黄河上游沙漠宽谷段高含沙支流对干流的淤堵影响

黄河上游沙漠宽谷段支流——十大孔兑的高含沙洪水多次淤堵黄河干流,造成灾害。基于实测水文泥沙资料和模型试验手段对孔兑高含沙洪水特点、淤堵干流过程和交汇区沙坝形态进行了研究。结果表明：孔兑洪水具有陡涨陡落、持续时间短、洪峰流量大、含沙量高、输沙量大等特点;孔兑洪水形成沙坝、淤堵黄河的过程可分为形成、相对稳定和冲刷3个阶段,支流洪水量级较大,形成的沙坝规模较大,沙坝冲刷历时也较长;孔兑高含沙水流入黄交汇区水流结构分区包括壅水区、回流区、高流速带、剪切层等组成要素,沙坝淤积形态模式包括壅水区、回流区、回流区下游淤积体和交汇区输水输沙窄槽;基于沙坝淤堵判别方法以及实测和试验数据提出了孔兑入黄交汇区沙坝判别条件。     

RIPARIAN VEGETATION VARIABILITY ALONG PERENNIAL STREAMS IN CENTRAL ARIZONA

The spatial distribution of riparian vegetation is closely allied to abiotic processes along streams and rivers. There are dynamic relations between physical process, fluvial forms, and biotic structures. Explanation of these associations is critical to scientific understanding and practical management of riverine environments. Therefore, this study determines what geophysical parameters lead to the spatial patterns found in species of warm interior and cold montane riparian deciduous forests in central Arizona. Five riparian vegetation populations were examined along five perennial streams in the transition zone of central Arizona. The populations included Populus angustifolia (narrowleaf cottonwood), two commonly associated species of willow Salix lasiandra (western black willow) and Salix lasiolepis (arroyo willow), Alnus oblongifolia (Arizona alder), and Platanus wrightii (Arizona sycamore). Canonical correspondence analysis (CCA) with a forward selection was used to assess quantitatively the role of stream power in riparian vegetation patterns. Results indicated 40% of the spatial variability in the riparian populations was explained by channel morphology and several other variables related to changing channel geometry. Although floods are linked to the formation of geomorphic surfaces and the regeneration of riparian vegetation, changing fluvial landforms and channel patterns were closely related to the riparian species patterns in central Arizona. [Key words: Mogollon Rim, channel morphology, multivariate statistics.]     

Spatial hydrogeomorphological influences on sediment and nutrient deposition in riparian zones: observations from the Garonne River,France

This paper investigates the influence of geomorphological setting on riparian zone sedimentation within a reach of the River Garonne, France, during three major floods. The sampling design was stratified to reflect landforms constructed by fluvial processes (e.g. floodplain, lateral benches, islands, side channels and point bars). Observed sedimentation varied significantly with flood event, planform context, landform type and associated vegetation cover and, in some cases, with sample location within the landform. Lowest sedimentation was associated with the flood with the smallest peak discharge, peak sediment concentration and sediment load. Sites under natural riparian vegetation experienced higher sedimentation than poplar plantations. Sites on concave (outer) banks received less sedimentation than those on convex (inner banks). Sedimentation on floodplain sites and higher benches was lower than on low benches, point bars and side channels. There was considerable interdependence among these patterns, reflecting the underlying geomorphological forms and processes. Meandering rivers tend to evolve through erosion of concave banks and deposition on convex banks. Point bar features tend to be built along convex banks, whilst concave banks are eroded into higher floodplain and bench features. As a result, concave banks tend to be bordered by higher riparian margins that are less frequently flooded than convex banks. Where river margins are developed for agriculture, the higher, less frequently flooded sites are preferentially selected.Analyses of the quantity, calibre, nutrient and carbon content of the deposited sediment reveal further significant relationships, which reflect the geomorphological structure of the riparian zone. Sediment particle size coarsens in locations with higher amounts of sedimentation. The quantities of total organic carbon (TOC), total organic nitrogen (TON) and total phosphorus (TP) all increase as the quantity of deposited sediment increases. The concentration of TOC and TON also increased significantly with an increase in the percentage of silt plus clay in the deposited sediments.Based upon the above observations, a conceptual model is proposed, which considers the spatial pattern in riparian zone sedimentation according to riparian morphology and flood magnitude. The implications of channel incision for the functioning of the model are also discussed.     

