravel injection upstream of a mountain river with no other gravel or cobble sediment inputs. The spatial covariance of flow width and bed elevation are strongly associated with the volume of gravel deposition and erosion, but this also changes depending on flow discharge as different topographic elements become more effective in controlling hydrodynamics. Similarly, analyzing changes in 2D model derived flow directions shows that different flows can effectively push gravel and cobble into bedrock protrusions that at higher flows, become disconnected from the main flow routing in the river canyon. Results from this study suggest that both existing channel boundary variability and input hydrologic variability work together to create hydrodynamics spatial patterns that control the fate and transport of sediments in mountain rivers and ultimately their spatial structure.

Copeia

Phenotypic variation within a species can be associated with genetic differentiation and varying ... more Phenotypic variation within a species can be associated with genetic differentiation and varying environmental conditions among populations and their habitats. The relationship between environmenta...

Earth-Science Reviews

Abstract There has been an increasing practice of creating Earth-like, realistic synthetic landsc... more Abstract There has been an increasing practice of creating Earth-like, realistic synthetic landscapes by Earth scientists and computer scientists for a variety of applications. While together these two fields have made significant scientific and social contributions to creating synthetic landscapes, it is presently infeasible to build artificial digital rivers that represent the diversity found on Earth. To understand and summarize the state of the science of rendering artificial river topography, we reviewed >225 scientific articles and produced a road map for artificial synthesis of digital river topography. We broadly classify methods of digital river synthesis by whether they are driven by expert-based decisions or are strategic in the use of rules for objective rendering, with some rules being physics-based theories of river morphogenesis. Expert approaches include map, brush, geometric and interactive design. Strategic approaches include deterministic equilibrium models, morphodynamic models, and stochastic approaches. For each approach we discuss the conceptual basis for each method and how they can be applied. Readers can then identify what methods can create different types of digital riverscapes. We close by discussing how cross pollination can serve geomorphology and computer science, the role of digital rivers in furthering geoscience progress, and future directions in digital river synthesis.

Past river classifications use incommensurate typologies at each spatial scale and do not capture... more Past river classifications use incommensurate typologies at each spatial scale and do not capture the pivotal role of topographic variability at each scale in driving the morphodynamics responsible for evolving hierarchically nested fluvial landforms. This study developed a new way to create geomorphic classifications using metrics diagnostic of individual processes the same way at every spatial scale and spanning a wide range of scales. We tested the approach on flow convergence routing, a geomorphically and ecologically important process with different morphodynamic states of erosion, routing, and deposition depending on the structure of nondimensional topographic variability. Five nondimensional landform types with unique functionality represent this process at any flow; they are nozzle, wide bar, normal channel, constricted pool, and oversized. These landforms are then nested within themselves by considering their longitudinal sequencing at key flows representing geomorphically ...

Earth Surface Dynamics Discussions

This paper demonstrates a relatively new method of analysis for stage dependent patterns in meter... more This paper demonstrates a relatively new method of analysis for stage dependent patterns in meter-scale resolution river DEMs, termed geomorphic covariance structures (GCSs). A GCS is a univariate and/or bivariate spatial relationship amongst or between variables along a pathway in a river corridor. Variables assessed can be flow independent measures of topography (e.g., bed elevation, centerline curvature, and cross section asymmetry) and sediment size as well as flow dependent hydraulics (e.g., top width, depth, velocity, and shear stress; Brown, 2014), topographic change, and biotic variables (e.g., biomass and habitat utilization). The GCS analysis is used to understand if and how the covariance of bed elevation and flow-dependent channel top width are organized in a partially confined, incising gravel-cobbled bed river with multiple spatial scales of anthropogenic and natural landform heterogeneity across a range of discharges through a suite of spatial series analyses on 6.4 k...

Understanding the spatial organization of river systems in light of natural and anthropogenic cha... more Understanding the spatial organization of river systems in light of natural and anthropogenic change is extremely important because it can provide information to assess, manage, and restore them to ameliorate worldwide freshwater fauna declines. For gravel-and cobble-bedded alluvial rivers studies spanning analytical, empirical and numerical domains suggest that at channel-forming flows there is a tendency towards covarying bankfull bed and width undulations amongst morphologic units such as pools and riffles, whereby relatively wide areas have relatively higher minimum bed elevations and relatively narrow areas have relatively lower minimum bed elevations. The goal of this study was to determine whether minimum bed elevation and flow-dependent channel top width are organized in a partially confined, incising gravel–cobbled bed river with multiple spatial scales of anthropogenic and natural landform heterogeneity across a range of discharges. A key result is that the test river exhi...

