The three Rs of river ecosystem resilience: Resources,recruitment, and refugia

Resilience in river ecosystems requires that organisms must persist in the face of highly dynamic hydrological and geomorphological variations. Disturbance events such as floods and droughts are postulated to shape life history traits that support resilience, but river management and conservation would benefit from greater understanding of the emergent effects in communities of river organisms. We unify current knowledge of taxonomic‐, phylogenetic‐, and trait‐based aspects of river communities that might aid the identification and quantification of resilience mechanisms. Temporal variations in river productivity, physical connectivity, and environmental heterogeneity resulting from floods and droughts are highlighted as key characteristics that promote resilience in these dynamic ecosystems. Three community‐wide mechanisms that underlie resilience are (a) partitioning (competition/facilitation) of dynamically varying resources, (b) dispersal, recolonization, and recruitment promoted by connectivity, and (c) functional redundancy in communities promoted by resource heterogeneity and refugia. Along with taxonomic and phylogenetic identity, biological traits related to feeding specialization, dispersal ability, and habitat specialization mediate organism responses to disturbance. Measures of these factors might also enable assessment of the relative contributions of different mechanisms to community resilience. Interactions between abiotic drivers and biotic aspects of resource use, dispersal, and persistence have clear implications for river conservation and management. To support these management needs, we propose a set of taxonomic, phylogenetic, and life‐history trait metrics that might be used to measure resilience mechanisms. By identifying such indicators, our proposed framework can enable targeted management strategies to adapt river ecosystems to global change.     

A customised approach to determining the geomorphic effectiveness of small flood events in a regulated river

The construction of dams significantly alters flow and sediment regimes with subsequent deleterious effects on the morphological and ecological character of rivers. Effective experimental floods can ameliorate the downstream geomorphic impacts of dams. The traditional view is that large floods are required to perform effective geomorphic work, and the geomorphic outcomes of small floods are often overlooked. Many river restoration frameworks do not consider small floods. Yet, there is evidence that the hydrological characteristics that ameliorate specific geomorphic impacts in a river are unique to each river, and a customised approach to setting the right mix of floods (including small experimental floods) is needed. In this study, we modify an existing flood effectiveness model developed for large floods, for determining the geomorphic effectiveness of small floods in a highly regulated Australian river. Two flood classes were added to the model (medium peak stream power and moderate total energy expenditure), and the flood power characteristics were rescaled to reflect the relative difference in the magnitude of the small floods and the magnitude of the geomorphic work performed. Using a step‐wise approach, this customised model determined the geomorphic effectiveness of small floods. The best flood for ameliorating the geomorphic impacts of flow regulation had medium to long duration (10 to 51 days), high peak unit stream power (77 to 123 Wm^?2) and moderate to large total energy expenditure (78,600 to 342,320 × 10³ J). This approach to determining flood effectiveness for small floods is applicable to other geomorphically impacted river channels downstream of dams and can be used to inform experimental flood releases for geomorphic outcomes.     

Long-term reconstruction of energy fluxes in an alpine river: Effects of flow regulation and restoration

Flow regulation of montane and alpine headwater streams can fundamentally alter food web structure and energy flows through changes in productivity, resource availability, and community assembly. Dam flow-release schemes can be used to mitigate the environmental impacts of flow regulation via environmental flows, which can increase discharge variability and other ecologically important hydrological properties. In particular, managed floods can reintroduce disturbance to the system and stimulate the reactivation of physical habitat dynamics. However, how managed floods might restore ecosystem processes is virtually unknown. In this study, we examined patterns in potential energy fluxes before, during and after a long-term experimental flood program on the river Spöl, a regulated alpine River in southeast Switzerland. We used benthic samples collected during long-term monitoring and stable isotope analysis (δ¹³C and δ¹⁵N) of macroinvertebrates and their potential food sources to reconstruct secondary production, and potential energy fluxes, over a 20-year study period. The experimental floods did not alter the relative importance of basal resources but resulted in a considerable decline in secondary production, which remained low after the discontinuation of the floods. Our data suggest that a lack of recolonization by mosses following the discontinuation of the experimental flood program on the river Spöl may have driven patterns in energy fluxes by limiting macroinvertebrates using mosses for habitat. The effects of environmental flows on energy flows in this system thus depend on flood disturbance and the environmental context following the discontinuation of floods.     

