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.     

Benchmarking Fluvial Dynamics for Process‐Based River Restoration: the Upper Rhine River (1816–2014)

Multi‐temporal analysis of river‐floodplain processes is a key tool for the identification of reference conditions or benchmarks and for the evaluation of deviations or deficits as a basis for process‐based river restoration in large modified rivers. This study developed a methodology for benchmarking fluvial processes at river segment level, focusing on those interrelations between morphodynamics (aggradation, erosion, channel shift) and vegetation succession (initial, colonization, transition) that condition habitat structure. Habitat maps of the free‐flowing Upper Rhine River downstream from Iffezheim dam (France–Germany border) were intersected with a geographic information system‐based approach. Patches showing trajectories of anthropization, changeless, progression and regression allowed for the identification of natural and human‐induced processes over almost 200 years. Before channelization, the riverine system was characterized by a shifting habitat mosaic with natural heterogeneity, high degree of surface water connectivity and equilibrium between progression and regression processes. On the other hand, the following 175 years of human interventions led to severe biogeomorphologic deficits evidenced by loss of natural processes and habitat heterogeneity, hydrological disconnection between the river and its floodplain and imbalance of progression versus regression dynamics. The main driving forces of change are found in hydromorphological impacts (channelization, regulation and hydropower plant construction). Regression processes are now almost absent and have to be the objective of process‐based river restoration measures for the studied river‐floodplain system. A sustainable view on water management and river restoration should aim at a more resilient riverine system by balancing the recovery of natural processes with societal needs. Copyright © 2016 John Wiley & Sons, Ltd.     

VEGETATION–LANDFORM ASSEMBLAGES ALONG SELECTED RIVERS IN THE CZECH REPUBLIC,A DECADE AFTER A 500‐YEAR FLOOD EVENT

Rivers with a natural flow regime strongly influence the dynamics of riparian plant communities through hydrological and geomorphological processes. In this study, associations between fluvial landforms and vegetation are investigated on three near‐natural rivers in the Czech Republic a decade after a 500‐year return period flood in July 1997. This extreme disturbance destroyed the anthropogenically modified river channels and created suitable conditions for a range of ecosystems with high diversity and ecological stability. Field surveys were conducted on fluvial landforms (bars, islands, banks, floodplains and terraces) along three ‘renaturalized’ rivers, where no technical modifications had subsequently been made to their channels outside urban areas and the floodplains had been left in a post‐flood state. Vegetation species abundance and 13 environmental variables (topographical, hydrological and soil) were investigated in summer 2007, 10 years after the extreme flood disturbance. The results suggest that the recently created fluvial geomorphic forms are key environmental determinants of riparian vegetation distribution patterns. A range of statistical analyses illustrate that some plant species show predictable patterns of occurrence that correspond with the fluvial forms, supporting a fourfold grouping of herbaceous and woody species and the identification of typical plant communities associated with gravel bars, islands, banks, floodplains and terraces. An investigation of the species richness found on different fluvial landforms showed that the highest number of species occurred on the floodplain and decreased gradually towards the channel bed and towards terraces. Investigation of existing conditions in reaches of rivers with natural dynamics of fluvial processes provides valuable information that can be used as an effective tool for planning restoration strategies and precise management. However, the most important finding of this study is the remarkable establishment of complex river corridor vegetation–landform associations within 10 years of a 500‐year flood that removed the heavily cultivated landscape that had existed before the event. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Historical changes in hydrology,geomorphology, and floodplain vegetation of the Willamette River,Oregon

