Pools And Riffles Research Articles

Changing Character of Pool –Riffle Sequence: A Quantitative Representation of Long Profile of Ichamati, India

2 Abstract: The river Ichamati, which drains the eastern part of the district North 24 Pgs. W.B., India, has been analyzed regarding its long profile of the two years( 2004 and 2012) based on two windows. This paper has focused on the character of long profile such as, pool-riffle sequence and deviation index, which reveal the evolutionary tendency of long profile. Bed deformation technique (bdt) has identified the number and position of the pool and riffle on the riverbed. The classification of pool and riffle has been done with the help of tolerance value (T). The magnitude of T values is different in two windows. According to tolerance value (T), the position and number of pool and riffle has been changed significantly in two windows in two years. The threshold value of T has also been differed from 2004 to 2012. All these indicate the gradual decay of the river Ichamati.

Mixing effects on nitrogen and oxygen concentrations and the relationship to mean residence time in a hyporheic zone of a riffle-pool sequence

Flow paths and residence times in the hyporheic zone are known to influence biogeochemical processes such as nitrification and denitrification. The exchange across the sediment-water interface may involve mixing of surface water and groundwater through complex hyporheic flow paths that contribute to highly variable biogeochemically active zones. Despite the recognition of these patterns in the literature, conceptualization and analysis of flow paths and nitrogen transformations beneath riffle-pool sequences often neglect to consider bed form driven exchange along the entire reach. In this study, the spatial and temporal distribution of dissolved oxygen (DO), nitrate ( NO3−) and ammonium ( NH4+) were monitored in the hyporheic zone beneath a riffle-pool sequence on a losing section of the Truckee River, NV. Spatially varying hyporheic exchange and the occurrence of multi-scale hyporheic mixing cells are shown to influence concentrations of DO and NO3− and the mean residence time (MRT) of riffle and pool areas. Distinct patterns observed in piezometers are shown to be influenced by the first large flow event following a steady 8 month period of low flow conditions. Increases in surface water discharge resulted in reversed hydraulic gradients and production of nitrate through nitrification at small vertical spatial scales (0.10–0.25 m) beneath the sediment-water interface. In areas with high downward flow rates and low MRT, denitrification may be limited. The use of a longitudinal two-dimensional flow model helped identify important mechanisms such as multi-scale hyporheic mixing cells and spatially varying MRT, an important driver for nitrogen transformation in the riverbed. Our observations of DO and NO3− concentrations and model simulations highlight the role of multi-scale hyporheic mixing cells on MRT and nitrogen transformations in the hyporheic zone of riffle-pool sequences.

Morphodynamic response of a variable‐width channel to changes in sediment supply

AbstractRiver channels commonly exhibit downstream variations in channel width, which can lead to the development of alternating shallow and deep areas known as riffle‐pool sequences. The response of these channels to variations in sediment supply remains largely unexplored. Here we investigate the morphodynamic response of a variable‐width channel to changes in sediment supply through laboratory experiments conducted in a straight flume in which we imposed sinusoidal variations in width. We first developed equilibrium conditions under a constant sediment supply and then eliminated the sediment feed to create a degraded, armored bed. This sediment‐starved bed was subjected to two types of sediment supply increases: a return to the initial constant supply, and the introduction of a well‐sorted sediment pulse (analogous to gravel augmentation). Riffles and pools formed in wide and narrow areas, respectively, and the location of and relief between riffles and pools remained the same throughout all experimental runs, regardless of the sediment supply. The primary channel response to changes in supply was adjustment of the overall slope. The sediment pulse evolved primarily through dispersion rather than translation, which contrasts with prior gravel augmentation experiments conducted in constant‐width channels and suggests that width variation and resulting riffle‐pool topography enhances pulse dispersion. Our results indicate that width variation is a primary control on the location and relief of riffles and pools in straight channels, and sediment supply changes are unlikely to affect riffle‐pool morphology when bank geometry is fixed and water discharge is steady.

