Multi‐scale Approach to Hydrological Classification Provides Insight to Flow Structure in Altered River System

Rivers are hierarchical systems exhibiting processes and patterns across spatial and temporal scales principally driven by changes in flow. Hydrological indices estimated with mean or median daily flow data (i.e. daily scale) may be insensitive to anthropogenic alteration that imparts sub‐daily variation to flow. Therefore, indices developed at multiple temporal resolutions may provide additional insight into the presence of flow patterns masked by traditional techniques. We characterized the flow regime along the longitudinal gradient of the Platte River, a large Great Plains USA river, using hydrological indices derived with mean daily and sub‐daily flow data and a combination of multivariate statistical techniques. Three unique flow units were evident using daily scale flow data, whereas six unique flow units were evident at the sub‐daily scale. Flow units at both scales were not static, but rather the presence and extent of flow units across the riverscape depended on climate, tributary inflows and human influence. Anthropogenic alteration including hydropeaking was evident at the sub‐daily scale but not at the daily scale. The full complement of flow structure within regulated rivers, therefore, may not be captured using mean or median daily discharge values alone. Inductive river classification studies may benefit from assessing hydrological indices at multiple scales, particularly when investigating river systems with anthropogenic modification such as hydropeaking. Copyright © 2016 John Wiley & Sons, Ltd.     

Water quality changes in hyporheic flow paths between a large gravel bed river and off‐channel alcoves in Oregon,USA

Changes in water quality that occur as water flows along hyporheic flow paths may have important effects on surface water quality and aquatic habitat, yet very few studies have examined these hyporheic processes along large gravel bed rivers. To determine water quality changes associated with hyporheic flow along the Willamette River, Oregon, we studied hyporheic flow at six‐bar deposit sites positioned between the main river channel and connected lentic alcoves. We installed piezometers and wells at each site and measured water levels and water quality in river, hyporheic and alcove water. Piezometric surfaces along with substrate characteristics were used to determine hyporheic flow path direction and hyporheic flow rate. At all sites, hyporheic flow moved from the river through bar deposits into alcove surface water. Stable isotope analysis showed little influence of upwelling groundwater. At a majority of sites, hyporheic dissolved oxygen and ammonium decreased relative to river water, and hyporheic specific conductance, nitrate and soluble reactive phosphorous increased relative to river water. At three sites, hyporheic temperature decreased 3–7°C relative to river water; there was less temperature change at the other three sites. At the two sites with the highest hyporheic flow rates, hyporheic cooling was propagated into the alcove surface water. Hyporheic changes had the greatest effect on alcove water quality at sites with highly permeable substrates and high‐hyporheic flow rates. The best approach to enhancing hyporheic flows and associated water quality functions is through restoring fluviogeomorphic channel processes that create and maintain high‐permeability gravel deposits conducive to hyporheic flow. Copyright © 2006 John Wiley & Sons, Ltd.     

The distribution and management of channelized streams in Denmark

The Danish Watercourse Law of 1982 states that major river works and maintenance procedures must be planned and undertaken with regard to water quality and the physical form of the river channel. Fluvial geomorphology is an essential component of alternative procedures of river management which work with nature rather than against it. A survey of Danish stream channels has been produced by collating data obtained from maps, field surveys and engineering documents. This indicates that 97.8 per cent of Danish watercourses have been artificially straightened and that only 2.2 per cent (880 km) have natural morphological characteristics. The density of channel works is 300 times greater than in the U.S.A. and 15 times greater than in England and Wales. It is suggested that since there are only a few naturally sinuous channels in Denmark these should be given special consideration with regard to stream management practices. Measurements undertaken at approximately 300 sites in Denmark indicate five major processes of adjustment within straightened reaches. These adjustments include degradation of the channel bed with associated bank slumping, armouring of the bed, the development of a sinuous thalweg by the deposition of sediment at low gradients, and restoration of meanders. Channel stability is related to specific stream power and the results may be useful in predicting potential adjustments following channel straightening. Deposition in natural reaches below straightened channels often obliterates pool and riffle sequences but several channel adjustments may provide a greater variability of physical habitats than the initial uniform straightened channel; their significance for the biology of the watercourse is best considered before traditional management practices are carried out to control the adjustment.     

