Total Stream Power Research Articles

Stream power curve–loop–spiral conceptual method and an application to rivers of Taiwan

Study regionEighteen main rivers in Taiwan. Study focusThe concept of total stream power (TSP) or specific stream power (SSP) has been investigated previously as an index for assessing the status of a river channel. Here, we introduce a new diagram called the “stream power loop” to review variations in fluvial conditions, morphology, and sediment transport potential from river source to estuary. Because of its physical basis, the loop diagram reflects fluvial morphology and watershed conditions. Additionally, we present a spiral diagram that depicts variations in the spatial gradients of TSP and SSP. The gradient implied the sediment transport trend, which from upstream to downstream changes gradually from scouring to siltation in ideal conditions. The loop and spiral diagrams provide an overview of the fluvial system. New hydrological insights for the regionThe loop diagram applications in Taiwan categorized the rivers into three types: simple loop, maple-leaf loop, and complex loop. Diverse geology increases the likelihood of large tributaries and abrupt changes in terrain, both of which affect the components of stream power–discharge and slope. The scatter distribution of all the data aggregated from the 18 rivers showed a shrinking radius and a counter-clockwise spiral pathway, passing through the mountain river zone, the braided river zone, and finally to the straight & meandering river zone. The result supported the concept of stream power loop and indicated that stream power loop can interpret the status of the river systems.

Multi-decadal changes in river morphology in an urbanizing watershed: Highland Creek, Toronto, Canada

Urban land development causes significant changes to river channel form and function. Outcomes are strongly conditioned by local circumstances although responses such as channel enlargement and incision are common but are highly variable in extent and detectability. Many case studies focus on small watersheds (or short reaches) and before-after conditions, and direct causation and temporal trajectories of change are not well known but often presumed to be a single step change and response. Well-documented examples in fully-urban, larger watersheds are rare, and rarer still are cases covering the full trajectory from pre-urban to fully urban land cover with accompanying data on the direct drivers of change such as discharge and stream power. Highland Creek, Toronto, has a drainage area of 102 km2 and between the early 1950s and 2000s underwent a land cover change from agricultural to completely urban in very low relief terrain with channels in narrow valleys incised locally into glacial sediments. Streamflow monitoring, land cover data and aerial photography over this seven-decade period, along with a calibrated runoff model and digital elevation data, give a complete picture over multiple epochs of the causes and changes in fluvial hydrology and geomorphology. The complete transformation of the flow regime, along with channel straightening and steepening, resulted in increases in average stream power (total and specific) of the order of ten and six times, respectively. The channel is now exceptionally energetic with maximum total and specific stream power for 2-year flood approaching 5000 Wm−1 and 300 Wm−2, respectively. The semi-alluvial and supply-limited character of the channels is also influential. Channel response is variable along the 20–25 km of channel measured and is notably different between the sub-watersheds, related to timing of development and its intersection with engineering intervention, and changing conceptions of the channel over time. In places the channel widened by a factor of four or five and changed from high sinuosity meanders to wide, bar-chute morphology, consistent with regime expectations for gravel-bed channels. In other places the process of urbanization has involved progressive engineering of the channel to convey stormwater (channelized), protect infrastructure and mitigate erosion, so that responses in these reaches are much more restricted and fluvial functions such as bend and bar development have been eliminated. Substantial channel transformation can be seen to have occurred episodically resulting from two major flood events as well as the increases in magnitude and frequency of peak flows, which limit post-flood recovery. Urbanization effects and consequences have developed gradually over a long period, rather than in a single urbanization step change, and they do not match simple models of channel evolution. In many ways urbanization of the river is ongoing, particularly in the form of geomorphically-referenced design of the channels as a response to damage from large floods and overall institutional watershed planning. The ongoing urbanization story of Highland Creek illustrates some extreme effects of urbanization as well as the complexity and contingency of response to the physical and institutional processes influencing the trajectory of urban river evolution.

