How Do Gravel-Bed Rivers Braid?

Sedimentary processes and bed forms leading to the onset of braiding were observed in small-scale hydraulic models of gravel-bed streams. The laboratory streams had a variety of combinations of (constant) discharge and slope but identical bed-material particle-size distributions. From initially straight channels, braiding occurred by four different processes: deposition and accumulation of a central bar, chute cutoff of point bars, conversion of single transverse unit bars to mid-channel braid bars, and dissection of multiple bars. In these experiments the chute cutoff mechanism was the most common, but the predominant braiding mechanism depends upon sediment mobility (excess bed shear stress) and the bed-form regime. At very low excess bed shear stress the central bar process dominates, but at higher excess bed shear stress slip-face unit bars are more common, bed scour at confluences is more pronounced, and propogation of alternate convergence (scour) and divergence (deposition) is more likely; thus chute cutoffs and bar conversion dominate. The multiple bar mechanism is restricted to channels with very high width/depth ratio. All of these processes, along with avulsion, are significant for maintenance of an established braided channel.The direct physical sedimentary cause of primary braiding is essentially the same in all these processes: local aggradation (often by stalling of bed-load sheets) and loss of competence in a lateral flow expansion. The chute cutoff process occurs in a morphologically distinctive setting and may be aided by other factors, but it is usually triggered by the local thalweg shoaling that is the fundamental physical mechanism causing the onset of braiding by the other processes. Local short-term pulses in bed-load supply are often the trigger for the initiation and maintenance of braiding, regardless of the exact braiding process

Flume Tests on Fluvial Erosion Mechanisms in Till-bed Channels

Semi-alluvial stream channels eroded into till and other glacial sediments are common in areas of extensive glacial deposition such as the Great Lakes region and northern interior plains of North America. The mechanics of erosion and erosional weakness of till results in the dominance of fluvial scour and spontaneous fracture at planes of weakness under shearing flow. There have been few controlled tests looking at erosional mechanisms and resistance of till in river channels. We subjected small blocks of till to unidirectional flows in a laboratory flume to measure the threshold shear stress for erosion and observed the erosion mechanics. Critical shear stress for erosion varied from 7 – 8 Pa for samples with initial saturated moisture content in which a combination of fluvial scour and mass cracking/block erosion dominated. When dried, micro-fissures occurred in the sample and erosional resistance of the till was extremely low at \u3c 1 Pa with erosion appearing to be by fluvial scour. When mobile gravel was added to the test conditions, the gravel reduced the erosion threshold slightly because of the enhanced scour around and below the gravel particles and the tendency for the gravel to aid in crack enlargement. Thus a partial or thin gravel cover over the till may provide no protection from erosion. The erosion processes and effects reflect the complex and contingent mechanics and properties of till, and suggest that the erosion characteristics of till bed semi-alluvial channels differ from abrasion or plucking dominated processes in more resistant bedrock

Towards a Sociogeomorphology of Rivers

While human impacts on rivers and other landforms have long been a component of geomorphic research, little of this work explicitly includes insights into human agency from social science or recognises that in many cases rivers can be considered to be hybrid coproductions or „socio-natures‟. A socio-geomorphic approach proposed here has parallels with some aspects of sociohydrology and can extend and enrich existing geomorphic explanations of the morphology of, for example, urban rivers by explicitly recognising and working with the coevolution of the human and natural systems. Examples from recent literature illustrate ways in which these relationships can be understood and analyzed, showing a range of socio-natural influences in particular contexts that have material consequences for river morphology and recognising that events in the system have many forms. The approach recognises the importance of contingency in time and place together with the role and nature of both local and global knowledge. An important element of this approach is that it provides ways for understanding the nature, position and intention of geomorphic and other scientific interventions as part of the system, for example in the case of river restoration. This also leads to the need for reflexivity by geomorphologists and reconsideration of the nature of geomorphological knowledge by those involved in such work and with respect to sociogeomorphology as a whole

Effective Discharge for Suspended Sediment Transport in Streams of the Saskatchewan River Basin

Effective discharge for suspended sediment load was determined for 21 sites in the Saskatchewan River basin at which sediment records range from 5 to 29 years in length. The drainage areas for these streams ranges from 10 to over 300,000 km2. The sediment discharge histograms have a variety of forms ranging from the classic unimodal form in which the peak occurs at discharges with a duration of 1–3% to those in which the effective discharge is the extreme event of record and cases in which a single effective discharge is difficult to define. The percentage duration of the effective-discharge ranges from less than 0.1% to over 15%, a greater range than previously has been reported. There is an obvious tendency for the percentage duration of the effective discharge to increase with drainage area and hence downstream through the drainage system

Spatial and Temporal Patterns of Suspended-Sediment Yield in the Saskatchewan River Basin

Long-term suspended-sediment concentration and load records are available for 23 Water Survey of Canada sediment-monitoring stations in the Saskatchewan River basin, where the drainage areas range from 10 to over 300 000 km2. Mean annual sediment yield is greatest in the western Alberta Plains along the Oldman and Red Deer rivers (over 100 t km−2 year−1) and tends to increase downstream along the North and South Saskatchewan rivers until major reservoirs in Saskatchewan intervene. Average sediment concentration shows a pattern of variation similar to that of yield. Temporal aspects of suspended-sediment transport vary along the drainage network. The range and skewness of the yield–duration and concentration–duration curves are greater in the intermediate-size basins close to the Rocky Mountains and in two small basins with Prairie sources than they are in the large Prairie streams with mountain sources and the glacier-fed upper North Saskatchewan River. Similarly, infrequent flows transport a larger proportion of the annual load in the smaller Foothills and western Plains basins than in the large Prairie streams because of differences in drainage area and discharge regime

