Valley Confinement Research Articles

An approach for measuring confinement and assessing the influence of valley setting on river forms and processes

AbstractValley setting and confinement (or lack thereof) are primary controls on river character and behaviour. Although there are various proxies for valley confinement, direct measures that quantify the nature and extent of confinement are generally lacking and/or inconsistently described. As such they do not lend themselves to consistent analysis over large spatial scales. Here we clearly define forms of confinement to aid in quantification of degrees of confinement. Types of margin that can induce confinement are differentiated as a valley margin, valley bottom margin, and/or anthropogenic margin. Such margins sometimes overlap and share the same location, and in other situations are separated, giving immediate clues as to the valley setting. We apply this framework to examples from Australia, United States and New Zealand, showing how this framework can be applied across the spectrum of river diversity. This method can help to inform interpretations of reach‐scale river behaviour, highlighting the role of antecedent controls on contemporary forms and processes. Clear definitions of confinement are shown to support catchment‐scale analysis of river patterns along longitudinal profiles, and appraisals of the geomorphic effectiveness of floods and sediment flux in catchments (e.g. process zone distribution, lateral sediment inputs and (dis)connectivity). Copyright © 2015 John Wiley & Sons, Ltd.

Development of sediment slug upstream from the Czorsztyn Reservoir (southern Poland) and its interaction with river morphology

The effects of base-level rising upstream of dam reservoirs on in-channel sedimentation and interaction of the stored sediments with the gravel-bed channel morphology have received little attention so far. Previous studies, however, suggested that the feedback mechanism between in-channel sedimentation and bank erosion may affect channel morphology. Here, the pattern of the bar area, bank erosion, and morphology of the gravel-bed Dunajec River upstream from the Czorsztyn Reservoir (CR), constructed in 1997 in southern Poland were analyzed from aerial images (1982–2012) and LiDAR data (2013). In the part of the post-dam period with a large flood, the average bar area increased significantly in the backwater section, and at some distance upstream, and then extended in the upstream direction at an average rate of above 40m/y, reaching 2.2km upstream from the CR in 2012. The bar area variation was 40% and 77% explained by local bank erosion in the periods of large and low to moderate floods, respectively. The sum of bank erosion from the post-dam period explained 80% of the variation in the present width/depth ratio, significantly increased in the backwater section. The results showed that the large floods in 1997, in conjunction with backwater inundation, initiated intensive bank erosion and bar growth. Subsequently, in the period with low and moderate floods, the localized bar-bank interaction, connected with the flow divergence around the deposited bar, led to localized bank erosion and additional bar growth promoting bend development. These processes, propagating in the upstream direction, were facilitated by the existence of a large amount of easily remobilized sediment stored in the floodplain connected with the sedimentation zone from the end of the nineteenth century. Bend development was controlled by valley confinement causing downstream bend translation in the narrower valley-confined section and its extension in the wider unconfined section of past sediment storage. The results obtained from this site-specific location imply that the occurrence of similar past sediment storage zones of easily remobilized coarse sediments may favor intensive bank erosion and bar growth during large floods, and in the case of low transport competence backwater zones of gravel-bed rivers where the remobilized sediments cause localized bar growth; it may promote morphologically effective bar-bank interaction in the period with low and moderate floods.

A multi-scale hierarchical framework for developing understanding of river behaviour to support river management

