Novel plant communities limit the effects of a managed flood to restore riparian forests along a large regulated river

Dam releases used to create downstream flows that mimic historic floods in timing, peak magnitude and recession rate are touted as key tools for restoring riparian vegetation on large regulated rivers. We analysed a flood on the 5th‐order Green River below Flaming Gorge Dam, Colorado, in a broad alluvial valley where Fremont cottonwood riparian forests have senesced and little recruitment has occurred since dam completion in 1962. The stable post dam flow regime triggered the development of novel riparian communities with dense herbaceous plant cover. We monitored cottonwood recruitment on landforms inundated by a managed flood equal in magnitude and timing to the average pre‐dam flood. To understand the potential for using managed floods as a riparian restoration tool, we implemented a controlled and replicated experiment to test the effects of artificially modified ground layer vegetation on cottonwood seedling establishment. Treatments to remove herbaceous vegetation and create bare ground included herbicide application (H), ploughing (P), and herbicide plus ploughing (H + P). Treatment improved seedling establishment. Initial seedling densities on treated areas were as much as 1200% higher than on neighbouring control (C) areas, but varied over three orders of magnitude among the five locations where manipulations were replicated. Only two replicates showed the expected seedling density rank of (H + P) > P > H> C. Few seedlings established in control plots and none survived 1 year. Seedling density was strongly affected by seed rain density. Herbivory affected growth and survivorship of recruits, and few survived nine growing seasons. Our results suggest that the novel plant communities are ecologically and geomorphically resistant to change. Managed flooding alone, using flows equal to the pre‐dam mean annual peak flood, is an ineffective riparian restoration tool where such ecosystem states are present and floods cannot create new habitat for seedling establishment. This problem significantly limits long‐term river and riparian management options. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of stream flow intermittency on riparian vegetation of a semiarid region river (San Pedro River,Arizona)

The San Pedro River in the southwestern United States retains a natural flood regime and has several reaches with perennial stream flow and shallow ground water. However, much of the river flows intermittently. Urbanization‐linked declines in regional ground‐water levels have raised concerns over the future status of the riverine ecosystem in some parts of the river, while restoration‐linked decreases in agricultural ground‐water pumping are expected to increase stream flows in other parts. This study describes the response of the streamside herbaceous vegetation to changes in stream flow permanence. During the early summer dry season, streamside herbaceous cover and species richness declined continuously across spatial gradients of flow permanence, and composition shifted from hydric to mesic species at sites with more intermittent flow. Hydrologic threshold values were evident for one plant functional group: Schoenoplectus acutus, Juncus torreyi, and other hydric riparian plants declined sharply in cover with loss of perennial stream flow. In contrast, cover of mesic riparian perennials (including Cynodon dactylon, an introduced species) increased at sites with intermittent flow. Patterns of hydric and mesic riparian annuals varied by season: in the early summer dry season their cover declined continuously as flow became more intermittent, while in the late summer wet season their cover increased as the flow became more intermittent. Periodic drought at the intermittent sites may increase opportunities for establishment of these annuals during the monsoonal flood season. During the late summer flood season, stream flow was present at most sites, and fewer vegetation traits were correlated with flow permanence; cover and richness were correlated with other environmental factors including site elevation and substrate nitrate level and particle size. Although perennial‐flow and intermittent‐flow sites support different streamside plant communities, all of the plant functional groups are abundant at perennial‐flow sites when viewing the ecosystem at broader spatial and temporal scales: mesic riparian perennials are common in the floodplain zone adjacent to the river channel and late‐summer hydric and mesic annuals are periodically abundant after large floods. Copyright © 2005 John Wiley & Sons, Ltd.     

