Bankfull Flood Research Articles

Macroscale controls determine the recovery of river ecosystem productivity following flood disturbances

River ecosystem function depends on flow regimes that are increasingly modified by changes in climate, land use, water extraction, and flow regulation. Given the wide range of variation in flow regime modifications and autotrophic communities in rivers, it has been challenging to predict which rivers will be more resilient to flow disturbances. To better understand how river productivity is disturbed by and recovers from high-flow disturbance events, we used a continental-scale dataset of daily gross primary production time series from 143 rivers to estimate growth of autotrophic biomass and ecologically relevant flow disturbance thresholds using a modified population model. We compared biomass recovery rates across hydroclimatic gradients and catchment characteristics to evaluate macroscale controls on ecosystem recovery. Estimated biomass accrual (i.e., recovery) was fastest in wider rivers with less regulated flow regimes and more frequent instances of biomass removal during high flows. Although disturbance flow thresholds routinely fell below the estimated bankfull flood (i.e., the 2-y flood), a direct comparison of disturbance flows estimated by our biomass model and a geomorphic model revealed that biomass disturbance thresholds were usually greater than bed disturbance thresholds. We suggest that primary producers in rivers vary widely in their capacity to recover following flow disturbances, and multiple, interacting macroscale factors control productivity recovery rates, although river width had the strongest overall effect. Biomass disturbance flow thresholds varied as a function of geomorphology, highlighting the need for data such as bed slope and grain size to predict how river ecosystems will respond to changing flow regimes.

Impacts of River Engineering on River Channel Behaviour: Implications for Managing Downstream Flood Risk

Although knowledge of sediment transport has improved over the last 25 years, our understanding of bedload transfer and sediment delivery is still based on a limited set of observations or on models that make assumptions on hydraulic and sediment transport processes. This study utilises repeat lidar survey data of the River Caldew above the City of Carlisle in the UK to investigate the balance of erosion and deposition associated with channel switching from an engineered and managed single thread channel to a naturalising incipient wandering system. Over the 11-year survey period (four bankfull flood events) around 271,000 m3 of sediment were delivered to the river and floodplain and 197,000 m3 eroded suggesting that storage rates of around 7000 m3/annum occurred. The balance of erosion and deposition is influenced by channelisation with very restricted overbank sedimentation and only limited local and transient in-channel bar deposition along the engineered reach (8000 m3 eroded). This contrasts with the activity of the naturalising reach downstream where a developing wandering channel system is acting to store coarse sediment in-stream as large bar complexes and the associated upstream aggrading plane bed reaches and overbank as splay deposits (87,000 m3 stored). Such behavior suggests that naturalisation of channelised systems upstream of flood vulnerable urban areas can have a significant impact on sediment induced flooding downstream. This conclusion must, however, be moderated in the light of the relatively small volumes of material needed to instigate local aggradation in over-capacity urban channels.

Flooding and hydrologic connectivity modulate community assembly in a dynamic river-floodplain ecosystem.

Braided river floodplains are highly dynamic ecosystems, where aquatic communities are strongly regulated by the hydrologic regime. So far, however, understanding of how flow variation influences assembly mechanisms remains limited. We collected benthic chironomids and oligochaetes over a year across a lateral connectivity gradient in the semi-natural Tagliamento River (Italy). Four bankfull flood events occurred during the study, allowing the assessment of how flooding and hydrologic connectivity mediate the balance between stochastic and deterministic community assembly. While invertebrate density and richness were positively correlated with connectivity, diversity patterns showed no significant correlation. Species turnover through time increased with decreasing connectivity. Contrary to expectations, hydrologic connectivity did not influence the response of community metrics (e.g. diversity, density) to floods. Invertebrate composition was weakly related to connectivity, but changed predictably in response to floods. Multivariate ordinations showed that faunal composition diverged across the waterbodies during stable periods, reflecting differential species sorting across the lateral gradient, but converged again after floods. Stable hydrological periods allowed communities to assemble deterministically with prevalence of non-random beta-diversity and co-occurrence patterns and larger proportion of compositional variation explained by local abiotic features. These signals of deterministic processes declined after flooding events. This occurred despite no apparent evidence of flood-induced homogenisation of habitat conditions. This study is among the first to examine the annual dynamic of aquatic assemblages across a hydrologic connectivity gradient in a natural floodplain. Results highlight how biodiversity can exhibit complex relations with hydrologic connectivity. However, appraisal of the assembly mechanisms through time indicated that flooding shifted the balance from deterministic species sorting across floodplain habitats, towards stochastic processes related to organisms redistribution and the likely resetting of assembly to earlier stages.

