Fraser Delta Research Articles

Turbidity currents on the open slope of the Fraser Delta

Turbidity currents have been observed and measured in submarine channels and canyons where the flows are confined, but with the exception of one serendipitous direct measurement, are only inferred to occur on unconfined delta slopes. In this paper, direct measurement of turbidity currents on the Fraser delta slope are reported and their dynamics compared to turbidity currents in channelized settings. Twelve turbidity current events occurring between May 2014 and June 2018 were identified from Acoustic Doppler Current Profiler instruments deployed at two sites (109 and 159 m water depth). All the events occurred in the months of May and June, in association with high spring river flow. Five of the events were recorded at both sites and the timing of first arrivals gives estimates of the current speed ranging from 0.4 to 3.6 m s−1. Direct current measurements confirm that most flow speeds exceed 1.5 m s−1, although one weak event did not exceed 0.4 m s−1. Current profiles indicate that the flows are of variable thickness, ranging from less than 3 m to 15 m. The turbidity current events varied in duration from 5 to 66 min and, in the cases where transit speeds were available, the durations were proportionally shorter when the transit speed was high. In stronger flows, the initial stage of the flow is highly variable, suggesting extreme turbulence, and the velocity structure of the flow is similar to previous experimental and field observations, with a distinctive relatively thin, high speed lower region being evident at times. Downslope translation of the instrument platform during two of the events suggest the presence of a dense basal layer up to 2.75 m thick, as suggested in the travelling wave model of Heerema et al. (2020). The fact that only approximately half the flows measured at the shallower site are observed at the deeper site suggest that the runout distances of open slope flows are limited. Weaker flows are barely distinguishable in speed and direction from ambient tidal velocities. Flow directions suggest that the open slope flows are unconfined and initiate in the upper slope area that is approximately the same as the area of upslope sediment waves. It is hypothesized that shallow gullies in the same area may be important for initiation and ignition of the flows that become unconfined on the lower part of the slope.

Sedimentary processes at the mouth of a tidally‐influenced delta: New insights from submarine observatory measurements, Fraser Delta, Canada

AbstractThis paper examines particle settling dynamics in a tidally‐influenced delta, including observations of convective settling, strongly modulated by seasonal discharge and tidal cycles. Long time series of Acoustic Doppler Current Profiler backscatter, a proxy for suspended sediment concentration in the water column, current and sediment accumulation measurements from the slope of the Fraser River delta show variability of sedimentary processes over timescales from semi‐diurnal to annual. Acoustic Doppler Current Profiler backscatter shows a lagged response to river discharge and strong diurnal/semi‐diurnal variability. Most of the tidal cycle is characterized by a buoyant effluent plume. During the ebbing phase of diurnal major tides, discrete settling events are observed extending through the entire water column. The pulse‐like nature of these events is the result of downward plunging fingers characteristic of convective settling, similar to those observed in laboratory flume experiments, and appear to be initiated at the interface between the surface buoyant plume and the underlying ambient water. Currents show strong variability in a three‐layer water column. The surface plume shows offshore and southward directed transport of sediments. The remaining water column is influenced by tidal flows, but near surface and intermediate water‐depth currents are out of phase by several hours, indicating a stratification of the basin water and decoupled propagation of surface and internal tides. This phase lag is unlikely to cause discernible facies changes, but may have implications for basin‐fill patterns in restricted basins with strong tidal range. The results of this work will help to better understand river mouth sedimentation processes and dispersal patterns in other modern deltas and in the geological record.