Beaver herbivory and its effect on cottonwood trees: influence of flooding along matched regulated and unregulated rivers

We compared beaver (Castor canadensis) foraging patterns on Fremont cottonwood (Populus deltoides subsp. wislizenii) saplings and the probability of saplings being cut on a 10 km reach of the flow‐regulated Green River and a 8.6 km reach of the free‐flowing Yampa River in northwestern Colorado. We measured the abundance and density of cottonwood on each reach and followed the fates of individually marked saplings in three patches of cottonwood on the Yampa River and two patches on the Green River. Two natural floods on the Yampa River and one controlled flood on the Green River between May 1998 and November 1999 allowed us to assess the effect of flooding on beaver herbivory. Independent of beaver herbivory, flow regulation on the Green River has caused a decrease in number of cottonwood patches per kilometre of river, area of patches per kilometre, and average stem density within cottonwood patches. The number of saplings cut per beaver colony was three times lower on the Green River than on the Yampa River but the probability of a sapling being cut by a beaver was still higher on the Green River because of lower sapling density there. Controlled flooding appeared to increase the rate of foraging on the Green River by inundating patches of cottonwood, which enhanced access by beaver. Our results suggest regulation can magnify the impact of beaver on cottonwood through interrelated effects on plant spatial distribution and cottonwood density, with the result that beaver herbivory will need to be considered in plans to enhance cottonwood populations along regulated rivers. Published in 2002 by John Wiley & Sons, Ltd.     

A dynamic model to predict responses of millets (Echinochloa sp.) to different hydrologic conditions for the Illinois floodplain–River

Millets grow on floodplain mud flats exposed when seasonal floods recede, and the seeds of this plant are an important food source for waterfowl during their spring and autumn migrations in the Mississippi Flyway. Productivity of millets along the Illinois River has declined because of unnaturally frequent floods that inundate the mud flats and drown the plants during the summer growing season. These small floods are caused by operation of the navigation dams on the main channel and by alterations of the floodplain and tributary watersheds and channels. Predictive models are needed to evaluate the most cost‐effective combination of approaches for restoring plant productivity. We developed a moist‐soil plant model that simulates millet growth on 1 m² in response to daily water levels during the summer growing season. The model responds to daily water depth, flood timing (within the growing season), and flood duration, and was qualitatively verified using historical (1938–1959) water levels and plant coverage for three areas along the Illinois River. In the absence of untimely floods, the model predicts net above‐ground primary productivity of ～500 g m^?2 yr^?1 and plant heights of up to 130 cm by the end of the growing season. As expected, growth declines with decreasing land elevation or with more frequent flooding (or a shorter duration of the dry period) at the same elevation. A dry period of >85 days is required to achieve at least 50% of maximum production during the growing season, which is somewhat longer than the 70‐day recommendation based on reported field observations. The model predictions of plant success or failure agree with historical observations, indicating that water regime is a major factor limiting plant success. Copyright © 2004 John Wiley & Sons, Ltd.     

Influence of the flood regime on the reproduction of fish species with different reproductive strategies in the Cuiabá River,Upper Pantanal,Brazil

This study evaluated the influence of the flood regime of the Cuiabá River on the reproductive dynamics of fish species with different reproductive strategies. Sampling was carried out at ten sites in the basin, between March 2000 and April 2004. The reproductive strategies evaluated were long‐distance migrant (LM), short‐distance migrant (SM), sedentary with parental care (PC) and sedentary or SM with internal fertilization (IF). Period, duration and intensity of floods were the flooding attributes considered. Duration and time of spawning were evaluated using the index of reproductive activity (IRA), and inferences concerning reproductive allocation were based on the analysis of gonad weight. Reproductive success was evaluated based on the annual catch of young‐of‐the‐year of each species. Reproductive dynamics and flood regime were closely correlated; the reproductive peaks of fishes using all four strategies always preceded flood peaks. Intense floods favoured gonadal development of LM and PC, but were less important for IF. In relation to juvenile survival, the occurrence of floods appeared to be crucial for the strategies of LM, PC and IF, because such floods increased fish survival in the period of initial development. In contrast, SM appeared to be less dependent on floods for reproduction. These results indicate that, except for SM, floods have an important role in the recruitment of species using other reproductive strategies, and influence spawning success as well as juvenile survival. Copyright © 2008 John Wiley & Sons, Ltd.     