Environmental Management, 2016

The presentation of Fig. 1e was incorrect in the original publication of the article. The correct... more The presentation of Fig. 1e was incorrect in the original publication of the article. The corrected Fig. 1 is given below. The original version of this article was revised.

In restoring channels and floodplains for ecological species, scientists and practitioners need t... more In restoring channels and floodplains for ecological species, scientists and practitioners need to be able to unify the analysis and design of these vital landforms. This talk will discuss how varying levels of channel and floodplain syncing can be assessed and designed using a suite of conventional and advanced tools. Channel-floodplain syncing is when topography and flow regime are coupled in a quasi-equilibrium state in harmony with the surrounding landscape and land use. From this, we can identify synchronous channel-floodplain systems, that is, those that are topographically linked to their flow and sediment regimes, from asynchronous ones. In this talk I will show several real world examples of synchronous and asynchronous channel-floodplain systems from California and abroad. Next, I will go over several ways we can assess how “synced” channels are with their floodplains. Beyond assessing channel-floodplain dynamics, we also need to apply our knowledge of functional floodplai...

Freshwater flora and fauna have been declining worldwide due to anthropogenic alterations to Eart... more Freshwater flora and fauna have been declining worldwide due to anthropogenic alterations to Earth’s rivers and streams. To address this, rivers and streams are often restored by modifying or removing human induced stressors. River restoration in the form of topographic manipulation is often needed to remove or modify constraints that limit the potential for natural adjustments in channel topography that create and maintain physical habitat. The practice of river restoration has become a billion dollar industry nationally in the US alone, but is highly criticized as being ineffective because often the scientific foundations of restoration actions are unclear. Process based river restoration is an ideology based on restoring the fundamental processes that shape and maintain river systems. While process based restoration has a strong conceptual basis in achieving successful restoration, it still lacks the specificity needed for restoring rivers and streams through topographic manipula...

ABSTRACT In linking flow and form fluvial geomorphologists have established that alluvial river t... more ABSTRACT In linking flow and form fluvial geomorphologists have established that alluvial river topography can have specific geometric configurations related to processes that shape these landforms. For lowland alluvial rivers where the flow depth associated with channel changing events is greater than the median particle size it is common that topographic high points have a tendency to be located in wider areas within the stream corridor, while topographic low points such as pools tend to be found in narrower locations. In mountain rivers that have bedrock controls or boulders there can be obvious exceptions to this, especially when large particles lock in constrictions, as the channel boundaries often have a resistance greater than that provided by flow. However, many of these observations are limited in that they are either based on detailed morphologic unit scale (e.g. several channel widths) observations or reach and segment scale observations that have considerably less detail and resolution. We hypothesize that mountain and lowland rivers show contrasting covariance and coherence of stage dependent width and bed elevation. To assess this hypothesis we present preliminary results from the analysis of the longitudinal bed elevation and stage dependent flow width spatial series extracted from a highly detailed 36-km stretch of the Lower Yuba River and 12-km of one of its tributaries of the South Fork Yuba River. These rivers represent contrasting physiographic environments with variations in flow hydrology and boundary materials making up the channel bed and bank resulting in varying stream morphologies. Spatial covariance between the thalweg series and width series at various water stages in both rivers along with power spectral density and coherence estimates are used to show that in alluvial and mountain rivers patterns of flow width and bed elevation can emerge but vary in there spatial consistency and frequency that they are engaged by the flow record. Preliminary results suggest that channel form can manifest varying degrees of geometric organization due to varying environmental controls and subsequent fluvial processes that operate in these settings.

