Effects of flooding on recruitment and abundance of Golden Perch (Macquaria ambigua ambigua) in the lower River Murray

Flooding is often considered a stimulus for production of fish in floodplain rivers. In the southern Murray–Darling Basin (MDB), Australia, however, few native fish species have been shown to use the floodplain for spawning, and recruitment has been positively and negatively associated with flooding. In 2010/11, extensive flooding in the lower River Murray provided an opportunity to investigate the recruitment response of Golden Perch (Macquaria ambigua ambigua) following 10 years of drought and floodplain isolation. Annual variation in Golden Perch abundance and recruitment were investigated in anabranch and main channel habitats at Chowilla in the floodplain geomorphic region of the lower River Murray over a 7‐year period incorporating the flood and 6 years of in‐channel flow. Spatial variation in recruitment in the lower River Murray was also investigated by comparing the age structure of Golden Perch in the swamplands/lakes, gorge and floodplain geomorphic regions. Golden Perch abundance in the Chowilla region increased significantly postflooding compared with drought years. Age structures indicated that increased abundance was due predominantly to fish spawned during the flood (2010/11) and the previous year (2009/10), which was characterised by in‐channel flows. Age structure was similar in the nearby Katarapko Anabranch system indicating a uniform postflood recruitment response in the floodplain geomorphic region. Juvenile Golden Perch from the 2010/11 and 2009/10 cohorts were less apparent in the gorge and swamplands/lakes regions. Golden Perch have flexible life histories and will spawn and recruit in association with in‐channel rises in flow and overbank flows, but significant increases in abundance in the lower River Murray may result from overbank flooding. Contemporary approaches to flow restoration in the MDB emphasise overbank flows and floodplain processes. We suggest, however, that environmental flow management that incorporates floodplain and in‐channel processes, at appropriate spatio‐temporal scales, will result in more robust populations of Golden Perch.     

Fish recruitment in a large,temperate floodplain: the importance of annual flooding,temperature and habitat complexity

1. Large river floodplains are considered key nursery habitats for many species of riverine fish. The lower Volga River floodplains (Russian Federation) are still relatively undisturbed, serving as a suitable model for studying the influence of flooding and temperature on fish recruitment in floodplain rivers. 2. We examined the interannual variability in recruitment success of young‐of‐the‐year (YOY) fish in the lower Volga floodplain in relation to flood pulse characteristics and rising water temperatures in the spring. We sampled four areas with different flooding regimes, in three consecutive years (2006–2008). 3. Extensive areas with a long duration of flooding accommodated high densities of young fish. This suggests that extended inundation improves the recruitment success of river fish. In areas with extensive flooding, the biomass of YOY of most fish species was about three times higher in 2006 and 2007 than in 2008. We hypothesise that low spring temperatures in 2008 may have caused this reduced recruitment and that a flood synchronised with rising temperature enhances recruitment success. 4. Extensive flooding was particularly favourable for species characterised by large body size, delayed maturation, high fecundity and low parental investment, such as pike Esox lucius, roach Rutilus rutilus and ide Leuciscus idus. Gibel carp Carassius gibelio, a species tolerant of high temperature and hypoxia, did particularly well in small waterbodies in the driest parts of the floodplain. 5. Structural characteristics of floodplain waterbodies explained much of YOY fish density. These species–environment associations varied from year to year, but some species such as common bream Abramis brama, roach and gibel carp showed consistent relationships with structural habitat characteristics in all years, despite large interannual fluctuations in flood pulse and spring temperature.     

