Mixed Bedrock-alluvial Channels Research Articles

Covariation in width and depth in bedrock rivers

AbstractThe width and depth of rivers are generally inversely related. For a given discharge, wider rivers tend to be shallower than narrower rivers, which are correspondingly deeper. This is particularly true in bedrock and mixed bedrock–alluvial channels, where deep pools occur downstream of lateral constrictions, downstream of which the channel becomes wider and shallower. However, covariation of width and depth in bedrock and mixed bedrock–alluvial rivers has never been explored due to the lack of field measurements. Here we present a 375 km survey of width and depth measurements in the Fraser Canyon, British Columbia, which alternates irregularly among alluvial (no bedrock exposed on either bank), bedrock‐constrained (bedrock exposed on one bank), and bedrock‐bound (bedrock exposed on both banks) sections. We find that bedrock‐bound reaches have the deepest and narrowest sections, followed by bedrock‐constrained reaches and alluvial reaches, which feature the shallowest and widest sections of channel. There is an inverse relation between width and depth for all the channels, with alluvial channels having the highest correlation between these two variables, and thus the greatest covariance. We further explore the relation between width and depth and the downstream hydraulic geometry of 42 individual bedrock‐bound canyons. There is an inverse relation between canyon width and depth, with substantial variation within individual canyons. The downstream hydraulic geometry for these bedrock‐bound canyons does not follow that typical of alluvial channels; depth is the only variable that adjusts substantially as a response to increasing discharge and upstream basin area.

Large wood and sediment storage in a mixed bedrock-alluvial stream, western Montana, USA

Sediment storage by instream wood in forested mountain streams mediates sediment movement from hillslopes through the channel network and can alter channel morphology at multiple spatial scales. Mixed bedrock-alluvial channels are prevalent in mountain stream networks, yet the distribution and geomorphic impact of large wood within these streams are poorly understood. To estimate how the distribution of large wood in a mixed bedrock-alluvial stream relates to sediment storage, we measured and characterized large wood, and we surveyed the volume of associated sediment within a stream in the Bitterroot Mountains, Montana. The upstream portion of the study reach is predominantly alluvial and the downstream portion has significant bedrock exposure along the channel bed and banks. Wood volume and sediment storage in the mixed bedrock-alluvial subreach are 50% and 15%, respectively, of those measured in the alluvial subreach. Most wood is organized into jams, and two channel-spanning jams within the upstream subreach account for 50% and 80% of the reach-averaged wood and sediment volume, respectively. The volume of sediment stored by wood in the full reach is the same order of magnitude as the estimated annual bedload export. Our study highlights the interactions among channel form and the fluxes and storage of wood and sediment. Wood that forms channel-spanning jams, which store a disproportionate amount of wood and sediment in the study reach, may accentuate differences between alluvial and bedrock-dominated subreaches in mountain stream networks, by promoting sediment deposition and storage upstream and by reducing sediment supply to downstream reaches.

Performance of bedload transport equations in a mixed bedrock–alluvial channel environment

Mixed bedrock–alluvial channel beds are common in mountain rivers. It is well-known that in such system the balance between sediment supply and transport capacity defines either the level of the exposure of bedrock (in the riverbed) or its coverage by alluvial material. We investigate whether applying the representation of the energy losses (caused by macro-roughness) developed for alluvial channels improve the capacity of bedload transport equations to predict the bedload transport in a mixed bedrock–alluvial stream. Therefore, the purpose of this study was to evaluate the performances of five bedload equations against field measurements, while considering the effects of macro-roughness on the equations and the limited availability of mobile sediment. We analyzed the data collected in a stream located in southern Brazil, which has a 2.3% gradient and mixed bed conditions. Optimal results in estimating the bedload transport rate were obtained when using the reduced shear stress τ' and reduction factor of the available bed material (Frm) together in the five tested equations. Analyzing the two approaches separately, the implementation of τ' proved to be more critical for improving the performance of the equations than using Frm alone. Among all the equations, Recking (2013) presented the best result for the case in which 85.2% of the estimates fell within one order of magnitude of the measured transport rates (0.1 < r > 10.0); this was followed by the Recking (2010) (70.4% of estimates within one order of magnitude) and Meyer-Peter and Müller (1948) (37% of estimates within one order of magnitude) equations. In conclusion, we observed that even by implementing flow resistance and with the limited availability of mobile sediment, the equations overestimated the bedload transport rate in the studied river reach, indicating an underestimation of energy loss in the flow resistance equations of mixed bedrock–alluvial channels.