ABSTRACT An emerging view in fluvial geomorphology is that rivers are systems with multiple scale... more ABSTRACT An emerging view in fluvial geomorphology is that rivers are systems with multiple scales of variability. This is especially true in mountain rivers that have spatially variable alluvial-bedrock boundaries as multiple scales of topography, from individual boulders to valley scale deposits, can steer flow paths affecting the erosion and deposition patterns of transported sediments. We hypothesize that depending on flow discharge and stage, different scales of channel topography can become dominant in routing sediment such that the resulting topography is a layered sequence of past flows. Here we evaluate gravel and cobble channel change associated with hydraulic unit to reach scale (e.g. 10-1-102 channel widths) changes in topographic and stage-dependent hydrodynamic variability in a mixed alluvial-bedrock river canyon. This study takes advantage of a unique opportunity where 4,535 metric tons of gravel ranging from 6-128 mm was augmented directly below a dam for spawning habitat rehabilitation in a 1,200 m long mountain river reach with no other sources of gravel sediment supply and an existing substrate of bedrock, large cobbles (>250mm), angular shot rock (>0.5m) and boulders (>1m). While the study site is a regulated river flows above 117 m3/s, just below the bankfull discharge, still overtop the dam so natural aspects of the hydrograph are still retained such that the reach still experiences large floods capable of considerable topographic change. We utilize kite-blimp aerial photography, kayak-based surveying, topographic change detection, and 2D modeling to understand how flow discharge can activate topographic features that ultimately control channel change following a controlled gravel injection upstream of a mountain river with no other gravel or cobble sediment inputs. The spatial covariance of flow width and bed elevation are strongly associated with the volume of gravel deposition and erosion, but this also changes depending on flow discharge as different topographic elements become more effective in controlling hydrodynamics. Similarly, analyzing changes in 2D model derived flow directions shows that different flows can effectively push gravel and cobble into bedrock protrusions that at higher flows, become disconnected from the main flow routing in the river canyon. Results from this study suggest that both existing channel boundary variability and input hydrologic variability work together to create hydrodynamics spatial patterns that control the fate and transport of sediments in mountain rivers and ultimately their spatial structure.

An Integrated Approach, 2013

Riffle construction is a common practice in river engineering, but insufficient science exists to... more Riffle construction is a common practice in river engineering, but insufficient science exists to guide objective design of riffle-pool relief and the three-dimensional forms of features smaller than the scale of channel width. In this study, numerical experimentation with two-dimensional hydrodynamic modeling and ecohydraulic analysis was used to evaluate the performance of six different configurations of a sequence of riffle-pool units typical of shallow, regulated gravel-bed rivers, emphasizing a range of riffle-pool amplitudes (e.g., low, intermediate with hybrid features, and high). Twenty-two specific performance indicators (16 for physical habitat and six sediment-transport regime) were used to compare designs. It was found that low riffle-pool relief yielded the best performance for the majority of physical habitat indicators and all of the sediment transport regime indicators. The spatial patterns of test metrics revealed the mechanisms responsible for the statistical outcomes. Methodologically, two-dimensional modeling and ecohydraulic analysis are vital tools in project design along with previously accepted hydrologic, geomorphic, and engineering analyses. Scientifically, low-relief riffle-pool units are indicated as the normative condition in gravel-bed rivers where forcing elements driving deep scour are not systematically controlling morphology.

Journal of Hydrology, 2014

The relationships between flow hydrology, topography, and channel change in mountain rivers is im... more The relationships between flow hydrology, topography, and channel change in mountain rivers is important to understanding landscape evolution, the structure and persistence of aquatic habitat, and also the physiochemical cycling of upstream derived organic and inorganic materials. There is a paucity of detailed studies that analyze the joint roles of hydrology and topography in controlling multiple mechanisms of channel change in mountain rivers. In this study, gravel and cobble channel change in a bedrock river canyon were analyzed in light of a controlled yet natural experiment where 4,491 metric tonnes of rounded gravel and cobble was augmented below a sediment-barrier dam in a 1,200 m long mountain river reach that had no prior sources of rounded gravel or cobble and still experiences floods above the bankfull discharge. The overall study goal was to investigate how flow hydrology can modulate multiple channel change processes depending on the topographic features engaged by the flow. Channel change was assessed via differencing of high resolution repeat topographic and bathymetric surveys, along with cm-scale aerial photography post injection. Statistical tests used to implicate topographic feature-specific mechanisms of channel change that vary with discharge included analyzing geomorphic covariance structures of flow dependent width, bed elevation, and channel change as well as autocorrelation of flow width spatial series. Stage dependent topographic steering was inferred from associations of erosion and deposition with changes in 2D model derived flow directions at multiple discharges. A variety of mechanisms of channel change were qualitatively and quantitatively confirmed including particle hiding, topographic steering, eddying, and flow convergence. No single mechanism explained the observed patterns of channel change but rather it is thought that process-blending occurs, as modulated by the interactions of flow hydrology with complex topography. Results from this study suggest that both existing channel boundary variability and input hydrologic variability work together to create hydrodynamic spatial patterns that control the fate and transport of sediments in mountain rivers and ultimately their spatial structure.