HYDROLOGICAL CONNECTIVITY OF ABANDONED CHANNEL WATER BODIES ON A COASTAL PLAIN RIVER

Oxbow lakes, sloughs and other floodplain depressions associated with former channel positions are critical elements of floodplain hydrology, geomorphology and ecology. They comprise key elements of wetland and aquatic habitats and have important influence on the storage and routing of floodwaters. The hydrological connectivity between active river channels and floodplain depressions varies considerably in a qualitative sense, even within a single fluvial system. Several oxbows, sloughs and paleochannels were examined on the lower Sabine River, Texas/Louisiana, during a period of high but sub‐bankfull flow as well as at lower flows. Six different types of surface water connectivity with the main, active channel were identified: (i) flow through—a portion of the river flow regularly passes through the feature and returns to the main channel; (ii) flood channel—there is no hydraulic connection at normal flows, but at high flows the channels convey discharge, at least part of which returns to the main channel; (iii) fill and spill—the features fill to a threshold level at high flows and then overflow (mainly via ephemeral channels) into flood basins; (iv) fill and drain—the features fill at high river discharges but do not (except in large floods) overflow because as river discharge declines, water drains back to the river; (v) tributary occupied—tributaries draining to the abandoned channel continue to occupy it, flowing through it to the active channel; and (vi) disconnected—no flow is exchanged except during large floods. The age or stage of infilling and the relative elevation of abandoned channels are important first‐order controls of hydrological connectivity, but the lateral distance from the active channel is poorly related. Other critical controls are whether the cutoff section receives tributary input and whether a tie channel forms. The alluvial valley geomorphic context—specifically the presence of a meander belt ridge and flood basins—is also critical. Copyright © 2011 John Wiley & Sons, Ltd.     

Decay state and inundation history control assemblage structure of log‐dwelling invertebrates in floodplain forests

Fallen timber (logs, large boughs) is recognized as having high ecological significance on forest floors. In floodplain forests, fallen timber potentially has even higher value for supporting biodiversity than upland forests because distinct faunal elements use the timber in the flooded and unflooded conditions. Invertebrates were sampled in logs of the river red gum (Eucalyptus camaldulensis Dehnh.) that had been inundated 1 year earlier and compared with invertebrates in logs that had not been inundated for many years. The invertebrate fauna in river red gum logs was relatively depauperate, probably reflecting the variable, sub‐humid conditions on the floodplain. The abundance and taxon richness of invertebrates was highest in logs with greater structural complexity and heterogeneity due to extensive decay. Recent inundation slightly reduced taxon richness. The succession of log‐dwelling invertebrates was tracked though transitions between terrestrial fauna and aquatic fauna in a spring/summer flood cycle. Transition between the two faunae was rapid. Logs were colonized by aquatic invertebrates within 2 weeks of immersion by floodwaters and recolonized by terrestrial invertebrates within 4 weeks of emersion. This faunal dynamism highlights the need to consider the entire flood cycle in decisions about the management of fallen timber on floodplains for biodiversity. Copyright © 2009 John Wiley & Sons, Ltd.     

Substrate indices as indicators of interstitial pore space in gravel‐bed channels

Streambed sediments that determine aquatic habitat quality are typically assessed using substrate indices. These indices are proxies for the amount of interstitial pore space that facilitates biological processes within the substrate. This study evaluates how well a suite of substrate indices characterize streambed porosity. Using the hybrid McNeil sampler, 37 cores were collected from six gravel‐bed rivers on the North Island of New Zealand. Central tendency indices correlate more strongly to porosity than critical size indices, but both types are imprecise indicators of the amount of interstitial pore space. Copyright © 2008 John Wiley & Sons, Ltd.     

