Unmanned aerial vehicles (UAVs) and structure-from-motion photogrammetry enable detailed quantification of geomorphic change. However, rigorous precision-based change detection can be compromised by survey accuracy problems producing systematic topographic error (e.g. 'doming'), with error magnitudes greatly exceeding precision estimates. Here, we assess survey sensitivity to systematic error, directly correcting topographic data so that error magnitudes align more closely with precision estimates. By simulating conventional grid-style photogrammetric aerial surveys, we quantify the underlying relationships between survey accuracy, camera model parameters, camera inclination, tie point matching precision and topographic relief, and demonstrate a relative insensitivity to image overlap. We show that a current doming-mitigation strategy of using a gently inclined (<15°) camera can reduce accuracy by promoting a previously unconsidered correlation between decentring camera lens distortion parameters and the radial terms known to be responsible for systematic topographic error. This issue is particularly relevant for the wide-angle cameras often integrated into current-generation, accessible UAV systems, frequently used in geomorphic research. Such systems usually perform on-board image pre-processing, including applying generic lens distortion corrections, that subsequently alter parameter interrelationships in photogrammetric processing (e.g. partially correcting radial distortion, which increases the relative importance of decentring distortion in output images). Surveys from two proglacial forefields (Arolla region, Switzerland) showed that results from lower-relief topography with a 10°-inclined camera developed vertical systematic doming errors > 0•3 m, representing accuracy issues an order of magnitude greater than precision-based error estimates. For higher-relief topography, and for nadir-imaging surveys of the lower-relief topography, systematic error was < 0•09 m. Modelling and subtracting the systematic error directly from the topographic data successfully reduced error magnitudes to values consistent with twice the estimated precision. Thus, topographic correction can provide a more robust approach to uncertainty-based detection of event-scale geomorphic change than designing surveys with small off-nadir camera inclinations and, furthermore, can substantially reduce ground control requirements.

Research in the 1990s showed that bed-material transport rates could be estimated at the reach scale in both onedimension and, over small spatial scales (10s of m), in two-dimensions. The limit on the latter was the spatial scale over which it was possible to obtain distributed data on morphological change. Here, we revisit the morphological method given progress in both topographical data acquisition and hydraulic modelling. The bed-material transport needed to conserve mass is calculated in both one and two dimensions for a 1600 m × 300 m Alpine braided river "laboratory". High-resolution topographical data were acquired by laser scanning to quantify Digital Elevation Models (DEMs), and morphological changes caused by the flushing of the water intake were derived from repeated surveys. Based on DEMs of differences, 1D bed-material transport rates were calculated using the morphological method. Then, a 2D hydraulic model was combined with a topographic correction to route sediment through the network of braided channels and to obtain a spatially variable estimate of transport in both downstream and cross-stream directions. Monte Carlo simulation was applied to the routing model parameters, allowing identification of the most probable parameter values needed to minimize negative transport. The results show that within-section spatial compensation of erosion and deposition using the 1D treatment leads to substantial local errors in transport rate estimates, to a degree related to braiding intensity. Even though the 2D application showed that a large proportion of the total transport was actually concentrated into one main channel during the studied low flow event, the proportion of transport in secondary anabranches is substantial when the river starts braiding. Investigations of the effects of DEM resolution, competent flow duration and survey frequency related to 'travelling bedload' and sequential erosion-deposition emphasized the critical importance of careful data collection in the application of the morphological method.

The temporal variability in sediment export yield from glaciers over a timescale of multiple glacial cycles (e.g. 1 × 102 − 1 × 106 years) is of interest for a wide range of applications in glaciology, sedimentology, geomorphology, climatology and environmental engineering. However, the time required for the products of glacial erosion to be transferred through glaciated catchments and the extent to which glacially-conditioned sediment can be transiently stored within them are still poorly constrained and a matter of debate within the community. We propose a conceptual model of the variability in sediment exported from glaciers over multiple glacial cycles based on a literature review. Sediment yield is likely to be highly variable through a glacial cycle, notably between phases of glacier advance, retreat and re-advance due to changes in ice velocity and erosion rates, ice and meltwater transport capacity, and in glacially-conditioned sediment accessibility at the bed. Typically, early phases of glacier retreat and re-advance are expected to lead to the highest increase in sediment yield due to the ease with which the products of bedrock erosion can be accessed and reworked. In contrast, later phases of glacial (re)advance, once glacially-conditioned sedimentary sources become exhausted, may be characterized by intermediate rates of sediment export yield maintained through bedrock erosion. The latest phases of deglaciation, once glacially-conditioned sedimentary sources are either exhausted, stabilized or disconnected from active processes of sediment transfer, are likely to have the lowest rate of export. The conceptual model proposed in this paper fills a gap in the literature by developing a continuous pattern of sediment yield rate variability over the course of multiple glacial cycles, with wider implications for future research. However, its systematic applicability to various glacier settings and glaciations needs more field and modeling data to validate it.

Braided gravel‐bed rivers show characteristic temporal and spatial variability in morphological change and bedload transport under steady flow and sediment supply rates. Their morphodynamic behavior and long‐term evolution in response to nonstationary external forcing is less well known. We studied daily morphological changes in a well‐constrained reach of an Alpine braided river that is subject to regulated sediment‐laden flows, associated with hydroelectric power exploitation, as well as occasional floods. We found that net reach erosion and deposition were forced by upstream sediment supply, albeit in a nonlinear fashion. The spatial distribution of morphological change and inferred spatially‐distributed sediment transport rates varied strongly along the braided reach and between successive sequences of flushing. Local morphological change was driven by two factors: (1) local relief, leading to the preferential filling of topographic lows and erosion of highs, particularly during longer duration floods, which allow braided dynamics to be maintained; and (2) system memory, leading to a negative autocorrelation in bed level changes where erosion was followed by deposition of similar magnitude and vice versa. This effect was associated with the temporary storage of high sediment loads from flushing due to the abrupt on‐off nature of these flows and reveals the relatively efficient transport of sediment in a river that is heavily impacted upon by flow abstraction. In general, the internal morphodynamics of the braided river condition their own response to external forcing events and thus sediment transfer.

Understanding and predicting bedload transport is an important element of watershed management. Yet, predictions of bedload remain uncertain by up to several order(s) of magnitude. In this contribution, we use a 5‐year continuous time series of streamflow and bedload transport monitoring in a 13.4‐km2 snow‐dominated Alpine watershed in the Western Swiss Alps to investigate hydrological drivers of bedload transport. Following a calibration of the bedload sensors, and a quantification of the hydraulic forcing of streamflow upon bedload, a hydrological analysis is performed to identify daily flow hydrographs influenced by different hydrological drivers: rainfall, snowmelt, and combined rain and snowmelt events. We then quantify their respective contribution to bedload transport. Results emphasize the importance of combined rain and snowmelt events, for both annual bedload volumes (77% on average) and peaks in bedload transport rate. A non‐negligible, but smaller, amount of bedload transport may occur during late summer and autumn storms, once the snowmelt contribution and baseflow have significantly decreased (9% of the annual volume on average). Although rainfall‐driven changes in flow hydrographs are responsible for a large majority of the annual bedload volumes (86% on average), the identified melt‐only events also represent a substantial contribution (14% on average). The results of this study help to improve current predictions of bedload transport through a better understanding of the bedload magnitude‐frequency relationship under different hydrological conditions. We further discuss how bedload transport could evolve under a changing climate through its effects on Alpine watershed hydrology.