ABSTRACT The description of fluvial form has evolved from anecdotal descriptions to artistic rend... more ABSTRACT The description of fluvial form has evolved from anecdotal descriptions to artistic renderings to 2D plots of cross section or longitudinal profiles and more recently 3D digital models. Synthetic river valleys, artificial 3D topographic models of river topography, have a plethora of potential applications in fluvial geomorphology, and the earth sciences in general, as well as in computer science and ecology. Synthetic river channels have existed implicitly since approximately the 1970s and can be simulated from a variety of approaches spanning the artistic and numerical. An objective method of synthesizing 3D stream topography based on reach scale attributes would be valuable for sizing 3D flumes in the physical and numerical realms, as initial input topography for morphodynamic models, stream restoration design, historical reconstruction, and mechanistic testing of interactions of channel geometric elements. Quite simply - simulation of synthetic channel geometry of prescribed conditions can allow systematic evaluation of the dominant relationships between river flow and geometry. A new model, the control curve method, is presented that uses hierarchically scaled parametric curves in over-lapping 2D planes to create synthetic river valleys. The approach is able to simulate 3D stream geometry from paired 2D descriptions and can allow experimental insight into form-process relationships in addition to visualizing past measurements of channel form that are limited to two dimension descriptions. Results are presented that illustrate the models ability to simulate fluvial topography representative of real world rivers as well as how channel geometric elements can be adjusted. The testing of synthetic river valleys would open up a wealth of knowledge as to why some 3D attributes of river channels are more prevalent than others as well as bridging the gap between the 2D descriptions that have dominated fluvial geomorphology the past century and modern, more complete, 3D treatments.

ABSTRACT An emerging view in fluvial geomorphology is that rivers are systems with multiple scale... more ABSTRACT An emerging view in fluvial geomorphology is that rivers are systems with multiple scales of variability. This is especially true in mountain rivers that have spatially variable alluvial-bedrock boundaries as multiple scales of topography, from individual boulders to valley scale deposits, can steer flow paths affecting the erosion and deposition patterns of transported sediments. We hypothesize that depending on flow discharge and stage, different scales of channel topography can become dominant in routing sediment such that the resulting topography is a layered sequence of past flows. Here we evaluate gravel and cobble channel change associated with hydraulic unit to reach scale (e.g. 10-1-102 channel widths) changes in topographic and stage-dependent hydrodynamic variability in a mixed alluvial-bedrock river canyon. This study takes advantage of a unique opportunity where 4,535 metric tons of gravel ranging from 6-128 mm was augmented directly below a dam for spawning habitat rehabilitation in a 1,200 m long mountain river reach with no other sources of gravel sediment supply and an existing substrate of bedrock, large cobbles (>250mm), angular shot rock (>0.5m) and boulders (>1m). While the study site is a regulated river flows above 117 m3/s, just below the bankfull discharge, still overtop the dam so natural aspects of the hydrograph are still retained such that the reach still experiences large floods capable of considerable topographic change. We utilize kite-blimp aerial photography, kayak-based surveying, topographic change detection, and 2D modeling to understand how flow discharge can activate topographic features that ultimately control channel change following a controlled gravel injection upstream of a mountain river with no other gravel or cobble sediment inputs. The spatial covariance of flow width and bed elevation are strongly associated with the volume of gravel deposition and erosion, but this also changes depending on flow discharge as different topographic elements become more effective in controlling hydrodynamics. Similarly, analyzing changes in 2D model derived flow directions shows that different flows can effectively push gravel and cobble into bedrock protrusions that at higher flows, become disconnected from the main flow routing in the river canyon. Results from this study suggest that both existing channel boundary variability and input hydrologic variability work together to create hydrodynamics spatial patterns that control the fate and transport of sediments in mountain rivers and ultimately their spatial structure.