A LIDAR‐DERIVED EVALUATION OF WATERSHED‐SCALE LARGE WOODY DEBRIS SOURCES AND RECRUITMENT MECHANISMS: COASTAL MAINE,USA

In‐channel large woody debris (LWD) promotes quality aquatic habitat through sediment sorting, pool scouring and in‐stream nutrient retention and transport. LWD recruitment occurs by numerous ecological and geomorphic mechanisms including channel migration, mass wasting and natural tree fall, yet LWD sourcing on the watershed scale remains poorly constrained. We developed a rapid and spatially extensive method for using light detection and ranging data to do the following: (i) estimate tree height and recruitable tree abundance throughout a watershed; (ii) determine the likelihood for the stream to recruit channel‐spanning trees at reach scales and assess whether mass wasting or channel migration is a dominant recruitment mechanism; and (iii) understand the contemporary and future distribution of LWD at a watershed scale. We utilized this method on the 78‐km‐long Narraguagus River in coastal Maine and found that potential channel‐spanning LWD composes approximately 6% of the valley area over the course of the river and is concentrated in spatially discrete reaches along the stream, with 5 km of the river valley accounting for 50% of the total potential LWD found in the system. We also determined that 83% of all potential LWD is located on valley sides, as opposed to 17% on floodplain and terrace surfaces. Approximately 3% of channel‐spanning vegetation along the river is located within one channel width of the stream. By examining topographic and morphologic variables (valley width, channel sinuosity, valley‐side slope) over the length of the stream, we evaluated the dominant recruitment processes along the river and often found a spatial disconnect between the location of potential channel‐spanning LWD and recruitment mechanisms, which likely explains the low levels of LWD currently found in the system. This rapid method for identification of LWD sources is extendable to other basins and may prove valuable in locating future restoration projects aimed at increasing habitat quality through wood additions. Copyright © 2011 John Wiley & Sons, Ltd.     

The Impacts of Natural Flood Management Approaches on In‐Channel Sediment Quality

Natural Flood Management (NFM) techniques aim to reduce downstream flooding by storing and slowing the flow of stormwater to river channels. These techniques include a range of measures, including setback stormwater outfalls and the physical restoration of channels and floodplains, to improve the natural functioning of catchments. An additional benefit of NFM measures is the potential reduction in sediment and pollutant delivery to the channel. Urban development releases a variety of heavy metal and nutrient pollutants that enter rivers through stormwater outfalls with adverse effects on the aquatic ecosystem. In this study, the influence of channel modification and quality of the river habitat on the sediment quality surrounding stormwater outfalls was assessed. Sediment samples were taken at several outfalls within the Johnson Creek catchment, Oregon, USA, and analysed for a variety of urban pollutants. The level of river habitat quality and modification at each site were assessed using a semi‐quantitative scoring methodology. Significant increases in pollutant levels were observed at outfalls, with a greater and more variable increase at direct compared to setback outfalls. Removal efficiency of certain pollutants was found to be significantly correlated to the level of habitat quality or modification (for Fe, Ba, Sn, Mg, P, K) indicating that more natural reaches had greater potential for pollutant removal. The findings highlight the multiple benefits associated with NFM and river restoration approaches in relation to sediment quality and pollutant content. © 2016 The Authors River Research and Applications Published by John Wiley & Sons, Ltd.     

Human influence on the morphological adjustment of river channels: The evolution of pertinent concepts in river science

Whereas 6 decades ago little attention was accorded to the influence of human activity on affecting river channel adjustment, great progress has been made subsequently, with many contributions published in River Research and Applications. Many concepts have been involved, and this paper considers how they have arisen and developed during a sequence of complementary research stages. The first stage during the 1970s and 1980s, including research by Geoff Petts, involved “recognition of change” and saw the emergence of two separate research strands focused on different timescales: over shorter timescales river channel adjustments affected by the impacts of dams and reservoirs, channelization, land use changes, and urban effects; over longer timescales investigations of river metamorphosis. The second “realization” stage involved significant advances in understanding by considering interaction between these two timescales, more detailed investigations of changing processes, a more holistic catchment‐based approach, and incorporation of ecological changes. These advances led into the third stage, “application” of results, stimulated by analysis of the effects of hard and soft engineering, development of international drivers such as the Water Framework Directive, and concern for what is “natural.” This paper evaluates this sequence of stages, the concepts that have emerged, the extent to which they are consistent and sustainable, and how they can provide the foundation for evaluation of channel adjustments. The effects of urbanization in Fountain Hills, Arizona, USA provide an illustration. Finally, the future requirement for adaptation of existing concepts and the possible development of new ones is briefly considered in the context of global environmental changes.     

