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.     

Hydromorphological alteration of a large Mediterranean river: Relative role of high and low flows on the evolution of riparian forests and channel morphology

This paper evaluates the causes and effects of the hydrogeomorphological alteration of the central reach of the Ebro River (NE Spain). The Ebro River is one of the largest Mediterranean rivers. In this reach, it develops a meandering planform in a wide floodplain. Geographic Information System (GIS) analyses of historic aerial photographs, analysis of hydrologic data and measurement of various indicators linked to the fluvial morphology and the structure and distribution of the riparian vegetation led to the establishment of the prevailing processes in the dynamics of this river. Statistical analyses conducted on some of the main components of the flow regime, including floods, droughts and flow duration curves, showed a role for these components in river dynamics. Similarly, a thorough analysis of the evolution of the aforementioned indicators was performed to identify and measure the effects of the hydrological regulation of the river. These indicators were measured in 1927, 1956 and 2003 for a 106 km reach. The geomorphic dynamics of the Ebro River in its central reach reflect a remarkable tendency for stabilization and rigidification of the channel. The active river corridor has largely been modified, primarily in the second half of the twentieth century. The corridor lost a huge portion of its width and extension, the channel suffered an intense narrowing and the natural mobility of the meander train decayed proportionally. The structure and distribution of the riparian vegetation were completely transformed. The riparian forest lost its original function, behaving as a linear corridor and was notably continuous and very close to the channel thalweg. The vegetation colonized most of the previously active channel, contributing to the loss of the natural dynamics of the river. The hydrological analyses suggest that the large morphological modification of the river planform and the parallel alteration of the riparian forests are not to be seen as a consequence of a loss of the attributes of natural floods. On the contrary, these extreme hydrological events only generate slight alterations due to river regulation and are not capable of enhancing the aforementioned evolution. Nevertheless, a profound change in the attributes of the low (summer) flows was found. The modification of the low flows was studied through its relationship with the global evolution of the geomorphic indicators and the riparian forest indicators. The results show the relative role played by high and low flows in the evolution of the river dynamics. These results are used to propose a future scenario of ecohydrological management in the central reach of the Ebro River. This scenario is intended to improve its ecological status and recover, at least partially, its natural dynamics. Copyright © 2010 John Wiley & Sons, Ltd.     

Instream flow and cottonwood growth in the eastern Sierra Nevada of California,USA

Dendro-ecological studies indicated that radial growth of Populus trichocarpa was significantly related to annual streamflow at 20 riparian sites in the eastern Sierra Nevada of California. The strength of the relationship varied among sites, depending on geomorphology and tree cover. The strongest correlation between streamflow and tree growth occurred at sites in wide, unconfined valleys, where alluvial groundwater typically fluctuates directly with surface water. In such areas, trees on streambanks and in the floodplain showed equally strong relationships between flow and growth. In narrow mountain canyons, relationships between tree growth and streamflow were weaker and showed more within-site variability. Streambank trees in the canyon settings generally showed stronger relationships with streamflow than did floodplain trees. These data suggest that P. trichocarpa trees in confined canyons, in comparison with those in wide alluvial valleys, may rely to a greater extent on water sources that are not in direct hydraulic connection with surface water. Flow-growth models were also stronger at sites where tree basal area and density were low, including sites where flow diversion has caused tree mortality. Sparse tree cover may allow for a greater expression of flow-growth relationships by minimizing the effects of competition for light and other resources, and allowing for greater control of growth by abiotic rather than biotic factors.     