Historical trends in hydrology, geomorphology, and floodplain vegetation provide fundamental contexts for designing future management of large rivers, an area of fluvial research extensively informed by studies of historical channel dynamics. Changes in hydrology, channel structure, floodplain forests, and large wood were documented for the 273‐km main stem of the Willamette River from 1850 to present. Reduced sediment supply and frequency and magnitude of floods have decreased channel mobility and incised channels, leading to fewer gravel bars, islands, and side channels. Human alteration of channel morphology, vegetation, and bank hardening has exacerbated channel simplification caused by reductions in floods, sediment supply, and inputs of wood. A substantial number of floodplain channels reoccupied remnants of previous active channels inundated during recent floods, demonstrating functional but often forgotten role of historical geomorphic structure in modern floodplains and flood processes. In most reaches, area of floodplain forests in 1990 was only 10% to 25% of the area of forests in 1850. Abundance of wood in the wetted channel was generally greater in reaches with higher abundances of floodplain forests. Future trajectories will be influenced by legacies of the historical river but increasingly will reflect evolution of a new river shaped by human development, changing climate, and emerging hydrogeomorphic and vegetation processes. Understanding historical characteristics and anticipating future rates and patterns of ecosystem change provide fundamental contexts for restoring biophysical processes and structure in a large floodplain river.     

Channel dynamics and geomorphic variability as controls on gravel bar vegetation; River Tummel,Scotland

This paper presents the results of an investigation into environmental controls on vegetation dynamics on gravel bars. Such environments are a hotspot of threatened plant biodiversity and the dynamics of their vegetation reflect a range of processes that should be indicative of the integrity of the wider floodplain ecosystem. The study was undertaken on a 2 ha mid‐channel gravel bar complex that evolved over two decades, in response to several high magnitude flood events (including two with a return period in excess of 25 years), on a ‘wandering’ reach of the River Tummel, Scotland. Over 180 plant species, including a number of national or regional scarcities, had colonized. The fluvial chronology of the site was documented via sequential sets of aerial photography that revealed a number of discrete surfaces created by individual floods. Environmental heterogeneity, both within and between fluvial units, was investigated by field sampling of vegetation and abiotic variables at 66 locations. The fluvial surfaces were assigned to five habitat classes that ranged in age from two to approximately 20 years, from fine gravel to cobbles, and maintained an elevation range of up to 2.5 metres above low flow river levels. Multivariate analysis highlighted the relative importance of elevation, grain size, moisture content and infiltration and trapping of fines in controlling plant species composition. After standardizing sampling effort the habitat mosaic was found to support on average 1.36 times more species than an equivalent sample of any one habitat. In terms of biodiversity and river management, our results emphasize the importance of sustaining fluvial processes that preserve the habitat mosaic in order to conserve the characteristic biota of gravel bar complexes and river channel islands. Copyright © 2006 John Wiley & Sons, Ltd.     

THE IMPORTANCE OF VARIABLE LATERAL CONNECTIVITY BETWEEN ARTIFICIAL FLOODPLAIN WATERBODIES AND RIVER CHANNELS

The rehabilitation of lowland rivers subjected to channelization and artificial levee construction should attempt to improve habitat heterogeneity and diversity of floodplain hydrological connectivity. However, rehabilitation efforts rarely consider the importance of variable lateral hydrological connectivity between floodplain waterbodies and main river channels (ranging from those permanently connected to those temporarily connected during river level rises), instead focusing on increasing individual floodplain waterbody connectivity. This study investigated the young‐of‐the‐year (YoY) fish communities in 10 artificial floodplain waterbodies of variable hydrological connectivity with the river Trent, England, between May and November 2006, inclusive. Floodplain waterbody connectivity to the main river was positively correlated with the number of species captured (alpha diversity), Shannon–Wiener diversity, Margalef's species richness index and the relative abundance of rheophilic species and negatively correlated with species turnover (beta diversity). YoY fish communities in poorly connected water bodies were most dissimilar to riverine communities. The results demonstrate the importance of variable lateral connectivity between artificial floodplain waterbodies and main river channels when rehabilitating lowland river fish communities. Copyright © 2011 John Wiley & Sons, Ltd.     