Copeia

Phenotypic variation within a species can be associated with genetic differentiation and varying ... more Phenotypic variation within a species can be associated with genetic differentiation and varying environmental conditions among populations and their habitats. The relationship between environmenta...

Earth-Science Reviews

Abstract There has been an increasing practice of creating Earth-like, realistic synthetic landsc... more Abstract There has been an increasing practice of creating Earth-like, realistic synthetic landscapes by Earth scientists and computer scientists for a variety of applications. While together these two fields have made significant scientific and social contributions to creating synthetic landscapes, it is presently infeasible to build artificial digital rivers that represent the diversity found on Earth. To understand and summarize the state of the science of rendering artificial river topography, we reviewed >225 scientific articles and produced a road map for artificial synthesis of digital river topography. We broadly classify methods of digital river synthesis by whether they are driven by expert-based decisions or are strategic in the use of rules for objective rendering, with some rules being physics-based theories of river morphogenesis. Expert approaches include map, brush, geometric and interactive design. Strategic approaches include deterministic equilibrium models, morphodynamic models, and stochastic approaches. For each approach we discuss the conceptual basis for each method and how they can be applied. Readers can then identify what methods can create different types of digital riverscapes. We close by discussing how cross pollination can serve geomorphology and computer science, the role of digital rivers in furthering geoscience progress, and future directions in digital river synthesis.

Past river classifications use incommensurate typologies at each spatial scale and do not capture... more Past river classifications use incommensurate typologies at each spatial scale and do not capture the pivotal role of topographic variability at each scale in driving the morphodynamics responsible for evolving hierarchically nested fluvial landforms. This study developed a new way to create geomorphic classifications using metrics diagnostic of individual processes the same way at every spatial scale and spanning a wide range of scales. We tested the approach on flow convergence routing, a geomorphically and ecologically important process with different morphodynamic states of erosion, routing, and deposition depending on the structure of nondimensional topographic variability. Five nondimensional landform types with unique functionality represent this process at any flow; they are nozzle, wide bar, normal channel, constricted pool, and oversized. These landforms are then nested within themselves by considering their longitudinal sequencing at key flows representing geomorphically ...

Earth Surface Dynamics Discussions

This paper demonstrates a relatively new method of analysis for stage dependent patterns in meter... more This paper demonstrates a relatively new method of analysis for stage dependent patterns in meter-scale resolution river DEMs, termed geomorphic covariance structures (GCSs). A GCS is a univariate and/or bivariate spatial relationship amongst or between variables along a pathway in a river corridor. Variables assessed can be flow independent measures of topography (e.g., bed elevation, centerline curvature, and cross section asymmetry) and sediment size as well as flow dependent hydraulics (e.g., top width, depth, velocity, and shear stress; Brown, 2014), topographic change, and biotic variables (e.g., biomass and habitat utilization). The GCS analysis is used to understand if and how the covariance of bed elevation and flow-dependent channel top width are organized in a partially confined, incising gravel-cobbled bed river with multiple spatial scales of anthropogenic and natural landform heterogeneity across a range of discharges through a suite of spatial series analyses on 6.4 k...

Understanding the spatial organization of river systems in light of natural and anthropogenic cha... more Understanding the spatial organization of river systems in light of natural and anthropogenic change is extremely important because it can provide information to assess, manage, and restore them to ameliorate worldwide freshwater fauna declines. For gravel-and cobble-bedded alluvial rivers studies spanning analytical, empirical and numerical domains suggest that at channel-forming flows there is a tendency towards covarying bankfull bed and width undulations amongst morphologic units such as pools and riffles, whereby relatively wide areas have relatively higher minimum bed elevations and relatively narrow areas have relatively lower minimum bed elevations. The goal of this study was to determine whether minimum bed elevation and flow-dependent channel top width are organized in a partially confined, incising gravel–cobbled bed river with multiple spatial scales of anthropogenic and natural landform heterogeneity across a range of discharges. A key result is that the test river exhi...

Environmental Management, 2016

The presentation of Fig. 1e was incorrect in the original publication of the article. The correct... more The presentation of Fig. 1e was incorrect in the original publication of the article. The corrected Fig. 1 is given below. The original version of this article was revised.