Geomorphic controls,riffle substrate quality,and spawning site selection in two semi‐alluvial salmon rivers in the Gaspé Peninsula,Canada

The relationships between valley and channel morphology, spawning substrate quality (content of fine sediment < 2 mm) and the selection of spawning sites by Atlantic salmon (Salmo salar) were investigated along 45 km of two semi‐alluvial, valley‐confined rivers in the Gaspé Peninsula, Canada. Linear and logistic regressions confirm that Atlantic salmon prefer spawning at riffles providing good rather than mediocre or poor spawning substrate, as defined by the percentage sand and the Sand Index of Peterson and Metcalfe. However, exceptionally large concentrations of redds were observed on the few riffles located at island heads, with sub‐optimal substrate quality. This observation suggests that, in addition to content of fine material in the substrate, the morphology of spawning reaches may be a significant factor controlling the intensity of inter‐gravel flow through redds and the consequent selection of spawning sites. In the study systems, the quality of spawning substrate was controlled by ‘large‐scale’ geomorphic attributes at the scale of valley segments (1–5 km here): segments located within a wide valley were actively meandering, had higher sinuosity and bank erosion rates, generally lower shear stresses and presented somewhat higher sand content than segments confined by a narrow valley. Although sand contents were significantly higher, laterally unstable segments in wide valleys still harboured good to excellent spawning substrate overall. The study data do not allow the roles of variations in levels of riffle‐zone shear stress to be distinguished from those of cut bank fines input, to explain the observed inter‐segment association between valley width and riffle fines content. Copyright © 2004 John Wiley & Sons, Ltd.     

Riparian vegetation and channel change in response to river regulation: a comparative study of regulated and unregulated streams in the Green River Basin,USA

The effects of river damming on geomorphic processes and riparian vegetation were evaluated through field studies along the regulated Green River and the free‐flowing Yampa River in northwestern Colorado, USA. GIS analysis of historical photographs, hydrologic and sediment records, and measurement of channel planform indicate that fluvial processes and riparian vegetation of the two meandering stream reaches examined were similar prior to regulation which began in 1962. Riparian plant species composition and canopy coverage were measured during 1994 in 36, 0.01 ha plots along each the Green River in Browns Park and the Yampa River in Deerlodge Park. Detrended correspondence analysis (DCA) of the vegetation data indicates distinctive vegetation differences between Browns Park and Deerlodge Park. Canonical correspondence analysis (CCA) indicates that plant community composition is controlled largely by fluvial processes at Deerlodge Park, but that soil chemical rather than flow related factors play a more important role in structuring plant communities in Browns Park. Vegetation patterns reflect a dichotomy in moisture conditions across the floodplain on the Green River in Browns Park: marshes with anaerobic soils supporting wetland species (Salix exigua, Eleocharis palustris, Schoenoplectus pungens, and Juncus nodosus) and terraces having xeric soil conditions and supporting communities dominated by desert species (Seriphidium tridentatum, Sarcobatus vermiculatus, and Sporobolus airoides). In contrast, vegetation along the Yampa River is characterized by a continuum of species distributed along a gradual environmental gradient from the active channel (ruderal species such as Xanthium struminarium and early successional species such as S. exigua, Populus deltoides subsp. wislizenii, and Tamarix ramossissima) to high floodplain surfaces characterized by Populus forests and meadow communities. GIS analyses indicate that the channel form at Browns Park has undergone a complex series of morphologic changes since regulation began, while the channel at Deerlodge Park has remained in a state of relative quasi‐equilibrium with discharge and sediment regimes. The Green River has undergone three stages of channel change which have involved the transformation of the historically deep, meandering Green River to a shallow, braided channel over the 37 years since construction of Flaming Gorge Dam. The probable long‐term effects of channel and hydrologic changes at Browns Park include the eventual replacement of Populus‐dominated riparian forest by drought tolerant desert shrublands, and the enlargement of in‐channel fluvial marshes. Copyright © 2000 John Wiley & Sons, Ltd.     