Pre- and postrestoration assessment of stream water–groundwater interactions: effects on hydrological and chemical heterogeneity in the hyporheic zone

Reach-scale stream restoration with natural channel design is often used to improve stream ecosystem structure and function. Some investigators have studied the effects of restoration on the hyporheic zone, but most used space-for-time substitution instead of comparing the same reach before and after restoration. We examined spatial patterns of hyporheic exchange rates and geochemistry during base flow in a 30-m pool–riffle–pool sequence before and 1 y after the stream was restored by installation of a cross-vane and engineered rock-riffle. Prerestoration vertical hyporheic exchange rates were relatively uniform across the riffle bedform. Average downwelling was 30 cm/d at or upstream of the riffle, and average upwelling was 30 cm/d downstream of the riffle. Downwelling hyporheic exchange rates increased up to an order of magnitude adjacent to the cross-vane and in the engineered rock-riffle. Prerestoration porewater [NO3−] was distributed along a wide and continuous gradient (0.1–3.8 mg/L), with higher concentrations in areas of downwelling and lower concentrations in areas of upwelling. Postrestoration [NO3−] was distributed bimodally with relatively high concentrations (>1.6 mg/L) adjacent to the cross-vane and low concentrations (<1 mg/L) elsewhere. These results suggest that rapid flushing of surface water through the subsurface creates residence times too short to yield net changes in [NO3−] along flow paths around the cross-vane. Distinct zones of groundwater upwelling were present before and after restoration, but the size of the zone of upwelling increased downstream of the cross-vane after restoration, possibly because higher permeability material was placed over the original, lower permeability sediment during restoration, creating an effective anisotropy that favored horizontal flow and produced larger apparent zones of groundwater discharge.

Applying fluvial geomorphological riffle-pool sequences concept when rebuilding the existing drop hydraulic structure

The paper deals with the problem of rebuilding the existing water straight drop structure in Brenna on the Brennica river (Polish Carpathian mountains), which was changed into the rapid hydraulic structure. The technical project was set up in 1988 and finished in the same year. The structure was rebuilt in the field in the early autumn of 1990. One of the concepts of applied fluvial geomorphological solution was used to improve the river channel bed condition. In that case it was found that the existing hydraulic structure reducing river slope and stabilizing river bed can be changed without any harm in to semi-natural riffle structure which could be tolerated by river and organisms living in. Artificial roughness of the slope plate of the rapid hydraulic structure was obtained by placing cobbles along all the slope apron of the structure. The diameter of cobbles was calculated applying various methods, and the optimum value for that dimension was chosen. The cobbles, used for rebuilding purposes, were taken directly from the riverbed, so that the structure is environmentally similar to the site. All work was done due to European Framework Directive for Rivers.

Coupling channel evolution monitoring and RFID tracking in a large, wandering, gravel-bed river: Insights into sediment routing on geomorphic continuity through a riffle–pool sequence

Bedload transport and bedform mobility in large gravel-bed rivers are not easily monitored, especially during floods. Large reaches present difficulties in bed access during flows for flow measurements. Because of these logistical issues, the current knowledge about bedload transport processes and bedform mobility lacks field-based information, while this missing information would precisely match river management needs. The lack of information linking channel evolution and particle displacements is even more striking in wandering reaches. The Durance River is a large, wandering, gravel-bed river (catchment area: 14,280km2; mean width: 240m), located in the southern French Alps and highly impacted by flow diversion and gravel mining. In order to improve current understanding of the link between sediment transport processes and river bed morphodynamics, we set up a sediment particle survey in the channel using Radio Frequency Identification (RFID) tracking and topographic surveys (GPS RTK and scour chains) for a 4-year recurrence interval flood. By combining topographic changes before and after a flood, intraflood erosion/deposition patterns from scour chains, differential routing of tracer particles, and spatial distribution of bed shear stress through a complex reach, this paper aims to define the critical shear stress for significant sediment mobility in this setting. Gravel tracking highlights displacement patterns in agreement with bar downstream migration and transport of particles across the riffle within this single flood event. Because no velocity measurements were possible during flood, a Telemac three-dimensional model helped interpret particle displacements by estimating spatial distribution of shear stresses and flow directions at peak flow. Although RFID tracking in a large, wandering, gravel-bed river does have some technical limitations (burial, recovery process time-consuming), it provides useful information on sediment routing through a riffle–pool sequence.