Absence of well‐developed floodplains along the lowland rivers and controls of the hydrogeomorphic conditions in the Western Ghat, India

AbstractLowland rivers across the world are characterized by accumulation of sediment due to a decrease in the channel slope. Despite the antiquity of the topography, the lowland part of the west‐flowing rivers of Western Ghat, India is characterized by poorly developed floodplains except for the southern part of the Ghat. This poses a question of the sediment transportation and deposition capacity in these lowland rivers, which in turn governs geomorphic characteristics. In this study, we adapted a novel method to integrate hydrological model‐derived discharge estimates, channel slope estimates from digital elevation models, bankfull channel width estimates from high‐resolution Google Earth satellite images and field observations. We modelled the downstream distribution of total stream power ( ), specific stream power ( ) and shear stress ( ) for four west‐flowing river systems from different lithological and climatic settings. The 75th percentile discharge from monthly time series data was used in modelling to assess channel processes. The peak values of are distributed all along the longitudinal profile, whereas the peak values of and are consistently higher in the escarpment. Scaling relationships between hydrogeomorphic parameters were also established to understand the inherent controls on the distribution of these driving parameters. We inferred that is sufficient to transport large grain sizes even in the low‐gradient lowland stretches. Hence sediments are mostly absent in the channel, and floodplains are not well developed. Despite having a low gradient, the lowland coastal rivers can efficiently evacuate most of the eroded sediments to the offshore region. Finally, the lithological control is manifested by the variability of channel width regulating the sediment transport in the coastal rivers of the Western Ghat.

Large-scale assessment of spatial variability in stream power distribution: An indicator of channel erosion and deposition potential

Channel morphodynamics, governed by erosion and deposition processes, constitute a relevant aspect of integrated water management planning, and development of sustainable fluvial projects. According to the Water Framework Directive, there is a need for hydromorphological recognition and channel geomorphic adjustment at the national level. Such a large scale excludes field measurements as the primary research method. Instead, methods that require less resources and are based on relatively simple process representations are required. The stream power (SP) concept, combining the effect of channel gradient and discharge, has been widely used for quantitative measurements in geomorphic works at different spatial scales, but not at the national level so far. The primary objective of this study was to estimate total and specific stream power, and detect erosion and deposition patterns along the river network in Poland. Given the dominant role of stream slope in SP spatial variations, the specific objective was to assess the vertical accuracy of high-resolution DEM representing water surface elevations along the river course, and to propose a method for extraction of elevation points along river reaches that increase the accuracy of DEM-derived stream slopes. We demonstrated that DEM generally overestimates the elevation by more than 0.5 m, significantly affecting the DEM-derived stream slope estimation. By extracting minimum water surface elevation from DEM within a 15 m radius from the target cell (river segment endpoint), a substantial improvement was achieved. The difference between the actual and DEM-derived water surface elevation dropped to 0.16 m. The results indicated that the distribution of dominating processes in Polish rivers is strongly governed by the character and origin of the terrain relief and lithological diversity. For regions with flat relief, most channels are generally in the state of equilibrium, whereas in upland and mountainous regions, erosion processes are dominant. This study demonstrates the potential use of the SP concept for mapping channel sensitivity to erosion and deposition dominance at a national scale, which can be relevant for integrated river catchment management.

Changes in the hydrological regime and channel morphology as the effects of dams and bridges in the Barakar River, India

The anthropogenic activities affect the river channel as well as the whole system in different magnitudes and dimensions. Barakar River, the main tributary to the Damodar River in eastern India, is modified by several engineering structures. Hydrological parameters, such as monthly discharge, peak flow discharge and geomorphological factors, such as gradient, width–depth ratio, grain size, braid–channel ratio, sinuosity ratio, riffle-pool sequence, and stream power are taken into consideration to highlight the significant alterations of the river due to dam and bridge construction. The alterations are assessed with the help of hydrological data, satellite images, and digital elevation data along with field survey. The downstream section of the dams, the river is characterised by high braiding, sinuous, total and unit stream power along with the presence of a box-shaped bedrock channel, high gradient, bed coarsening and armouring due to the release of high-velocity sediment-free ‘hungry water’. In the upstream reach, the grain size decreases towards the dam, and it increases suddenly with poor sorting at the immediate downstream regime of the dam. The effects of bridges on the Barakar river morphology include an increase of gradient, width and depth of the river channel at the downstream of the bridges. The construction of bridges influences riffle-pool sequences. Thereby, the pool depth spacing is greater than the riffle crest spacing. However, the integration of natural as well as human-induced factors can be the best approach to understand the anthropogenic alteration of the river. Moreover, construction of some check dams at the upper section of the tributaries of the Barakar River can be very effective for morphological stability.