Do dams and levees impact nitrogen cycling? Simulating the effects of flood alterations on floodplain denitrification

A fundamental challenge in understanding the global nitrogen cycle is the quantification of denitrification on large heterogeneous landscapes. Because floodplains are important sites for denitrification and nitrogen retention, we developed a generalized floodplain biogeochemical model to determine whether dams and flood‐control levees affect floodplain denitrification by altering floodplain inundation. We combined a statistical model of floodplain topography with a model of hydrology and nitrogen biogeochemistry to simulate floods of different magnitude. The model predicted substantial decreases in NO₃‐N processing on floodplains whose overbank floods have been altered by levees and upstream dams. Our simulations suggest that dams may reduce nitrate processing more than setback levees. Levees increased areal floodplain denitrification rates, but this effect was offset by a reduction in the area inundated. Scenarios that involved a levee also resulted in more variability in N processing among replicate floodplains. Nitrate loss occurred rapidly and completely in our model floodplains. As a consequence, total flood volume and the initial mass of nitrate reaching a floodplain may provide reasonable estimates of total N processing on floodplains during floods. This finding suggests that quantifying the impact of dams and levees on floodplain denitrification may be possible using recent advances in remote sensing of floodplain topography and flood stage. Furthermore, when considering flooding over the long‐term, the cumulative N processed by frequent smaller floods was estimated to be quite large relative to that processed by larger, less frequent floods. Our results suggest that floodplain denitrification may be greatly influenced by the pervasive anthropogenic flood‐control measures that currently exist on most majors river floodplains throughout the world, and may have the potential to be impacted by future changes in flood probabilities that will likely occur as a result of climate shifts.     

Optimising environmental watering of floodplain wetlands for fish

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Effects of flooding on recruitment and dispersal of the Southern Pygmy Perch (Nannoperca australis) at a Murray River floodplain wetland

Summary With limited evidence linking Australia's Murray‐Darling Basin fish species and flooding, this study assessed annual variation in abundance and recruitment levels of a small‐bodied, threatened floodplain species, the Southern Pygmy Perch (Nannoperca australis), in floodplain habitats (creeks, lakes and wetlands) in the Barmah‐Millewa Forest, Murray River, Australia. Spring and summer sampling over a 5‐year period encompassed large hydrological variation, including 1 year of extended floodplain inundation which was largely driven by an environmental water release, and 2 years of severe regional drought. Recruitment and dispersal of Southern Pygmy Perch significantly increased during the floodplain inundation event compared with the other examined years. This study provides valuable support for an environmental water allocation benefiting a native species, and explores the link between flooding and its advantages to native fish. This suggests that the reduced flooding frequency and magnitude as a result of river regulation may well be a major contributing factor in the species’ decline in the Murray‐Darling Basin.     

The Significance of Meander Restoration for the Hydrogeomorphology and Recovery of Wetland Organisms in the Kushiro River,a Lowland River in Japan

The meanders and floodplains of the Kushiro River were restored in March 2011. A 1.6‐km stretch of the straightened main channel was remeandered by reconnecting the cutoff former channel and backfilling the straightened reach, and a 2.4‐km meander channel was restored. Additionally, flood levees were removed to promote river–floodplain interactions. There were four objectives of this restoration project: to restore the in‐stream habitat for native fish and invertebrates; to restore floodplain vegetation by increasing flooding frequency and raising the groundwater table; to reduce sediment and nutrient loads in the core wetland areas; to restore a river–floodplain landscape typical to naturally meandering rivers. In this project, not only the natural landscape of a meandering river but also its function was successfully restored. The monitoring results indicated that these goals were likely achieved in the short term after the restoration. The abundance and species richness of fish and invertebrate species increased, most likely because the lentic species that formerly inhabited the cutoff channel remained in the backwater and deep pools created in the restored reach. In addition, lotic species immigrated from neighboring reaches. The removal of flood levees and backfilling of the formerly straightened reach were very effective in increasing the frequency of flooding over the floodplains and raising the water table. The wetland vegetation recovered rapidly 1 year after the completion of the meander restoration. Sediment‐laden floodwater spread over the floodplain, and approximately 80–90% of the fine sediment carried by the water was filtered out by the wetland vegetation.     