Virtual manipulation of topography to test potential pool–riffle maintenance mechanisms

In this study, numerical experimentation with two-dimensional hydraulic modeling of pool–riffle river topography drawing on the testbed data from the classic Keller (1971) study was used to investigate the effect of synthetically manipulating topography on the occurrence and magnitude of velocity and Shields stress reversals in a pool–riffle sequence. Reversals in velocity and shear stress have been used to explore mechanisms of pool–riffle maintenance, while Shields stress (a combined measure of transport capacity and substrate erodibility) is emerging in importance. The original site topography was modeled alongside six altered ones to evaluate the sensitivity of hydraulic reversals to subtle morphology — five incrementally wider pools and a filled pool. The Caamaño (2009) criterion, a simplified geometric threshold for predicting velocity reversals, was applied to each terrain to evaluate its utility. The original pool–riffle topography was just over the threshold for a velocity reversal and well over the threshold for a strong Shields stress reversal. Overall, pool widening caused a predominantly local response, with change to pool hydraulics and no change in section-averaged velocity in the riffle beyond the initial widening of 10%. Filling in the pool significantly increased the magnitude of reversals, whereas expanding it eliminated the occurrence of a reversal in mean velocity, though the Shields stress reversal persisted because of strong differentiation in bed material texture. Using Shields stress as a reversal parameter enabled the quantification of pool modification effects on pool–riffle resiliency. The Caamaño (2009) criterion accurately predicted reversal occurrence for the altered terrains with exaggerated effects, but failed to predict the weak reversal for the original topography. Two-dimensional modeling coupled with previously accepted hydrologic, geomorphic, and engineering analyses is vital in project design and evaluation prior to construction.

Hyporheic zone exchange fluxes and residence times inferred from riverbed temperature and radon data

Summary Vertical profiles of temperature, radon and electrical conductivity are used to characterise downwelling, neutral and upwelling hyporheic zones along a pool–riffle sequence in the Haughton River in north-eastern Australia. Water residence times and vertical fluxes are derived from temperature and radon data and then directly compared for downwelling profiles. Temperature and radon-derived fluxes in downwelling zones ranged from 0.02 to 24 m day −1 with a mean of 1.69 m day −1 while residence times across the study site ranged from tens of minutes to greater than 15 days. The radon approach has the lowest uncertainty for residence times between 0.1 and 15 days while the uncertainty of the temperature approach (using a diel river signal) is lowest for residence times that are less than a few days. For 83% of depths in downwelling profiles, radon-derived residence times were greater (some up to two orders of magnitude greater) than temperature-derived residence times. When the error bounds of the residence time estimates were accounted for, 57% of radon-derived residence times were considerably greater than temperature-derived residence times in downwelling profiles. We suggest that this disparity is due to the different influence of small scale heterogeneity on temperature and radon transport. These field based results indicate that small scale heterogeneity may play a far more important role than has been previously considered in groundwater–surface water interaction studies.

Restoring riffle-pool structure in an incised, straightened urban stream channel using an ecohydraulic modeling approach

Streams in urban and urbanizing watersheds are impacted by altered runoff hydrology and sediment yields, floodplain modifications, and channel relocations. One morphological response to urbanization is degradation of riffle-pool sequences. Pools and riffles are fundamental mesohabitat units where many lotic biota have evolved to occupy preferentially, and support diverse biological communities. Restoring self-maintaining pool-riffle structures is essential to the ecological rehabilitation of urban streams when lost. However restoring these structures can be problematic in straightened urban streams constrained by civil infrastructure preventing channel re-meandering. The project goal was to utilize geomorphic, hydraulic, and ecological principles to rehabilitate a study reach in a confined geomorphic setting with a primary emphasis on developing a stable riffle design that improves habitat patch dynamics. A 270-m tree-lined study site was selected on Beaver Creek, Knox County, East Tennessee. Through experimental 3D and 2D hydraulic modeling, a riffle-pool design was developed consisting of removing trees at expanded channel locations, placing 3.8-cm gravel substrate for the riffle bed, and deepening the pool prior to riffle entrance. Riffle-pool maintenance processes of the proposed design included occurrence of shear stress reversal between low- and high-flows, and high-flow acceleration-deceleration from pools to riffles. Cobble was interspersed on riffle surfaces for leaf pack formation. Root wads were positioned at bank locations potentially vulnerable to erosion. River2D provided useful design information to assess pre-construction channel stability and habitat quality. In this case study, an ecohydraulic modeling approach to urban stream restoration is described. Construction of four riffle-pool structures was completed in March 2012, and a geomorphic survey completed in April 2013 observed the riffle structures remained stable even with the project site experiencing eight bankfull events. Post-construction monitoring has shown that the unique design for planform-constrained urban channels has promise for increasing hydraulic habitat diversity and improving biotic integrity in these stressed environments.