Investigating the turbulent flow characteristics in an open channel with staggered vegetation patches

In the present study, flow around circular and staggered vegetation patches was investigated numerically. For turbulence modelling, the Reynolds‐averaged Navier–Stokes technique and Reynolds stress model were adopted. The numerical model was validated with the experimental data using varying vegetation density and flow velocities. The simulated results of mean stream‐wise velocities were in close agreement with the experimental results. The results show that the mean stream‐wise velocity in the downstream regions of vegetation patches were reduced, whereas the velocity in the free stream regions were increased. The influence of neighbouring and staggered vegetation patches on the flow was observed. The vegetation patches with larger nondimensional flow blockage (aD = 2.3, where a is the frontal area per volume of patches, and D is the diameter of vegetation patches) offered more turbulence when compared to the patches with a smaller flow blockage (aD = 1.2). Larger turbulence in the form of kinetic energy and turbulent intensity was recorded within the vegetation as well as the regions directly behind the patches. Negative Reynolds stresses were observed at the top of submerged vegetation. The turbulence characteristics peaked at the top of vegetation, that is, z/h = 1.0 (where z is the flow depth, and h is the vegetation height), which may be migrated vertically as the frontal area of the vegetation patch is increased. This high frontal area also increased stream‐wise velocity above the vegetation, leading to an increased variation in turbulence around the vegetation canopy.     

Post‐flooding distribution and characteristics of large woody debris piles along the semi‐arid Sabie River,South Africa

The formation of large woody debris (LWD) piles has a profound impact on channel patterns and riparian succession in temperate rivers. The opportunity to study LWD along the Sabie River, a river in the semi‐arid region of Kruger National Park, South Africa, arose in February 2000 after a significant flood (c. 100‐year return interval) removed a large proportion of the fully mature riparian forest and other plant communities. Much of the uprooted vegetation was deposited as LWD piles (woody vegetation accumulations deposited on the ground > 0.1 m³) throughout the riparian and upland zones. In this article we describe the spatial distribution patterns of LWD as related to geomorphic channel type and flood frequency zone, and assess pile composition characteristics six months after the flood. Within the areas surveyed there were 68 LWD piles per hectare, the median size of LWD piles was 4.6 m³ but pile sizes (by volume) varied widely. Pool/rapid geomorphic channel types had the highest density of LWD piles (79 ha^?1) and the largest piles (by volume) were in the bedrock anastomosing channels (mean = 124 m³). Piles were larger in the seasonal and ephemeral flood frequency zones (mean = 54 m³ and 55 m³) than piles in the active zone (c. 2 m³). The patterns of distribution and volume of LWD will affect the subsequent development of vegetation communities as debris piles form a mosaic of patches of surviving organisms and propagules that can strongly influence the initial trajectory of succession. The amount, distribution, and subsequent decomposition of LWD are different from that reported for temperate rivers, suggesting that the role of LWD may be different on non‐floodplain rivers such as the Sabie in semi‐arid South Africa. Copyright © 2004 John Wiley & Sons, Ltd.     

Effect of flood regime on tree growth in the floodplain and surrounding uplands of the Wisconsin River

Flood regime and vegetation flood tolerance interact to influence tree growth in riverine landscapes. We studied tree growth in floodplain and upland forests of the Wisconsin River. About a century ago, levees set back from the river were constructed on this floodplain. The levee restricts some floodplain area from overbank flood events, but leaves a portion of active floodplain still inundated by floods. We addressed two questions: (1) how do growth rates of flood‐tolerant and flood‐intolerant tree species in the floodplain differ with flood regime? (2) At the stand level, how does growth rate differ with flood regime and between floodplain and upland areas? Annual tree growth rates from 1991 to 2000 were determined from tree increment cores for both individual species and stands. Tree growth rates of individual species varied between flood regimes. The most flood‐tolerant species (Betula nigra and Fraxinus pennsylvanica) grew faster in areas with active flooding, while the growth of less flood‐tolerant species (Quercus velutina and Q. ellipsoidalis) was depressed in swales and active floodplain. However, stand‐level tree growth did not differ between the floodplain and upland, or between flood regimes within the floodplain. Therefore, variation in the growth of individual species may not scale up to create differences in stand‐level tree growth because forest community composition varies spatially with flood regime. We suggest that growth rates are similar among sites because each community comprises of species adapted to their current flood regime. Copyright © 2008 John Wiley & Sons, Ltd.     