Spangled perch (Leiopotherapon unicolor) in the southern Murray‐Darling Basin: Flood dispersal and short‐term persistence outside its core range

The spangled perch Leiopotherapon unicolor is considered a rare vagrant in the southern Murray‐Darling Basin, Australia, due to its intolerance of the relatively cool water temperatures that prevail during winter months. This study details 1342 records of the species from 68 locations between 2010 and 2014 outside its accepted ‘core adult range’ following widespread flooding during 2010 and 2011. Although records of the species declined over 2013, L. unicolor remained resident in the southern Murray‐Darling Basin as of April 2014. The species persisted in several locations for three consecutive winters with recruitment documented at two sites. This study represents the first identification of the dispersal of large numbers of L. unicolor into the southern Murray‐Darling Basin, persistence beyond a single winter, and recruitment by the species in habitats south of its recognized ‘core adult range’. Targeted research would determine the potential for predicted environmental changes (artificially warmer drainage wetlands, climate change and greater floodplain connectivity) to facilitate longer term persistence and range expansion by the species in the southern Murray‐Darling Basin.     

Tree health and regeneration response of Black Box (Eucalyptus largiflorens) to recent flooding

Black Box (Eucalyptus largiflorens) is a dominant floodplain tree in the Murray–Darling Basin. In northern Victoria, the health of Black Box woodlands has declined as a consequence of river regulation and drought. This has raised concern about the long‐term survival of populations. Although tree health and regeneration of this species are entwined with flooding, there is limited knowledge of its response to environmental watering. This inhibits the effective implementation of management actions to secure the long‐term survival of floodplain Black Box populations. We investigated the effect of flooding history on Black Box tree health, population status and regeneration at 26 sites within the Murray Sunset National Park, Victoria. Sites were assessed under two flooding treatments: (1) ‘recently flooded’ (sites frequently flooded within the last 5 years and (2) ‘historically flooded’ (sites not flooded since 1993). Black Box populations in recently flooded sites had a greater range of life stages present and fewer dead trees, indicating a healthier and more sustainable population structure. In addition, trees were in better health with higher canopy condition and reproductive output (e.g. flowers and fruits), and the average tree diameter was greater than in historically flooded sites. Seedlings and saplings were present only at recently flooded sites, indicating that water availability and tree health are a strong determinant of regeneration in these landscapes. Flooding is an important factor in the sustainability of Black Box populations in this Victorian semi‐arid floodplain. A key recommendation is that managed flows should be implemented to target populations in poor condition. Although regular flooding is required to maintain or improve the health of Black Box populations, developing the optimum flow regime (timing, frequency and duration) to facilitate this outcome requires further investigation.     

Ecological impacts of dams,water diversions and river management on floodplain wetlands in Australia

Australian floodplain wetlands are sites of high biodiversity that depend on flows from rivers. Dams, diversions and river management have reduced flooding to these wetlands, altering their ecology, and causing the death or poor health of aquatic biota. Four floodplain wetlands (Barmah‐Millewa Forest and Moira Marshes, Chowilla floodplain, Macquarie Marshes, Gwydir wetlands) illustrate these effects with successional changes in aquatic vegetation, reduced vegetation health, declining numbers of water‐birds and nesting, and declining native fish and invertebrate populations. These effects are likely to be widespread as Australia has at least 446 large dams (>10 m crest height) storing 8.8 × 10⁷ ML (10⁶ L) of water, much of which is diverted upstream of floodplain wetlands. More than 50% of floodplain wetlands on developed rivers may no longer flood. Of all of the river basins in Australia, the Murray‐Darling Basin is most affected with dams which can store 103% of annual runoff and 87% of divertible water extracted (1983–84 data). Some floodplain wetlands are now permanent storages. This has changed their biota from one tolerant of a variable flooding regime, to one that withstands permanent flooding. Plans exist to build dams to divert water from many rivers, mainly for irrigation. These plans seldom adequately model subsequent ecological and hydrological impacts to floodplain wetlands. To avoid further loss of wetlands, an improved understanding of the interaction between river flows and floodplain ecology, and investigations into ecological impacts of management practices, is essential.     