Associations between the plant communities of floodplain wetlands,water regime and wetland type

Understanding how floodplain wetland vegetation is influenced by water regimes can inform the management of regulated river systems by targeting appropriate environmental water allocations. In this study, we examined plant community structure in 21 floodplain wetlands adjacent to the Murray River between Hume Reservoir and Tocumwal, south‐eastern Australia. Correlations between the water regime of the previous 25 years, and wetland type were investigated. We found the structure of plant communities, as assessed by the richness and percentage cover of plants, to be related to water regime, with clear differences between the communities of wetlands with historical ‘Wet’, ‘Dry’ and ‘Intermediate’ water regimes. Plant community structure was also related to wetland type, with differences being found between the communities of floodplain depressions, flood‐runners and cut‐off meanders. Managers of riverine/floodplain ecosystems need to consider both wetland type and water regime when planning strategies for the restoration or conservation of floodplain wetland vegetation in regulated river systems. Copyright © 2009 John Wiley & Sons, Ltd.     

黄河下游洪峰流量对断面塑造作用的试验研究

根据泥沙动床模型试验结果,探讨了黄河下游不同洪水洪峰流量下断面的冲淤及形态响应、洪峰含沙量对河槽塑造过程中水流运动、泥沙冲淤、主槽形态及过流输沙能力影响等问题,研究结果表明,断面响应与洪峰流量大小、历时、来沙系数及初始断面形态有关,大漫滩与一般漫滩洪峰流量对断面塑造特性有明显的不同,弯顶断面和顺直断面具有不同的冲淤特性,研究得到大漫滩、一般漫滩及不漫滩洪峰流量下断面的响应模式,建立了典型断面形态参数对一般漫滩及不漫滩洪峰流量下水量、来沙系数参数响应的经验关系式.     

Discontinuity of trichopteran (caddisfly) communities in regulated waters of the clearwater river,Idaho, U.S.A.

Riverine trichopteran (caddisfly) communities were sampled seasonally at eight locations over a two-year period to determine if a multiple-level release hydroelectric dam created a faunal discontinuity along the longitudinal profile of the Clearwater River in northern Idaho. The confluence of a non-regulated river with a regulated reach provided an unusual opportunity to examine the recovery of community parameters in a semi-regulated river. A marked discontinuity in the caddisfly community occurred in the regulated reach immediately below the dam, reflected by greatly reduced diversity and abundance values, and by shifts in functional relationships. These faunal changes are primarily attributable to the modified temperature, discharge, and food resources induced by river regulation. The non-regulated river exerted a major mitigative effect on the trichopteran fauna below its confluence with the regulated reach, despite maintenance of semi-regulated conditions. Trichopteran community parameters approached near normal levels concomitant with partial reductions in flow fluctuations, and partial recovery of temperature and food resources. In addition, the non-regulated river provided an important colonization pathway for riverine species to enter the semi-regulated lower reaches.     

浅议桃源水电站坝址河段洪水位波动现象

桃源水电站经历的2场大洪水("2014-07-17"洪水和"2017-07-02"洪水)的实际检验,通过对2场实测洪水参数的对比分析,发现洪水位出现了波动现象.从认识流域洪水特性、调查历史洪水资料、回顾枢纽洪水设计、追踪河道行洪断面变化过程等方面,全面查找分析坝址河段洪水位波动的原因.经分析,导致坝址河段洪水位波动的根...     

长江中下游设计洪水分析

长江是一条雨洪河流,流域源远流长,大暴雨洪水时有发生.干流自宜昌出三峡后进入长江中下游平原水网区,河道穿行于广阔的冲积平原,水网交错、湖泊毗连,江湖关系复杂,两岸堤防高筑.遇大洪水时期,堤防的溃决或有意识的分蓄洪,以及江湖对洪水调蓄作用的变迁,致使中下游控制站实测洪水系列具有明显不一致性.在分析长江中下游洪水特性基础上,针对中下游河道行洪特征,采用总入流分析法,分析了中下游主要控制站设计洪水.经合理性分析表明,分析的总入流设计洪水成果可靠,能适应防洪规划的需要.     