Numerical modeling of bed form induced hyporheic exchange

The hyporheic zone is a region beneath and alongside a stream, river, or lake bed where shallow groundwater and surface water mix. Field and experimental observations, along with modeling studies, indicate that hyporheic exchange occurs mainly in response to pressure gradients driven by the geomorphological features of stream beds. Flow over a pool-riffle sequence creates an irregular pressure gradient that drives hyporheic exchange. Currently to analyze the overall flow pattern in different types of pool-riffle structures, hyporheic exchange flow was analyzed using a fully coupled hydro-dynamic model. Simulation results showed that recirculation zones and stagnation points in the pool-riffle structures dominantly controlled the upwelling and downwelling patterns. Numerical simulations were analyzed for the velocity distribution, velocity vectors, and the streamline and flux of groundwater and surface water. Upwelling flow was dominated by a pressure gradient generated by the apex of riffles. Downwelling flow patterns were affected by the flow pattern formed in pools, which was related to the geometric shapes of the pools. The mixing pattern in the groundwater was also affected by the pool shape. The results will be applicable for river restoration projects and stream ecology related to hyporheic exchange, in the prediction and management of upwelling and downwelling flow induced by bed forms.

The Effect of Road Crossing on River Morphology and Riverine Aquatic Life: A Case Study in Kunur River Basin, West Bengal

Road crossings play important role in land transport system, but improper designed stream crossing may alter the form and function of stream geomorphology, floodplain ecosystem, habitat and movement of aquatic organisms. The objective of this paper is to explain the local level impact of a road crossing on channel morphology and aquatic lives of the Kunur River, based on geomorphic survey and stream classification. This study shows a comparison between upstream and downstream reaches of Kunur River where the State Highway-14 crossed the river over a bridge. There are changing fluvio-geomorphic indicators like mean and maximum bankfull channel depth, cross-section area, flow velocity, entrenchment ratio, channel bed gradient, water level and depth, braided index (BI), pool-riffle sequence, area and shape of bars, and others taken into consideration. Results are temporally increases of BI and expansions of bars. In the downstream, river channel is deeply incised by massive bed degradation and highly entrenched with high flow velocity and steep channel bed gradient, low entrenchment ratio (&lt;2.2), low width-depth ratio (11.26) and increasing inter pool-riffle distance. Present study reveals that bank failure is a common phenomenon here with deep scours and making unsuitable environment for fish habitat. Alternatively, it has appeared to be greater deposition of fine sediment in the upstream reaches with high braiding index, area of bars. High entrenchment ratio (&gt;2.2) and width-depth ratio (&gt;35) also indicates well developed floodplain ecosystem here.Key Words: Anthropogenic feature; channel morphology; bed degradation; bank failure; fine sediment

The hydro-cryologic continuum of a steep watershed at freezeup

• Freezeup is a very dynamic period of the year. • Ice dams store and release substantial volumes of water in the drainage system . • Stage and discharge variations can be associated with specific weather conditions. • Ice processes in small channels significantly affect the discharge of larger channels. • Monitoring simple parameters can improve our discharge estimations at freezeup. Ice processes that occur in steep (gravel bed) channels are completely different from those occurring in mild-gradient rivers. As such, the impact of ice on water level and discharge in steep streams is different than what is usually found documented for large channels. Whereas in large mild rivers, the development of a surface ice cover generates a nominal increase in stage in the order of 30%, in steep channels, the formation of series of ice dams caused the water level to rise by up to 500%. The increase in stage caused a maximum estimated ice-induced discharge depression of 50%. The ice-dam stage may represent the highest level of the year whereas the freezeup discharge may temporarily drop to its annual minimum. Ice formations significantly alter local channel hydraulic conditions (bed elevations, cross-sectional area, riffle-pool sequences, etc.) and both stage and discharge vary drastically in a short time. ‘Dynamic’ ice processes (particularly the formation of ice dams) in steep channels depend on morphology and are driven by heat fluxes into and out of the system. Based on the authors observations of many steep channels over many winters, the inter-relationships (synchronicity) between air temperature fall (and rise), ice dam formation (and breaching), water level rise (and drop) and discharge depletion (and increase) are documented here. This paper presents air and water temperatures, water stages, discharges, and ice processes taking place on an hourly basis in four channels of increasing Strahler order. The data and hydro-crologic continuum analyses presented significantly improve our understanding and hence our ability to interpret freezeup water level signals. The paper also discusses how hydrographers may use this understanding and additional data to provide better estimates of discharge in steep channels at freezeup.