Riparian Vegetation and Sediment Supply Regulate the Morphodynamic Response of an Experimental Stream to Floods

Feedbacks between woody plants and fluvial morphodynamics result in co-development of riparian vegetation communities and channel form. To advance mechanistic knowledge regarding these interactions, we measured the response of topography and flow to the presence of riparian tree seedlings with contrasting morphologies in an experimental, field-scale, meandering stream channel with a mobile sand bed. On a convex point bar, we installed seedlings of Tamarix spp. (tamarisk) and Populus fremontii (cottonwood) with intact roots and simulated a bankfull flood, with each of eight runs varying sediment supply, plant density, and plant species. Vegetation reduced turbulence and velocities on the bar relative to bare-bed conditions, inducing sediment deposition when vegetation was present, regardless of vegetation density or species. Sediment supply also played a dominant role, and eliminating sediment supply reduced deposition regardless of the presence of vegetation. Unexpectedly, plant density and species architecture (shrubby tamarisk versus single-stemmed cottonwood) had only a secondary influence on hydraulics and sediment transport. In the absence of plants, mobile bedforms were prominent across the bar, but vegetation of all types decreased the height and lateral extent of bedforms migrating across the bar, suggesting a mechanism by which vegetation modulates feedbacks among sediment transport, topography, and hydraulics. Our measurements and resulting insights bridge the gap between laboratory conditions and real dryland sand-bed rivers and motivate further morphodynamic modeling.

Dynamics of metals bound to suspended sediments in floods and on channel banks of the ephemeral Wadi Sekher, northern Negev desert, Israel

This study examines the variation in the concentration of eight heavy metals (Cu, Ni, Pb, Cd, Cr, Co, V, Sc) bound on suspended sediments during flow events and those deposited in channel banks. The study was undertaken in Wadi Sekher, an ephemeral flash-flood channel draining the Neot Hovav industrial chemical complex in the semi-arid northern Negev, Israel, at sites varying with bank height and distance from the industrial complex.Metal concentrations were found to be higher in the initial flow and recession than during peak flow. Bound metal concentrations were mainly affected by suspended sediment composition and concentrations. During peak flow the suspended sediment concentration rises and quartz sand is a major constituent. This leads to a decrease in bound metal concentrations, as the larger particles have a weaker affinity to binding. In six sediment samples, taken during the first 15 min of two floods, the bound metal concentrations changed similarly. These changes are suggested to represent variations in the source and cannot be attributed to dilution or binding effects.Six banks were sampled along Wadi Sekher, from the Neot Hovav hydrometric reach downstream to the confluence of Wadi Sekher and Wadi Beer-Sheva, at specified heights, following a bank-full flood. In every cross-section, a significant difference (p < 0.01) was seen in Cu, Cd, Co, V and Sc concentrations with bank height, with highest concentrations at bank-bed intersection. Downstream variations in bound metal concentrations along Wadi Sekher were statistically (p > 0.05) undifferentiated, indicating that the industrial area is not a source point for these metals, and that a constant input exists into the channel from tributaries along the channel.Due to the difficulty in sampling flash floods in ephemeral channels, existing data are not abundant, and our understanding of these environments is therefore limited. This study improves knowledge of interrelated variations in bound metal concentrations in fluvial deposits, in suspended sediment and in water during flash floods.