Groundwater change forecasts widespread forest dieback across an extensive floodplain system

1. Regulation of rivers for human demands has led to extensive forest dieback on many floodplains. If these important ecosystems are to be maintained under future drier climates, we need accurate tools for predicting forest dieback. In the absence of spatially explicit flooding histories for many floodplains, changes in groundwater conditions may be a good indicator of water availability and, therefore, an important environmental indicator. 2. Eucalyptus camaldulensis forests of Australia are an acute example of forest dieback, with 70% of the Victorian Murray River floodplain dying back. We quantified the relationship between forest dieback and ground water across this extensive floodplain (c. 100 000 ha of forest over 1500 km of river length). 3. A combination of extensive ground surveys, remotely sensed data and modelling methods was used to predict forest dieback at the time of the survey and in the past. This approach provides a valuable tool for accurately monitoring forest condition over large spatial scales. Forest dieback was estimated to have increased from 45 to 70% of the floodplain between 1990 and 2006. 4. Accurate groundwater data (depth and salinity) over a 20‐year period were obtained for 289 bores and summarised using nonlinear regression. Groundwater depth and salinity were strong predictors of stand condition. This suggests that changes in groundwater conditions could be used to signal areas vulnerable to forest dieback and prioritise the limited water available for managed flooding. 5. In the upper Murray, where ground water is predominantly fresh (<15 mS cm^?2), dieback increased with increasing groundwater depth. In contrast, the condition of stands in the lower Murray improved with increases in groundwater depth due its high salinity (>30 mS cm^?2). These regional differences in response of the same tree species to groundwater conditions show that our understanding of the drivers of forest dieback is best achieved at spatial and temporal scales representative of the problem.     

Floodplain ants show a stronger response to an extensive flood than to variations in fallen‐timber load

Understanding how species respond to differences in resource availability is critical to managing biodiversity under the increasing pressures associated with climate change and growing human populations. Over the last century, the floodplain forests of Australia's largest river system, the Murray‐Darling Basin, have been much affected by intensive harvesting of timber and firewood, and increasingly stressed by river regulation and, recently, an extended drought. Fallen timber – logs and shed branches – is known to play a key role in the ecology of several important species on these floodplains. Here, we monitored the response of the ant assemblages of a floodplain forest along the Murray River to a large‐scale (34 ha) experimental manipulation of fallen‐timber load (0 to 80 t ha^?1) over 4 years. The forest was subjected to an incidental, extensive flood that enabled us to examine how two important stressors (timber removal and river regulation) affect ant assemblages. Ants showed little response to the proximity of fallen timber within plots, prior to the flood, or to different loads among plots, unlike other floodplain biota. After the flood, both ant abundance and species richness increased and species composition changed. However, this increase in species richness after flooding was less pronounced in plots with higher amounts of fallen timber. Managing river red gum forest using a mosaic of flood regimes, more representative of historical conditions, is likely to be the most effective way to maintain and enhance the diversity of ants and other biota on these important floodplains.     