Hydraulic resistance in mixed bedrock-alluvial meandering channels

ABSTRACT We present an experimental analysis of hydraulic roughness variations due to changes in alluvial cover in a mixed bedrock-alluvial meandering channel with larger bedrock roughness than alluvial roughness. Three sets of experiments were conducted in a highly sinuous flume: one with bare bedrock, and two with enough sediment to cover 21% and 78% of the bedrock respectively. We compare our results with data from experiments previously conducted in the same flume with flat- and smooth-bed, and data from experiments with fully alluvial conditions. Our results show that: (1) hydraulic resistance in a mixed bedrock-alluvial channel changes with the degree of alluviation; (2) hydraulic resistance is greater for bare-bedrock conditions, and decreases as sediment supply increases; (3) if bedforms appear, hydraulic resistance may be larger than for bare bedrock conditions due to form drag; (4) fluctuations in alluvial cover due to freely-migrating bars lead to instantaneous changes in hydraulic resistance.

Coarse sediment storage and connectivity and off-highway vehicle use, Board Camp Creek, Arkansas

Upper Board Camp Creek (BCC) in western Arkansas drains the Wolf Pen Gap (WPG) Trail Complex, a designated Off-Highway Vehicle (OHV) area in use since the 1990s. The mixed bedrock-alluvial channel is quite active, with extensive bars and eroding banks present within the higher-order, main-valley channels. This study was conducted to determine the relative magnitudes of sediment storage and production within the channel, and whether geomorphic changes are synchronous with establishment and increasing OHV use levels on the trail system. Fourteen geomorphically active reaches within the main-valley channels, representing the range of conditions of BCC within the WPG complex, were examined in detail. All sites had significant alluvial storage in the form of point, lateral, or mid-channel bars dominated by cobble and gravel. Sediment storage volumes ranged from 140 to nearly 10,000 m3 per kilometer of channel, with a mean of about 3400. Eleven of 14 reaches also had actively eroding banks. Ten reaches (71%) exhibited net sediment storage. Two are possible net sources, and two may be in approximate steady state (storage ≈ erosion). The imbalance between local bank erosion sources and in-channel storage, and the evidence of activity and mobility of most of the bars indicates a connected system, with coarse sediment mobile during banktop flow events, and no evidence of sediment starvation. Finer (<8 mm) sediment from the trail system does not seem to be accumulating in the stream, suggesting that most is either sequestered before reaching BCC Creek, transported downstream, or deposited on floodplains during overbank flow. Many of the channel bars predate the trail complex, and most are active. This suggests that these features constitute mainly transient storage and are an inherent feature of the channel. At only two reaches could geomorphic changes be confidently attributed to the trail system. Like many streams, BCC has an active channel, independent of the WPG trail system. These results highlight the difficulty of attributing fluvial change to specific causes or forcings in active fluvial systems.

Quantifying and contextualising cyclone-driven, extreme flood magnitudes in bedrock-influenced dryland rivers

In many drylands worldwide, rivers are subjected to episodic, extreme flood events and associated sediment stripping. These events may trigger transformations from mixed bedrock-alluvial channels characterised by high geomorphic and ecological diversity towards more dominantly bedrock channels with lower diversity. To date, hydrological and hydraulic data has tended to be limited for these bedrock-influenced dryland rivers, but recent advances in high-resolution data capture are enabling greater integration of different investigative approaches, which is helping to inform assessment of river response to changing hydroclimatic extremes. Here, we use field and remotely sensed data along with a novel 2D hydrodynamic modelling approach to estimate, for the first time, peak discharges that occurred during cyclone-driven floods in the Kruger National Park, eastern South Africa, in January 2012. We estimate peak discharges in the range of 4470–5630 m3 s−1 for the Sabie River (upstream catchment area 5715 km2) and 14,407–16,772 m3s−1 for the Olifants River (upstream catchment area 53,820 km2). These estimates place both floods in the extreme category for each river, with the Olifants peak discharge ranking among the largest recorded or estimated for any southern African river in the last couple of hundred years. On both rivers, the floods resulted in significant changes to dryland river morphology, sediment flux and vegetation communities. Our modelling approach may be transferable to other sparsely gauged or ungauged rivers, and to sites where palaeoflood evidence is preserved. Against a backdrop of mounting evidence for global increases in hydroclimatic extremes, additional studies will help to refine our understanding of the relative and synergistic impacts of high-magnitude flood events on dryland river development.