The Selwyn River of New Zealand: a benchmark system for alluvial plain rivers

Most of the world's alluvial plain rivers have undergone hydrological and geomorphical modifications due to water abstraction, dam and levee construction, gravel mining and other human activities. Some of these rivers function as benchmark systems for identifying and quantifying the ecological responses to hydrological and geomorphological changes. Benchmark systems are critical for understanding these responses, for predicting the effects of future changes, and for trialling restoration and mitigation measures. The Selwyn River of New Zealand is a benchmark system for undammed alluvial rivers that are under intense pressure for water abstraction, and are subject to large flow fluctuations. The Selwyn is a remarkably complex river, and increased understanding of this system will provide insight for understanding and managing other rivers in its class. Hydrological properties that characterize the Selwyn include strong surface water–groundwater interactions, contiguous ephemeral, intermittent, perennial‐losing and perennial‐gaining reaches and an expanding and contracting dry segment that persists for most of the year. The dry segment, in combination with broad spatial variation in aquifer structure and rainfall, cause the upstream (runoff‐fed) and downstream (groundwater‐fed) river sections to function very differently. These sections are also dissimilar in channel morphology; the upstream section has a braided planform, with mobile bars, and abundant islands and remnant channels, and the downstream section has a single, meandering channel, stable bars and no islands. As in many alluvial plain rivers, large floods drive reach‐scale channel evolution. This paper introduces a long‐term research program that is underway at the Selwyn River, and explores the hydrological and morphological dynamics that characterize the river. We focus on groundwater–surface water interactions, flow‐permanence patterns and flood‐dependent geomorphology. Hydrological and meteorological data are summarized in a conceptual model of relationships between prevailing weather systems, runoff, aquifer recharge and river flow. The physical template described in this paper governs ecological processes such as dispersal, succession and nutrient cycling. A conceptual model is proposed to organize predictions about dispersal in response to changes in hydrological connectivity. Copyright © 2007 John Wiley & Sons, Ltd.     

Targeting lateral connectivity and morphodynamics in a large river‐floodplain system: The upper Rhine River

As a consequence of historical damming and channelization, most large rivers are disconnected from their floodplains, which therefore endure severe deficits in fluvial dynamics. Regaining some degree of lateral connectivity can lead to improved geomorphological and biological interactions. Yet, it is necessary to take into account limitations posed by current uses and legislation. This study presents a methodological approach to the selection of a realistic restoration target for a heavily modified large river segment, the free‐flowing Upper Rhine River downstream of Iffezheim dam (France–Germany border), based on the analysis of the existing biogeomorphic deficits, constraints set by human uses, and previous restoration experiences. To achieve the selected restoration target, proposed scenarios include embankment removal, bank lowerings, and side channel widenings with the aim of increasing lateral hydrological connectivity and promoting morphodynamics (bank erosion in lateral channels) that allow for the renewal of floodplain habitats. Results from 2‐D hydraulic simulations allow for a sensitivity analysis, comparing the current situation with the proposed scenarios, through parameters such as shore length of side channels actively connected at both ends to the main channel (eupotamon), and shear stress as a proxy for initiation of gravel erosion. Outcomes indicate that the two proposed restoration scenarios would succeed in reconnecting side channels and in increasing areas prone to substrate erosion, while maintaining flood protection and the heaviest navigation use among European rivers. The presented approach aids in the assessment of potential large river restoration scenarios and, thus, in the discussion of water management strategies.     