Response of wetland plant communities to inundation within floodplain landscapes

Summary Many floodplain wetlands in south‐eastern Australia have become isolated from the main river channel as a consequence of reduced high flows and associated flood events following river regulation. In the Central Murray region of south‐eastern Australia, many temporary wetlands would have received water once every five years or so, with large floods maintaining floodplain connectivity every decade, under natural conditions. Now, the River Murray is highly regulated and many of these wetland areas have not been flooded for periods of up to 30 years. Consequently, these wetlands are becoming degraded and the biodiversity of the area is in decline. From 2001–2003, 21 Black Box depression wetlands in the Central Murray region were each watered once. Plant communities in each wetland were monitored for changes in abundance (assessed as percentage cover) before and during the wetting and drying phases. Wetlands were watered during spring or early summer with the length of inundation ranging from 6 to 19 weeks. After watering, the percentage cover of native plant taxa and native plant functional groups in most wetlands increased. In general, there was a decrease in the percentage number of terrestrial plants present and an increase in the percentage cover of aquatic plants. Introduced species were a minor component. Although these wetlands are all located in the Central Murray region, individual wetlands developed plant communities that contained taxa specific to individual wetlands despite initial similarities. These results indicate that wetland plant biodiversity within the landscape can be promoted and maintained by ensuring there is a diversity of wetlands with varying flood regimes within the landscape.     

The relative contributions of refugium types to the persistence of benthic invertebrates in a seasonal snowmelt flood

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Impacts of hydrological restoration on lowland river floodplain plant communities

Many lowland floodplain habitats have been disconnected from their rivers by flood defence banks. Removing or lowering these banks can reinstate regular flooding and thus restore these important wetland plant communities. In this study we analyse changes in wetland hydrology and plant community composition following the lowering of flood defence banks at a floodplain of the River Don in the United Kingdom (UK). The aim of the restoration project was to improve the quality of “floodplain grazing marsh” habitat, which is a group of wetland communities that are of conservation interest in the UK. We analyse changes in species richness and community composition over a period of 6 years, and compare the presence of indicator species from the target floodplain grazing marsh plant communities. The lowering of the flood banks increased the frequency of flood events, from an estimated average of 1.7 floods per year to 571 floods per year. The increased flooding significantly increased the proportion of time that the wetland was submerged, and the heterogeneity in hydrological conditions within the floodplain. There were significant differences in composition between the pre-restoration and restored plant communities. Plants with traits for moisture tolerance became more abundant, although the communities did not contain significantly more ‘target’ floodplain grazing marsh species at the end of the study period than prior to restoration. Colonisation by floodplain grazing marsh species may have been limited because environmental conditions were not yet suitable, or because of a shortage of colonising propagules. While the desired target plant community has not been achieved after 5 years, it is encouraging that the community has changed dynamically as a result of hydrological changes, and that moisture-tolerant species have increased in occurrence. Over the next few decades, the restored flood regime may cause further environmental change or colonisation events, thus helping increase the occurrence of desired floodplain grazing marsh indicator species.

     

Fish diet composition in floodplain lagoons of an Australian dryland river in relation to an extended dry period following flooding

Floodplain rivers worldwide are threatened by loss of connectivity to their floodplains and hence reduced benefits from floodplain energy subsidies. Dryland rivers with ‘boom and bust’ ecological responses to flooding and extended dry periods may be particularly vulnerable. This paper describes variations in dietary composition of three fish species of contrasting trophic position in dryland floodplain lagoons with variable flood inundation and drying histories. The study species were Ambassis agassizii - a microphagic carnivore, Leiopotherapon unicolor - a carnivore/omnivore, and Nematalosa erebi - an algivore/detritivor. Despite the range of food items recorded in fish guts, each species fed mostly on relatively few food categories and few food items within each category. Most of the spatial (i.e. among lagoons) and temporal dietary variation was associated with different proportional contributions of these food items. Given the absence or low magnitude of flooding during the study period, temporal changes in diets of the three species are probably the result of successional changes in composition of invertebrate prey as the dry season progressed. The focus of each fish species on relatively few food categories and a few reliable food items within each category may be the most profitable foraging strategy when food resources are limiting in progressively drying floodplain lagoons.     