The influence of coarse particle mobility on scour depth in salmonid spawning habitat

AbstractThis study examined the influence of flow hydraulics and coarse particle mobility on bed scour adjacent to coho salmon (Oncorhynchus kisutch) redds in a coastal California watershed for a bankfull flood. It was theorized that coarse particle mobility (i.e., mobility of particles larger than the median bed particle size, D50) exerts a strong control on bed scour depth. Maximum scour depth at the study sites was found to be negatively correlated with flow shear stress, which is dissimilar to findings from previous scour studies in spawning reaches. This resulted from a relatively similar coarse particle size (D84) for all study sites and a negative relationship between shear stress and coarse particle exposure to flow (or the D84/D50 ratio), which together caused sites with low shear stress to have a high degree of localized coarse particle mobility and an associated high maximum scour depth. This study provides new insights into the vulnerability of spawning reaches with low flow energy to redd scour and highlights the need to consider the mobility of coarse particle sizes explicitly when examining the dominant controls on redd scour.

Bank erosion and large wood recruitment along a gravel bed river

Riverine environments can be very dynamic and complex systems, particularly because of the interaction between active channel and riparian land during flood events of different magnitude. In recent years increasing attention has been paid to large wood (LW), focusing on its role and impact along riverine systems and fluvial landscapes. This research aims to analyze the characteristics of LW recruitment as a consequence of a flood event along a reach of a gravel-bed river. The study was conducted on a 3 km-long reach located in the middle course of the Piave River (north-eastern Italian Alps). A 20 m-wide buffer zone was considered along the floodplains and islands. Every standing tree in this buffer with diameter ≥0.10 m was measured manually (diameter breast height; height), whereas shrubs were not considered. The most common species in the study area are: &lt;em&gt;Populus&lt;/em&gt; sp., &lt;em&gt;Salix&lt;/em&gt; sp., &lt;em&gt;Alnus&lt;/em&gt; sp., &lt;em&gt;Carpinus&lt;/em&gt; sp., &lt;em&gt;Fraxinus&lt;/em&gt; sp., &lt;em&gt;Pinus&lt;/em&gt; &lt;em&gt;sylvestris&lt;/em&gt; and &lt;em&gt;Robinia&lt;/em&gt; &lt;em&gt;pseudoacacia&lt;/em&gt;. An over bankfull flood (Q=1329 m&lt;sup&gt;3&lt;/sup&gt;s&lt;sup&gt;–1&lt;/sup&gt;; recurrence interval=6 years) in November 2014 caused erosions along the floodplain (15,565.5 m&lt;sup&gt;2&lt;/sup&gt;), pioneer islands (25.2 m&lt;sup&gt;2&lt;/sup&gt;) and building islands (2085.6 m&lt;sup&gt;2&lt;/sup&gt;), recruiting 690 trees. Four of these trees were recruited from the pioneer islands (0.16 tree m&lt;sup&gt;–2&lt;/sup&gt;), 79 from building islands (0.04 tree m&lt;sup&gt;–2&lt;/sup&gt;) and 607 from floodplains (0.04 tree m&lt;sup&gt;–2&lt;/sup&gt;). Accurate dendrometric measurements were used to define the input volume of LW from the floodplains (86.25 m&lt;sup&gt;3&lt;/sup&gt;), pioneer islands (0.14 m&lt;sup&gt;3&lt;/sup&gt;) and building islands (6.62 m&lt;sup&gt;3&lt;/sup&gt;). The maximum distance traveled by LW recruited from the floodplain, pioneer and building islands was 8927, 1021 and 3727 m, respectively. Statistical analysis showed no significant relationship between the displacement and LW characteristics considered (diameter, length, volume, density). These results demonstrate that the recruitment and subsequent transport of LW is a complex mechanism that requires further study. To better characterize these mechanisms, it appears clear that it is important to consider not only the LW characteristics but also the connection between LW, morphological settings, and flood event characteristics.

