Abstract Open framework gravels (OFGs) are an inherent textural component of alluvial gravel outwash deposited by braided river systems. Being exceptionally permeable, they play an important part in facilitating the transmission of water and contaminants through alluvial gravel aquifers. Understanding how connected OFG facies are is helpful in making informed predictions about groundwater flow and contaminant transport through such aquifer systems. This work examined a section of the Rakaia fan, Canterbury, New Zealand. A 3 × 3 grid of large diameter auger holes was drilled in close proximity to a sea cliff, which provided very good three-dimensional exposure of the fan architecture. A novel smoke tracing experiment and water tracing field tests were conducted to measure the dynamic connectivity of the OFG facies. Smoke proved to be an effective tracer for measuring the interconnectedness of OFGs over set distances of 5 m. The water tracing tests confirmed that OFGs are connected across much longer distances – in excess of 18 m. Results from both tests revealed how rapid, and non-uniform, aqueous transport can be through alluvial outwash materials. The connectivity information will be used to improve realizations of the heterogeneity of the Canterbury Plains aquifer and inform future hydrogeological modelling.

Abstract Gravel-bed river floodplains are dynamic landscapes that support a high level of ecosystem biocomplexity and biodiversity in large part because of the continual physical turnover of habitat. We evaluated the potential of a gravel-bed river to do geomorphic work on a series of floodplains below a dam by linking airborne hyperspectral imagery with corresponding groundtruth measures of flow velocity, water depth, floodplain surface topography, and vegetative cover. These data were analyzed in a geographic information system to map the spatial distribution of potential stream power over a range of discharge regimes. Nodes of flow separation at specific discharges that co-occurred with zones of high stream power were used as a metric to determine potential geomorphic threshold levels and location of channel avulsions. In order to address discharge duration as a factor affecting geomorphic change on the study floodplains, we established the relationship between discharge and total cumulative power applied to a single key floodplain and then used that relationship to examine historical discharge records and changes in flow release in terms of total cumulative power. We used the assumption that similar levels of total cumulative stream power, above a minimum geomorphic threshold, would produce similar levels of geomorphic work as a higher-magnitude, short-duration flood event. These results form the basis of an objective approach to evaluating flow releases needed from dams to maintain the dynamic structure and ecological function of gravel-bed river flood-plains. Moreover, the methodologies presented herein lend themselves to quantitative investigation of potential geomorphic changes related to complete dam removal and return of normalized flow of water and materials through the river system.

Regional high-magnitude rainstorms have produced several large floods in north coastal California during the last century, which resulted in extensive mass-movement activity and channel aggradation. Channel monitoring in Redwood Creek, through the use of cross-sectional surveys, thalweg profiles, and pebble counts, has documented the persistence and routing of channel-stored sediment following these large floods in the 1960s and 1970s. Channel response varied on the basis of timing of peak aggradation. Channel-stored sediment was evacuated rapidly from the upstream third of the Redwood Creek channel, and the channel bed stabilized by 1985 as the bed coarsened. Currently only narrow remnants of flood deposits remain and are well vegetated. In the downstream reach, channel aggradation peaked in the 1990s, and the channel is still incising. Channel-bed elevations throughout the watershed showed an approximate exponential decrease with time, but decay rates were highest in areas with the thickest flood deposits. Pool frequencies and depths generally increased from 1977 to 1995, as did median residual water depths, but a 10 yr flood in 1997 resulted in a moderate reversal of this trend. Channel aggradation generated during 25 yr return interval floods has persisted in Redwood Creek for more than 30 yr and has impacted many life cycles of salmon. Watershed restoration work is currently focused on correcting erosion problems on hillslopes to reduce future sediment supply to Redwood Creek instead of attempting in-channel manipulations.

