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.     

Bed-material transport estimated from channel surveys: Vedder River,British Columbia

This study investigates the possibility to estimate bed-material transfer in gravel-bed rivers by analysis of morphological changes along Vedder River, British Columbia. Data from repeated cross-section surveys are used to estimate volume changes along the length of an 8 km reach. Gravel budgets are based on a continuity approach. An error analysis is performed to evaluate the uncertainty in the best estimate of transport rates. The mean annual gravel transport into the reach over a 9 year period was estimated to be 36600 ± 5600 m³ yr^?1. The sediment transport regime along the length of the river is evaluated and examined in relation to peak flood flows. Significant spatial and temporal variability in transport rates is demonstrated, making dubious the generalization of transport estimates from hydraulic calculations, or from sample measurements at a single cross-section. The assumptions, procedures and limitations of the ‘morphological approach’ to sediment transport analysis are discussed. It is concluded that this approach provides information of quality comparable or superior to that of direct measurements of transport, yet requires less field effort. It also provides additional information about river morphological changes, making it a preferred method for geomorphlogical investigations and for many river management concerns.     

Play: The handmaiden of work

The natural habitat of geomorphologists is at play in the fields of the Lord. Picnics, not the work ethic, often motivate the most productive. As in the field, so too in the mind, creative insights derive from the play of ideas. As in much of science today, Earth scientists are called upon to ‘solve’ immediate problems, but problems whose solutions lie at the cutting edge of the research frontier. The impact of anthropogenic activities in the context of natural processes, the playground where many geomorphologists work, requires continuous reciprocal exchange between research and application. Apologies are not needed for choices of orientation, but only for destructive separation.     

Turbulent flow structure in a gravel-bed river: Markov chain analysis of the fluctuating velocity profile

Three electromagnetic current meter probes were deployed in a Canadian gravel-bed river to obtain simultaneous records at 10 Hz of streamwise (u) and vertical (v) velocity components at three heights above the bed. By looking at the positive and negative signs of the instantaneous fluctuations from the time-average values of each velocity component at each height, the fluctuating velocity profile of u or v can be treated as a Markov chain with eight states and its statistical properties can be tested against null hypotheses based on the absence of spatial structure. We report results of this novel approach. The most common states of the u profile were those with either higher-than-average or lower-than-average velocities at all heights; these ‘high speed’ and ‘low speed’ states persisted for up to 3 s. The most common v profiles were all-upwards or all-downwards, but these persisted for shorter times than the high speed and low speed u profiles. Analysis of transition probabilities shows statistically significant tendencies for acceleration from the low speed u profile, and change from all-upwards to all-downwards v profile, to take place progressively from the uppermost probe downwards, in a sweep-like way. Deceleration from the high speed to low speed u profile and change from all-downwards to all-upwards v profile (burst-like behaviour) do not show such clear patterns. The results are interpreted in terms of the advection of inverted wedges of relatively high-momentum fluid, followed by more chaotic structures. A separate set of flow visualization experiments over a mixed gravel bed in a flume supports the presence of advected wedge structures, the decelerating part of the sequence corresponding to irregular ejections of near-bed fluid.     

Monitoring alluvial sand suspension by eddy correlation

Eddy correlation techniques are standard tools in micrometeorology and oceanography to measure momentum and contaminant transport across turbulent boundary layers. They can, in theory, be used to estimate the net vertical suspended sediment flux directly over different areas of an alluvial channel boundary, and thus disclose ongoing erosion/deposition patterns. The basic principles and main problems in applying the technique to alluvial suspension are first introduced. Results from a trial application of the method in a large sand bed river are then presented; the focus of the analysis is on the substantial (and surprising) contributions of multi-minute flow fluctuations to suspension work in the study environment. The data were collected in a 10 m deep channel of the Fraser River near Mission, British Columbia, Canada. Turbulent fluctuations of flow components streamwise and normal to the bed, along with the output of an optical suspended sediment sensor, were monitored over 7 h, 1 m above the bed. Flow velocities averaged 0·9 ms^?1 and mean suspended sediment concentrations 500 mgl^?1, at sensor level above 1–5 dm high dunes. Spectral analysis of the records reveals that approximately 30 per cent of the vertical suspended sand mixing across the sensor level (and roughly as much of the momentum exchange) was linked to gradual flow oscillations with periods between 1 and 13·6 min (underlying briefer, turbulent fluctuations). Extended periods of sediment-rich, slightly upward directed but slower mean flow alternated with periods of sediment-poor, slightly downward and faster mean flow; these slow fluctuations involved 10–20 cms^?1 changes in 5 min average flow speed, 2–4· changes in vertical flow angle and 100 mgl^?1 changes in mean sand concentration. To obtain accurate eddy-correlation estimates of the vertical suspension flux in the study conditions, hour-scale flow and turbidity records that include many of these multi-minute cycles appear to be necessary. The spectra of the Fraser River near-bed signals do not conspicuously differ in overall shape (in terms of low-frequency content and location of peak) from turbulent spectra encountered in some atmospheric boundary layers. Nonetheless, the long period fluctuations observed on the Fraser River may not be turbulent; rather they may reflect slowly evolving perturbations in the near-bed streamlines, caused by bedform translation or gradual fluctuation within the large-scale streamwise cells of the secondary flow.     