CHANNEL ADJUSTMENTS AND ISLAND DYNAMICS IN THE BRENTA RIVER (ITALY) OVER THE LAST 30 YEARS

Many gravel bed rivers in the European Alpine area suffered different ranges and types of human pressure that modified their morphology and altered their processes. This work presents the case of the middle portion of the Brenta River, historically impacted by human activities such as floodplain occupations, bank protection, gravel mining, hydropower schemes and water diversion. Dam operation and gravel mining have produced considerable modifications in the natural sediment regime generating important morphological channel responses (narrowing and incision). Large areas of the former active channel have been colonized by riparian vegetation, both as islands and as marginal woodlands. Overall, the river changed its morphological pattern from braided to wandering. The present study analyses the timing and extent of the planform morphological changes that occurred over the last 30 years along the middle portion of the river (20 km long) through the examination of aerial photos, repeated topographic measurements and hydrological data. A series of recent aerial photos (1981, 1990, 1994, 1999, 2003, 2006, 2008, 2010 and 2011) have been used to assess the medium and short‐term morphological changes of the floodplains and the active channel area. As to the medium‐term modification, the recent changes in in‐channel gravel mining have determined a new trend of active channel widening through erosion of vegetated areas. The analysis has also allowed to assess the morphological effect of single flood events. Only floods with recurrence interval higher than 8–10 years appear to be able to determine substantial erosion of floodplain and island margins. Copyright © 2013 John Wiley & Sons, Ltd.     

The hydraulic impact and performance of a lowland rehabilitation scheme based on pool–riffle installation: the River Waveney,Scole, Suffolk,UK

Pool–riffle installation is increasingly becoming the standard form of river habitat enhancement undertaken, largely for the benefit of fisheries. This study documents the effect of riffle installation on the morphological and hydraulic diversity of a low gradient engineered river. Despite their prevalence there have to date been few published studies of the impacts of these features on channel hydraulics, despite concerns as to their potential impact on flood levels. In this paper the impacts of the installation of gravel bedforms on water surface elevations and flow resistance are considered. The performance of the riffle–pool sequences is assessed against a set of criteria derived from the scientific literature. The analysis reveals that the gravel bedforms do display the hydraulic functionality associated with natural pool–riffle sequences. At bankfull discharge, water surface elevation is not significantly increased over those existing prior to installation, and physical habitat is shown to be more diverse following rehabilitation. The stability and appropriate classification of the gravel bedforms created in the scheme are discussed, together with the implications for floodplain and river rehabilitation in general. Copyright © 2004 John Wiley & Sons, Ltd.     

IDENTIFICATION OF FLUVIAL WOOD USING GOOGLE EARTH

Fluvial wood has long been known to enhance stream complexity by creating aquatic habitat and by increasing complexity in channel hydraulics and morphology. Although the presence and dynamics of large wood in river floodplains have been studied in a multitude of settings due to its importance in monitoring and managing ecohydrologic systems, limitations occur when studying fluvial wood on a basin scale. I postulate that with the employment of Google Earth, satellite images may be used to identify large wood and measure floodplain width across broader spatial scales previously inhibited by expensive and incomplete geospatial data. In this study, large wood was correctly identified within the floodplain of the Queets River, Washington, USA, through Google Earth; however, correct identification within the wetted channel was only possible during low flow if at least 50% was above water level. Within the study area, fluvial channel widths are measured as well. Google Earth proves to be an effective tool to discern large wood across greater spatial scales if the high‐resolution imagery is available for the study area. Results of statistical analyses derived from the downstream hydraulic geometry of the river reveal that this channel is influenced by bankfull width, the orientation of the wood to the channel, and whether it is located on a bar or within the wetted channel. In addition, wood counts analyzed in the context of the geometry of the river indicate that the fluvial wood has an influence on overall channel behavior. Copyright © 2013 John Wiley & Sons, Ltd.     

Habitat loss as the main cause of the slow recovery of fish faunas of regulated large rivers in Europe: the transversal floodplain gradient