Morphologic and hydraulic variability of small bedrock and alluvial channels in relation to lithological controls, Upper Ogun River Basin, Southwestern Nigeria

ABSTRACT The influence of lithology on morphological variability across space presents a unique approach to understand how morphological variables and domain units respond to lithological variations. This study examines variation existing in channel morphological variables across domain units within a basement complex. Eighty-three reaches were surveyed across six third-order basins within the Upper Ogun River basin, Nigeria. Each of the six third-order basins is heterogeneously underlain by various lithological formations. Fourteen morphological variables (bankfull; depth, width, cross-sectional area, width-depth ratio, wetted perimeter, hydraulic radius, discharge, total stream power, specific stream power, shear stress, velocity, dimensionless stream power, Darcy-Weisbach friction factor and channel gradient) were measured and estimated across the 83 reaches. Analysis of Variance revealed that significant variation occurs in channel morphological properties between schist and granite and between granite and migmatite. Also, bankfull width showed exponent values of wbf ~ Qbf 0.21 for migmatite; wbf ~ Qbf 0.11 for granites; and wbf ~ Qbf 0.34 for schists. Channel unit sequence using Markov chain presents two transitions identified as bedrock to plane-bed and step-pools to plane-bed. The absence of transitions as observed is closely tied to the occurrence of non-cyclic patterns.

How do stream processes affect hazard exposure on alluvial fans? Insights from an experimental study

Alluvial fans are among the most privileged settlement areas in many mountain regions. These landforms are particularly dynamic being episodically affected by distributary processes generated by extreme flood events. Addressing risk assessment entails determining hazard exposure and unravelling how it might be related to process loading and to process dynamics once the flow becomes unconfined on the surface of alluvial fans. By following a ‘similarity of process concept’, rather than by attempting to scale a real-world prototype, we performed a set of 72 experimental runs on an alluvial fan model. Thereby, we considered two model layouts, one without a guiding channel and featuring a convex shape and the other one with a guiding channel, a bridge, and inclined but planar overland flow areas as to mirror an anthropic environment. Process magnitude and intensity parameters were systematically varied, and the associated biphasic distributary processes video recorded. For each experiment, the exposure was detected by mapping the exposed area in a GIS, thereby discerning between areas exposed to biphasic flows and the associated depositional phenomena or to the liquid flow phase only. Our results reveal that total event volume, sediment availability and stream power in the feeding channel, as well as depositional effects, avulsion, and channelization on the alluvial fan concur to determine the overall exposure. Stream process loading alone, even when rigorously defined in terms of its characterizing parameters, is not sufficient to exhaustively determine exposure. Hence, further developing reliable biphasic simulation models for hazard assessment on settled alluvial fans is pivotal.

Spatio-temporal variability in stream power distribution in the Upper Kosi River basin, Central Himalaya: Controls and geomorphic implications

River processes in a drainage basin are strongly governed by the spatio-temporal variability in stream power. Recently, various approaches have been developed to derive the spatial variability of stream power in a river basin; however, the temporal variability in stream power due to hydrological changes remains poorly understood. An assessment of the temporal variability of stream power is critical for river studies in monsoonal regimes because significant geomorphic work is performed during this period, particularly in the Himalayan rivers. This paper presents a novel methodology to derive the stream power distribution at a basin-scale using a physically-based-hydrological model, the Soil and Water Assessment Tool (SWAT). Hydrological changes in the upper Kosi (also called Koshi) basin in the Central Himalaya were analysed and integrated with slope variability and channel width to obtain the basin-scale distribution of total stream power and specific stream power. Most of the tributaries of the Kosi River are characterized by a multi-peak distribution of stream power. Stream power distribution in the basin shows no systematic downstream increase or decrease of either total stream power or specific stream power. Total and specific stream power peaks during the monsoon period are 100–1500 times higher compared to the non-monsoon period and exceed the threshold for erosion and sediment transport processes. The SWAT model also helped to assess controls on the stream power distribution in the basin. The downstream distribution of stream power between confluence points is influenced by slope variability, while major increases at confluence points are due to discharge variability. Geological characteristics control the spatial distribution of stream power while the annual rainfall distribution influences the seasonal variability of stream power.