Using heat as a tracer to estimate spatially distributed mean residence times in the hyporheic zone of a riffle-pool sequence

[1] Biochemical reactions that occur in the hyporheic zone are highly dependent on the time solutes that are in contact with sediments of the riverbed. In this investigation, we developed a 2-D longitudinal flow and solute-transport model to estimate the spatial distribution of mean residence time in the hyporheic zone. The flow model was calibrated using observations of temperature and pressure, and the mean residence times were simulated using the age-mass approach for steady-state flow conditions. The approach used in this investigation includes the mixing of different ages and flow paths of water through advection and dispersion. Uncertainty of flow and transport parameters was evaluated using standard Monte Carlo and the generalized likelihood uncertainty estimation method. Results of parameter estimation support the presence of a low-permeable zone in the riffle area that induced horizontal flow at a shallow depth within the riffle area. This establishes shallow and localized flow paths and limits deep vertical exchange. For the optimal model, mean residence times were found to be relatively long (9–40.0 days). The uncertainty of hydraulic conductivity resulted in a mean interquartile range (IQR) of 13 days across all piezometers and was reduced by 24% with the inclusion of temperature and pressure observations. To a lesser extent, uncertainty in streambed porosity and dispersivity resulted in a mean IQR of 2.2 and 4.7 days, respectively. Alternative conceptual models demonstrate the importance of accounting for the spatial distribution of hydraulic conductivity in simulating mean residence times in a riffle-pool sequence.

Temporal and spatial response of hyporheic zone geochemistry to a storm event

AbstractAlthough there has been recent focus on understanding spatial variability in hyporheic zone geochemistry across different morphological units under baseflow conditions, less attention has been paid to temporal responses of hyporheic zone geochemistry to non‐steady‐state conditions. We documented spatial and temporal variability of hyporheic zone geochemistry in response to a large‐scale storm event, Tropical Storm Irene (August 2011), across a pool–riffle–pool sequence along Chittenango Creek in Chittenango, NY, USA. We sampled stream water as well as pore water at 15 cm depth in the streambed at 14 locations across a 30 m reach. Sampling occurred seven times at daily intervals: once during baseflow conditions, once during the rising limb of the storm hydrograph, and five times during the receding limb. Principal component analysis was used to interpret temporal and spatial changes and dominant drivers in stream and pore water geochemistry (n = 111). Results show the majority of spatial variance in hyporheic geochemistry (62%) is driven by differential mixing of stream and ground water in the hyporheic zone. The second largest driver (17%) of hyporheic geochemistry was temporal dilution and enrichment of infiltrating stream water during the storm. Hyporheic sites minimally influenced by discharging groundwater (‘connected’ sites) showed temporal changes in water chemistry in response to the storm event. Connected sites within and upstream of the riffle reflected stream geochemistry throughout the storm, whereas downstream sites showed temporally lagged responses in some conservative and biogeochemically reactive solutes. This suggests temporal changes in hyporheic geochemistry at these locations reflect a combination of changes in infiltrating stream chemistry and hyporheic flowpath length and residence time. The portion of the study area strongly influenced by groundwater discharge increased in size throughout the storm, producing elevated Ca2+ and concentrations in the streambed, suggesting zones of localized groundwater inputs expand in response to storms. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Spatial variations in surface sediment structure in riffle–pool sequences: a preliminary test of the Differential Sediment Entrainment Hypothesis (DSEH)