Late Holocene upper bounds of flood magnitudes and twentieth century large floods in the ungauged, hyperarid alluvial Nahal Arava, Israel

The impact of large twentieth century floods on the riparian vegetation and channel morphology of the relatively wide anabranching and braided Nahal Arava, southern Israel, was documented as part of developing tools to (a) identify recent large floods, (b) determine these flood's respective magnitudes in alluvial ungauged streams, and (c) determine long-term upper bounds to flood stages and magnitudes. Along most of its course Nahal Paran, a major tributary that impacts the morphology, floods and sediments of Nahal Arava at the study reach, is a coarse-gravel, braided ephemeral stream. Downstream of the Arava–Paran confluence, aeolian and fluvial sand delivered from eastern Arava valley alters the channel morphology. The sand has accreted up to 2.5 m above the distinct current channels, facilitating the recording of large floods. This sand enhances the establishment of denser riparian vegetation (mainly Tamarix nilotica and Haloxylon persicum) that interacts with floods and affects stream morphology. A temporal association was found between specific floods recorded upstream and tree-ring ages of re-growth of flood-damaged tamarix trees (‘Sigafoos trees’) in the past 30 years. This association can be utilized for developing a twentieth century flood chronology in hyperarid ungauged basins in the region. The minimum magnitude of the largest flood that covered the entire channel width, estimated from flood deposits, is approximately 1700–1800 m³s^− 1. This is a larger magnitude than the largest gauged flood of 1150 m³s^− 1 that occurred in 1970 about 30 km upstream in Nahal Paran. Our estimation agrees with flood magnitude estimated from the regional envelope curve of the largest floods. Based on Holocene alluvial stratigraphy and OSL dating in the study reach we also conclude that flood stages did not reach the late Holocene ( 2.2 ka) surface and therefore we estimate a non-exceedance upper bound of  2000 m³s^− 1 flood magnitudes for Nahal Arava during that interval. This study indicates that in unfavorable areas the combination of hydrology, fluvial morphology and botanic evidence can increase our understanding of ungauged basins and give information crucial for hydrology planning.     

Woody vegetation and channel morphogenesis in low-gradient, gravel-bed streams in the Ozark Plateaus, Missouri and Arkansas

Woody vegetation affects channel morphogenesis in Ozark streams of Missouri and Arkansas by increasing local roughness, increasing bank strength, providing sedimentation sites, and creating obstructions to flow. Variations in physiographic controls on channel morphology result in systematic changes in vegetation patterns and geomorphic functions with increasing drainage basin area. In upstream reaches, streams have abundant bedrock control and bank heights that typically are less than or equal to the rooting depth of trees. In downstream reaches where valleys are wider and alluvial banks are higher vegetation has different geomorphic functions. At drainage areas of greater than 100–200 kM², Ozarks streams are characterized by longitudinally juxtaposed reaches of high and low lateral channel migration rates, referred to as disturbance reaches and stable reaches, respectively. Whereas stable reaches can develop stable forested floodplains (if they are not farmed), disturbance reaches are characterized by dynamic vegetation communities that interact with erosion and deposition processes.Disturbance reaches can be subdivided into low-gradient and high-gradient longitudinal zones. Low-energy zones are characterized by incremental, unidirectional lateral channel migration and deposition of gravel and sand bars. The bars are characterized by prominent bands of woody vegetation and ridge and swale topography. Channel monitoring data indicate that densely vegetated bands of woody vegetation formed depositional sites during bedload-transporting events. The same floods caused up to 20 m of erosion of adjacent cutbanks, scoured non-vegetated areas between vegetation bands, and increased thalweg depth and definition. In high-energy (or riffle) zones, channel movement is dominantly by avulsion. In these zones, vegetation creates areas of erosional resistance that become temporary islands as the channel avulses around or through them. Woody vegetation on islands creates steep, root-defended banks that contribute to narrow channels with high velocities.Calculation of hydraulic roughness from density and average diameter of woody vegetation groups of different ages indicates that flow resistance provided by vegetation decreases systematically with group age, mainly through decreasing stem density. If all other factors remain constant, the stabilizing effect of a group of woody vegetation on a gravel bar decreases with vegetation age.     