Floodplain forest dynamics: Half‐century floods enable pulses of geomorphic disturbance and cottonwood colonization along a prairie river

In dry ecoregions, trees are restricted to river valley floodplains where river water supplements the limited local precipitation. Around the Northern Hemisphere, cottonwoods, riparian poplars, are often predominant trees in floodplain forests and these ecological specialists require floods that create and saturate sand and gravel bars, enabling seedling recruitment. By pairing the interpretation of aerial photographs at approximately decade intervals with dendrochronology, we explored the coordination between river floods, geomorphic disturbance and colonization of plains cottonwoods (Populus deltoides) over eight meanders along the Red Deer River in the semi‐arid prairie of western Canada. This river has a relatively natural flow regime and minimal human alteration through the World Heritage Site of Dinosaur Provincial Park. We found that the 50‐year flood of 1954 increased channel migration and produced extensive accretion with downstream expansion of meander lobes and some channel infilling, which was followed by prolific cottonwood colonization. Those processes accompanied the major flood, while bank erosion and cottonwood losses were more gradual and continuous over the past half‐century. Results indicated even greater floodplain and woodland development after an earlier 100‐year flood in 1915. Each flood produced an arcuate band of mature cottonwoods and there were five to seven progressively older woodland bands across the floodplain, with each cottonwood age grouping increasing by about a half‐century. The 700 m wide floodplain was progressively reworked by the river through pulses of channel movement and floodplain and woodland development over approximately 250 years and correspondingly, the oldest cottonwoods were about 250 years old.     

江口水电站设计洪水分析研究

在分析芙蓉江流暴雨的洪水特性的基础上，采用多种方法插补展延了洪水资源系列，并充分利用历史洪水调查资料，使设计洪水分析计算采用的资料具有一定的代表性，可靠性，其中坝堤百年一遇设计洪水洪峰流量为12000m^3/s,72h洪量为10.2亿m^3，对江口水电站设计洪水成果，通过与上下游，干支流各站的设计洪水参数及与长江流域的部分大中型水利水电工程设计水成果比较，进行了合理性分析。     

Extreme floods and river values: A social–ecological perspective

The social–ecological status of rivers is particularly pronounced during extreme flood events. Extreme floods are a substantial threat to people, infrastructure, and livelihoods. Efforts to address the threats of extreme floods are aligned largely with social values of flood risk mitigation, flood preparation, and avoidance of loss. However, extreme floods are also a fundamental driver of river ecosystems, aligned with ecological (biophysical) values of event effectiveness, river change, disturbance, biotic response, and heterogeneity. A survey of the public perceptions of extreme floods revealed that participants generally understood the ecological values of extreme floods through concepts of naturalness, climate change, and knowledge production. However, participants had less understanding of how river integrity might influence the response of rivers to extreme floods. Resilience can be used as a framework for uniting the social and ecological values of extreme floods because it embodies a common language of change, disturbance, and adaptation and complements the socially dominated discourse of risk and emergency management. Three strategies are given for river scientists to frame ecological values in parallel with the paradigms of the socially dominated discourse of extreme floods: be prepared to act following an extreme flood disaster, learn and use the language of the flood risk and emergency management sector, and undertake assessments of the ecological values of extreme floods to highlight the threats to those values that may occur with climate change and river modification.     

淮河入江水道行洪能力分析

入江水道是淮河下游最大的泄洪河道,承担着淮河上中游70%以上的洪水泄入长江。根据1961—2018年大洪水期间的实测资料,利用水位流量法计算分析入江水道的泄洪能力和防洪能力。结果表明:因历史客观条件限制以及4个梯级控制河段整治的难度与复杂性,在不同时期各控制河段的行洪能力呈现各自不同的特点;经过多年持续有效治理,河道行洪能力整体得到提高;由近年来实测资料推算,各控制河段的行洪能力基本达到设计要求。对入江水道行洪能力的分析为淮河下游区的防汛抗洪和降低特大洪水威胁提供借鉴和参考,对区域经济社会又好又快发展具有现实意义。     

黄河小北干流和渭河揭河底冲刷现象分析

黄河1964～1977年六次揭河底冲刷洪水来源,除1970年8月一次洪水来源为府谷吴堡区间的孤山川、窟野河、秃尾河、佳芦河粗沙区暴雨洪水外,其它五次均为吴堡龙门区间的暴雨洪水。渭河1964～1992年七次揭河底冲刷洪水均为泾河的高含沙量洪水,主要来自马连河、蒲河和红河。黄河、渭河的揭河底冲刷的共同点是高含沙量大洪水,长河段的自上而下的沿程主河槽冲刷。黄河小北干流主河槽宽浅,单宽输沙率小,冲刷河段短,均未超过潼关;渭河主河槽窄,单宽输沙率大,冲刷河段长,1966年、1977年均发展到潼关和与溯源冲刷交会,导致潼关1000m3/s水位降低1.0～2.3m。     

贺江流域特大暴雨洪水特性分析

贺江流域"1994.7","2002.7"相继发生了特大洪水,给沿江两岸造成了严重的洪涝灾害.对这两场暴雨洪水的成因、雨洪特点、暴雨洪水过程、洪水组成、重要水利工程排洪等进行分析,探索出贺江流域特大洪水的成因及特性,为该流域水利工程建设和防洪规划提供科学的依据.     