ABSTRACTRiffle–pool sequences are maintained through the preferential entrainment of sediment grains from pools rather than riffles. This preferential entrainment has been attributed to a reversal in the magnitude of velocity and shear stress under high flows; however the Differential Sediment Entrainment Hypothesis (DSEH) postulates that differential entrainment can instead result from spatial sedimentological contrasts. Here we use a novel suite of in situ grain‐scale field measurements from a riffle–pool sequence to parameterize a physically‐based model of grain entrainment. Field measurements include pivoting angles, lift forces and high resolution digital elevation models (DEMs) acquired using terrestrial laser scanning, from which particle exposure, protrusion and surface roughness were derived. The entrainment model results show that grains in pools have a lower critical entrainment shear stress than grains in either pool exits or riffles. This is because pool grains have looser packing, hence greater exposure and lower pivoting angles. Conversely, riffle and pool exit grains have denser packing, lower exposure and higher pivoting angles. A cohesive matrix further stabilizes pool exit grains. The resulting predictions of critical entrainment shear stress for grains in different subunits are compared with spatial patterns of bed shear stress derived from a two‐dimensional computational fluid dynamics (CFD) model of the reach. The CFD model predicts that, under bankfull conditions, pools experience lower shear stresses than riffles and pool exits. However, the difference in sediment entrainment shear stress is sufficiently large that sediment in pools is still more likely to be entrained than sediment in pool exits or riffles, resulting in differential entrainment under bankfull flows. Significantly, this differential entrainment does not require a reversal in flow velocities or shear stress, suggesting that sedimentological contrasts alone may be sufficient for the maintenance of riffle–pool sequences. This finding has implications for the prediction of sediment transport and the morphological evolution of gravel‐bed rivers. Copyright © 2012 John Wiley &amp; Sons, Ltd.

MAGNITUDE, FREQUENCY AND DURATION OF INSTREAM FLOWS TO STIMULATE AND FACILITATE CATADROMOUS FISH MIGRATIONS: AUSTRALIAN BASS (MACQUARIA NOVEMACULEATA PERCIFORMES, PERCICHTHYIDAE)

ABSTRACTThe migratory response and behaviour of catadromous Australian bass with regard to hourly mean river flows and water temperatures was assessed over 15 months. Fish movement was assessed using a 75‐km passive acoustic telemetry array in the regulated Shoalhaven River below Tallowa Dam, NSW, Australia. The majority (62%) of downstream pre‐spawning migrations from freshwater to estuarine habitats were stimulated by a series of flow pulses from April to September, but a proportion of fish (38%) commenced downstream migrations under regulated baseflow conditions after a sustained decrease in water temperature to below 15°C in late autumn. Equal numbers of fish undertook post‐spawning upstream return migrations during flow pulses and during regulated baseflow conditions, with regulated baseflow migrants exhibiting a preference for dusk–dawn passage through freshwater pool–riffle sequences. The median magnitude of flow pulses at the time of commencement of downstream and upstream freshwater migrations by Australian bass was not large, equivalent to natural (in the absence of river regulation) flows equalled or exceeded for 56% and 48% of time, respectively. There was no evidence for increased numbers of migrants with increasing flow pulse magnitude, with individual fish ignoring some flow pulses but responding to subsequent events. In regulated rivers, the release of more frequent flow pulses with peak magnitudes approximating the natural 50th flow duration percentile may be more effective in stimulating greater numbers of Australian bass to undertake pre‐spawning and post‐spawning migrations between freshwater and estuarine habitats than the release of a single, larger event. The propensity of Australian bass to also undertake spawning migrations under regulated baseflow conditions emphasizes the need for provision of baseflow regimes in regulated rivers that can facilitate migrations by large bodied fishes. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Hydro-morphologically related variance in benthic drift and its importance for numerical habitat modelling

Numerical fish-habitat modelling on various scales is considered to be state of the art in river management. However, most of the concepts applied use steady-state hydraulic parameters such as flow velocity and water depth. Herein we present analysis and discussion of the possibility of including a drift-feeding parameter (SIF) into habitat evaluations based on multiplying suitability indices. “Sources” and “sinks” of benthic drift were identified according to both the zero-crossing and hydraulic-threshold methods in an alpine gravel-bed river. Minor differences could be determined between the two methods in a well-developed riffle–pool section. Macroinvertebrates, used for simulating benthic drift, were collected by multi-habitat sampling and appraised according to their critical threshold (τcr) for motion on the bed surface and sinking velocity (vs). The findings of the calculation of drift rates using one- (1D) and two-dimensional (2D) hydrodynamic numerical models highlight a specification of best feeding position for drift-feeding fish (i.e. brown trout, grayling) considering the SIF parameter. Riffle–pool sequences are characteristic of pristine alpine streams; our findings underline their importance as production (riffles) and consumption areas (pools) in terms of holistic river function. Moreover, the results indicate that (artificial) lateral obstruction (e.g. dams) may lead to a reduced transport rate of benthic organisms due to low bottom shear stress (<0.25 N m−2). Thus, deposition of drifting macroinvertebrates occurs in backwaters, with downstream impacts on benthic and fish communities.