Vegetation structure and dynamics of a floodplain wetland along a subtropical regulated river

This paper documents vegetation changes in a floodplain area lying next to a newly constructed reservoir on the River Yamuna (near Delhi), about a kilometre downstream of an older, silted‐up reservoir. The study site was a rectangular depression bounded by dykes on three sides and agricultural fields on the fourth. The composition and abundance of species in the plant community were observed over a ten year period (1986–1996) and changes in water level both at the study site and in the reservoir were followed. Site hydrology was governed by water level changes caused by reservoir operation with effect from 1990, when it was first filled to capacity and water began to seep through the dyke. The study area experienced increasing depth, duration and frequency of flooding. Species richness peaked in 1992, and the plant community developed four distinct zones closely associated with the hydrological gradient. Patchiness also increased though Typha angustata patches merged over time to form a continuous expanse. The microtopography of the study site, and hydrological and plant‐induced changes were largely responsible for community changes. Dyke compaction over time resulted in cessation of seepage and the study site gradually dried up by 1998, with a consequent loss of plant species. The study concludes that the hydrological regime, rather than physical connectivity with the river, may play the dominant role in developing and maintaining plant community structure in floodplain wetlands. Copyright © 2005 John Wiley & Sons, Ltd.     

Flooding tolerance of Sagittaria latifolia and Sagittaria rigida under controlled laboratory conditions

Pool‐scale growing‐season water‐level reductions (drawdowns) have been implemented on the Upper Mississippi River in an effort to improve fish and wildlife habitat. Aquatic vegetation is a key habitat component, with perennial emergent species, such as Sagittaria latifolia and Sagittaria rigida, especially important. River managers have assumed the need for continuous drawdown during the growing season with limited reflooding and used this guidance in assessing the potential for an ecologically successful drawdown. However, information on the effects of growing‐season flooding episodes on survival and growth of Sagittaria is limited. To assess the flooding tolerance of S. latifolia and S. rigida, we evaluated multiple levels of timing, duration, and depth on survival and productivity of plants. Plants were produced from S. latifolia and S. rigida seeds and S. latifolia tubers; all were reared under moist‐soil or shallow‐flooded rearing conditions. Mortality of plants was low (2%) among plants from large tubers, low (7%) among seedlings (and largely associated with early flooding treatments), and modest (11%) among plants from small tubers (with no clear effects of inundation). Flooding treatments generally had a positive effect on biomass production from seedlings, particularly when treatments occurred early, were relatively shallow, and were short in duration. There were no clear effects of depth, duration, or timing components of flooding treatments on plant biomass arising from tubers. This experiment indicates that S. latifolia and S. rigida are relatively tolerant of flooding events during the growing season and may actually benefit from some level of inundation.     

Community resilience following extreme disturbances: The response of ground beetles to a severe summer flood in a Central European lowland stream

Extreme environmental events are predicted to increase in future due to global climate change. However, their effects on biodiversity still remain insufficiently understood because of the rarity and consequently the difficulty of studying the effects of extreme events. Here, we investigate the impacts on ground beetles of an unpredictable catastrophic flood event of the Elbe River in Germany in the year 2002 using pre‐ and post‐flood data. We analysed the response of grassland communities differentially exposed to flooding and focused on the question of how long their response lagged behind this extreme flood. Ground beetles were sampled from 1998 to 1999 (pre‐flood period) and from 2002 to 2006 (post‐flood period) on 48 floodplain grassland plots with a stratified randomized sampling design. Community resilience was quantified by calculating changes in species richness, species abundances, Simpson diversity and beta diversity of ground beetle assemblages. Ground beetles showed low resistance but high resilience to the extreme flood. Species richness decreased strongly immediately after the flood but reached pre‐flood values 2 years later. However, beta diversity remained relatively high in the subsequent years indicating persistent shifts in species composition and abundances. Contrary to our expectation, assemblages inhabiting plots prone to flooding, expected to be less sensitive to floods, did not recover faster than those on rarely inundated plots. We considered both the timing and the long duration of the flood as main reasons for the low community resistance to the flood. Strategies related to dispersal and habitat generality are identified to be crucial for the quick community recovery following the extreme flood. Our results endorse that extreme floods are integral parts of functioning floodplain ecosystems and that species can cope well even with such unpredictable extreme events, although recovery time tends to be longer than after normal floods. Copyright © 2010 John Wiley & Sons, Ltd.     