In large European rivers the chemical water quality has improved markedly in recent decades, yet the recovery of the fish fauna is not proceeding accordingly. Important causes are the loss of habitats in the main river channels and their floodplains, and the diminished hydrological connectivity between them. In this study we investigate how river regulation has affected fish community structure in floodplain waterbodies of the rivers Rhône (France), Danube (Austria), Rhine and Meuse (The Netherlands). A typology of natural and man‐made aquatic habitats was constructed based on geomorphology, inundation frequency and ecological connectivity, along the transversal river–floodplain gradient, i.e. perpendicular to the main stream of the river. Fish species were classified in ecological guilds based on their flow preference, reproduction ecology and diet, and their status on national red lists was used to analyse the present state of the guilds and habitats. Ecological fish guilds appear to be good indicators of ecological integrity and functioning of river–floodplain systems. A transversal successional gradient in fish community structure that bears some resemblance to the gradient found in natural rivers can still be discerned in heavily regulated rivers. It resembles the longitudinal river gradient; even some predictions of the River Continuum Concept are applicable. Overall, richness and diversity of species and ecological guilds decrease with decreasing hydrological connectivity of floodplain waterbodies. Anthropogenic disturbances have affected fish species unevenly: guilds of specialized species that are highly adapted to specifically riverine conditions have declined far more than generalist species. Fish habitats in the main and secondary channels have suffered most from regulation and contain the highest percentage of threatened species. Rheophilic fishes have become rare because their lotic reproductive habitats are severely degraded, fragmented, absent or unreachable. Limnophilic fishes have become rare too, mainly as a result of eutrophication. Eurytopic fishes have become dominant everywhere. Copyright © 2003 John Wiley & Sons, Ltd.     

SHORT‐TERM IMPACTS OF LATERAL HYDROLOGICAL CONNECTIVITY RESTORATION ON AQUATIC MACROINVERTEBRATES

In floodplain ecosystems, the lateral hydrological connectivity between the main river channel and the secondary channels plays a major role in shaping both the habitat conditions and the macroinvertebrate diversity. Among other threats, human activities tend to reduce the lateral connectivity, which increases floodplain terrestrialization and induces a loss of aquatic biodiversity. Consequently, the restoration of lateral connectivity is of growing concern. We studied four secondary channels of the Rhône floodplain that were subjected either to no restoration or to three different restoration measures (river flow increase only, flow increase plus dredging and flow increase plus reconnection to the river). Macroinvertebrate and environmental data were analysed one year before and during a period of five years after restoration. We expected a progressive increase of lateral connectivity according to the type of restoration. Changes in macroinvertebrate assemblages were predicted to be towards more rheophilic communities and proportionally related to the changes in lateral connectivity. In the reconnected channel, lateral connectivity increased and remained high five years after restoration. In the dredged channel, the immediate increase of the lateral connectivity metric induced by sediment removal was followed by a rapid decrease. In the unrestored channel and the channel only influenced by flow increase, the metric remained constant in time. The macroinvertebrate composition and the rarefied EPT richness changes were proportionally related to the changes in lateral connectivity. Alien species richness and densities increased progressively in all channels after restoration. Our results showed that modifications of the lateral connectivity lead to predictable changes in macroinvertebrate diversity. Synergistic interactions between restoration and longer‐term changes (e.g. climatic change, invasion of alien species) encourage long‐term monitoring to assess the durability and trends of restoration measures. Copyright © 2012 John Wiley & Sons, Ltd.     

DISCONNECTING THE FLOODPLAIN: EARTHWORKS AND THEIR ECOLOGICAL EFFECT ON A DRYLAND FLOODPLAIN IN THE MURRAY–DARLING BASIN,AUSTRALIA

Globally, dams and water extractions are well‐recognised disruptors of flow regimes in floodplain wetlands, but little is known of the hydrological and ecological impacts of floodplain earthworks constructed for irrigation, flood mitigation and erosion control. We mapped the distribution of earthworks with high‐resolution SPOT (Système Probatoire d'Observation de la Terre) imagery in an internationally recognised Ramsar wetland, the Macquarie Marshes of the Murray–Darling Basin, Australia. There were 339 km levees, 1648 km channels, 54 off‐river storages and 664 tanks (0.5–5 m high), detected within the 4793 km² floodplain study area. Earthworks reduced localised flooding compared with undeveloped sites. The most pronounced disconnection of the original floodplain (73.0%) occurred where earthworks were most concentrated compared with areas with few earthworks (53.2%). We investigated relationships between hydrological connectivity and mortality of the perennial flood‐dependent river red gum Eucalyptus camaldulensis at 55 floodplain sites (225 × 150 m). Over half of the river red gums were dead at 21.8% of the sites. Earthworks blocked surface flows to flood‐dependent vegetation and drowned vegetation in artificially inundated off‐river storages. Mortality was due to impacts of earthworks and potentially exacerbated by effects of river regulation, water extraction and climate. River red gums were healthiest in narrow river corridors where earthworks confined flows and flows could recede freely. Rehabilitation of flood‐dependent ecosystems should focus on reinstating lateral connectivity and protecting environmental flows. Copyright © 2011 John Wiley & Sons, Ltd.     