This paper introduces this special issue of Aquatic Sciences. It outlines a multi-scale, hierarchical framework for developing process-based understanding of catchment to reach hydromorphology that can aid design and delivery of sustainable river management solutions. The framework was developed within the REFORM (REstoring rivers FOR effective catchment Management) project, funded by the European Union’s FP7 Programme. Specific aspects of this ‘REFORM framework’ and some applications are presented in other papers in this special issue. The REFORM framework is founded on previous hierarchical frameworks, sixteen examples of which are reviewed. However, the REFORM framework has some particular properties that reflect the European context for which it was developed. The framework delineates regional landscapes into nested spatial units at catchment, landscape unit, segment, reach, geomorphic unit and finer scales. Reaches, regardless of their ‘naturalness’, are assigned to a river type based on valley confinement, planform and bed material. Indicators are quantified at each spatial scale to feed three groups of assessments. First, contemporary indicators at reach and geomorphic unit scales investigate present processes, forms and human pressures within each reach. These feed assessments of present reach hydromorphological function/alteration, including whether the reach is functioning appropriately for its type; riparian corridor function and alteration; and hydromorphological adjustment. Second, indicators at catchment to segment scales investigate water and sediment production and delivery to reaches and how these are affected by human pressures now and in the past. These are used to construct an inventory of changes over space and time. Third, historical reach and geomorphic unit scale indicators are used to construct the trajectory of reach-scale changes. Contemporary reach-scale assessments, space–time inventory, and trajectory of changes are then combined to establish how river reaches of different type, subject to different human pressures, and located in different environmental contexts behave in response to changes at all considered spatial scales. These support forecasts of the likely responses of reaches to future scenarios (e.g., changes in climate, land cover, channel interventions).

Monitoring channel responses to flood events of low to moderate magnitudes in a bedrock-dominated river using morphological budgeting by terrestrial laser scanning

Changes to channel morphology reflect geomorphic work by flood events of differing magnitude and frequency. Advances in remote sensing and digital terrain processing now allow for sophisticated analysis of spatial and temporal changes in erosion and deposition. Although the morphological budgeting approach using digital elevation models of difference has been widely applied to track volume estimation of changes to erosion and deposition over time, appraisals of geomorphic effectiveness in high-energy confined and partly confined channels are still lacking. This study applied terrestrial laser scanning to monitor three reaches of the Liwu River, a bedrock-dominated river in eastern Taiwan, from 2009 to 2012, to investigate channel responses to flood events of low to moderate magnitude and appraise their geomorphic effectiveness. Variability in geomorphic effectiveness reflected valley confinement and the composition/configuration of geomorphic units on the channel bed. Annual low magnitude flood events reworked gravel and sand deposits, creating local scour and fill (<0.5m in depth) in the confined and in the unconfined reaches. Lower unit stream power in the wider, less-confined reach resulted in longer intervals between phases of boulder reworking relative to the confined reach. Bedrock exposure and stable sediment storage units in the confined reach restricted changes to channel pattern. Successive moderate and low magnitude events in 2012 created an evident erosion of 7556m3 (~1m in depth) and were able to modify channel configuration in partly confined and unconfined reaches. Frequent changes of scour/fill position on the channel bed indicate that the Liwu River is highly sensitive to flood events of low to moderate magnitude. This reflects an active orogenic river system that is characterized by a steep channel and narrow valley floors with limited accommodation space but abundant sediment.

Predicting spatial distribution of postfire debris flows and potential consequences for native trout in headwater streams

Habitat fragmentation and degradation and invasion of nonnative species have restricted the distribution of native trout. Many trout populations are limited to headwater streams where negative effects of predicted climate change, including reduced stream flow and increased risk of catastrophic fires, may further jeopardize their persistence. Headwater streams in steep terrain are especially susceptible to disturbance associated with postfire debris flows, which have led to local extirpation of trout populations in some systems. We conducted a reach-scale spatial analysis of debris-flow risk among 11 high-elevation watersheds of the Colorado Rocky Mountains occupied by isolated populations of Colorado River Cutthroat Trout (Oncorhynchus clarkii pleuriticus). Stream reaches at high risk of disturbance by postfire debris flow were identified with the aid of a qualitative model based on 4 primary initiating and transport factors (hillslope gradient, flow accumulation pathways, channel gradient, and valley confinement). This model was coupled with a spatially continuous survey of trout distributions in these stream networks to assess the predicted extent of trout population disturbances related to debris flows. In the study systems, debris-flow potential was highest in the lower and middle reaches of most watersheds. Colorado River Cutthroat Trout occurred in areas of high postfire debris-flow risk, but they were never restricted to those areas. Postfire debris flows could extirpate trout from local reaches in these watersheds, but trout populations occupy refugia that should allow recolonization of interconnected, downstream reaches. Specific results of our study may not be universally applicable, but our risk assessment approach can be applied to assess postfire debris-flow risk for stream reaches in other watersheds.