Improving bio‐diagnostic monitoring using simple combinations of standard biotic indices

Invertebrate biotic indices are used widely to assess river quality. However, because reduced values can have many potential causes, general biotic indices have limited value in diagnosing reasons for impairment. Here, we investigate whether simple combinations of biotic indices can improve diagnostic capability. In the catchment of the Welsh River Wye, invertebrates varied significantly among groups of 55 streams in taxonomic composition and in index scores representing acidification (AWIC), mild eutrophication/organic pollution (BMWP/ASPT), and flow (LIFE). Although sites impacted by different forms of pollution tended to have reduced BMWP scores, acidified and enriched sites became distinguishable from each other, and from unimpaired streams, when classified on a combination of these indices. Combined indices also differentiated among competing explanations for trends in biological quality through time by revealing how increasing BMWP at some sites reflected local reductions in eutrophication. These data illustrate how simple univariate indices, calibrated to respond to specific pressures, have bio‐diagnostic capability when used together even in a relatively unpolluted catchment such at the Wye. In this Special Area of Conservation, they identified specific management needs in different locations—respectively to mitigate acidification in upland base‐poor tributaries and to reduce diffuse nutrients in the lower catchment. We advocate (i) the development of more pressure‐specific indices, for example to detect morphological modification, sedimentation and metal impacts; and (ii) further exploration of combined indices from one or more groups of organisms (e.g., diatoms and invertebrates) to increase bio‐diagnostic capability in river monitoring. Copyright © 2008 John Wiley & Sons, Ltd.     

Assessment of the value of rivers for Otters (Lutra lutra)

The British otter Lutra lutra L. requires good water quality, an adequate supply of fish and certain types of vegetation or cavities (holts) adjacent to water bodies for resting and breeding sites. In the U.K. otters and their holts are legally protected. Trained surveyors are needed to confirm the presence of otters on a river as the animals are rarely seen, but their tracks and droppings (spraints) may be detected. To evaluate a river for otters, information is required on water quality, fish species and biomass, as well as available habitat. Pioneer evaluation work based on the River Corridor Survey techniques has been done on Welsh rivers with good otter populations. Riparian vegetation, especially trees, is shown to be particularly important. This work is now being extended to areas on the margins of otter distribution such as the lower river Wye in England and Wales. Such surveys provide important management information for water authorities and give opportunities for enhancement work to benefit this animal.     

Spatio‐temporal habitat dynamics in a changing Danube River landscape 1812—2006

Modern, holistic concepts dealing with river/floodplain ecology recognize the key role of hydromorphological turnover processes for the development of distinct habitat patterns. Such patterns, in turn, are a vital basis for the extraordinary biodiversity of riverine systems. Natural braided/anabranched river systems in particular are characterised by high turnover rates; in the mid‐term, however, they are thought to stay in dynamic equilibrium (shifting‐mosaic steady‐state) as long as the physical framework conditions remain unchanged. This study analyses both the historical composition and the spatio‐temporal development of riverine habitats associated with an anabranched section of the Austrian Danube River. A habitat age model was used to analyse the age structure of the different habitat types. The results for the period 1812–1821 prior to channelization indicate that terrestrialization and habitat ageing were almost balanced with habitat regeneration and rejuvenation. Even though intensive morphological changes occurred, the Danube here largely persisted in dynamic equilibrium. The first channelization measures between 1821 and 1838, when 21% of the main channel banks were embanked, slightly promoted habitat regeneration. From 1859 onwards (80% embanked) until 1925, the natural habitat life time cycle was disrupted and the regenerating processes almost ceased. Altogether, human interferences led to a river landscape in a morphologically static state governed by significant terrestrialization (habitat succession) and habitat ageing. Without natural disturbances or, alternatively, targeted habitat management strategies, such an ecosystem soon lacks morphologically young habitats and adequate site conditions for a river/floodplain type‐specific biocoenosis. Copyright © 2010 John Wiley & Sons, Ltd.     