Flooding and Surface Connectivity of Taxodium‐Nyssa Stands in a Southern Floodplain Forest Ecosystem

An understanding of the factors controlling the permanent and episodic links between the main stem of a river and the ecosystems of its alluvial floodplain is necessary for evaluating the influence of modern river processes on floodplain ecology and habitat diversity and for the successful implementation of flow regimes that meet human needs for water in a manner that sustains the ecological integrity of affected systems. In this study, we examined relationships between river hydrology and lateral hydrological connectivity, which is crucial to directing fluxes of water, material, and organisms into and across a floodplain. We did so by translating measures of river discharge for the Congaree River into high resolution maps of flood conditions for the floodplain at Congaree National Park using a 2D flood inundation model. Utilizing a graph network approach, we then analyzed the connectivity of a key wetland ecosystem, Taxodium‐Nyssa forested swamps, to the main stem river and to each other under different flows. Our results underscore that floodplain connectivity is initiated at sub‐bankfull discharges and does not depend on levee overtopping, while also clarifying that various sources of connectivity are triggered at different flow levels in specific reaches. Further, our findings demonstrate the sensitive and non‐linear response of floodplain connectivity to river flows and provide useful information to facilitate the management of flood processes in the Congaree River watershed. Copyright © 2014 John Wiley & Sons, Ltd.     

The effect of altered flow regime on the frequency and duration of bankfull discharge: Murrumbidgee River,Australia

On meandering rivers with well‐developed floodplains, bankfull stage has geomorphological and ecological significance because it approximates the level of connection between the channel and the floodplain. As a river rises to bankfull stage, sediment begins to be deposited on the floodplain, wetlands are progressively inundated and organisms migrate between the channel and floodplain habitats. On many rivers large headwater dams have reduced the frequency and duration of floodplain inundation downstream. However, the lack of reliable pre‐regulation flow data has made it difficult to quantify the effects of river regulation. This study used historical regulated and modelled natural flow data to determine the effects of regulation on the frequency and duration of bankfull flows on the Murrumbidgee River, one of Australia's largest and most heavily regulated rivers. In combination with floodplain surveys the flow data show that regulation has halved the frequency and duration of bankfull flows. This reduction in channel–floodplain connection has implications for the ecological health of the Murrumbidgee River. Copyright © 2005 John Wiley & Sons, Ltd.     

Island dynamics in a braided river from analysis of historical maps and air photographs

An analysis of island and active corridor dynamics is presented for a 16 km island‐braided reach of the gravel‐bed Tagliamento River (Italy) based upon information extracted, geocorrected and registered to a common base from three map (1803, 1833, 1927) and nine aerial photograph sources (1944/6, 1954, 1970, 1986, 1991, 1996, 1997, 1999, 2005). The active corridor width showed a general decline over the study period but with some recent widening. Adjustments in active corridor width were achieved through processes of floodplain avulsion, island attachment and progressive encroachment of the edge of the active corridor across gravel areas. These adjustments were accompanied by the preferential creation of dissection (floodplain avulsion) islands during periods of widening and the construction of mid islands within the corridor during periods of narrowing. Changes in island extent were achieved by rapid island turnover, which reached a maximum rate of over 50% per annum when corridor narrowing was most rapid between 1970 and 1991. Very few island surfaces were found to persist for more than 24 years. Despite this enormous dynamism and apparent cyclic behaviour, between 1944/6 and 2005 the ratio of island area to active corridor area remained relatively constant at around 0.08 and supported a consistently high bankfull shoreline to downstream length ratio of around 6 km · km^?1. These intrinsic properties of the dynamics of the study reach and other island‐braided channels need to be recognized and maintained by river managers because they represent a characteristic habitat dynamism that is crucial to the maintenance of ecological integrity. Copyright © 2008 John Wiley & Sons, Ltd.     