Identifying the natural flow regime and the relationship with riparian vegetation for two contrasting western Australian rivers

The natural flow regime and the relationship between flows and riparian vegetation are described for sites on both the Blackwood River in south‐western Australia and the Ord River in north‐western Australia. Analysis of long‐term flow data showed the historic mean monthly river discharge for the Blackwood River is strongly seasonal and highly predictable with generally low variability each month. The Ord River showed a strong seasonality of flows with about 92% of the (total) yearly flow occurring between December and March. Flow variability was very high (e.g. coefficient of variation >100% for all months) but highly predictable, with this mostly attributed to low but constant dry‐season flows. Water depth, duration of flood events and the number of flood events per year show a significant correlation with aspects of the riparian vegetation within experimental vegetation plots. Results highlight the strong relationship between floristics, life form structure and population dynamics with stream hydrology. On the Blackwood River, species richness and cover of shrubs reduced with increased duration and frequency of flooding, while cover of exotic species and annual herbs increased with increased flooding. Germination of tree seedlings was not influenced by flood regime but size class of tree species increased with flooding frequency. On the Ord River, species richness was not influenced by flooding regime. However, cover of perennial grasses increased with flooding frequency whilst cover of shrubs decreased. There was no relationship between flooding and seedling establishment whilst tree size class decreased with increased flooding. The methods described here can be used to compare the response of different components of the riparian vegetation to different fluvial regimes (e.g. because of impoundment and abstraction). This technique can be expanded for the management of riparian zones and planning rehabilitation programmes. It may also be useful for improving the ecological knowledge base for setting environmental flows in regulated systems. Copyright © 2001 John Wiley & Sons, Ltd.     

Environmental and biological responses of former channels to river incision: A diachronic study on the upper rhǒne river

The impacts of river incision should be a lowering of the river level, which would increase the rate of water flow from the hillslope aquifer to the river and its former channels. If this aquifer is nutrient-poor, it should favour the oligotrophication of the former channels. However, if the lowering of the water-table exceeds the depth of the former channels, then it should lead to the drainage of these channels. These hypotheses were tested on two former channels located in a degrading reach of the Rhǒne River (France); the former channels were influenced by floods, seepage water from the river and hillslope groundwater in 1989. The rate of river incision in this reach has increased since 1980 because of gravel extraction from the river bed (0.5 m between 1989 and 1993). Between 1989 and 1993, the reduction of river infiltration and the increase in the amount of water from the hillslope aquifer in the former channels were demonstrated by the decrease in the phosphate and ammonia content of the water, and its increased alkalinity and conductivity. The responses of aquatic vegetation to river incision depended on the vegetation zone investigated. Two floristic zones that were frequently flooded and species-poor in 1989 remained species-poor in 1993. Two floristic zones, which were species-rich and patchily organized, became dry or very shallow in 1993; as a consequence, aquatic vegetation disappeared, and was replaced by helophytes and terrestrial species. In the last vegetation zone, the increase in the amount of water coming from the hillslope aquifer caused the appearance and development of Chara hispida, an oligotraphent species. Unforeseen impacts of river incision were a decrease in the spatial and temporal heterogeneity of the former channels and lower aquatic macrophyte richness.     