Vegetation effects on sediment connectivity and processes in an ephemeral channel in SE Spain

High sediment delivery due to catchment and channel erosion can impact on waterway health and reservoir sedimentation. Methods that decrease sediment connectivity within the system are needed to address these potential impacts. Vegetation has the potential to decrease channel erosion and sediment transfers in dryland environments by increasing channel bed resistance and roughness. Detailed studies of vegetation and process interactions were undertaken within an ephemeral channel in SE Spain at three scales (channel network, reach and patch) by repeat surveys and mapping after floods. Connectivity mapping showed variations in vegetation type and density along the channel and that these influence sediment retention and channel erosion. Hydraulic calculations of vegetation roughness effects at 25 cross-sections down the channel indicate the influence of different vegetation type and characteristics. Particular attention was paid to the perennial grass Lygeum spartum because of its potential benefits in channel stabilisation. Sedimentation takes place within these reaches, mainly by trapping within and downstream of individual plants. Check dams have a profound influence on vegetation and sediment distribution in this channel but use of vegetated zones may be a more sustainable strategy of sediment control.     

Geomorphic effects of rural-to-urban land use conversion on three streams in the Central Redbed Plains of Oklahoma

This research evaluates the impact of rural-to-urban land use conversion on channel morphology and riparian vegetation for three streams in the Central Redbed Plains geomorphic province (central Great Plains ecoregion) of Oklahoma. The Deep Fork Creek watershed is largely urbanized; the Skeleton Creek watershed is largely rural; and the Stillwater Creek watershed is experiencing a rapid transition from rural to urban land cover. Each channel was divided into reaches based on tributary junctions, sinuosity, and slope. Field surveys were conducted at transects in a total of 90 reaches, including measurements of channel units, channel cross-section at bankfull stage, and riparian vegetation. Historical aerial photographs were available for only Stillwater Creek watershed, which were used to document land cover in this watershed, especially changes in the extent of urban areas (impervious cover).The three streams have very low gradients (< 0.001), width-to-depth ratios < 10, and cohesive channel banks, but have incised into red Permian shales and sandstone. The riparian vegetation is dominated by cottonwoods, ash, and elm trees that provide a dense root mat on stream banks where the riparian vegetation is intact. Channels increased in width and depth in the downstream direction as is normally expected, but the substrate materials and channel units remained unchanged. Statistical analyses demonstrated that urbanization did not explain spatial patterns of changes in any variables. These three channels in the central Redbed Plains are responding as flumes during peak flows, funneling runoff and the wash-load sediment downstream in major runoff events without any effect on channel dimensions. Therefore, local geological conditions (similar bedrock, cohesive substrates and similar riparian vegetation) are mitigating the effects of urbanization.     

Complex channel responses to changes in stream flow and sediment supply on the lower Duchesne River, Utah

Channel responses to flow depletions in the lower Duchesne River over the past 100 years have been highly complex and variable in space and time. In general, sand-bed reaches adjusted to all perturbations with bed-level changes, whereas the gravel-bed reaches adjusted primarily through width changes. Gravel-bed reaches aggraded only when gravel was supplied to the channel through local bank erosion and degraded only during extreme flood events.A 50% reduction in stream flow and an increase in fine sediment supply to the study area occurred in the first third of the 20th century. The gravel-bed reach responded primarily with channel narrowing, whereas bed aggradation and four large-scale avulsions occurred in the sand-bed reaches. These avulsions almost completely replaced a section of sinuous channel about 14 km long with a straighter section about 7 km long. The most upstream avulsion, located near a break in valley slope and the transition from a gravel bed upstream and a sand bed downstream, transformed a sinuous sand-bed reach into a braided gravel-bed reach and eventually into a meandering gravel-bed reach over a 30-year period. Later, an increase in flood magnitudes and durations caused widening and secondary bed aggradation in the gravel-bed reaches, whereas the sand-bed reaches incised and narrowed. Water diversions since the 1950s have progressively eliminated moderate flood events, whereas larger floods have been less affected. The loss of frequent flooding has increased the duration and severity of drought periods during which riparian vegetation can establish along the channel margins. As a result, the channel has gradually narrowed throughout the study area since the late 1960s, despite the occasional occurrence of large floods. No tributaries enter the Duchesne River within the study area, so all reaches have experienced identical changes in stream flow and upstream sediment supply.     