Characteristics of a Mixed Bedrock-Alluvial Channel in a Plateau and Plateau Fringe Region: A Study on the Barakar River of the Chotanagpur Plateau, India

The rivers that bear the properties of both alluvial and bedrock character are considered as bedrock and alluvial mixed character rivers. They are characterised by stable planform, erosion resistant bank, rock outcrop, local variability in channel bed gradient and variable flow regime. The Barakar River in the Chotanagpur plateau and its fringe region of India has the distinctive features of bedrock and gravel mixed river with sinuous and braided nature in the upper and the lower sections of the river, respectively. To investigate and extract the geomorphic features of the bedrock and gravel mixed character of the Barakar River, planform view and behaviour of the sediments were considered. The sinuous bedrock and braided reaches have been analysed using the Sinuosity Index and the Braid-channel Ratio, respectively. The upper sinuous bedrock section is marked by a steep-sided, narrow valley with the presence of incised channel. In the downstream reach of the Maithon Dam, the river channel is a box-shaped bedrock channel with a low form ratio. Potholes and grooves of varying size and shape and inner channel are the main features at the mouth zone. While, in the lower section of the river, the wide and straight braided reach is characterised by higher form ratio and higher gradient than the sinuous reach. The riffle-pool sequence in the braided section is not developed significantly like in the alluvial channel. In the braided reach, the sediment grain size is continuously decreased towards the Maithon Dam and the sediment grains are well sorted. The lithology and hydraulic processes of the river determine the mixed character of the river that favours the fluvial system to maintain its lateral stability.

Finite amplitude bars in mixed bedrock-alluvial channels

We present a nonlinear asymptotic theory of fully developed flow and bed topography in a wide channel of constant curvature to describe finite amplitude perturbations of bottom topography, subject to an inerodible bedrock layer. The flow field is evaluated at the leading order of approximation as a slowly varying sequence of locally uniform flows, slightly perturbed by a weak curvature-induced secondary flow. Using the constraint of constant fluid discharge and sediment flux, we calculate an analytical solution for the cross-sectional profile of flow depth and bed topography, and we determine the average slope in the bend necessary to transport the sediment supplied from a straight, alluvial, upstream reach. Both fully alluvial bends and bends with partial bedrock exposure are shown to require a larger average slope than a straight upstream reach; the relative slope increase is much larger for mixed bedrock-alluvial bends. Curvature and sediment supply are shown to have a strong effect on the characteristics of the point bars in mixed bedrock-alluvial channels. Higher curvature bends produce bars of larger amplitude and more bedrock exposure through the cross section, and increasing the sediment supply leads to taller and wider point bars. Differences in the relative roughness of sediment and bedrock have a smaller, secondary effect on point bar characteristics. Our analytical approach can potentially be extended to the case of arbitrary, yet slowly varying, curvature, and should ultimately lead to an improved understanding of the formation of meanders in bedrock channels.

Punctuated sand transport in the lowermost Mississippi River

[1] Measurements of sand flux and water flow in the Mississippi River are presented for a portion of the system 35–50 km upstream from the head of its subaerial delta. These data are used to provide insight into how nonuniform flow conditions, present in the lower reaches of large alluvial rivers, affect the timing and magnitude of sand transport near the river outlet. Field surveys during both low and high water discharge include (1) sequential digital bathymetric maps defining mobile river bottom topography which were used to estimate bed material flux, (2) multiple water velocity profiles, and (3) multiple suspended sediment profiles collected using a point-integrated sampler. These data show that total sand transport increases by two orders of magnitude over the measured range in water discharge (11,300 to 38,400 m3 s−1). During low water discharge no sand is measured in suspension, and sand discharge via bed form migration is minimal. During high water discharge 54% of the sand discharge is measured in suspension while 46% of the sand discharge is part of bed form migration. The component of boundary shear stress associated with moving this sediment is estimated using a set of established sediment transport algorithms, and values for the total boundary shear stress are predicted by fitting logarithmic velocity functions to the measured profiles. The estimates of boundary shear stress, using measurements of suspended sand transport, bed form transport, and downstream oriented velocity profiles are internally consistent; moreover, the analyses show that boundary shear stress increases by nearly 10-fold over the measured water discharge range. We show how this increase in shear stress is consistent with backwater flow arising where the river approaches its outlet. The hydrodynamic properties of backwater flow affect the timing and magnitude of sand flux and produce punctuated sand transport through the lowermost Mississippi River. Our field data are used to evaluate the influence of this sand transport style on development of the mixed bedrock alluvial channel for the lowermost Mississippi River.