Short- and long-term impacts of a major flood event on crayfish (Orconectes obscurus) in a forested stream

Due to their large, blunt bodies, crayfish are particularly vulnerable to dislodgment at high flows associated with flood events. However, little is known about their resistance/resilience to intense spates. In this study, we examined short- and long-term size-dependent recovery by a lotic crayfish (Orconectes obscurus) population following a high discharge flood event associated with Hurricane Katrina (2005). Short-term population recovery was assessed using mark-recapture sampling in a riffle-shallow pool-riffle sequence. Crayfish were marked with Visible Implant Elastomer tags the morning of the rainstorm and recapture attempts occurred 2, 8, 10, and 16 days post-flood. The crayfish population in riffles returned to pre-flood densities within 10―16 days, however, the shallow pool population did not. Recovery was size-dependent with small crayfish returning to pre-flood densities within 16 days, while large crayfish did not recover during this time interval. Although crayfish showed short-term effects, this flood event had limited impact on crayfish densities on a longer time scale (assessed using seasonal density estimates collected on a reach scale). For example, summer 2005 (∼2 months pre-flood) and summer 2006 (9 months post-flood) densities were not statistically different from each other for either small or large crayfish. Small crayfish resilience/resistance may result from movement into areas of lower hydraulic stress to avoid high current velocities, whereas large crayfish may require a complete growing season for recovery. Resistance to flooding by small crayfish may be the key component to long-term population persistence of crayfish populations in flood-prone streams.

Flow phenomena and mechanisms in a field-scale experimental meandering channel with a pool-riffle sequence: Insights gained via numerical simulation

[1] Large-eddy simulation of turbulent flow through a natural-like meandering channel with pool-riffle sequences installed in the St. Anthony Falls Laboratory Outdoor StreamLab is carried out to elucidate the hydrodynamics at bankfull flow condition. It is shown that the shallow flow in the riffle is dominated by the presence of large-scale roughness elements that enhance turbulent mixing; increase turbulence anisotropy; and induce multiple, streamwise secondary cells driven by turbulence anisotropy. The flow in the pool, on the other hand, is dominated by the formation and interaction of the center region and outer bank secondary flow cells and the large horizontal recirculation regions along the inner bank. The collision of the counterrotating center region and outer bank cells at the water surface gives rise to a line of three-dimensional separation (flow convergence) in the time-averaged streamlines at the surface and the associated strong downward flow toward the bed that redistributes streamwise momentum and increases the bed shear stress along the channel thalweg. Intense turbulence is produced along the line of separation due to highly anisotropic velocity fluctuations. Our results make a strong case that the center region cell is driven by the curvature effects while the outer bank cell is driven by the combined effects of turbulence anisotropy and the curvature-induced centrifugal force. The inner bank horizontal recirculation zone consists of multiple eddies, which collectively span the entire point bar. A striking finding is that the center of the primary eddy is located directly above the crest of the point bar.

Do seasonal changes in habitat features influence aquatic macroinvertebrate assemblages in perennial versus temporary Mediterranean streams?

In this study we examined the importance of seasonal changes in habitat features and aquatic macroinvertebrate responses in temporary and perennial streams from two different catchments in the Western Mediterranean region in Spain. Macroinvertebrate sampling was spatially intensive to account for the relative frequency of meso- (i.e., riffles and pools) and micro-habitats (i.e., different mineral and organic-based substrata) at each site. Samples were collected at two distinctly different phases of the hydrograph: (1) during the flowing period, when pool-riffle sequences were well-established, and (2) during the dry phase, when only isolated pools were expected to occur in the temporary streams. During the dry season, both a reduction in the available total habitat and in microhabitat diversity in all sites studied was observed. As a result, taxon richness decreased in all streams, but more dramatically at temporary stream sites and particularly so in the infrequently remaining discontinuous riffles. Macroinvertebrate assemblages differed among catchments (i.e., geographical identity) and sites (perennial vs. temporary). Invertebrate differences were also strong within and among meso- and micro-habitats, particularly mineral and organic microhabitat patches, and differences were due to both loss of taxa from some habitats and some taxa exhibiting certain habitat affinities.