Lithologic and hydrologic controls of mixed alluvial–bedrock channels in flood-prone fluvial systems: Bankfull and macrochannels in the Llano River watershed, central Texas, USA

The rural and unregulated Llano River watershed located in central Texas, USA, has a highly variable flow regime and a wide range of instantaneous peak flows. Abrupt transitions in surface lithology exist along the main-stem channel course. Both of these characteristics afford an opportunity to examine hydrologic, lithologic, and sedimentary controls on downstream changes in channel morphology. Field surveys of channel topography and boundary composition are coupled with sediment analyses, hydraulic computations, flood-frequency analyses, and geographic information system mapping to discern controls on channel geometry (profile, pattern, and shape) and dimensions along the mixed alluvial–bedrock Llano River and key tributaries. Four categories of channel classification in a downstream direction include: (i) uppermost ephemeral reaches, (ii) straight or sinuous gravel-bed channels in Cretaceous carbonate sedimentary zones, (iii) straight or sinuous gravel-bed or bedrock channels in Paleozoic sedimentary zones, and (iv) straight, braided, or multithread mixed alluvial–bedrock channels with sandy beds in Precambrian igneous and metamorphic zones. Principal findings include: (i) a nearly linear channel profile attributed to resistant bedrock incision checkpoints; (ii) statistically significant correlations of both alluvial sinuosity and valley confinement to relatively high f (mean depth) hydraulic geometry values; (iii) relatively high b (width) hydraulic geometry values in partly confined settings with sinuous channels upstream from a prominent incision checkpoint; (iv) different functional flow categories including frequently occurring events (<1.5-year return periods) that mobilize channel-bed material and less frequent events that determine bankfull channel (1.5- to 3-year return periods) and macrochannel (10- to 40-year return periods) dimensions; (v) macrochannels with high f values (mostly ≥0.45) that develop at sites with unit stream power values in excess of 200watts per square meter (W/m2); and (vi) downstream convergence of hydraulic geometry exponents for bankfull and macrochannels, explained by co-increases of flood magnitude and noncohesive sandy sediments that collectively minimize development of alluvial bankfull indicators. Collectively, these findings indicate that mixed alluvial–bedrock channels exhibit first-order lithologic controls (lithologic resistance and valley confinement) of channel geometry, second-order hydrologic (flow regime) control of channel dimensions, and third-order sedimentary controls that exert subsidiary influence on channel shape and bed configuration.

Planform architecture, stratigraphic signature and morphodynamics of an exhumed Jurassic meander plain (Scalby Formation, Yorkshire, UK)

AbstractModern fluvial meander plains exhibit complex planform transformations in response to meander‐bend expansion, downstream migration and rotation. These transformations exert a fundamental control on lithology and reservoir properties, yet their stratigraphic record has been poorly evaluated in ancient examples due to the lack of extensive three‐dimensional exposures. Here, a unique exhumed meander plain exposed to the north of Scarborough (Yorkshire, UK) is analysed in terms of architecture and morphodynamics, with the aim of developing a comprehensive model of facies distribution. The studied outcrop comprises tidal platforms and adjacent cliffs, where the depositional architecture of un‐tilted deposits was assessed on planform and vertical sections, respectively. In its broader perspective, this study demonstrates the potential of architectural mapping of extensive planform exposures for the reconstruction of ancient fluvial morphodynamics. The studied exhumed meander plain is part of the Scalby Formation of the Ravenscar Group, and originally drained small coastal incised valleys within the Jurassic Cleveland Basin. The meander plain is subdivided into two storeys that contain in‐channel and overbank architectural elements. In‐channel elements comprise expansional and downstream‐migrating point bars, point‐bar tails and channel fills. Overbank elements comprise crevasse complexes, levées, floodplain fines and lake fills. The evolution of the point bars played a significant role in dictating preserved facies distributions, with high flood‐stage nucleation and accretion of meander scrolls later reworked during waning flood‐stages. At a larger scale, meander belt morphodynamics were also a function of valley confinement and contrasts in substrate erodibility. Progressive valley infilling decreased the valley confinement, promoting the upward transition from prevalently downstream migrating to expansional meander belts, a transition associated with enhanced preservation of overbank elements. Strikingly similar relations between valley confinement, meander‐bend transformations and overbank preservation are observed in small modern meandering streams such as the Beaver River of the Canadian prairies and the Powder River of Montana (USA).