Adaptive adjustment of channel geometry

This paper constructs a model of channel geometry composed of three subsections: a steady-state submodel, a gradedstate submodel, and a stochastic error submodel. With the aid of this representation of the morphology of channels, the at-a-point changes in geometry can be reproduced by a simple recursive equation of autoregressive, moving-average form which is derived from methods used in the statistical analysis of time series. A set of height loss data for three Japanese rivers derived from Yatsu's (1955) paper is used to determine the effect of bed material changes on adaptations within the graded-state submodel of the channel. The results of analyzing the autocorrelation function, spectrum, and adaptive parameter shifts within this model can be used to infer that significant changes in the amplitude of height change variation, and a shift to higher frequency oscillations of bed forms are associated with the shift in bed material conditions.     

Flood-borne sediment analysis of the Hoanib River, northwestern Namibia

The suspended sediment contained in six flood events in the Hoanib River catchment during the wet season of 1999/2000 were analysed for volatile (or low molecular weight) fine particulate organic material (FPOM), higher molecular weight FPOM and the total organic carbon (TOC). Samples were evaporated to dryness at 80 °C, then heated in a high temperature oven to 250, 550 and 850 °C. The results show that the amount of sediment carried during the flood events varied between 1.54 and 31.27 g/L, with an average of 14.62 g/L. The amount of suspended sediment in the flood-waters (2.789–616.359×10⁶ kg) was found to be dependent on the timing and duration of the flood event. The largest percentage of organic material was found in the volatile FPOM fraction (5.77±6.26%), with lesser amounts being found in the higher molecular weight FPOM (3.36±0.62%) and TOC (4.93±1.86%) fractions.     

中国国际河流研究进展

1IntroductionThe increasing momentum of economic globalization and regional geopolitical cooperation no longer stand as the major drivers of a new world order. Resource and environmental diplomacy are now regarded as new elements establishing future international patterns. Transboundary resources and environmental issues in international river basins have attracted increasing global concern and professional attention. The impact of such issues penetrates into national security, international e…     

Further perspectives on the evolution of bed material waves in alluvial rivers

In one‐dimensional mathematical models of ?uvial ?ow, sediment transport and morphological evolution, the governing equations based on mass and momentum conservation laws constitute a hyperbolic system. Succinctly, the hyperbolic nature excludes dispersion or diffusion operators, which is well known in the context of differential equations. There is no doubt that the so‐called ‘dispersion’ argument for bed material wave evolution is questionable, as we have explicitly asserted. Surprisingly, in a recent communication, the authors of the ‘dispersion’ argument suggest that dispersion is not precluded in hyperbolic systems. We provide herein further perspectives to help explain that the dispersion argument is neither appropriate nor necessary for interpreting bed material wave evolution. Also the continuity equations involved are addressed to prompt wider understanding of their signi?cance. In particular, the continuity equation of the water–sediment mixture proposed by the authors of the ‘dispersion’ argument is proved to be incorrect, and inevitably their reasoning based on it is problematic. Copyright © 2005 John Wiley & Sons, Ltd.     

Particle clusters in gravel‐bed rivers: an experimental morphological approach to bed material transport and stability concepts

Structured gravel river beds clearly exert a major influence on bed stability. Indexing structural stability by field measurements of bed strength neglects the processes operating to entrain and transport bed material in different parts of each structure. This study takes a morphological approach to interpreting the critical processes, using particle tracing to determine the movement of individual cluster particles over a range of flood event magnitudes and durations. The experiment was carried out on the River South Tyne, UK; it uses flow hydrographs measured nearby and also benefits from previous studies of historical development, channel morphology and sediment transport at the same site. More than 30 clusters were monitored over a seven‐month period during which clusters occupied 7–16 per cent of the bed. Threshold flows delimiting three apparently contrasting bed sediment process regimes for cluster particles are tentatively set at 100 m³ s^?1 and 183 m³ s^?1; durations of flow at these levels are critical for cluster development, rather than flow peak values. Wake particles are transported most easily. Flow straightening in the wandering channel planform reduces the stability of clusters, since mechanical strength is markedly reduced by this change of direction. The overall area covered by clusters between significant transport events varies little, implying a dynamic equilibrium condition. Copyright © 2005 John Wiley & Sons, Ltd.