The geomorphological characteristics of the Mekong River in northern Cambodia: A mixed bedrock–alluvial multi-channel network

The controls on the development of channel morphology of bedrock-constrained rivers are poorly known. The relative importance of lithological and structural control on channel alignment and character in comparison with the role of hydraulic erosion of the substratum is unclear. In addition, bedrock rivers often have a variable sediment fill and can be described as mixed bedrock–alluvial systems. The Mekong River in northern Cambodia is an anastomosed mixed bedrock–alluvial channel, but little and poorly researched. In this paper information has been gathered from diverse literature sources; digital data sets showing topography, hydrology, geology and land cover; past aerial photographs; and maps. Such data, together with field survey, provide a clear picture of the Mekong River in this region.The channels may be classified into three types: primary, secondary, cross channels. The varying characteristics of these three help towards understanding the evolution of the modern Mekong. Similarly the two-fold classification of the islands reveals the relationship between island inundation characteristics and the annual monsoon flood cycle. The associated riparian vegetation ecotones include a rare and unusual seasonally-inundated forest. Spatial variations in lithology and structure, when combined with maps of river networks, reveals that the channel alignments locally reflect the geological factors that cause the regional topographical gradient. Fault-line constraints on the local slopes of the channel may induce backwater effects and consequent sedimentation patterns (alluvial overprints) or alternatively, steepening of the channels with concomitant reduction in sedimentation. These structural constraints, taken together, lead to the identification of a mixed bedrock–alluvial five-fold geomorphological zonation within the study area. The general absence of paleochannels, and terraces close to the modern river, indicates that the Mekong is laterally stable with a dominating channel that downcuts. Consequently, a flood surface of limited lateral extent constrains entire floods.

The influence of large landslides on river incision in a transient landscape: Eastern margin of the Tibetan Plateau (Sichuan, China)

Deep landscape dissection by the Dadu and Yalong rivers on the eastern margin of the Tibetan plateau has produced high-relief, narrow river gorges and threshold hillslopes that frequently experience large landslides. Large landslides inundate river valleys and overwhelm channels with large volumes (>10 5 m 3 ) of coarse material, commonly forming stable landslide dams that trigger extensive and prolonged aggradation upstream. These observations suggest that strong feedbacks among hillslope processes, channel morphology, and incision rate are prevalent throughout this landscape and are likely characteristic of transient landscapes experiencing large increases in local relief, in general. Landslide effects are a by-product of rapid incision initiated by regional uplift. However, over timescales relevant to landscape evolution (>10 4 yr), large landslides can also act as a primary control on channel morphology and longitudinal river profiles, inhibiting incision and further preventing the complete adjustment of rivers to regional tectonic, climatic, and lithologic forcing. We explore a probabilistic, numerical model to provide a quantitative framework for evaluating how landslides influence bedrock river incision and landscape evolution. The time-average number of landslide dams along a river course, and thus the magnitude of the landslide influence, is set by two fundamental timescales—the time it takes to erode landslide deposits and erase individual dams and the recurrence interval of large landslides that lead to stable dams. Stable, gradually eroding landslide dams create mixed bedrock-alluvial channels with spatial and temporal variations in incision, ultimately slowing long-term rates of river incision, thereby reducing the total amount of incision occurring over a given length of river. A stronger landslide effect implies that a higher percentage of channel length is buried by landslide-related sediment, leading to reduced river incision efficiency. The longer it takes a river channel to incise into a landslide dam and remove all landslide-related sediment, the more control these events have on the evolution of the river profile and landscape evolution. This can be the result of slow erosion of stable dams, or a higher frequency of large events.