In restoring channels and floodplains for ecological species, scientists and practitioners need t... more In restoring channels and floodplains for ecological species, scientists and practitioners need to be able to unify the analysis and design of these vital landforms. This talk will discuss how varying levels of channel and floodplain syncing can be assessed and designed using a suite of conventional and advanced tools. Channel-floodplain syncing is when topography and flow regime are coupled in a quasi-equilibrium state in harmony with the surrounding landscape and land use. From this, we can identify synchronous channel-floodplain systems, that is, those that are topographically linked to their flow and sediment regimes, from asynchronous ones. In this talk I will show several real world examples of synchronous and asynchronous channel-floodplain systems from California and abroad. Next, I will go over several ways we can assess how “synced” channels are with their floodplains. Beyond assessing channel-floodplain dynamics, we also need to apply our knowledge of functional floodplai...

Freshwater flora and fauna have been declining worldwide due to anthropogenic alterations to Eart... more Freshwater flora and fauna have been declining worldwide due to anthropogenic alterations to Earth’s rivers and streams. To address this, rivers and streams are often restored by modifying or removing human induced stressors. River restoration in the form of topographic manipulation is often needed to remove or modify constraints that limit the potential for natural adjustments in channel topography that create and maintain physical habitat. The practice of river restoration has become a billion dollar industry nationally in the US alone, but is highly criticized as being ineffective because often the scientific foundations of restoration actions are unclear. Process based river restoration is an ideology based on restoring the fundamental processes that shape and maintain river systems. While process based restoration has a strong conceptual basis in achieving successful restoration, it still lacks the specificity needed for restoring rivers and streams through topographic manipula...

ABSTRACT In linking flow and form fluvial geomorphologists have established that alluvial river t... more ABSTRACT In linking flow and form fluvial geomorphologists have established that alluvial river topography can have specific geometric configurations related to processes that shape these landforms. For lowland alluvial rivers where the flow depth associated with channel changing events is greater than the median particle size it is common that topographic high points have a tendency to be located in wider areas within the stream corridor, while topographic low points such as pools tend to be found in narrower locations. In mountain rivers that have bedrock controls or boulders there can be obvious exceptions to this, especially when large particles lock in constrictions, as the channel boundaries often have a resistance greater than that provided by flow. However, many of these observations are limited in that they are either based on detailed morphologic unit scale (e.g. several channel widths) observations or reach and segment scale observations that have considerably less detail and resolution. We hypothesize that mountain and lowland rivers show contrasting covariance and coherence of stage dependent width and bed elevation. To assess this hypothesis we present preliminary results from the analysis of the longitudinal bed elevation and stage dependent flow width spatial series extracted from a highly detailed 36-km stretch of the Lower Yuba River and 12-km of one of its tributaries of the South Fork Yuba River. These rivers represent contrasting physiographic environments with variations in flow hydrology and boundary materials making up the channel bed and bank resulting in varying stream morphologies. Spatial covariance between the thalweg series and width series at various water stages in both rivers along with power spectral density and coherence estimates are used to show that in alluvial and mountain rivers patterns of flow width and bed elevation can emerge but vary in there spatial consistency and frequency that they are engaged by the flow record. Preliminary results suggest that channel form can manifest varying degrees of geometric organization due to varying environmental controls and subsequent fluvial processes that operate in these settings.