The variability in the morphological active width: Results from physical models of gravel‐bed braided rivers

AbstractThe morphological active width, defined as the lateral extent of bed material displacement over time, is a fundamental parameter in multi‐threaded gravel‐bed rivers, linking complex channel dynamics to bedload transport. Here, results are presented from five constant discharge experiments, and three event hydrographs, covering a range of flow strengths and channel configurations for which morphological change, bedload transport rates, and stream power were measured in a physical model. Changes in channel morphology were determined via differencing of photogrammetrically‐derived digital elevation models (DEMs) of the model surface generated at regular intervals over the course of ~115 h of experimental runs. Independent measures of total bedload output were made using downstream sediment baskets. Results indicate that the morphological active width increases with total and dimensionless stream power and is strongly and positively correlated with bulk change (total volume of bed material displaced over time) and active braiding intensity (ABI). Although there is considerable scatter due to the inherent variability in braided river morphodynamics, the active width is positively correlated with independent measurements of bedload transport rate. Active width, bulk change, and bedload transport rates were all negligible below a dimensionless stream power threshold value of ~ 0.09, above which all increase with flow strength. Therefore, the active width could be used as a general predictor of bulk change and bedload transport rates, which in turn could be approximated from total and dimensionless stream power or ABI in gravel‐bed braided rivers. Furthermore, results highlight the importance of the active width, rather than the morphological active depth, in predicting volumes of change and bedload transport rates. The results contribute to the larger goals of better understanding braided river morphodynamics, creating large high‐resolution datasets of channel change for model calibration and validation, and developing morphological methods for predicting bedload transport rates in braiding river systems. Copyright © 2018 John Wiley & Sons, Ltd.

Stream energy distribution below Eleyele Dam in Southwestern Nigeria

Expressing the rate of energy dissipation either within a basin or in a downstream manner is important in understanding occurrence and association between various morphological variables. This study focuses on the downstream variation in stream power and its relationship with morphological variates along the main channel of River Ona, Ibadan, Nigeria. River Ona stretches for 20.5 km downstream of Eleyele Dam. Twenty‐seven (27) points were sampled along this stretch. The 27 points were averaged into nine (9) reaches. At each cross‐section, channel characteristics were observed and measured. These include: width (W), depth (D), velocity (V), slope (S), Cross‐sectional area (A), discharge (Q), total stream power (TSP), specific stream power (SSP), width‐depth ratio (W/D), hydraulic radius (R) and wetted perimeter (P). This study confirmed that the influence of a dam present upstream of a river channel significantly alters the behaviour of slope and this in essence affects the distribution of energy along the river channel. Slope produces a logarithmic relationship with increasing distance downstream (Y = ‐32.79‐0.70InS, r = 0.70, r2 = 0.49, p < 0.05) while there was no significant relationship between Q and distance downstream.

Stream power framework for predicting geomorphic change: The 2013 Colorado Front Range flood

The Colorado Front Range flood of September 2013 induced a diverse range of geomorphic changes along numerous stream corridors, providing an opportunity to assess responses to a large flood in a semiarid landscape. We defined six classes of geomorphic change related to peak unit stream power and valley confinement for 531 stream reaches over 226km, spanning a gradient of channel scales and slope. Geomorphic change was generally driven by erosion of channel margins in confined reaches and by a combination of deposition and erosion in unconfined reaches. The magnitude of geomorphic change typically increased with unit stream power (ω), with greater responses observed in unconfined channels. Cumulative logit modeling indicated that total stream power or unit stream power, unit stream power gradient, and valley confinement are significant predictors of geomorphic response for this flood event. Based on this dataset, thresholds for geomorphic adjustment were defined. For channel slopes<3%, we noted a credible potential for substantial channel widening with ω>230W/m2 (16lb/ft-s; at least 10% of the investigated sites experienced substantial channel widening) and a credible potential for avulsions, braiding, and loss of adjacent road embankments associated with ω>480W/m2 (33lb/ft-s; at least 10% of the investigated sites experienced such geomorphic change). Infrequent to numerous eroded banks were very likely with ω>700W/m2 (48lb/ft-s), with substantial channel widening or major geomorphic change shifting from credible to likely. Importantly, in reaches where there were large reductions in ω as the valley form shifted from confined to relatively unconfined, large amounts of deposition-induced, reach-scale geomorphic change occurred in some locations at relatively low ω. Additionally, alluvial channels with slopes>3% had greater resistance to geomorphic change, likely caused by armoring by larger bed material and increased flow resistance from enhanced bedforms. Finally, we describe how these results can potentially be used by practitioners for assessing the risk of geomorphic change when evaluating current or planned conditions.