Flow structure and channel change in a sinuous grass‐lined stream within an agricultural drainage ditch: Implications for ditch stability and aquatic habitat

Many drainage ditches in the Midwest have developed a geomorphological configuration characterized by vegetated bars, or benches, on the bottom of the ditch and a stream flowing within a channel inset into these bars or benches. Past work has focused on the sedimentology of the benches and the depositional processes involved in bench development. This study investigates the three‐dimensional flow structure and short‐term channel change in a small grass‐lined stream flanked by benches at the bottom of an agricultural drainage ditch in east central Illinois, USA. In particular, it focuses on the influence of channel curvature and bank vegetation on flow through the inset channel at two different stages and explores how the structure of the flow is related to documented patterns of channel change. Results indicate that the mean flow is characterized by a submerged high‐velocity core that mainly is confined to the centre of the channel by near‐bank zones of flow stagnation/separation induced by abrupt changes in channel alignment and by strong frictional effects of grasses extending into the flow along the channel margins. Where the high‐velocity core is close to the channel margins, minor erosional adjustments of the inset channel can occur in the form of bank erosion. Patterns of turbulence kinetic energy reflect the development of shear layers near the channel margins and surrounding the submerged high‐velocity core. Locations with strong turbulence also correspond to locations of minor bank erosion. The results indicate that the inset channel is a relatively stable feature, especially where the alignment of this channel is straight, but that erosion‐control treatments may be necessary locally where the inset channel impinges on the ditch bank. Although the development of benches, a geomorphic response to ditch maintenance, is commonly viewed as a threat to drainage efficacy, preserving or constructing benches and associated inset channels in drainage ditches can enhance aquatic habitat and water quality. Copyright © 2010 John Wiley & Sons, Ltd.     

An Impact Assessment Method of Dam/Sluice on Instream Ecosystem and its Application to the Bengbu Sluice of China

A method is presented for impact evaluation of dams/sluices on their downstream aquatic ecosystems in the absence of detailed long-sequence ecology ecosystem data. It is based on the analysis of relations between indices of water quality and aquatic organisms. It can be applied to a river section absent of long-sequence ecology ecosystem data. For this study it was used to evaluate the effects of the Bengbu Sluice (BS), a typical sluice in the Huai River in China. Results indicate that in April and May the impact of BS on different downstream aquatic organisms varied. The most significant impact was on phytoplankton, with 91.7% of phytoplankton indices showing a 100% increase in quantity and biomass. The second most significant was on zooplankton, with 60.0% of zooplankton showing greater increase in quantity and biomass. The least significant impact was on zoobenthos, with only 33.3% showing greater increase in quantity and biomass. Most of these organisms have higher pollution-resistibility. These findings suggest that the present operation scheme will gradually lead to biodiversity decrease in the downstream aquatic ecology system, which will harm the instream aquatic ecology. Overall, BS has a negative impact on the downstream aquatic ecology system. It is argued that, in order to reduce the negative impact on the downstream aquatic ecology system, the operation of BS needs to be improved so that the discharge into the downstream river course is increased. Data comparison of the aquatic ecosystem condition between 1982 and 2006 also demonstrates the decreasing aquatic ecosystem quality downstream of BS.     

Rapid Response of a Sand‐Dominated River to Installation and Removal of a Temporary Run‐of‐the‐River Dam

Run‐of‐the‐river dams (RORDs) comprise the vast majority of dams on river systems and are commonly removed as a part of stream restoration strategies. Although these dams are routinely removed, few studies have documented the geomorphological responses of sand‐bed rivers to the removal of RORDs. We examined the response of a large sand‐bed river located in South‐Central Kansas, USA, to the installation and removal of a dam that is installed annually for seasonal recreational purposes. Channel adjustments were tracked using cross‐sections sampled over the course of 7 months as the dam was installed and subsequently removed. Multivariate spatiotemporal analysis revealed emergence of channel stability when the dam was in place for most cross‐sections, except for those immediately adjacent to or at great distances from the dam. Our results provide an approximation for how sand‐bed rivers respond to RORD construction and removal and are useful for guiding management decisions involving preservation or restoration of connectivity. Results of this study suggest that sand‐bed rivers are resilient and recover quickly when transient RORDs are removed. Copyright © 2014 John Wiley & Sons, Ltd.     