Hydrodynamic simulation of the operational management of a proposed flood emergency storage area at the Middle Elbe River

Emergency storage areas can be an effective structural flood protection measure. By their controlled flooding the risk of inundation for downstream areas with higher vulnerability can be reduced. In the present study, the flooding and emptying process of a proposed storage area at the Middle Elbe River is simulated. The storage area has a maximum capacity of 40 million m³ and is divided into two polder basins. It is designed for the attenuation of extreme floods of 100 years or more return period. A one‐dimensional hydrodynamic model is set up for a 20 km reach of the Elbe River, wherein the storage area is schematised by two storage cells each representing one polder basin. Flow between the storage cells and the Elbe River is controlled by adjustable gates, which operate based on the pre‐defined conditions. Four flood scenarios which differ in flood magnitude and hydrograph shape are simulated. The scenarios are derived from analyses of a 70 years discharge record. Furthermore, for each flood scenario two gate control strategies are investigated. The results show that during large floods the utilization of the storage area with controlled gate operations significantly reduces the Elbe River peak discharges. However, the magnitude of the attenuation depends on the steepness of the flood hydrograph and the applied control strategy with well‐timed gate operations. Copyright © 2008 John Wiley & Sons, Ltd.     

CATCHMENT CHARACTERISTICS AND PLANT RECRUITMENT FROM SEDIMENT IN STREAM AND MEADOW HABITATS

Streams and rivers constitute a dense network with a large interface to the surrounding landscape and are thus highly susceptible to anthropogenic pressures related to land‐use activities in adjacent riparian and upland areas. In the present study, we investigated the influence of catchment characteristics on potential propagule and species recruitment from sediment in lowland stream ecosystems. We tested the following hypotheses: (1) catchment characteristics affect species recruitment from stream sediment in both stream and riparian habitats and (2) recruitment of species associated with undisturbed fen‐meadow habitats is higher in places with natural vegetation in the riparian zones. A large number of wetland species emerged from the stream sediment and sediment recruitment and therefore can act as an important dispersal corridor for common species in stream ecosystems. The recruited propagules were dominated by terrestrial species, but amphibious and aquatic species also appeared, particularly in the artificial stream channels. These included among others species within the genera Ranunculus sp., Callitriche sp. and Potamogeton sp. The large between‐site differences in land‐use characteristics in the riparian zones of the studied stream reaches, both locally and along upstream reaches, were not reflected in species recruitment from the stream sediments. Thus, most recruited species were common and widely distributed, and they were dominated by species with ruderal and competitive life history strategies, whereas only few species associated with fen‐meadow vegetation were recruited. From these findings, we infer not only that hydrochorous dispersal of species can be a potential efficient dispersal vector in agricultural landscapes but also that limitations can exist as to which species can be recruited. We suggest that further studies are performed to elucidate this issue further. Copyright © 2012 John Wiley & Sons, Ltd.     

Analysis and management of unseasonal flooding in the Barmah–Millewa Forest,Australia

The Barmah‐Millewa Forest is the largest red gum forest in the world and lies adjacent to the middle reaches of Australia's River Murray. Regulation of the River Murray, to supply water for irrigation, has changed the watering regime of the forest and thus is degrading its environmental values. The watering regime has been changed in two ways: (1) there are now fewer large winter/spring events that inundate extensive areas because these floods are mitigated by irrigation storages; and (2) there are more small summer/autumn events that flood low‐lying areas and are caused by the way the river is operated to supply irrigation demand. The increased frequency of these small unseasonal floods is the subject of this paper. During the irrigation season, water to meet irrigation requirements must be released four days in advance to allow for travel time from storages to irrigation areas upstream of the Barmah‐Millewa Forest. If there is heavy summer rainfall, irrigators cancel their orders so the flow that would have been diverted, remains in the river and causes a small ‘rain rejection’ flood. At the same time, river freshets from unregulated tributaries can also increase river flows. The River Murray channel in this area has low capacity and these high flows result in water spilling into the forest. Based on analysis of pre‐regulation conditions (1908–1929) and current conditions (1980–2000), forest flooding has increased from 15.5% of days to 36.5% of days between December and April. In particular, small, localized floods, which cover less than 10% of the forest, occur at least eight times more frequently now, than before regulation. Work by others has related these hydrologic changes to tree death and changes in floristic structure in wetland systems. There are also economic costs because much of the water that spills into the forest is not available for irrigation. Two solutions to unseasonal flooding are described in this paper. One is to limit the maximum flow in the river during the irrigation season so there is capacity to convey at least some of the rain rejection flows without spilling water into the forest. The other is to maintain airspace in a diversion weir (Lake Mulwala) upstream of the forest to store the surplus water when orders are cancelled. Preliminary economic analysis shows the preferred option is to increase airspace in Lake Mulwala which provides net benefits of at least Aus$1.4 million per year along with unquantified environmental benefits from decreased unseasonal forest flooding. Copyright © 2003 John Wiley & Sons, Ltd.     