Estimating bankfull discharge and depth in ungauged estuaries

AbstractIt is difficult to measure river discharge accurately in an estuary, and particularly, in the region where the tidal flow dominates over the river discharge. River discharge is important for the morphology and hydrodynamics of estuaries as it influences the salt intrusion process, tidal dynamics, freshwater supply (water resources management), and the occurrence of floods. Here we try to derive river regime characteristics from the seaward end: the estuary. It is found that there are empirical relationships that link the geometry of an estuary to its river regime, which can be used to estimate river discharge characteristics with the least of data available. The aims of this study are: (1) to derive empirical relations between geometrical characteristics of estuaries and the bankfull discharge; (2) to explore a physical explanation for this relation; and (3) to estimate the bankfull discharge in estuaries. The physical connection between an estuary and its river regime is found by combining estuary shape analysis, tidal dynamic analysis, and Lacey's hydraulic geometry theory. The relationships found between the estuary depth, width, and bankfull river discharge have been tested in 23 estuaries around the world (including seven recently surveyed estuaries). From the analysis, it shows that the depth of an estuary is a function of the bankfull flood discharge to the power of 1/3, which is in agreement with Lacey's formula. This finding not only provides a method to estimate estuary depth, it also allows estimating flood discharge characteristics from readily available estuary shape indicators.

Analysis of flow characteristics around islands due to semi-diurnal currents in the Han River, Korea

In this study, an unsteady 2D depth-averaged flow model by Petrov-Galerkin stabilization was developed to investigate flow characteristics caused by semi-diurnal current at the tidal reach of the Han River. In order to analyze the velocity field around islands located in the mid-course of the Han River, simulations of the transient flow were conducted with principal lunar and solar semidiurnal constituent conditions. In addition, the influence of an artificial island construction located at the tributary confluence was quantitatively examined. The results show that, under normal flow condition, around Bam Island, main flow was inclined to the left half of the river span, and the velocity reversal was occurred during the flood tide which would result in the deposition sediment to form the Island. Under bank-full flood condition, velocities around No-deul Island were higher than those around Seon-yu Island reaching 3.5 m/s in maximum. The construction of the artificial island at the Jung-nang Stream confluence caused the significant reduction in the flood conveyance of the Han River and 90% increase in maximum velocity during the bank-full flood flow condition.

Survival of Casuarina cunninghamiana on a recovering sand-bed stream in the Wollombi Valley of coastal New South Wales.

The successful management and restoration of riparian corridors in Australia is currently hindered by our poor understanding of the links between hydrology, fluvial geomorphology and plant population dynamics. The aim of the current study was to determine the survival rates of Casuarina cunninghamiana subsp. cunninghamiana Miq. (family Casuarinaceae) on a sand-bed stream in the Wollombi Valley, a tributary of the Hunter River system, in coastal New South Wales (lat 33°02'S; long 151°10'E). A population planted on the banks of Dairy Arm in 1987, as part of river training works, was used to determine the long-term (24-year) survival rate. A bankfull flood in June 2011 provided an opportunity to examine the survival response of seedlings regenerating naturally within the channel. We found that 24% of the seedlings planted on the banks in 1987 had survived to adulthood. The bankfull flood significantly decreased (d.f. = 14; W= - 30; p = 0.0024) median seedling density within the channel from 12 to 2 individuals per 100 m 2 . Seedling survival varied with height, with seedlings > 15 cm more likely to survive the bankfull flood. T he percentage of seedlings partially buried by sediment was significantly higher (d.f. = 14; W = 13; p = 0.016) after flood compared to before flood. Seedling density was positively correlated with the amount of bare ground prior to the flood (r = 0.61; p = 0.02), but this relationship was no longer significant after flood (r = 0.18; p = 0.53). 37% of the seedlings surveyed showed evidence of grazing. Our results confirm that hydrogeomorphic processes associated with a bankfull flood affect the survival of Casuarina cunninghamiana seedlings. The management implications of our findings are discussed in terms of riparian revegetation techniques and the geomorphic recovery of over-widened sand-bed streams.