Channel-reach morphology controls of headwater streams based in flysch geologic structures: An example from the Outer Western Carpathians, Czech Republic

A detailed measurement of 93 channel reaches that were classified with an adjusted Montgomery–Buffington (1997) reach-scale system provided comprehensive information of approximately 9 at-a-reach parameters: the channel gradient, the bankfull width, the bankfull depth, d90, the percentage of resistant rocks in the bed sediment, the number of pieces of large woody debris, valley confinement, direct sediment inputs and the presence of fluvial accumulations in the stream channel. In addition, the quantified intensity of sediment transport (i.e. ratio between sediment supply and transport capacity in longitudinal stream profiles) during flood events has been estimated by the one-dimensional bedload transport model (TOMSED), which was validated in two local streams. The principal component analysis of the at-a-reach parameters did not reveal significant groups of channel-reach morphologies; thus, the selected parameters that exclude sediment transport dynamics within stream longitudinal profile cannot reliably distinguish or predict individual channel reach morphologies. Nevertheless, the channel gradient represented the most significant single explanatory variable for stepped-bed morphologies. The addition of bedload transport parameters demonstrated that limited sediment supply streams and streams with limited transport capacities featured different successions of the channel reach morphologies in terms of the channel gradient and, subsequently, the fluvial continuity. The bedrock-cascades and step-pools were significant for the first case, whereas cascade and step-rapid morphology often occurred in higher sediment supply conditions.

Potential and actual geomorphic complexity of restored headwater streams in northern Sweden

Stream restoration usually relies on ecological theories presuming that increased habitat heterogeneity leads to higher biodiversity. However, to test this hypothesis a quantitative metric of overall geomorphic complexity is needed. We quantified geomorphic complexity using 29 metrics over five dimensions (sediment distribution, longitudinal profile, cross section, planform, and instream wood) of headwater streams in northern Sweden. We examined reaches with four different restoration statuses after a century of timber floating (channelized, restored, demonstration restored, and unimpacted) to determine (1) whether restoration increases complexity in all dimensions, (2) whether a complexity gradient can be quantified and which metrics can serve as proxies for the gradient, and (3) levels of potential complexity based on large-scale controls (drainage area, glacial legacy sediment, valley slope, valley confinement, old-growth forest/buffer zone, and beaver activity). We found a significantly higher complexity in unimpacted and demonstration restoration sites than in channelized sites in all five dimensions except the cross section (based on the two metrics quantifying variability in the cross section). Multivariate analyses were able to elucidate an apparent complexity gradient driven by three complexity metrics: longitudinal roughness, sediment sorting, and cross section chain and tape ratio. The large-scale factors of valley and channel gradient as well as median grain size, along with restoration status, drive differences in complexity composition. Restoring a reach to its potential complexity is beneficial in regions without reference systems or sufficient data to model flow and sediment processes. Unimpacted and demonstration restoration reaches displayed not only more intrareach variability than channelized reaches but also greater interreach heterogeneity in complexity composition, which supports a focus on reach-scale controls on potential complexity and a landscape-scale view on restoration.