ABSTRACT An emerging view in fluvial geomorphology is that rivers are systems with multiple scale... more ABSTRACT An emerging view in fluvial geomorphology is that rivers are systems with multiple scales of variability. This is especially true in mountain rivers that have spatially variable alluvial-bedrock boundaries as multiple scales of topography, from individual boulders to valley scale deposits, can steer flow paths affecting the erosion and deposition patterns of transported sediments. We hypothesize that depending on flow discharge and stage, different scales of channel topography can become dominant in routing sediment such that the resulting topography is a layered sequence of past flows. Here we evaluate gravel and cobble channel change associated with hydraulic unit to reach scale (e.g. 10-1-102 channel widths) changes in topographic and stage-dependent hydrodynamic variability in a mixed alluvial-bedrock river canyon. This study takes advantage of a unique opportunity where 4,535 metric tons of gravel ranging from 6-128 mm was augmented directly below a dam for spawning habitat rehabilitation in a 1,200 m long mountain river reach with no other sources of gravel sediment supply and an existing substrate of bedrock, large cobbles (>250mm), angular shot rock (>0.5m) and boulders (>1m). While the study site is a regulated river flows above 117 m3/s, just below the bankfull discharge, still overtop the dam so natural aspects of the hydrograph are still retained such that the reach still experiences large floods capable of considerable topographic change. We utilize kite-blimp aerial photography, kayak-based surveying, topographic change detection, and 2D modeling to understand how flow discharge can activate topographic features that ultimately control channel change following a controlled gravel injection upstream of a mountain river with no other gravel or cobble sediment inputs. The spatial covariance of flow width and bed elevation are strongly associated with the volume of gravel deposition and erosion, but this also changes depending on flow discharge as different topographic elements become more effective in controlling hydrodynamics. Similarly, analyzing changes in 2D model derived flow directions shows that different flows can effectively push gravel and cobble into bedrock protrusions that at higher flows, become disconnected from the main flow routing in the river canyon. Results from this study suggest that both existing channel boundary variability and input hydrologic variability work together to create hydrodynamics spatial patterns that control the fate and transport of sediments in mountain rivers and ultimately their spatial structure.

An Integrated Approach, 2013

Riffle construction is a common practice in river engineering, but insufficient science exists to... more Riffle construction is a common practice in river engineering, but insufficient science exists to guide objective design of riffle-pool relief and the three-dimensional forms of features smaller than the scale of channel width. In this study, numerical experimentation with two-dimensional hydrodynamic modeling and ecohydraulic analysis was used to evaluate the performance of six different configurations of a sequence of riffle-pool units typical of shallow, regulated gravel-bed rivers, emphasizing a range of riffle-pool amplitudes (e.g., low, intermediate with hybrid features, and high). Twenty-two specific performance indicators (16 for physical habitat and six sediment-transport regime) were used to compare designs. It was found that low riffle-pool relief yielded the best performance for the majority of physical habitat indicators and all of the sediment transport regime indicators. The spatial patterns of test metrics revealed the mechanisms responsible for the statistical outcomes. Methodologically, two-dimensional modeling and ecohydraulic analysis are vital tools in project design along with previously accepted hydrologic, geomorphic, and engineering analyses. Scientifically, low-relief riffle-pool units are indicated as the normative condition in gravel-bed rivers where forcing elements driving deep scour are not systematically controlling morphology.

Journal of Hydrology, 2014

The relationships between flow hydrology, topography, and channel change in mountain rivers is im... more The relationships between flow hydrology, topography, and channel change in mountain rivers is important to understanding landscape evolution, the structure and persistence of aquatic habitat, and also the physiochemical cycling of upstream derived organic and inorganic materials. There is a paucity of detailed studies that analyze the joint roles of hydrology and topography in controlling multiple mechanisms of channel change in mountain rivers. In this study, gravel and cobble channel change in a bedrock river canyon were analyzed in light of a controlled yet natural experiment where 4,491 metric tonnes of rounded gravel and cobble was augmented below a sediment-barrier dam in a 1,200 m long mountain river reach that had no prior sources of rounded gravel or cobble and still experiences floods above the bankfull discharge. The overall study goal was to investigate how flow hydrology can modulate multiple channel change processes depending on the topographic features engaged by the flow. Channel change was assessed via differencing of high resolution repeat topographic and bathymetric surveys, along with cm-scale aerial photography post injection. Statistical tests used to implicate topographic feature-specific mechanisms of channel change that vary with discharge included analyzing geomorphic covariance structures of flow dependent width, bed elevation, and channel change as well as autocorrelation of flow width spatial series. Stage dependent topographic steering was inferred from associations of erosion and deposition with changes in 2D model derived flow directions at multiple discharges. A variety of mechanisms of channel change were qualitatively and quantitatively confirmed including particle hiding, topographic steering, eddying, and flow convergence. No single mechanism explained the observed patterns of channel change but rather it is thought that process-blending occurs, as modulated by the interactions of flow hydrology with complex topography. Results from this study suggest that both existing channel boundary variability and input hydrologic variability work together to create hydrodynamic spatial patterns that control the fate and transport of sediments in mountain rivers and ultimately their spatial structure.