Transport Capacity of Overland Flow for Sediment Mixtures

AbstractSediment transport capacity of overland flows is crucial to understanding and modeling the erosion and deposition processes. The transport capacity for sediment mixtures is particularly complex because of the need for an algorithm to allocate the total available shear stress or stream power to individual size classes. The transport capacity for sediment mixtures may not even be unique, depending on whether the flow is eroding or depositing sediments unless all sediments are of a single size class. Using data from a set of flume experiments involving steep slopes and high sediment concentrations, a formula was proposed for the sediment transport capacity limited by available stream power and available accommodation space for sediments. Sediments used for the experiment were further sieved into five distinct size classes. The sediment transport capacity was measured for 25 flow-slope combinations to validate a new formula for individual size classes, and to propose and test a method to compute the s...

Fire and flood expand the floodplain shifting habitat mosaic concept

The floodplain shifting habitat mosaic concept suggests that habitat patch dynamics are influenced by hydrologic disturbances driven by flood pulses of sufficient power to initiate incipient motion of the substratum and maintain cut-and-fill alluviation of the channel and banks. However, floodplain habitat mosaics are subject to other important landscape-scale disturbance regimes. In the Rocky Mountains of the USA and Canada, fire also affects floodplain habitat patch composition. The floodplain exists at the intersection of disturbance regimes that shape the riverscape and those that shape the landscape. We extended the shifting habitat mosaic concept by examining the effects of multiple disturbance elements on habitat patch composition across the aquatic–terrestrial ecotone of the North Fork of the Flathead River, a free-flowing river in southeastern British Columbia, Canada, and northwestern Montana, USA. We used remotely sensed imagery, meteorological records, empirical and modeled rainfall-runoff data, extent and frequency of past wildfires, and anthropogenic landuse data for 1991–2013 to examine the relationships among hydrology, fire, landuse, geomorphic position, and floodplain habitat patch dynamics. Exploratory path analysis revealed that fire had the strongest total effect and stream power and geomorphic position had moderate total effects on the variability of floodplain habitat patch composition along the North Fork. These 3 factors explained 13 to 26% of the variance in floodplain habitat patch composition between study reaches across all years. We used graphical analysis to examine the locations and intensity of disturbance and recovery pathways across floodplain transition zones throughout the study period. Our results support the hypothesis that hydrologic and fire disturbances and recovery pathways maintain the shifting habitat mosaic across the floodplains of this river system.

Gradients in stream power influence lateral and downstream sediment flux in floods

Locations of landslides, bank failures, and floodplain deposition during recent intense flooding in Vermont and Colorado (USA) were spatially nonuniform, indicating that some reaches are more prone to these types of geologic hazards. These three key flood effects signal redistribution of sediment across landscapes, reflecting hillslope-channel coupling and the sources and/or sinks of material. We show that spatial gradients in total stream power (Ω) provide critical additional information beyond at-a-point Ω magnitudes for predicting which reaches are likely to be susceptible to these hazards during floods. Field tests in four rivers (watershed areas 0.8–180 km 2 ) indicate that downstream increases in Ω coincide with erosion and mass wasting into channels, and downstream decreases in Ω are associated with floodplain deposition. Our analytical approach, supported by field evidence, predicts geologic hazards and, more broadly, sources and sinks of material along rivers. These are critical concerns from a practical and theoretical standpoint.

Floodplain persistence and dynamic-equilibrium conditions in a canyon environment

Canyon river systems are laterally constrained by steep walls, strath terraces, and bedrock intrusions; however, semialluvial reaches are nested within these environments as discontinuous floodplains along the river margins. These semialluvial floodplains provide an example of dynamic-equilibrium set within high-energy fluvial systems, marking areas where the river is free to alter its boundary conditions. Most research has focused on hydraulic conditions necessary for floodplain formation and persistence in unconfined systems, whereas little is known about canyon streams. This paper focuses on (1) characterizing dynamic-equilibrium, (2) describing the controls on floodplain formation and distribution, and (3) evaluating the performance of extremal hypotheses to identify dynamic-equilibrium and floodplain persistence in high-energy, semiconfined canyon environments.These objectives were addressed with field and numerical data derived from a one-dimensional hydraulic model for bankfull and 100-year return interval flood events, supported by closely spaced cross sections for the lower 38-km canyon reach of the Deadwood River (Idaho). Under bankfull conditions, critical energy thresholds for equilibrium floodplain persistence at this study site present the upper limits of: slope=0.018, shear stress=175N/m2, and specific stream power=400W/m2. Channel and floodplains near equilibrium, quantified with a near-zero sediment transport divergence (Exner equation), were successfully identified by the minimum unit stream power extremal hypothesis and to a lesser degree by the other extremal hypotheses that minimize energy expenditure (minimum specific stream power, minimum total stream power, and minimum Froude number), provided backwater environments and major tributaries could be identified. Extremal results were compared to hydraulic geometry relations to evaluate how closely equilibrium floodplains approached values for unconfined alluvial river systems.