Predicting natural channel patterns based on landscape and geomorphic controls in the Columbia River basin, USA

[1] Based on known relationships of slope, discharge, valley confinement, sediment supply, and sediment caliber in controlling channel patterns, we developed multivariate models to predict natural channel patterns across the 674,500 km2 Columbia River basin, USA. We used readily available geospatial data sets to calculate reach slopes, 2 year flood discharge, and valley confinement, as well as to develop hypothesized landscape-level surrogates for sediment load and caliber (relative slope, percent of drainage area in alpine terrain, and percent of drainage area in erosive fine-grained lithologies). Using a support vector machine (SVM) classifier, we found that the four channel patterns were best distinguished by a model including all variables except valley confinement (82% overall accuracy). We then used that model to predict channel pattern for the entire basin and found that the spatial distribution of straight, meandering, anabranching, and braided patterns were consistent with regional topography and geology. A simple slope-discharge model distinguished meandering channels from all other channel patterns, but did not clearly distinguish braided from straight channels (68% overall accuracy). Addition of one or more of the hypothesized sediment supply surrogates improved prediction accuracy by 4–14% over slope and discharge alone. Braided and straight channels were most clearly distinguished on an axis of relative slope, whereas braided and anabranching channels were most clearly distinguished by adding percent alpine area to the model.

Modeling freshwater mussel distribution in relation to biotic and abiotic habitat variables at multiple spatial scales

The habitat requirements of many native freshwater mussels remain unclear despite their imperiled status and ecological importance. To explore scale-specific habitat associations in the three genera of mussels found in the western United States (Anodonta, Gonidea, and Margaritifera) we used a multiscale random forest modeling approach to assess functional habitat parameters throughout a 55 km segment of the upper Middle Fork John Day River in northeastern Oregon. We characterized mussel occurrence and density with respect to the hierarchical, hydrogeomorphic structure by sampling reaches of varying valley confinement and channel units nested within individual reaches. Each genus exhibited unique longitudinal trends and channel unit-use patterns. In particular, the large-scale longitudinal trends in Margaritifera occurrence were associated with hydrogeomorphic characteristics at the reach and channel unit scale, with Margaritifera densities peaking in narrow valley segments and in riffles and runs. At the scale of the channel unit, all mussel genera responded to variation in physical habitat characteristics, particularly those that indicated more stable parts of the channel. Our results suggest that spatial patterns in freshwater mussels are associated with the hierarchical structuring of the lotic ecosystem and may provide guidance to restoration efforts.

Assessing stream bank condition using airborne LiDAR and high spatial resolution image data in temperate semirural areas in Victoria, Australia

Stream bank condition is an important physical form indicator for streams related to the environmental condition of riparian corridors. This research developed and applied an approach for mapping bank condition from airborne light detection and ranging (LiDAR) and high-spatial resolution optical image data in a temperate forest/woodland/urban environment. Field observations of bank condition were related to LiDAR and optical image-derived variables, including bank slope, plant projective cover, bank-full width, valley confinement, bank height, bank top crenulation, and ground vegetation cover. Image-based variables, showing correlation with the field measurements of stream bank condition, were used as input to a cumulative logistic regression model to estimate and map bank condition. The highest correlation was achieved between field-assessed bank condition and image-derived average bank slope (R2 1/4 0.60, n 1/4 41), ground vegetation cover (R2 1/4 0.43, n 1/4 41), bank width/height ratio (R2 1/4 0.41, n 1/4 41), and valley confinement (producer's accuracy 1/4 100%, n 1/4 9). Crossvalidation showed an average misclassification error of 0.95 from an ordinal scale from 0 to 4 using the developed model. This approach was developed to support the remotely sensed mapping of stream bank condition for 26,000 km of streams in Victoria, Australia, from 2010 to 2012.