Geomorphic Controls on Historical Channel Planform Changes on the Lower Pages River,Hunter Valley,Australia

Historical planform changes in a 14.7 km reach of the lower Pages River were determined to assess whether they were autogenic (inherent in the river regime) or allogenic (driven by external changes) in nature so as to better focus river management activities and river restoration works. A pattern metamorphosis or complete change in river morphology occurred during the February 1955 flood. The peak discharge of this event exceeded the slope and grain size (intrinsic) threshold for braiding, converting the narrow, slightly sinuous stream to a wide, braided-like river. Five subsequent intrinsic threshold-exceeding floods did not cause further bar development because an over-widened channel already existed. Autogenic channel planform changes included sinuosity variations due to lateral migration and pattern metamorphosis due to the exceedance of a discharge–slope–grain size geomorphic threshold. Allogenic channel planform changes included: (1) realignment/channel straightening and artificial cutoffs by river training works; (2) lateral migration by increased bank erodibility due to riparian vegetation clearing; (3) lateral migration by the operation of a transitive geomorphic threshold involving the onset of a flood-dominated regime after 1946 and increased catchment runoff after 1830 due to large-scale clearing of catchment vegetation; and (4) the occurrence of a large flood in February 1955. Multiple forcing factors have clearly caused historical channel planform changes of the lower Pages River, making the design of river management and restoration works a complex matter outside the scope of simple formulaic protocols.     

Interacting effects of multiple factors on the morphological evolution of the meandering reaches downstream the Three Gorges Dam

Elucidating the influence of dams on fluvial processes can inform river protection and basin management. However, relatively few studies have focused on how multiple factors interact to affect the morphological evolution of meandering reaches. Using hydrological and topographical data, we analyzed the factors that influence and regulate the meandering reaches downstream the Three Gorges Dam (TGD). Our conclusions are as follows. (1) The meandering reaches can be classified into two types based on their evolution during the pre-dam period: G1 reaches, characterized by convex point bar erosion and concave channel deposition (CECD), and G2 reaches, characterized by convex point bar deposition and concave channel erosion (CDCE). Both reach types exhibited CECD features during the post-dam period. (2) Flow processes and sediment transport are the factors that caused serious erosion of the low beaches located in the convex point bars. However, changes in the river regime, river boundaries and jacking of Dongting Lake do not act as primary controls on the morphological evolution of the meandering reaches. (3) Flood discharges ranging from 20,000 to 25,000 m³/s result in greater erosion of convex point bars. The point bars become scoured if the durations of these flows, which are close to bankfull discharge, exceed 20 days. In addition, the reduction in bedload causes the decreasing of point bar siltation in the water-falling period. (4) During the post-dam period, flood abatement, the increased duration of discharges ranging from 20,000 to 25,000 m³/s, and a significant reduction in sediment transport are the main factors that caused meandering reaches to show CECD features. Our results are relevant to other meandering reaches, where they can inform estimates of riverbed change, river management strategies and river protection.     

Regional and local controls on the spatial distribution of bedrock reaches in the Upper Guadalupe River, Texas

While studies on gravel mantled and mixed alluvial bedrock rivers have increased in recent decades, few field studies have focused on spatial distributions of bedrock and alluvial reaches and differences between reach types. The objective of this work is to identify the spatial distribution of alluvial and bedrock reaches in the Upper Guadalupe River. We compare reach length, channel and floodplain width, sinuosity, bar length and spacing, bar surface grain size, and slope in alluvial and bedrock reaches to identify whether major differences exist between channel reach types. We find that local disturbances, interaction of the channel and valley sides, variation in lithology, and regional structural control contribute to the distribution of bedrock reaches in the largely alluvial channel. Alluvial and bedrock channel reaches in the Upper Guadalupe River are similar, particularly with respect to the distribution of gravel bars, surface grain size distributions of bars, and channel slope and width. Our observations suggest that the fluvial system has adjusted to changes in base level associated with the Balcones Escarpment Fault Zone by phased incision into alluvial sediment and the underlying bedrock, essentially shifting from a fully alluvial river to a mixed alluvial bedrock river.     