The effects of rock uplift and rock resistance on river morphology in a subduction zone forearc, Oregon, USA

AbstractRelationships between riverbed morphology, concavity, rock type and rock uplift rate are examined to independently unravel the contribution of along‐strike variations in lithology and rates of vertical deformation to the topographic relief of the Oregon coastal mountains. Lithologic control on river profile form is reflected by convexities and knickpoints in a number of longitudinal profiles and by general trends of concavity as a function of lithology. Volcanic and sedimentary rocks are the principal rock types underlying the northern Oregon Coast Ranges (between 46°30′ and 45°N) where mixed bedrock–alluvial channels dominate. Average concavity, θ, is 0·57 in this region. In the alluviated central Oregon Coast Ranges (between 45° and 44°N) values of concavity are, on average, the highest (θ = 0·82). South of 44°N, however, bedrock channels are common and θ = 0·73. Mixed bedrock–alluvial channels characterize rivers in the Klamath Mountains (from 43°N south; θ = 0·64). Rock uplift rates of ≥0·5 mm a−1, mixed bedrock–alluvial channels, and concavities of 0·53–0·70 occur within the northernmost Coast Ranges and Klamath Mountains. For rivers flowing over volcanic rocks θ = 0·53, and θ = 0·72 for reaches crossing sedimentary rocks. Whereas channel type and concavity generally co‐vary with lithology along much of the range, rivers between 44·5° and 43°N do not follow these trends. Concavities are generally greater than 0·70, alluvial channels are common, and river profiles lack knickpoints between 44·5° and 44°N, despite the fact that lithology is arguably invariant. Moreover, rock uplift rates in this region vary from low, ≤0·5 mm a−1, to subsidence (&lt;0 mm a−1). These observations are consistent with models of transient river response to a decrease in uplift rate. Conversely, the rivers between 44° and 43°N have similar concavities and flow on the same mapped bedrock unit as the central region, but have bedrock channels and irregular longitudinal profiles, suggesting the river profiles reflect a transient response to an increase in uplift rate. If changes in rock uplift rate explain the differences in river profile form and morphology, it is unlikely that rock uplift and erosion are in steady state in the Oregon coastal mountains. Copyright © 2006 John Wiley &amp; Sons, Ltd.

The form and function of headwater streams based on field and modeling investigations in the southern Appalachian Mountains

AbstractHeadwater streams drain the majority of most landscapes, yet less is known about their morphology and sediment transport processes than for lowland rivers. We have studied headwater channel form, discharge and erosive power in the humid, moderate‐relief Valley and Ridge and Blue Ridge provinces of the Appalachian Mountains. Field observations from nine headwater (&lt;2 km2 drainage area), mixed bedrock–alluvial channels in a variety of boundary conditions demonstrate variation with respect to slope‐area channel initiation, basic morphology, slope distribution, hydraulic geometry, substrate grain size and role of woody debris. These channels display only some of the typical downstream trends expected of larger, lowland rivers. Variations are controlled mainly by differences in bedrock resistance, from the formation level down to short‐wavelength, outcrop‐scale variations. Hydrologic modeling on these ungauged channels estimates the recurrence of channel‐filling discharge and its ability to erode the channel bed. Two‐year recurrence discharge is generally larger and closer to bankfull height in the Valley and Ridge, due to low soil infiltration capacity. Discharge that fills the channel to its surveyed bankfull form is variable, generally exceeding two‐year flows at small drainage areas (&lt;0·5 km2) and being exceeded by them at greater drainage areas. This suggests bankfull is not controlled by the same recurrence storm throughout a channel or physiographic region. Stream power and relative competence are also variable. These heterogeneities contrast relations observed in larger streams and illustrate the sensitivity of headwater channels to local knickpoints of resistant bedrock and armoring of channels by influx of coarse debris from hillslopes. The general lack of predictable trends or functional relationships among hydraulic variables and the close coupling of channel form and function with local boundary conditions indicate that headwater streams pose a significant challenge to landscape evolution modeling. Copyright © 2005 John Wiley &amp; Sons, Ltd.