Geomorphic effects, flood power, and channel competence of a catastrophic flood in confined and unconfined reaches of the upper Lockyer valley, southeast Queensland, Australia

Flooding is a persistent natural hazard, and even modest changes in future climate are believed to lead to large increases in flood magnitude. Previous studies of extreme floods have reported a range of geomorphic responses from negligible change to catastrophic channel change. This paper provides an assessment of the geomorphic effects of a rare, high magnitude event that occurred in the Lockyer valley, southeast Queensland in January 2011. The average return interval of the resulting flood was ~2000years in the upper catchment and decreased to ~30years downstream. A multitemporal LiDAR-derived DEM of Difference (DoD) is used to quantify morphological change in two study reaches with contrasting valley settings (confined and unconfined). Differences in geomorphic response between reaches are examined in the context of changes in flood power, channel competence and degree of valley confinement using a combination of one-dimensional (1-D) and two-dimensional (2-D) hydraulic modelling. Flood power peaked at 9800Wm−2 along the confined reach and was 2–3 times lower along the unconfined reach. Results from the DoD confirm that the confined reach was net erosional, exporting ~287,000m3 of sediment whilst the unconfined reach was net depositional gaining ~209,000m3 of sediment, 70% of the amount exported from the upstream, confined reach. The major sources of eroded sediment in the confined reach were within channel benches and macrochannel banks resulting in a significant increase of channel width. In the unconfined reach, the benches and floodplains were the major loci for deposition, whilst the inner channel exhibited minor width increases. The presence of high stream power values, and resultant high erosion rates, within the confined reach is a function of the higher energy gradient of the steeper channel that is associated with knickpoint development. Dramatic differences in geomorphic responses were observed between the two adjacent reaches of contrasting valley configuration. The confined reach experienced large-scale erosion and reorganisation of the channel morphology that resulted in significantly different areal representations of the five geomorphic features classified in this study.

Late Quaternary floodplain reworking and the preservation of alluvial sedimentary archives in unconfined and confined river valleys in the eastern interior of South Africa

In this study, geomorphological, sedimentological, and geochronological work was conducted on unconfined and confined reaches of three rivers in the eastern interior of South Africa in order to quantify the relative rates of floodplain reworking and alluvial preservation along river courses with variable valley confinement and lithology. Using optically stimulated luminescence (OSL) dating techniques, new chronologies for the Schoonspruit and Mooi River were created and the existing Klip River chronology was expanded. The results suggest that floodplains in both unconfined and confined reaches preserve complex spatial and temporal patterns of alluviation, although differences in boundary conditions lead to variation in the processes and rates of floodplain construction and reworking. On the Klip and Mooi Rivers where local base levels are stable, channels in unconfined reaches rework floodplain sediments through slow lateral migration punctuated by local erosion during avulsion events. On the Schoonspruit where base level has lowered, the channel in the unconfined reach is incised, the floodplain is abandoned, and a large gully has formed. In the confined reaches of all three rivers, the narrow floodplains are reworked by scour and fill activity and limited lateral migration. The OSL results suggest that unconfined reaches preserve relatively continuous alluvial records that extend into the Pleistocene, while the floodplains in the confined reaches preserve relatively discontinuous alluvial records biased toward the late Holocene. The alluvial geochronologic records in these systems preserve signals of changes in local base level controlled by variation in lithology and incision rather than climate change. By defining the processes, rates, and patterns of floodplain reworking in reaches with different degrees of valley confinement and channel incision, the findings contribute to understanding how rivers build, modify, and preserve alluvial sedimentary archives.