Interactions between river flows and colonizing vegetation on a braided river: exploring spatial and temporal dynamics in riparian vegetation cover using satellite data

ABSTRACTField, laboratory, and numerical modelling research are increasingly demonstrating the potential of riparian tree colonization and growth to influence fluvial dynamics and the evolution of fluvial landforms. This paper jointly analyses multi‐temporal, multispectral ASTER data, continuous river stage and discharge data, and field observations of the growth rates of the dominant riparian tree species (Populus nigra) along a 21 km reach of the Tagliamento River, Italy. Research focuses on the period 2004–2009, during which there was a bankfull flood on 24 October 2004, followed by 2 years with low water levels, nearly 2 years with only modest flow pulses, and then a final period from 15 August 2008 that included several intermediate to bankfull flow events. This study period of increasing flow disturbance allows the exploration of vegetation dynamics within the river's active corridor under changing flow conditions. The analysis demonstrates the utility of ASTER data for investigating vegetation dynamics along large fluvial corridors and reveals both spatial and temporal variations in the expansion, coalescence, and erosion of vegetated patches within the study reach. Changes in the extent of the vegetated area and its dynamics vary along the study reach. In sub‐reaches where riparian tree growth is vigorous, the vegetated area expands rapidly during time periods without channel‐shaping flows, and is subsequently able to resist erosion by bankfull floods. In contrast, in sub‐reaches where tree growth is less vigorous, the vegetated area expands at a slower rate and is more readily re‐set by bankfull flood events. This illustrates that the rate of growth of riparian trees is crucial to their ability to contribute actively to river corridor dynamics. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Late Quaternary climates and river channels of the Atlantic Coastal Plain, Southeastern USA

Late Quaternary river patterns and sedimentation styles reflect past climate changes in Atlantic drainages of eastern Georgia and the Carolinas. Past climate episodes for the region are summarized from pollen and eolian sedimentary records that exhibit correlation to major changes in river channel patterns. Records of past river responses are derived from luminescence and radiocarbon dated landforms and strata. Slow tectonic uplift does not explain temporal changes in channel patterns. Drainage networks have not received meltwater from continental or mountain glaciers. Eustatic influence would have been most pronounced on the now-submerged continental shelf prior to 6 ka and currently manifests only within 60–80 km of the present highstand shoreline. Human impact overshadows climatic changes during historic time, but was negligible during prehistoric time. The late Quaternary fluvial system primarily responded to climatic fluctuations that drove changes in vegetation, precipitation, and runoff patterns. Prehistoric meandering paleochannels of the Holocene stand as important references to guide “natural” channel rehabilitation efforts. During the coldest climate phases of the late Wisconsin stage (ca. 30–16 ka), when forest vegetation was sparse and interspersed with grasses and herbs, many rivers and streams were braided with eolian dunes on the floodplains. The terminal Wisconsin stage (ca. 16–11 ka) exhibited rapid warming and forest expansion characterized by pronounced change from braided to meandering patterns with intermediate forms of sandy scroll-bar patterns. The Younger Dryas cold excursion is not readily apparent in the regional fluvial record. Unusually large meandering channels of the terminal Wisconsin stage continue into the first half of the Holocene and indicate significantly larger bankfull flood discharges during those times than during the last half of the Holocene. Bankfull channels during the last half of the Holocene retained meandering patterns, but decreased to modern-like dimensions, suggesting more modern-like flood regimes. No clear fluvial response occurs to the presumed middle Holocene climatic optimum (Hypsithermal), the Medieval warm period and Little Ice Age, or to recent instrumentally-observed fluctuations in climate, but little research has been done in these realms.