Streamflow regulation and multi-level flood plain formation: channel narrowing on the aggrading Green River in the eastern Uinta Mountains, Colorado and Utah

The style and degree of channel narrowing in aggrading reaches downstream from large dams is dependent upon the dominant geomorphic processes of the affected river, the magnitude of streamflow regulation, and the post-dam sediment transport regime. We measured different magnitudes of channel adjustment on the Green River downstream from Flaming Gorge Dam, UT, USA, that are related to these three factors. Bankfull channel width decreased by an average of about 20% in the study area. In reaches with abundant debris fans and eddy deposited sand bars, the amount of channel narrowing was proportional to the decrease in specific stream power. The fan–eddy-dominated reach with the greatest decrease in stream power narrowed by 22% while the reach with the least decrease in stream power narrowed by 11%. In reaches with the same magnitude of peak flow reduction, meandering reaches narrowed by 15% to 22% and fan–eddy-dominated reaches narrowed by 11% to 12%. Specific stream power was not significantly affected by flow regulation in the meandering reaches.In the diverse array of reach characteristics and deposit types found in the study area, all pre- and post-dam deposits are part of a suite of topographic surfaces that includes a terrace that was inundated by rare pre-dam floods, an intermediate bench that was inundated by rare post-dam floods, and a post-dam floodplain that was inundated by the post-dam mean annual flood. Analysis of historical photographs and tree-ring dating of Tamarix sp. shows that the intermediate bench and post-dam floodplain are post-dam landforms in each reach type. Although these two surfaces occur at different levels, they are forming simultaneously during flows of different magnitude. And while the relative elevation and sedimentologic characteristics of the deposits differ between meandering reaches and reaches with abundant debris fans and eddies, both reach types contain deposits at all of these topographic levels.The process of channel narrowing varied between fan–eddy-dominated and meandering reaches. In the meandering reaches, where stream power has not changed, narrowing was accomplished by essentially the same depositional processes that operated prior to regulation. In fan–eddy-dominated reaches, where significant reductions in stream power have occurred, channel narrowing has been accompanied by a change in dominant depositional processes. Mid-channel sand deposits are aggrading on deposits that, in the pre-dam era, were active gravel bars. These deposits are creating new islands and decreasing the presence of open-framework gravel bars. In eddies, bare sand bars are replaced with vegetated bars that have a simpler topography than the pre-dam deposits.     

多因素对三峡水库下游弯曲河型演变的迭加效应（英文）

Elucidating the influence of dams on fluvial processes can inform river protection and basin management.However,relatively few studies have focused on how multiple factors interact to affect the morphological evolution of meandering reaches.Using hydrological and topographical data,we analyzed the factors that influence and regulate the meandering reaches downstream the Three Gorges Dam(TGD).Our conclusions are as follows.(1)The meandering reaches can be classified into two types based on their evolution during the pre-dam period:G1 reaches,characterized by convex point bar erosion and concave channel deposition(CECD),and G2 reaches,characterized by convex point bar deposition and concave channel erosion(CDCE).Both reach types exhibited CECD features during the post-dam period.(2)Flow processes and sediment transport are the factors that caused serious erosion of the low beaches located in the convex point bars.However,changes in the river regime,river boundaries and jacking of Dongting Lake do not act as primary controls on the morphological evolution of the meandering reaches.(3)Flood discharges ranging from 20,000 to 25,000 m3/s result in greater erosion of convex point bars.The point bars become scoured if the durations of these flows,which are close to bankfull discharge,exceed 20 days.In addition,the reduction in bedload causes the decreasing of point bar siltation in the water-falling period.(4)During the post-dam period,flood abatement,the increased duration of discharges ranging from 20,000 to 25,000 m3/s,and a significant reduction in sediment transport are the main factors that caused meandering reaches to show CECD features.Our results are relevant to other meandering reaches,where they can inform estimates of riverbed change,river management strategies and river protection.     