The influence of landscape configuration upon patterns of sediment storage in a highly connected river system

Orthoimages are used to analyze the influence of valley width, channel slope, and the area of landslides that deliver sediment to valley floors upon the downstream pattern of sediment storage in the Liwu basin, eastern Taiwan. Confined valley settings dominate this steep, deeply dissected, uplifting mountainous basin. Although they occupy 82% of the whole channel network, sediment storage is limited in these reaches. Partly confined valley settings are restricted to relatively short reaches of two tributary systems. Over 95% of sediment storage in the Liwu River is found within the short alluvial section of laterally unconfined valley close to the river mouth. Lateral bars are the primary features associated with sediment storage in confined reaches (54% in total volume), while floodplains dominate the partly confined and laterally unconfined reaches (95% in total volume). Downstream patterns of sediment storage are controlled primarily by valley confinement (i.e., valley width) and landscape configuration. Channel slope and locations of contemporary landslides are not key determinants of sediment storage patterns. Sediment accumulation at confluence zones is not determined by the catchment area of tributaries or by landslide area; rather, it is determined by valley confinement of the trunk stream at the confluence and the associated downstream sequence of river types. Sediment conveyance is very effective within this highly connected river system.

AN ASSESSMENT OF VARIANTS IN THE PROFESSIONAL JUDGEMENT OF GEOMORPHOLOGICALLY BASED CHANNEL TYPES

ABSTRACTRiver classification is a useful tool for researchers and managers wishing to organise, to simplify and to understand the forms and processes within freshwater systems. Many classifications require surveyors to classify reaches into specific channel types in a field environment. Channel types should be identifiable on the basis of a field surveyor's judgement of channel characteristics and landscape settings; these include channel planform, valley confinement, dominant bed material and/or instream geomorphic features (e.g. gravel bars). An accurate classification of reaches into the correct channel type is important to ensure consistency in management strategies and to assess the impact of engineering activities on the physical and ecological status of rivers. In this article, we examine the variation in professional judgement of geomorphologically based channel types by scientists with different disciplinary backgrounds and varying levels of involvement in classification systems using a photo‐questionnaire. Results indicate that there can be a large level of discrepancy in typing rivers; the choice of the modal channel type for each reach varied between 25.9% and 75.1% of the respondent selections. There were also differences in the level of agreement between earth scientists (with hydrogeomorphological or geological training), ecological scientists (with freshwater biology training) and practitioners involved in river conservation and management. A high level of experience in classification systems translates to a lower number of channel types being chosen per reach. In response to these results, the use of a photographic approach to typing needs to be fully tested and users fully trained before operational use. Furthermore, we advocate that designers of geomorphic typologies should aim to have a representative and workable number of classes within a typology with an emphasis for rationalisation of classes rather than expansion of numbers. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Geomorphometric floodplain classification in a hill region of Hungary

A sufficiently detailed floodplain typology is assumed to be useful in the estimation of local flood risk, planning flood control and river restoration measures and for the survey of landscape ecological patterns. Although floodplains are defined by a range of hydrological, geomorphological and geoecological criteria, floodplain classification should start with geomorphometric characterization of stream channels and floodplains in a valley setting. The study areas selected for the geomorphometric analyses are two representative catchments in the hill regions of Southwest-Hungary: the Kapos River, a river of medium size by Hungarian standards (a tributary of the Sió Canal and of the Danube), and a small tributary in the Drava River system, the Bükkösd Stream. For the delimitation and characterization of their floodplains Digital Elevation Models (DEMs), archive military surveys, and topographic maps have been used. The diagnostic parameters selected for the classification are floodplain width, floodplain (or valley floor) slope, valley confinement (expressed by a longitudinal profile index, LPI), channel sinuosity (indicating river mechanism), and valley depth. They have been analyzed in relationship with the distribution of tributary confluences along the river longitudinal profile. Currently, a large majority of watercourses in the region are channelized and some are dammed for fish-ponds, but the present-day floodplains had been built earlier by seminatural streams. Therefore, the classification is based on pre-regulation floodplain conditions (reconstructed from archive sources), which should be target conditions for any future management or restoration efforts. A visual comparison of the distribution of floodplain parameter values along the longitudinal profile shows that the LPI index is a useful tool for the characterization of the floodplain segments. The floodplain types are classified as 1) no floodplain (LPI values are >0.25, mostly >0.5); 2) gradually broadening floodplain (LPI is between 0.1 and 0.25); 3) partially confined floodplain (LPI ranging between 0.05 and 0.1) and 4) mostly unconfined floodplain with bottleneck reaches (LPI fluctuating more widely, from 0.05 to 0.25).