Gravity Currents Facilitate Formation of High‐Frequency Internal Solitons and Bores at the Base of the Fraser Delta in the Southern Strait of Georgia

AbstractWe use velocity and acoustic backscatter records from downward looking acoustic Doppler current profilers moored 47 m above the seafloor, 20 km apart in water depths of 100 and 240 m to examine internal wave dynamics near the seabed in the southern Strait of Georgia. The observations reveal that the lower layer of the strait becomes a seasonal (wintertime) waveguide for energetic internal “solibores”—internal bores trailed by large‐amplitude internal solitons—that propagate up the flank of the Fraser Delta. The rotor‐like bores have shoreward bottom velocities of 0.2 m s−1 and vertical velocities of 0.05 m s−1; the trailing wave trains attain amplitudes of tens of meters, periods as short as 10 min and durations of several hours. Establishment of the seabed waveguide required to support the high‐frequency internal motions begins in summer, when gravity currents originating from the shallow (~100 m) Skipjack‐Java sill in neighboring Boundary Pass, descend deep into the strait at monthly intervals. During the ensuing winter, lower‐density gravity currents form over the sill and subsequently override the denser bottom layer in the strait, increasing the buoyancy frequency of the guide. Based on the direction of solibore propagation, we speculate that the winter gravity currents also initiate generation of the internal waves during their descent over the rapidly deepening seafloor leading to the central strait. We further propose that the solibores are contributing to the formation and/or shoreward propagation of the O(10) m‐high sediment waves found on the delta foreslope.

Cyclic steps on a glacifluvial delta, Howe Sound, British Columbia

Howe Sound is a deep, steep-walled fjord on Canada's west coast. The primary sediment source is the glacially fed Squamish River. In the inner fjord, turbidity currents flow from the sandy Squamish River Delta at the fjord head reaching as far as the Porteau Creek Sill 15 km to the south. Recent observations from the Squamish Delta have shown that turbidity currents of various volumes are generated daily throughout almost a month of the summer meltwater period and flow through very active submarine channels characterized by active upstream-migrating cyclic step bedforms (Hughes Clarke et al. 2012 a ). Cyclic step bedforms are observed in other turbidity-current environments including the Fraser Delta (Hill 2012; Lintern et al. 2016) and Monterey Canyon (Fildani et al. 2006). The upper foreset slope of the Squamish Delta, is characterized by five incised submarine channels (Fig. 1a). The three northernmost of these channels are active while the two southernmost channels appear to have been abandoned after the artificial diversion of the Mamquam River in 1971. The younger channels and the …

Tide-supported gravity flows on the upper delta front, Fraser River delta, Canada

Three anomalous events (AEs) were recorded off the Fraser River, Canada during the freshet of 2008, and are ascribed to quasi-continuous gravity flows. These flows transport warm, low-salinity Fraser River waters to the upper delta front of the Fraser delta to at least 50m water depth. All flows occurred during the freshet and spring ebbing tide, and were characterized by deposition of fine-grained sand and silt beds with likely high-water contents. These sediments were susceptible to resuspension by the subsequent flood-tide current, producing very high near bed suspended sediment concentrations (SSC) with enough excess density to generate hyper-concentrated flows. Our observations show that these episodic events result from the combination of high river discharge, high SSC, and strong tides, and therefore, are likely to occur on the delta fronts of other large tide-dominated and tide-influenced rivers. In shallow-water basins (< 50m water depth), these tide-supported gravity flows can affect sediment deposition on both the delta front and prodelta.

Neoichnology of the lower delta plain: Fraser River Delta, British Columbia, Canada: Implications for the ichnology of deltas

Neoichnological (i.e., burrows, tracks and trails made by extant organisms) distributions are mapped across the active lower delta plain of the Fraser River Delta, British Columbia, Canada. The neoichnological character of the lower delta plain is best described as a moderate diversity trace assemblage mainly comprising simple, vertical burrows of filter-feeding animals and fewer dwellings of surface deposit feeders. Neoichnological trends generally parallel the coast, and bivalve-generated structures dominate in both muddy and sandy sediments. Given that all of the bivalves in sand-dominated sediments of the lower delta plain tidal flats are filter-feeding animals, the occurrence of large numbers of bivalve-generated structures in deltaic tidal flats may be indicative of at least moderate turbidity and possibly hypopycnal flow. These observations are not applicable to muddy substrates, however, as bivalve-generated structures in mud are made by both filter-feeding and deposit-feeding animals. Neoichnological trends on the active lower delta plain are compared to the neoichnological character of the effectively abandoned part of the lower delta plain at Boundary Bay (considered to be a fully marine equivalent of the lower delta plain). In comparison to Boundary Bay, there is a significant decrease in the diversity of infauna from Boundary Bay to the tidal flats on the active lower delta plain. Bivalves thrive in the more turbid waters of the active lower delta plain, whereas many large polychaetes cannot survive the low salinity conditions of this subenvironment. The Fraser delta is a tidally asymmetric delta with a net northward transport of sediment. However, at shallow depths (less than 10 m water depth relative to low low tide), the delta exhibits a reversed asymmetry in its ichnological character. Waves exert a greater control on sediment and animal distributions in shallow water, such that the updrift tidal flats (those that are south of the main distributary channel) experience higher sedimentation, salinity, and turbidity stresses than the downdrift tidal flats. This is manifested by a reduced diversity of traces and intensity of burrowing on the tidal flats updrift of the main Fraser distributary. In addition, there is a higher proportion of filter-feeding animal burrows on the downdrift side of the main distributary.