Seasonal Dependent Effects of Flooding on Plant Species Survival and Zonation: a Comparative Study of 10 Terrestrial Grassland Species

Past research has provided compelling evidence that variation in flooding duration is the predominant factor underlying plant species distribution along elevation gradients in river floodplains. The role of seasonal variation in flooding, however, is far from clear. We addressed this seasonal effect for 10 grassland species by testing the hypothesis that all species can survive longer when flooded in winter than when flooded in summer. We carried out an inundation experiment under simulated conditions of summer and winter flooding in the greenhouse. The results showed that all species survived longer under winter floods than under summer floods. However, responses upon flooding were species-specific. All summer flood-tolerant species had high tolerance for winter floods as well, but summer flood sensitive species survived either a little longer, or dramatically longer when flooded under simulated winter conditions. Next, we examined whether winter or summer survival best predicted the lower distribution limits of the species as measured in a natural flooding gradient after an extremely long winter flood. We found a strong significant relationship between the lower distribution limits of species in the field and their tolerance to summer floods, although we measured the lower limits 14 years after the latest major summer flood. In contrast, no such significant relationship existed with species tolerance to winter floods. Some relatively intolerant species occurred at much higher floodplain elevations as was expected from their tolerance to winter inundation in the experiments. This suggests that zonation patterns as created by occasional summer floods may be maintained for a long time, probably due to the limited ability of species to re-colonise lower positions in the floodplain.     

Moist-Soil Plants as Ecohydrologic Indicators for Recovering the Flood Pulse in the Illinois River

The hydrologic regime of the Illinois River has been substantially altered by locks and dams, floodplain levees, water diversion, and development of the watershed over the past 100 years. The natural flood pulse, a fundamental rhythm to which the plants and animals of both the river and its floodplain had adapted, has been disrupted. State, federal, and non‐governmental organizations are currently trying to naturalize the Illinois floodplain‐river system. Little, however, is known about how to recover those elements of the flood pulse essential to the native biota. In this study we propose moist‐soil plants, whose life history is dependent upon flood pulsing, as ecohydrologic indicators of the flood pulse. We explain how moist‐soil plants are supported by the natural flood pulse and present a conceptual framework that links the flooding regimes of the river and the reproductive success of the plants. Successful germination and full growth of moist‐soil plants can be a useful indicator for optimum naturalization of flood regimes. The framework also shows how the interdisciplinary linkages between hydrology, ecology, and spatial analysis assist in predicting, measuring, and comparing consequences of alternative naturalization scenarios. A new ecohydrologic parameter, lowest elevation for successful moist‐soil plant production, is presented.     

Nitrate elimination by denitrification in hardwood forest soils of the Upper Rhine floodplain – correlation with redox potential and organic matter

Denitrification in floodplains is a major issue for river- and groundwater quality. In the Upper Rhine valley, floodplain forests are about to be restored to serve as flood retention areas (polders). Besides flood attenuation in downstream areas, improvement of water quality became recently a major goal for polder construction. Redox potential monitoring was suggested as a means to support assessment of nitrogen elimination in future floodplains by denitrification during controlled flooding. To elucidate the relationship between redox potential and denitrification, experiments with floodplain soils and in situ measurements were done. Floodplain soil of two depth profiles from a hardwood forest of the Upper Rhine valley was incubated anaerobically with continuous nitrate supply. Reduction of nitrate was followed and compared with redox potential and organic matter content. The redox potential under denitrifying conditions ranged from 10 to 300 mV. Redox potential values decreased with increasing nitrate reduction rates and increasing organic matter content. Furthermore, a narrow correlation between organic matter and nitrate reduction was observed. Experiments were intended to help interpreting redox potentials generated under in situ conditions as exemplified by in situ observations for the year 1999. Results obtained by experiments and in situ observations showed that monitoring of redox potential could support management of the flooding regime to optimize nitrogen retention by denitrification in future flood retention areas.     