长江科学院长江泥沙若干问题研究与实践

简要介绍了近50年来,长江河道水流泥沙运动、河道演变、河道整治及水利枢纽泥沙问题研究情况与所取得的进展,提出了需要继续深入研究的若干重大问题,包括长江水土资源综合利用、干支流水库群联合运用对水沙及江湖演变的影响与对策、长江中下游河道和湖泊及长江口治理研究等。     

Bank stabilization,riparian land use and the distribution of large woody debris in a regulated reach of the upper Missouri River,North Dakota,USA

Large woody debris (LWD) is an important component of ecosystem structure and function in large floodplain rivers. We examined associations between LWD distribution and riparian land use, bank stabilization (e.g. riprap revetment), local channel geomorphology, and distance downriver from the dam in the Garrison Reach, a regulated reach of the upper Missouri River in North Dakota, USA. We conducted a survey of shoreline‐associated LWD in the reach during typical summer flow conditions. Reach‐wide LWD density was 21.3 pieces km^?1 of shoreline, of which most pieces (39% ) were ‘beached’ between the waterline and the bankfull level, 31% of pieces had evidence of originating at their current location (anchored), 18% of pieces were in deep water (>1 m), and 13% were in shallow water. LWD density along unstabilized alluvial (sand/silt) shorelines (27.3 pieces km^?1) was much higher than along stabilized shorelines (7.2 pieces km^?1). LWD density along forested shorelines (40.1 pieces km^?1) was higher than along open (e.g. rangeland, crop land; 9.2 pieces km^?1) or developed (e.g. residential, industrial; 7.8 pieces km^?1) shorelines. LWD density was highest overall along unstabilized, forested shorelines (45 pieces km^?1) and lowest along open or developed shorelines stabilized with a blanket‐rock revetment (5.5 pieces km^?1). Bank stabilization nearly eliminated the positive effect of riparian forest on LWD density. A predicted longitudinal increase in LWD density with distance from the dam was detected only for deep LWD (including snags) along unstabilized alluvial shorelines. Partial resurvey in the summer following the initial survey revealed a reduction in total LWD density in the reach that we attribute to an increase in summer flow between years. Changes in riparian management and land use could slow the loss of LWD‐related ecosystem services. However, restoration of a natural LWD regime in the Missouri River would require naturalization of the hydrograph and modification of existing bank stabilization and channel engineering structures. Copyright © 2004 John Wiley & Sons, Ltd.     

Managing rivers under changing environmental and societal boundary conditions,Part 1: National trends and U.S. Army Corps of Engineers reservoirs

Most major rivers in the United States are managed by a system of reservoirs; many of which were built more than a half century ago. These reservoirs were designed based on environmental, societal, and regulatory assumptions at the time of construction. Since then, we have learned that climate is not stationary, population growth is being decoupled from energy needs and water demand, and new regulations (such as the Clean Water Act and Endangered Species Act) affect how river systems are managed. This study explores changing environmental, societal, and regulatory conditions relevant to the design and operation of U.S. Army Corps of Engineers reservoirs across the conterminous United States. Results demonstrate large geographic variability in how these conditions have changed over time. In the south‐western United States, there is an amplified trend towards drier conditions and less reservoir flexibility with warmer temperatures, less precipitation, high sedimentation rates, and large population growth. In the north‐eastern United States, the impacts of increased temperature on reservoirs may be masked by greater precipitation and lower water demand. Environmental, societal, and regulatory changes can reduce the flexibility of reservoir operations and, in some instances, make it challenging for the reservoir to meet its intended purpose as designed decades ago. This study is the first step towards formalizing a process for monitoring broad trends relevant to water resources management for the purpose of moving towards adaptation of infrastructure. An interactive tool was developed for each condition: https://nicholasinstitute.duke.edu/reservoir‐national‐trends/ .     

黄河入海流路改走北汊1时机分析研究

从黄河现行入海流路的演变发展及近期北汊河的基本情况来看，清７断面以下现行流路的行河年限比原规划的要延长。在控制西河口水位不超过１２．００ｍ，确保防洪安全，并兼顾河口地区工农业发展及油田开采的基础上，总结河口治理措施，尽量延长清７以下河道行河年限，稳定黄河入海流路，科学掌握改汊时机，避免过早或意外地改走北汊１。