Geomorphic changes in reconfigured reaches of three Colorado rivers in response to floods in 2005 provide a benchmark for “restoration” assessment. Sediment-entrainment potential is expressed as the ratio of the shear stress from the 2 yr, 5 yr, 10 yr, and 2005 floods to the critical shear stress for sediment. Some observed response was explained by the excess of flood shear stress relative to the resisting force of the sediment. Bed-load entrainment in the Uncompahgre River and the North Fork Gunnison River, during 4 and 6 yr floods respectively, resulted in streambed scour, streambed deposition, lateral-bar accretion, and channel migration at various locations. Some constructed boulder and log structures failed because of high rates of bank erosion or bed-material deposition. The Lake Fork showed little or no net change after the 2005 flood; however, this channel had not conveyed floods greater than the 2.5 yr flood since reconfiguration. Channel slope and the 2 yr flood, a surrogate for bankfull discharge, from all three reconfigured reaches plotted above the Leopold and Wolman channel-pattern threshold in the “braided channel” region, indicating that braiding, rather than a single-thread meandering channel, and midchannel bar formation may be the natural tendency of these gravel-bed reaches. When plotted against a total stream-power and median-sediment-size threshold for the 2 yr flood, however, the Lake Fork plotted in the “single-thread channel” region, the North Fork Gunnison plotted in the “multiple-thread” region, and the Uncompahgre River plotted on the threshold. All three rivers plotted in the multiple-thread region for floods of 5 yr recurrence or greater.

Understanding and quantifying fluvial transport and bedrock abrasion processes have become major concerns in modeling landform response to tectonic and climatic forcing. Recent theoretical and experimental investigations have in particular stressed the importance of sediment supply and size in controlling bedrock incision rate. Many studies on the downstream evolution of pebble size have focused on unraveling the respective roles of selective sorting and abrasion, without paying much attention to sediment sources. In order to track sediment supply and characteristics from source to sink in an active tectonic setting, where long-term selective deposition can be excluded, we systematically measured sediment size and lithology on gravel bars along the Marsyandi River and its tributaries (Himalayas of central Nepal), and also in sediment source material from hillslopes (landslides, moraines, terrace deposits). The downstream evolution in lithological distribution is found to be in close agreement with common views on pebble abrasion and present views on denudation in the range: (1) pebbles from the more rapidly uplifted and eroded Higher Himalayan gneissic units are over-represented, due to their major contribution to sediment influx, (2) easily erodible lithologies such as schists, sandstones, and limestone are under-represented relative to resistant rock types like quartzite. More surprisingly, we observe a general downstream coarsening of gravel bar material along the middle and lower Marsyandi River, whereas downstream sediment fining is typical of most river systems. A simple integrative model that tracks pebbles from hillslope to the main stem of the river and includes abrasion coefficients for the different Himalayan lithologies and size distribution of hillslopes sediment supplies accounts for both changing lithologic proportion along the Marsyandi and for the downstream coarsening of gravel bar material. This coarsening mainly results from differences in sediment sources along the Marsyandi Valley, in particular from differences in size distributions of landslide and moraine material. However, the median pebble size of subsurface material in gravel bars is coarser than median size of the blocky material in the source. The choice of the measurement methods and their potential bias are discussed but cannot explain this surprising feature displayed by our measurements. We suspect that due to sediment transport modalities in active tectonic settings, the subpavement grain-size distribution on gravel bars is not representative of the average bed-load size distribution. Consequently, pebble abrasion is more easily demonstrated by description of pebble lithology than by the downstream evolution of pebble size. Our study also shows, in contrast with previous studies, that experimentally derived abrasion coefficients can account for the downstream evolution of pebbles without calling for additional fining processes. We conclude that the eroded lithology and hillslope sediment source exert a major influence on the downstream evolution of sediment characteristics, on bedload ratio, and probably on bedrock erosion efficiency. These conclusions have important implications in terms of river profile evolution, landscape denudation, internal erosion coupling, and the response of the fluvial network to glacial-interglacial fluctuations.