Strategies to restore floodplain vegetation after abandonment of human activities

Restoring the flooding‐related disturbance regime by removing and setting back flood defenses (channel widening) is the most efficient strategy for recovering riparian plant communities in floodplains formerly impacted by human activities such as agriculture, mining, and forestry. Removing flood defenses is generally not socially accepted, and alternative recovery strategies must be explored. We assessed vegetation establishment on 33 sites in the floodplains of the Middle Ebro River and three of its tributaries (NE Spain) where restoration approaches applied in the last 20 years include channel widening, floodplain excavation, and abandonment of agriculture, gravel extraction, and hybrid poplar plantations, with and without plantation of native species. Using analysis of similarity and ordinations, we found that channel widening led to plant communities closely resembling those found on natural gravel bars, including new recruits of keystone tree species. Excavation of the floodplain as the restoration approach resulted in pioneer, non‐strictly riparian communities. Abandonment of agricultural land or clearing of poplar plantations resulted in alternative stable states predominating, regardless of time elapsed since restoration and whether poles of native species were planted. However, forest‐like communities relatively similar to mature, natural riparian references were attainable when hybrid poplars were allowed to resprout after clear‐cutting, or after the human activities were abandoned and trees were not cut. Combining channel widening and assisted revegetation where flood defenses cannot be altered may partially reproduce a mosaic of habitats typical of natural floodplains.     

Importance of low-flow and high-flow characteristics to restoration of riparian vegetation along rivers in arid south-western United States

1. Riparian vegetation in dry regions is influenced by low‐flow and high‐flow components of the surface and groundwater flow regimes. The duration of no‐flow periods in the surface stream controls vegetation structure along the low‐flow channel, while depth, magnitude and rate of groundwater decline influence phreatophytic vegetation in the floodplain. Flood flows influence vegetation along channels and floodplains by increasing water availability and by creating ecosystem disturbance. 2. On reference rivers in Arizona's Sonoran Desert region, the combination of perennial stream flows, shallow groundwater in the riparian (stream) aquifer, and frequent flooding results in high plant species diversity and landscape heterogeneity and an abundance of pioneer wetland plant species in the floodplain. Vegetation changes on hydrologically altered river reaches are varied, given the great extent of flow regime changes ranging from stream and aquifer dewatering on reaches affected by stream diversion and groundwater pumping to altered timing, frequency, and magnitude of flood flows on reaches downstream of flow‐regulating dams. 3. As stream flows become more intermittent, diversity and cover of herbaceous species along the low‐flow channel decline. As groundwater deepens, diversity of riparian plant species (particularly perennial species) and landscape patches are reduced and species composition in the floodplain shifts from wetland pioneer trees (Populus, Salix) to more drought‐tolerant shrub species including Tamarix (introduced) and Bebbia. 4. On impounded rivers, changes in flood timing can simplify landscape patch structure and shift species composition from mixed forests composed of Populus and Salix, which have narrow regeneration windows, to the more reproductively opportunistic Tamarix. If flows are not diverted, suppression of flooding can result in increased density of riparian vegetation, leading in some cases to very high abundance of Tamarix patches. Coarsening of sediments in river reaches below dams, associated with sediment retention in reservoirs, contributes to reduced cover and richness of herbaceous vegetation by reducing water and nutrient‐holding capacity of soils. 5. These changes have implications for river restoration. They suggest that patch diversity, riparian plant species diversity, and abundance of flood‐dependent wetland tree species such as Populus and Salix can be increased by restoring fluvial dynamics on flood‐suppressed rivers and by increasing water availability in rivers subject to water diversion or withdrawal. On impounded rivers, restoration of plant species diversity also may hinge on restoration of sediment transport. 6. Determining the causes of vegetation change is critical for determining riparian restoration strategies. Of the many riparian restoration efforts underway in south‐western United States, some focus on re‐establishing hydrogeomorphic processes by restoring appropriate flows of surface water, groundwater and sediment, while many others focus on manipulating vegetation structure by planting trees (e.g. Populus) or removing trees (e.g. Tamarix). The latter approaches, in and of themselves, may not yield desired restoration outcomes if the tree species are indicators, rather than prime causes, of underlying changes in the physical environment.