ASSESSMENT OF RIPARIAN VEGETATION SENSITIVITY TO RIVER HYDROLOGY DOWNSTREAM OF A MAJOR TEXAS DAM

Dams may impact the health of downstream riparian vegetation communities through flow modifications such as decreased flood frequency and duration. Without historical vegetation data, however, it is difficult to relate changes in vegetation composition to hydrology patterns downstream of dams. We studied bottomland hardwood forests downstream of Toledo Bend Dam on the Sabine River in Texas and Louisiana to determine their sensitivity to minor changes in river hydrology with a particular focus on floods. Current riparian vegetation was characterized within three topographic zones at three selected sites below the dam. Using 80 years of hydrologic records from two gauging stations downstream of the dam, we evaluated trends in flood frequency, flood duration, peak discharge and total flood discharge in those periods before (1926?1965) and after (1971?2005) dam construction, as well as related flood stage to floodplain elevations to link topography to flood frequency. Plant species diversity in this system is highly dependent on minor changes in elevation, and the proportion of wetland‐dependent species changes rapidly with only a few centimeters difference in elevation. Although 50% of trees, shrubs and herbs in the sloughs were wetland adapted, their numbers were only 21% in the levees (74–284 cm higher in elevation) and 14% in the mid‐floodplains. Since dam construction, total flood discharge and duration at the most upstream gauge on the Sabine River decreased by 49%. At both gauges, mean discharge was also altered with higher summer flows. Patterns of tree regeneration point to less recruitment by wetland‐dependent species in the years following dam construction. These results suggest that minor changes in flood magnitude might limit occurrence of wetland species to the lowest topographic zones and illustrate the need to analyse sensitivity of plants to minor changes in flood characteristics when historical data for the vegetation community are lacking. Copyright © 2012 John Wiley & Sons, Ltd.     

Changes in the hydrological regime and invasions by plant species along riparian systems of the Adour River,France

Changes in the hydrological regime affect the phenomenon of invasion by plant species along riparian systems. The dynamics of exotic (non-native) and native species were examined at five sites that differed in exposure to hydrological disturbance (floods) during three consecutive years (medium, wet and dry years). When considering the disturbance gradient, exotic plants were favoured by direct exposure to floods (main channel) and by high flood frequencies. The response to year to year changes in hydrology was rapid for both native and exotic communities. However, the exotic plants responded more rapidly and were favoured by a dry year. A general framework including human and natural factors involved in invasions by exotic plants along rivers is presented.     

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.     

Channel recovery in a regulated river: Effects of an experimental and natural flood in the Snowy River,SE Australia

Experimental floods, generated downstream of dams, are used to recover specific bio‐geomorphic functions in regulated rivers. Studies of the effects of experimental floods vary in their objective, location, and the hydrological and bio‐geomorphic variables used to quantify recovery. Measurements of geomorphic change are required to guide future release strategies. The focus of this study was to determine if a large experimental flood in the Snowy River Australia, could promote geomorphic recovery of the river channel downstream of Jindabyne Dam following 35 years of flow regulation. The objectives of the release were to deepen, widen, and increase channel capacity and coarsen the riverbed substratum in the Jindabyne Gorge and Dalgety Uplands sections of the Snowy River. Data from the release were compared with that of a natural flood event that occurred after the experimental flow event. Both events showed channel adjustments and a degree of geomorphic recovery, but this varied between the two river sections. Marked channel adjustments occurred in the Dalgety Uplands reach following both the experimental and natural flood event and in the Jindabyne Gorge section following the natural flood event. Geomorphic changes were related to the hydrological character of each flood event. The number of flood peaks, the sequence of peaks, the flood duration, and the total energy expenditure differed markedly between the two events, and these four flood hydrological characteristics explained the greater geomorphic recovery associated with the natural flood event in the Jindabyne Gorge. No clear hydro‐geomorphic relationship was derived for channel change in the Dalgety Uplands where existing morphological constraints limit flood effectiveness.     

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.     

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.