Modelling suspended sediment concentration and load in a transport‐limited alluvial gully in northern Queensland, Australia

ABSTRACTAlluvial gullies are often formed in dispersible sodic soils along steep banks of incised river channels. Field data collected by Shellberg et al. (Earth Surface Processes and Landforms 38: 1765–1778, 2013) from a gully outlet in northern Australia showed little hysteresis between water discharge and fine (&lt;63 µm) and coarse (&gt;63 µm) suspended sediment, indicating transport‐limited rather than source‐limited conditions. The major source of the fine (silt/clay) component was the sodic soils of upstream gully scarps, and the coarser (sand) component was sourced locally from channel bed material. In this companion paper at the same study site, a new method was developed for combining the settling velocity characteristics of these two sediment source components to estimate the average settling velocity of the total suspended sediment. This was compared to the analysis of limited sediment samples collected during flood conditions. These settling velocity data were used in the steady‐state transport limit theory of Hairsine and Rose (Water Resources Research 28: 237–243, 245–250, 1992) that successfully predicted field data of concentrations and loads at a cross‐section, regardless of the complexity of transport‐limited upstream sources (sheet erosion, scalds, rills, gullies, mass failure, bank and bed erosion, other disturbed areas). The analysis required calibration of a key model parameter, the fraction of total stream power (F ≈ 0.025) that is effective in re‐entraining sediment. Practical recommendations are provided for the prediction of sediment loads from other alluvial gullies in the region with similar hydrogeomorphic conditions, using average stream power efficiency factors for suspended silt/clay (Fw ≈ 0.016) and sand (Fs ≈ 0.038) respectively, but with no requirement for field data on sediment concentrations. Only basic field data on settling velocity characteristics from soil samples, channel geometry measurements, estimates of water velocity and discharge, and associated error margins are needed for transport limit theory predictions of concentration and load. This theory is simpler than that required in source‐limited situations. Copyright © 2015 John Wiley &amp; Sons, Ltd.

The efficacy of stream power and flow duration on geomorphic responses to catastrophic flooding

Geomorphologists have long studied the impacts of extreme floods, yet the association between the magnitude of flow parameters (discharge, velocity, shear stress, or stream power) and resulting geomorphic effectiveness remains vague and non-deterministic. Attempts have been made to include flow duration and total expenditure of stream power, in combination with peak unit stream power, as important variables, but there has been minimal exploration of this hydraulic combination. Taking advantage of Tropical Storm Irene's rapid track through eastern Vermont (USA) in late summer 2011, this paper presents the array of geomorphic responses to a short duration (time to peak of <8h) but high magnitude flood that was the twentieth century flood of record for numerous watersheds. We present herein the geomorphic imprint of Tropical Storm Irene flooding within a larger context of fluvial theory concerning the role of, and trade-off between, the magnitude of energy expenditure during a flood and its duration. Focusing on a detailed field effort within the 187-km2 Saxtons River basin in southeastern VT, augmented by select sites along the adjacent lower gradient Williams River (291-km2), we elucidate (1) the geomorphic effects of a short duration flood in a humid, well-vegetated landscape; (2) the relationship between geomorphic response and (a) peak stream power, (b) total stream power, and (c) flow duration of stream power above a critical threshold; and (3) the spatial variation of geomorphic effects relative to reach-scale geologic and geomorphic controls. Flooding associated with Tropical Storm Irene ranged from the 1000year recurrence interval (RI) flood (based on Weibull flood frequency analysis) to the 300year RI flood (log Pearson Type III). Discharges spawned a peak unit stream power of 712W/m2 (Saxtons River) and 361W/m2 (Williams River), with total energy expenditure throughout the event of ~16,000×103 and 15,000×103J, respectively. For the Saxtons River, channel widening was spatially infrequent and limited in magnitude; however, other geomorphic effects were profound (1) the entrainment, transport, and deposition of extremely coarse material; (2) stripping of floodplain surfaces; (3) channel avulsions and incision into Pleistocene-aged material; and (4) deposition of coarse material across floodplains. Based on our extensive field data and hydrologic/hydraulic analyses, we contend that short duration, high energy flows can have profound sedimentological effects but have limited erosive, channel widening impacts. Gravel entrainment and deposition of a catastrophic nature can certainly occur under these flow regimes, but the impacts of these extreme flows on channel geometry may have limited expression.