Evolution of grain size distributions and bed mobility during hydrographs in gravel-bed braided rivers

Evolution of bed material mobility and bedload grain size distributions under a range of discharges is rarely observed in braiding in gravel-bed rivers. Yet, the changing of bedload grain size distributions with discharge is expected to be different from laterally stable, threshold, channels on which most gravel bedload theory and observation are based. Here, simultaneous observations of flow, bedload transport rate, and morphological change were made in a physical model of a gravel-bed braided river to document the evolution of grain size distributions and bed mobility over three experimental event hydrographs. Bedload transport rate and grain size distributions were measured from bedload samples collected in sediment baskets. Morphological change was mapped with high-resolution (~1 mm precision) digital elevation models generated from close-range digital photogrammetry. Bedload transport rates were extremely low below a discharge equivalent to ~50 % of the channel-forming discharge (dimensionless stream power ~70). Fractional transport rates and plots of grain size distributions indicate that the bed experienced partial mobility at low discharge when the coarsest grains on the bed were immobile, weak selective mobility at higher discharge, and occasionally near-equal mobility at peak channel-forming discharge. The transition to selective mobility and increased bedload transport rates coincided with the lower threshold for morphological change measured by the morphological active depth and active width. Below this threshold discharge, active depths were of the order of D90 and active widths narrow (\u3c 3% of wetted width). Above this discharge, both increased so that at channel forming discharge, the active depth had a local maximum of 9D90 while active width was up to 20% of wetted width. The modelled rivers approached equal mobility when rates of morphological change were greatest. Therefore, changes 38 in the morphological active layer with discharge is directly connected to the conditions bed mobility, and strongly correlated with bedload transport rate

The variability in the morphological active width: Results from physical models of gravel‐bed braided rivers

The morphological active width, defined as the lateral extent of bed-material displacement over time, is a fundamental parameter in multi-threaded gravel-bed rivers, linking complex channel dynamics to bedload transport. Here, results are presented from 5 constant discharge experiments, and three event hydrographs, covering a range of flow strengths and channel configurations for which morphological change, bedload transport rates, and stream power were measured in a physical model. Changes in channel morphology were determined via differencing of photogrammetrically-derived digital elevation models (DEMs) of the model surface generated at regular intervals over the course of ~115 hours of experimental runs. Independent measures of total bedload output were made using downstream sediment baskets. Results indicate that the morphological active width increases with total and dimensionless stream power and is strongly and positively correlated with bulk change (total volume of bed-material displaced over time) and active braiding intensity (ABI). Although there is considerable scatter due to the inherent variability in braided river morphodynamics, the active width is positively correlated with independent measurements of bedload transport rate. Active width, bulk change, and bedload transport rates were all negligible below a dimensionless stream power threshold value of ~ 0.09, above which all increase with flow strength. Therefore, the active width could be used as a general predictor of bulk change and bedload transport rates, which in turn could be approximated from total and dimensionless stream power or ABI in gravel-bed braided rivers. Furthermore, results highlight the importance of the active width, rather than the morphological active depth, in predicting volumes of change and bedload transport rates. The results contribute to the larger goals of better understanding of braided river morphodynamics, creating large high-resolution datasets of channel change for model calibration and validation, and developing morphological methods for predicting bedload transport rates in braiding river systems

Expanding the Active Layer

Church and Haschenburger (2017) make helpful distinctions around the issue of defining the active layer, with which we agree. We propose expanding discussion and definition of the ”active layer” in fluvial bedload transport to include the concept of the “morphological active layer”. This is particularly applicable to laterally unstable rivers (such as braided rivers) in which progressive morphological change over short time periods is the process by which much of the bedload transport occurs. The morphological active layer is also distinguished by variable lateral and longitudinal extent continuity over a range of flows and transport intensity. We suggest that the issue of forms of active layer raised by Church and Haschenburger opens up an important discussion on the nature of bedload transport in relation to river morpho-dynamics over the range of river types

Technical note: Ground-based remote sensing of a mountain stream: measuring stage and water width using a simple time-lapse camera.

Remote sensing applied to river monitoring adds complementary information useful for understanding the system behaviour. In this paper, we present a method for visual stage gauging and water surface width measurement using a ground-based time-lapse camera and a fully automatic image analysis algorithm for flow monitoring at a river cross section of a steep, bouldery channel. The remote stage measurement was coupled with a water level logger (pressure transducer) on site and shows that the image-based method gives a reliable estimate of the water height variation and daily flow record when validated against the pressure transducer (R = 0.91). From the remotely sensed pictures, we also extracted the water width and show that it is possible to correlate water surface width and stage. The images also provide valuable ancillary information for interpreting and understanding flow hydraulics and site weather conditions. This image-based gauging method is a reliable, informative and inexpensive alternative or adjunct to conventional stage measurement especially for remote sites