Study of impacts of floods on the water quality in an arid zone: the case of the Tarim River in Northwest China

This paper presents the results of the study undertaken at the Tarim River Basin in Northwest China to analyze impacts of flooding on water quality. It was shown that irregular rainfall was the cause of flash floods that affected many ecosystems and eroded soils. Simulation results and the existence of relationships between flood volume and flood peak allowed potential model application that included flood peak estimation. The analysis of water pollution through sample sediment was helped by spectroscopy techniques and it was found that the flood was the main cause of many chemical elements in water. The floods affected the quality of water in the Tarim River where it was slightly basic with pH = 8.1 before flooding and acidic with pH = 6.9 after flooding.     

Effects of flooding regime upon the decomposition of Eichhornia azurea (Sw.) Kunth measured on a tropical,flow‐regulated floodplain (Paraná River,Brazil)

Decomposition of macrophytes is an important process in river‐floodplain systems, especially in the Upper Paraná River floodplain, given that this ecosystem receives high inputs of detritus from this vegetation. Release of nutrients by decomposition is essential in this floodplain because it is located downstream from a reservoir chain where nutrients are being trapped. Water level fluctuations are considered one of the most important aspects that affect macrophyte decomposition. Anthropogenic alterations, such as the control of flooding regimes, observed in this floodplain, could change the dynamic of this process. To evaluate the influence of the hydrological cycle upon the decomposition of Eichhornia azurea (an aquatic macrophyte that has high biomass values in this ecosystem), litter bags with senescent leaves and petioles of this plant were submitted to four different flooding treatments, which differed in time of flooding and exposure to dry conditions. The decomposition rates and the detritus chemical composition (nitrogen and phosphorus concentrations) were measured over 113 days. There were significant effects of the flooding treatments and time upon all parameters. The materials that decomposed with flood conditions showed the greatest decomposition rates. The quality of the detritus seems to be highly linked with the flooding regime, showing highest releases of phosphorus in the submerged treatments. It was shown that floods, even the short duration ones, increase the decomposition velocity and the nutrient cycling relative to dry conditions. Thus, investigations that assess the nutrient budgets on the Upper Paraná River floodplain and the role of nutrients in its productivity should consider the detritus compartment and the effects of flood regimes upon its dynamics. Copyright © 2006 John Wiley & Sons, Ltd.     

A review of hydroecological results of the Northern River Basins Study,Canada. Part 2. Peace–Athabasca Delta

The Peace–Athabasca Delta (PAD) in northern Canada is one of the world's largest freshwater deltas. Concern developed over the ecological health of this system in the early 1970s following regulation of its main headwater tributary, the Peace River. Continued drying of the delta into the 1990s resulted in the initiation of two major science programs, the Northern River Basins Study and the Peace–Athabasca Delta Technical Studies. Recognizing the importance of water to restoring and maintaining biological productivity and diversity of the PAD, a series of studies was initiated to explain the reasons for the protracted drying and to design methods to restore flooding. These studies demonstrated that open‐water floods from the Peace River were unlikely to flood the ecologically sensitive perched basins within the PAD. Moreover they discovered that most large‐scale overbank flooding resulted from ice‐jams formed during spring break‐up. Increases in freeze‐up ice levels due to enhanced winter flows from the reservoir and a decrease in spring snowmelt runoff from downstream tributaries were suggested as being responsible for a decline in the frequency and severity of ice‐jam floods. Based on results from numerical modelling studies of ice‐jams, a flow augmentation strategy was designed to aid the formation of ice‐jams near the PAD. Results of a test trial based on this strategy are presented. An update is also provided about ecological studies conducted since the delta was recharged by floodwaters in 1996. Copyright © 2002 Environment Canada. Published by John Wiley & Sons, Ltd.