Controls on the distribution of channel reach morphology in selectively glaciated catchments

To assess the controls on the distribution of channel reach morphology in a selectively glaciated landscape, we used field mapping and a geographical information system (GIS) in the River Dee catchment, northeast Scotland. Controls on channel morphology were investigated using (1) continuous longitudinal assessment of channel morphology distribution in relation to geology, glacial history, topography, and total stream power (Ω) in two subcatchments, and (2) slope (S), Ω, and a slope–drainage area (S–A) framework to understand the occurrence of 173 widely distributed bedrock, mixed bedrock–alluvial, and alluvial (three different types) reaches. The S–A framework used indicators of transport capacity (Qc) and sediment supply (Qs) to differentiate channel types. The study highlights the disjointed nature of channel reach distribution at the river scale that reflects variable lithology and glacial modification. Because of the subdued topography in contrast to other regions, colluvial forcing of channel morphology in the headwaters was lacking. However, in common with other glaciated landscapes, repeated sequences of channel reach type progression determined by valley steps were evident. The S–A analysis successfully discriminated 87.2% of alluvial and 91.4% of bedrock reaches despite the variable land use and glacial modification. Discrimination of the full range of channel types using S, Ω, or the S–A framework was poor however. Notably, a third of the transport alluvial reaches were located in the bedrock S–A domain, and the majority of mixed reaches were widely distributed mostly within the bedrock domain and not close to the critical slope (Sc). In comparison to other regions, the Sc above which Qc>Qs and bedrock reaches dominate, was notably higher. We hypothesise that a drier climate and the higher entrainment threshold of coarse, granite-dominated bed materials create a higher Sc.

The Use of Stream Power as an Indicator of Channel Sensitivity to Erosion and Deposition Processes

Stream power is a measure of the main driving forces acting in a channel and determines a river's capacity to transport sediment and perform geomorphic work. Recent digital elevation models allow the calculation of channel gradient and consequently stream power at unprecedented spatial resolution, opening promising and novel opportunities to investigate river geomorphic processes and forms. The present paper investigates the suitability of map-derived information on total and specific stream power (SSP) to identify dominant processes within the channel (i.e. erosion, transport or deposition). SSP has been already used to identify a threshold for channel stability. This paper tests this knowledge and investigates whether or not attributes of stream power profiles are statistically correlated with distinctive field morphological forms. Two gravel bed single-thread English rivers are used as case studies, the Lune and the Wye. Available deposition and erosion features surveyed in the field from 124 different locations are used to classify channel reaches as erosion, transport or deposition dominated. Meaningful patterns emerge between the stream power attributes and the field-based channel classification. An SSP threshold, which erosion is triggered, compares favourably with the ones in the literature. Information about upstream stream power profiles helps to determine the dominant processes. The joint configuration of local and upstream stream power information uniquely classifies reaches into four classes of different sensitivity to erosion and deposition. Copyright © 2013 John Wiley & Sons, Ltd.

Search for the optimality signature of river network development

Whether the evolution of natural river networks pursues a certain optimal state has been a most intriguing and fundamental question. There have been many optimality hypotheses proposed but it has yet to be proved which of these best serves as a quantitative signature of river network development. Here, this fundamental question is investigated for the five hypotheses of "minimum total energy expenditure," "minimum total energy dissipation rate," "minimum total stream power," "minimum global energy expenditure rate," and "minimum topological energy." Using simple example landscapes, I examined whether any of these hypotheses pursues both the treelike river network formation and the concave stream longitudinal profile, the two characteristic patterns of natural landscapes. It is found that none of these hypotheses captures both patterns under the steady-state condition where the balance between tectonic uplift and sediment loss is satisfied. These findings are further verified through simulations of landscapes that satisfy given optimality criteria using an optimization method.