Downstream effects of reservoirs in areas suffering from water deficiency-the case of Hutuo River, China

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Morphodynamics of a pseudomeandering gravel bar reach

A large number of rivers in Tuscany have channel planforms, which are neither straight nor what is usually understood as meandering. In the typical case, they consist of an almost straight, slightly incised main channel fringed with large lateral bars and lunate-shaped embayments eroded into the former flood plain. In the past, these rivers have not been recognised as an individual category and have often been considered to be either braided or meandering. It is suggested here that this type of river planform be termed pseudomeandering.A typical pseudomeandering river (the Cecina River) is described and analysed to investigate the main factors responsible for producing this channel pattern. A study reach (100×300 m) was surveyed in detail and related to data on discharge, channel changes after floods and grain-size distribution of bed sediments. During 18 months of topographic monitoring, the inner lateral bar in the study reach expanded and migrated towards the concave outer bank which, concurrently, retreated by as much as 25 m. A sediment balance was constructed to analyse bar growth and bank retreat in relation to sediment supply and channel morphology. The conditions necessary to maintain the pseudomeandering morphology of these rivers by preventing them from developing a meandering planform, are discussed and interpreted as a combination of a few main factors such as the flashy character of floods, sediment supply (influenced by both natural processes and human impact), the morphological effects of discharges with contrasting return intervals and the short duration of flood events. Finally, the channel response to floods with variable sediment transport capacity (represented by bed shear stress) is analysed using a simple model. It is demonstrated that bend migration is associated with moderate floods while major floods are responsible for the development of chute channels, which act to suppress bend growth and maintain the low sinuosity configuration of the river.     

Biogeomorphic Relationships and Riparian Vegetation Changes Along Altered Ephemeral Stream Channels: Florence to Marana,Arizona

This study examines riparian vegetation cover changes along ephemeral channels due to the emplacement of the Central Arizona Project (CAP) canal. Two research questions examined are the following: (1) How has riparian vegetation changed over the course of twenty-eight years due to altered flow conditions? (2) How has channel morphology affected changes in vegetation cover? Five Landsat TM images acquired in 1982, 1989, 1996, 2003, and 2010 were classified. The average change of vegetation cover per 0.5-km section over the twenty-eight-year period is approximately 100,436 m² over 25.5-km length of the canal on the upstream section. In addition, the total amount of vegetation cover increase in the twenty-eight years over the 25.5-km length of the canal is approximately 5,122,239 m². Larger streams experienced a greater increase in vegetation cover upslope than smaller streams. In addition, streams of similar width dimensions that were completely closed off resulted in greater vegetation cover than streams that were semiconnected. A significant relationship between changes in vegetation green-up and channel widths was examined. Results from this study suggest that there is a quasi-linear relationship between channel widths and increases in vegetation cover for altered and impounded channels due to the presence of the CAP canal.     

EQUILIBRIUM AND NONEQUILIBRIUM CONDITIONS IN DRYLAND RIVERS

Rivers in drylands typically are characterized by extreme flow variability, with long periods of little or no flow interspersed with occasional large, sometimes extreme, floods. Complete adjustment of river form and process is sometimes inhibited, resulting in a common assumption that equilibrium conditions may rarely, if ever, exist in dryland rivers, and that transient and unstable (nonequilibrium) behavior is the norm. Examples from the Channel Country and the Northern Plains in central Australia challenge that notion. Along the middle reaches of these intermediate and large, low-gradient rivers, where long duration floods generate moderate to low unit stream powers and boundary resistance is high as a result of indurated alluvial terraces, cohesive muds or riparian vegetation, there is evidence that: (1) channels have remained essentially stable despite large floods; (2) sediment transport discontinuities, while present at a catchment scale, are largely insignificant for channel form and process in individual reaches; (3) there are strong correlations between many channel form and process variables; and (4) many rivers appear to be adjusted to maximum sediment transport efficiency under conditions of low gradient, abundant within-channel vegetation and declining downstream discharge. In these middle reaches, rivers are characterized by equilibrium conditions. However, in the aggradational lower reaches of rivers on the Northern Plains, where upstream terraces are buried by younger sediments and channels are less confined, nonequilibrium conditions prevail. Here, channels sometimes undergo sudden and substantial changes in form during large floods, sediment transport discontinuities are readily apparent, and landforms such as splays remain out-of-balance with normal flows. Hence, dryland rivers can exhibit both equilibrium and nonequilibrium conditions, depending on factors such as catchment size, channel gradient, flood duration, unit stream power, channel confinement, sediment cohesion, and bank strength. [Key words: dryland rivers, floods, equilibrium, nonequilibrium, central Australia.]     