Tree establishment on bars in low‐order gravel‐bed mountain streams

ABSTRACTIn many large alluvial rivers, trees often recruit and survive along laterally accreted sediments on bars. This produces a gradient of tree ages and composition with distance from the active channel. However, in low‐order, gravel‐bed mountain streams, such as the stream investigated in this study, it is suggested that vertical accretion results in sediment deposition patterns on bars that are often highly patchy. Consequently, tree species and ages are also heterogeneously distributed, rather than having distinct linear or arcuate banding patterns with distance from the channel. In addition, overall age patterns of trees on these bars follow the distribution of floods, with numerous young trees and few older trees. Recruitment is fairly continuous on these bars and is not correlated with high water years, suggesting that even flows close to bankfull levels are capable of transporting fine sediment to the bars on which trees establish. This pattern of sediment deposition/erosion and the resulting tree recruitment and survival seem to be a result of valley confinement and the lack of lateral accretion in these smaller, mountainous channels. Copyright © 2011 John Wiley &amp; Sons, Ltd.

A geophysical analysis of hydro-geomorphic controls within a headwater wetland in a granitic landscape, through ERI and IP

Abstract. Wetlands are undergoing considerable degradation in South Africa. As interventions are often technical and costly, there is a requirement to develop conceptual process models for these wetland systems so that rehabilitation attempts will be successful. This paper presents an approach using the geophysical methods of Electrical Resistivity Imaging (ERI) and Induced Polarization (IP) to delineate sub-surface hydro-geomorphic controls that maintain equilibrium disconnectivity of wetland-catchment processes, which through gully erosion are increasing the catchments connectivity through loss of water and sediment. The findings presented here give insight into the geomorphic processes that maintain the wetland in an un-degraded state, this allows for the development of a conceptual model outlining the wetland forming processes. The analysis suggests that sub-surface clay-plugs, within an otherwise sandy substrate are created by illuviation of clays from the surrounding hillslopes particularly at zones of valley confinement.

Valley confinement as a factor of braided river pattern for the Platte River

The Platte River in Nebraska has evolved in the twentieth century from a predominantly braided river pattern to a mélange of meandering, wandering, anastomosed, island braided, and fully braided reaches. Identifying the factors that determine the occurrence of a fully braided main channel was the objective of this study. Aerial photography, gage flow data, ground-surveyed cross sections, bed material samples, and the results of sediment transport modeling were used to examine factors that control spatial change in main river pattern of the central Platte River. Valley confinement is identified as the determining factor of braided river in nine of eleven divisions of the central Platte River. Flow reduction and the interruption of sediment supply are identified as determining factors preventing fully braided river in the remaining two of eleven reaches. Valley confinement, the topography which limits the width of the floodplain, was initially measured as width between historical banks (predevelopment river banks). This metric was later refined to width between confining features (historical banks, remnant bars, bridge abutments, protected banks and levees). Under existing conditions, the main channel of the central Platte River is fully braided when valley confinement (width between confining features) is 600 m or less and begins to divide into the multiple channels of an anastomosed pattern when valley confinement (width between confining features) exceeds 600 m When Platte River flow is divided between two to four major anabranches, a fully braided pattern in the main channel of the main anabranch requires a more confined valley of 400 m or less. Valley confinement is demonstrated to be the dominant factor in determining river pattern in the central Platte River, although this factor is not normally considered in the continuum of channel pattern model. Conclusions from this study can be used to increase the occurrence of fully braided main channel in the central Platte River, to aid habitat recovery for endangered or threatened bird species that favor this river pattern. Consideration of valley confinement with river continuum factors can aid river managers by improving predictions of river pattern in response to management actions.