Dynamics of river fish populations in response to hydrological conditions: a simulation study

Increasing multi‐sectoral demands on water resources have led to water abstraction and transfer activities, and the construction of dams and embankments that have significantly altered the flood regimes of rivers throughout the world resulting in the loss of fish production and biodiversity. The current emphasis on sustainable development and biodiversity conservation is leading efforts to mitigate these impacts by means of interventions such as the release of artificial floods downstream of dams and the manipulation of water levels within impounded floodplains. Whilst much work has been done to determine the hydrological requirements for the maintenance of salmonid populations, few equivalent studies are available from which to develop criteria for the management of hydrological regimes for fishes and fisheries in large floodplain–river systems such as the Mekong. The population dynamics of fish in such rivers are believed to respond to hydrological conditions in a density‐dependent manner. An age‐structured population dynamics model incorporating sub‐models describing density‐dependent growth, mortality and recruitment was used to explore how hydrological conditions within a theoretical floodplain–river system affect the dynamics of a common floodplain–river fish species. Graphical summaries of the response of exploitable biomass to a range of different drawdown rates, dry and flood season areas and volumes, and flood season durations are presented under five different model assumptions concerning density‐dependent processes. Optimal flooding patterns are also described for the model species and theoretical river system. The patterns of predictions that emerge from the simulations provide guidelines for managing or manipulating hydrological conditions in river systems for both fixed and variable volume hydrological scenarios. As a general rule of thumb, exploitable biomass is maximized by minimizing the rate of drawdown and maximizing the flood duration and flood and dry season areas or volumes. However, experiences from dam and other hydraulic engineering projects suggest that these predictions should be treated with caution until we better understand the influence of hydrology on spawning behaviour, system primary production, and critical habitat availability. Copyright © 2004 John Wiley & Sons, Ltd.     

Spatial distribution and geomorphic condition of fish habitat in streams: an analysis using hydraulic modelling and geostatistics

Reach‐scale physical habitat assessment scores are increasingly used to make decisions about management. We characterized the spatial distribution of hydraulic habitat characteristics at the reach and sub‐reach scales for four fish species using detailed two‐dimensional hydraulic models and spatial analysis techniques (semi‐variogram analyses). We next explored whether these hydraulic characteristics were correlated with commonly used reach‐scale geomorphic assessment (RGA) scores, rapid habitat assessment (RHA) scores, or indices of fish biodiversity and abundance. River2D was used to calculate weighted usable areas (WUAs) at median flows, Q₅₀, for six Vermont streams using modelled velocity, depth estimates, channel bed data and habitat suitability curves for blacknose dace (Rhinichthys atratulus), brown trout (Salmo trutta), common shiner (Notropis cornutus) and white sucker (Catostomus commersoni) at both the adult and spawn stages. All stream reaches exhibited different spatial distributions of WUA ranging from uniform distribution of patches of high WUA to irregular distribution of more isolated patches. Streams with discontinuous, distinct patches of high score WUA had lower fish biotic integrity measured with the State of Vermont's Mixed Water Index of Biotic Integrity (MWIBI) than streams with a more uniform distribution of high WUA. In fact, the distribution of usable habitats may be a determining factor for fish communities. A relationship between predicted WUAs averaged at the reach scale and RGA or RHA scores was not found. Future research is needed to identify the appropriate spatial scales to capture the connections between usable patches of stream channel habitat. Copyright © 2008 John Wiley & Sons, Ltd.     

Geomorphic Responses to a Large Check‐Dam Removal on a Mountain River in Taiwan

The ability to understand and predict the impacts of dam removal in river systems is important, especially as dam decommissioning is becoming increasingly popular. In this study, we document the morphological and sediment impact of the removal of Chijiawan Check Dam in May 2011; a 13‐m‐high dam located on a coarse‐grained, steep mountain river channel in Taiwan. An estimated 0.2 million m³ of sediment had accumulated within the impoundment before its removal. Longitudinal and bankfull cross‐sectional surveys and a detailed sediment textural survey were undertaken along a 3.2‐km study reach of the Chijiawan Creek between 2010 and 2012. A rotating knickpoint with migration rates of up to 22 m/day was observed along the study reach, following dam removal. The rate and character of channel change, associated with the dam removal, appear to be driven as much by channel morphology and distance from the dam as by the hydrology variability. Our results suggested that relatively small amounts of sediment were eroded during the first 3 weeks following dam removal because of low discharge conditions. However, after 1 and 15 months, 10 and 75% of the sediment that had accumulated within former impounded was eroded, respectively. Sites near the former dam had a sediment texture that reflected the transport of released sediment, and this suggested that basin‐wide sediment processes exerted a strong influence. The removal of Chijiawan Dam offers unique insight on how sediment processes can drive river channel responses to sediment pulses may vary with discharge and sediment load, in areas subject to remarkably high flows and sediment loads. Copyright © 2015 John Wiley & Sons, Ltd.