The braided Milk River, northern Montana, fails the Leopold–Wolman discharge-gradient test

The Milk River, the northernmost tributary to the Missouri–Mississippi River system, exhibits an anomalous sand-bed braiding reach in an otherwise meandering system. Shortly after leaving Alberta and entering Montana the river suddenly changes to braiding and maintains this pattern for 47 km before entering Fresno Reservoir. Measured stream gradient and bankfull discharge in the braiding reach severely fail the Leopold and Wolman [U.S. Geol. Surv. Prof. Pap. 282B (1957) 39] slope–discharge test for differentiating channel patterns. While channel slope has long been regarded as one of the primary variables associated with braiding, our data from the sand-bed Milk River do not support this relationship. Instead, the data show that the braiding reach has a lower channel slope (0.00047) than the meandering reach (0.00055). Coupled with a constant discharge the unit length stream power is comparable between the two reaches. At the morphologic transition between meandering and braiding, a dramatic reduction in channel bank strength occurs where the sampled silt–clay content declines from 65% in the meandering reach to 18% in the braiding. This enables channel widening which is reflected in a 60% reduction in unit area stream power in the braiding reach. Thus, sediment transport capacity declines and channel bars are deposited. During waning flows, these bars are dissected, producing a braiding morphology. We suggest that for sand-bed braiding rivers the silt–clay percentage in the channel banks may be more important than slope. A review of the original Leopold and Wolman [U.S. Geol. Surv. Prof. Pap. 282B (1957) 39] dataset, and many subsequent analyses, reveals that most braided rivers studied were gravel-bed. As a result, causal variables associated with braiding in sand-bed environments may need a thorough evaluation.     

Floods,drought, and seed mass of riparian plant species

Plant communities in dryland riparian ecosystems are influenced by flood disturbance and water availability. As global climate change alters stream flow regimes, there is increasing need to understand which traits allow plant species to persist under increased stress and disturbance. Small seed mass is part of a strategy that adapts for disturbance, but the role of seed mass as an adaptation for drought is less well documented. For dryland riparian plant communities, we asked, does seed mass vary with water availability and flood frequency? We compared community seed mass between sites that vary in flow permanence (longitudinal water gradient) and between hydrogeomorphic surfaces within sites (lateral gradients of moisture and disturbance). Using data from four rivers in Arizona, we contrasted seed mass between plant groups. We found community seed mass to be greater at sites with ephemeral than perennial flow, and to increase laterally from wet, frequently-flooded channel edges to dry, less disturbed terraces. Seed mass varied by moisture group (smallest for hydroriparian species) and by disturbance group (smallest for disturbance species), and showed a trend for being greater in introduced species. We conclude that small seed mass is independently associated with wet and disturbed conditions in dryland riparian ecosystems.     

The impact of the Parangana Dam on the River Mersey,Tasmania

By diverting the waters of the River Mersey into a neighbouring basin, the Parangana Dam has changed the hydrological character of the downstream river. The flow records of three gauging stations, located at distances of 10, 67 and 88 km below the dam, provide the basis for identifying the changes. Intermediate discharges have decreased the most and even the furthest downstream station are 50% lower than before. The flow extremes, at both ends of the scale, have been less affected, and particularly the less frequent flood discharges which are augmented by overspill. Indeed the highest flood on record, with a recurrence interval of over 500 years, occurred in the post-dam period. The entry of unregulated tributaries in the middle section separates a much changed upstream hydrology from a downstream one able to mitigate the worst effects of the dam except at intermediate discharges.Field survey, air and ground photograph analysis are used to assess the impact of impoundment on the downstream channel. The main changes involve the expansion of existing lateral bars and the growth of new ones, but such deposition and the consequent narrowing of the channel are rather localized. The invasion of bar surfaces by vegetation could accelerate the process, but the general coarseness of the channel bed and the lack of fine material for bank building will severely delay adjustment of the Mersey channel. The chemical and biological character of the river could be adversely affected by the altered flow regime, particularly in those reaches immediately below the dam.