Ecological connectivity in alluvial river ecosystems and its disruption by flow regulation

The dynamic nature of alluvial floodplain rivers is a function of flow and sediment regimes interacting with the physiographic features and vegetation cover of the landscape. During seasonal inundation, the flood pulse forms a ‘moving littoral’ that traverses the plain, increasing productivity and enhancing connectivity. The range of spatio-temporal connectivity between different biotopes, coupled with variable levels of natural disturbance, determine successional patterns and habitat heterogeneity that are responsible for maintaining the ecological integrity of floodplain river systems. Flow regulation by dams, often compounded by other modifications such as levee construction, normally results in reduced connectivity and altered successional trajectories in downstream reaches. Flood peaks are typically reduced by river regulation, which reduces the frequency and extent of floodplain inundation. A reduction in channel-forming flows reduces channel migration, an important phenomenon in maintaining high levels of habitat diversity across floodplains. The seasonal timing of floods may be shifted by flow regulation, with major ramifications for aquatic and terrestrial biota. Truncation of sediment transport may result in channel degradation for many kilometres downstream from a dam. Deepening of the channel lowers the water-table, which affects riparian vegetation dynamics and reduces the effective base level of tributaries, which results in rejuvenation and erosion. Ecological integrity in floodplain rivers is based in part on a diversity of water bodies with differing degrees of connectivity with the main river channel. Collectively, these water bodies occupy a wide range of successional stages, thereby forming a mosaic of habitat patches across the floodplain, This diversity is maintained by a balance between the trend toward terrestrialization and flow disturbances that renew connectivity and reset successional sequences. To counter the influence of river regulation, restoration efforts should focus on reestablishing dynamic connectivity between the channel and floodplain water bodies.     

Depositional response of a headwater stream to channelization,East Central Illinois,USA

Channelization typically modifies the energy regime and sediment transport capacity of rivers, triggering morphologic adjustments. Most past studies of channelization have focused on erosional responses involving channel incision and widening. Depositional adjustments to channelization, although noted in previous work, have not been documented in detail. This study investigates the depositional response of the Spoon River, a headwater agricultural stream in Illinois, USA, to channelization. Historical aerial and ground‐based photography show that channelization of the Spoon River in the early 20th century produced a wide, deep trapezoidal drainage channel. Following this channelization, unvegetated alternate and mid‐channel bars developed on the bottom of the ditch. Sedimentological analysis of bar stratigraphy indicates that the bars grew through vertical accretion of horizontal sheets of sand‐and‐gravel bedload and organic‐rich drapes of fine‐grained suspended load. The horizontal sheets of sand and gravel are consistent with the braided conditions shown on historical photographs. Late‐stage bar growth appears to have been dominated by progressive overbank deposition of suspended load as indicated by the presence of a thick, fine‐grained organic‐rich A horizon immediately below the surface of each bar. The development of a soil layer also suggests that the bars are stable—an inference supported by the thick grass cover on the bar surfaces. The net result of the depositional response is the formation of a meandering channel flanked by a discontinuous floodplain on the bottom of the ditch. The construction of a wide ditch relative to original stream size is a key factor promoting a depositional response to channelization. Allowing or actively promoting floodplain elements to form in overwidened ditches may be a viable management option for improving the environmental quality of channelized agricultural streams. The Spoon River has a diverse fish community compared to channelized streams in East Central Illinois that lack a meandering low‐flow channel. This morphological configuration apparently enhances geomorphological and ecological variability while sustaining the drainage function of the ditch. Copyright © 2003 John Wiley & Sons, Ltd.