Hydrodynamics of a forced riffle pool in a gravel bed river: 1. Mean velocity and turbulence intensity

Riffle pools are fundamental units of many gravel bed rivers. Considerable debate exists as to an appropriate hydrodynamic model for this bed form type. Sampling designs in previous studies have not always anticipated the degree of spatial variability of flow parameters in riffle pools nor considered the effect of the nonuniform boundary on hydrodynamics. The objective of this paper is to detail the distribution of mean velocities and turbulence intensities in a forced riffle pool. While limited by the lack of lateral velocity measurements, the data set is of high quality and includes measurements during a bankfull flood. Key observations include the perturbation of velocity profiles from what is observed in uniform flow, lateral convergence and divergence of flow, and the generation of turbulence away from the channel boundary. In the thalweg of the pool head, velocities are high near the water surface and low near the bed. Turbulence intensities are relatively high near the bed and may be significant for sediment entrainment. Higher mean velocities occur over the side bar and could indicate that sediment is routed around the thalweg. The midpool is characterized by the lateral constriction of flow and a shear zone downstream of the forcing element. The pool tail has very high velocities and turbulence intensities near the bed. A velocity reversal occurs in the pool tail at bankfull flow but does not occur in the pool head. A consideration of deceleration and acceleration as a result of vertical expansion and contraction is shown to explain many of the key observations.

Response of bankfull flood magnitudes to Holocene climate change, Uinta Mountains, northeastern Utah

Long-term variations in Holocene fl ood magnitude were quantifi ed from the bankfull dimensions of abandoned channels preserved on flsurfaces in the northern Uinta Mountains of northeastern Utah. Cross-sectional areas of abandoned channels were reconstructed, and relationships derived from the modern gage records were used to estimate bankfull discharges from bankfull cross-section areas. The results indicate systematic (nonrandom) variations of bankfull fl oods in the northern Uinta Mountains. Large fl oods, as much as 10%‐15% greater than modern, dominated from 8500 to 5000 calendar yr B.P., and again from 2800 to 1000 cal yr B.P. Small fl oods, as much as 15%‐20% less than modern, characterize the periods from 5000 to 2800 cal yr B.P., and from 1000 cal yr B.P. to near present. The middle and late Holocene record of bankfull fl ood magnitude compares well with independent evidence for climatic variation in the area. The early Holocene record indicates that larger than modern bankfull fl oods coincide with warmer than modern mean annual temperature. We hypothesize that an increased range of magnitude for seasonal solar radiation during the early Holocene favored the accumulation and rapid melting of deep snowpacks in the high Uinta Mountains, thus producing large fl oods despite warmer mean annual temperatures. The episode of smaller than modern bankfull fl oods between 5000 and 2800 cal yr B.P. coincides with records of increased forest fi re frequency in the northern Uintas. Larger than modern fl oods from 2800 to 1000 cal yr B.P. coincide with a local decrease in forest fi re frequency and evidence for minor local glacial readvances. The decrease in fl magnitudes following 1000 cal yr B.P. corresponds to numerous local and regional records of warming during the Medieval Climatic Anomaly.

The Vulnerability of Lower Englishman River to Modelled Climate Change

It is generally accepted in scientific circles that the earth’s atmosphere is warming, and that this warming trend is projected to increase as a result of atmospheric carbon dioxide doubling, by the end of this century. Such warming will affect regional precipitation patterns and thus river hydrology. This study focuses on the effects of modelled regional climate change on the frequency and magnitude of flooding along the floodplain of the Englishman River on the east coast of central Vancouver Island, British Columbia. Using the results of a regional climate model (Reynolds, 2002), we found that there will be changes to the flood regime of the river. Peak annual flows may be 8% larger by 2020, 14% larger by 2050 and 17% larger by 2080. This means that an increase in the frequency and magnitude of flows is likely in the future. For example by 2020, the 15-year flood is expected to have a slightly greater magnitude than the current 20-year flood, and by 2080, the 10-year flood is expected have the same magnitude as the current 20-year flood. The changes in flood magnitudes will have significant impacts on people living on the floodplain. Large areas of the floodplain are currently occupied by houses, and much of the remaining area is zoned for further subdivisions. The current bankfull flood is predicted to increase, and for this to occur there will be a change in the morphology of the channel.

Rolling Stones and Mosses: Effect of Substrate Stability on Bryophyte Communities in Streams

Effects of 3 disturbance parameters (% of the streambed in motion at bankfull discharge [P BF ], catchment-specific discharge [SPECQ BF ], and frequency of bankfull floods [N BF ] on aquatic bryophyte richness and community composition were assessed. Relationships between taxonomic richness and P BF and SPECQ BF were strongly curvilinear, such that richness was low in both stable and highly unstable streams, and in streams of both low and high SPECQ BF . Richness was highest in streams with moderate P BF and SPECQ BF . Interspecific competition may have reduced species richness at stable sites. In contrast, only a few bryophyte taxa were able to grow at unstable sites. Low richness at sites with a low SPECQ BF most likely reflects the intolerance of some species to desiccation. Low richness at sites with high SPECQ BF may reflect differences in the resistance of different taxa to high discharges. Community compo...

Influence of large woody debris and a bankfull flood on movement of adult resident coastal cutthroat trout (Oncorhynchus clarki) during fall and winter

To improve understanding of the significance of large woody debris to stream fishes, we examined the influence of woody debris on fall and winter movement by adult coastal cutthroat trout (Oncorhynchus clarki) using radiotelemetry. Fish captured in stream pools containing large woody debris moved less than fish captured in pools lacking large woody debris or other cover. Fish from pools lacking cover commonly moved to habitats with large boulders or brush, particularly during the day. Movements by fish over 1-day periods were strongly influenced by large woody debris or other elements providing cover. Fish initially found in habitats lacking large woody debris, large boulders, or brush cover moved the most extensively, while fish initially found in pools with large woody debris moved the least. Fish did not move extensively in response to a bankfull flood, although some moved to habitat downstream of large woody debris in tributaries or secondary channels. Habitat downstream of woody debris in the main channel was not used during the flood, apparently because of extreme turbulence. Overall, these observations provide additional evidence for the value of habitat complexity to some stream fishes and support previous observations of minimal effects of flooding on adult fish.

Influence of large woody debris and a bankfull flood on movement of adult resident coastal cutthroat trout (&lt;i&gt;Oncorhynchus clarki&lt;/i&gt;) during fall and winter

Influence of large woody debris and a bankfull flood on movement of adult resident coastal cutthroat trout (&lt;i&gt;Oncorhynchus clarki&lt;/i&gt;) during fall and winter

SEDIMENT TRANSPORT PROCESSES IN POOL–RIFFLE SEQUENCES

This paper reviews the published data for sediment transport in pool–riffle sequences which suggests that a velocity or shear stress reversal hypothesis does not explain all of the published evidence of sediment transport. This conclusion is explored in more detail using observations of sediment transport from the regulated River North Tyne. Sediment transport rates at discharges of <30 per cent bankfull are initially highest on riffles. As discharge increases, sediment transport rates in pools rise sharply, and values locally exceed those on riffles. Tracing experiments using a combination of magnetic and painted tracers are used to show the downstream dispersal of fine and coarse sediments through the pool–riffle sequence. Coarser particles experience longer transport paths and faster virtual rates of travel in pools during bankfull floods. Analysis of entrainment data reveals evidence for additional restraint operating on riffles and causing higher dimensionless entrainment thresholds. An appraisal of the possible mechanisms controlling sediment entrainment, transport and deposition in pool–riffle sequences is made which reveals evidence for the operation of a combination of hydraulic, sedimentological and interactive processes. A descriptive model of sediment transport processes in pool–riffle sequences is presented.

Fine sediment infiltration into gravel spawning beds within a regulated river experiencing floods: Ecological implications for salmonids

AbstractInfiltration rates into eight gravel spawning beds were monitored over a range of discharges including compensation flows, peak hydropower generation and floods of up to bankfull capacity. The results indicate that the interrelationship of sediment supply and discharge influence both the rate and grain size composition of infiltrated sediments, with maximum rates experienced during bankfull floods when sediments are scoured from upstream pools. The effects of river regulation for hydropower are shown to produce a finer matrix infill in the absence of unregulated tributary sources, although the rates of infiltration are much lower than for sites downstream of unregulated tributaries. Values for infiltration rates are shown to be detrimental for small salmonid spawning redds during post‐flood conditions when sediment transport is not restrained by supply.