Two Middle Holocene Marker Beds in Vertically Accreted Floodplain Deposits, Lower Fraser River, British Columbia

Des carottes recueillies dans la partie est du delta du Fraser et de la plaine d'inondation adjacente montrent deux lits repères datant du milieu de l'Holocène, un lit de tourbe datant d'à peu près 6000 BP et un lit de tephra, correspondant à l'éruption du mont Mazama (6800 BP). Ces lits repères, ainsi que les datations au radiocarbone obtenues dans les sédiments de crue, montrent que la plaine d'inondation du Fraser a progressé en réponse à une hausse du niveau marin survenue entre environ 8000 et 2250 BP et que l'aggradation suivait la hausse du niveau marin. L'accumulation de sédiments, de forme triangulaire, est composée principalement de limons riches en matière organique et de sables fins d'origine alluviale. Le triangle s'étend jusqu'à au moins 20 km dans la vallée inférieure du Fraser. La conservation des lits repères démontre la grande stabilité des biefs du Fraser inférieur depuis environ 7000 ans.

The circulation and residence time of the strait of Georgia using a simple mixing‐box approach

New observations in the Strait of Georgia, British Columbia, Canada show that temperature and dissolved oxygen have a pronounced seasonal cycle, with a spatially varying phase. Phase lags in oscillating systems arise due to internal time scales which can be interpreted in fluid systems as residence times. Exploiting phase we construct a quantitative and internally consistent circulation scheme for this body of water after dividing it into four regions: the Fraser River plume, the surface waters down to 50 m, the intermediate waters down to 200 m, and the deep water. In this scheme the intermediate water, the largest region by volume, is continually renewed, and its characteristics change in response to continuous changes in the characteristics of source waters. The dependence of the estuarine circulation on variations in fresh inflow is weak. The deep water is volumetrically less important, but seasonal changes in the density of oceanic source waters can produce a variation in the overall circulation by driving an additional inflow which leads to both deep renewal and increased upwelling. In turn, this increased upwelling results in lower surface temperatures than might otherwise be expected. Intermediate water residence times are about 160 days. Deep water is renewed once per year in summer and is affected only by vertical diffusion during the rest of the year. Surface water residence times for the entire Strait are a few months at most, but the Fraser River plume has a freshwater residence time of approximately 1 day. In addition, we find that the residence time of oceanic source waters in the Strait is 1.7 years due to a substantial recirculation in Haro Strait. Other consequences of this scheme are consistent with independent estimates of horizontal transports, air‐sea heat fluxes, subsurface oxygen (O2) utilization, and primary production. Finally, analysis of the spatial phase variations suggests that the intermediate inflow enters the Strait as a boundary current along the slopes of the Fraser delta.

First Records of the Yellow Bullhead, &lt;em&gt;Ameiurus natalis&lt;/em&gt;, a Loricariid Catfish, &lt;em&gt;Panaque suttonorum&lt;/em&gt;, and a Silver Pacu, &lt;em&gt;Piaractus&lt;/em&gt; cf. &lt;em&gt;P. brachypomus&lt;/em&gt;, in British Columbia

In Canada, there are no native catfish west of the continental divide and until recently, the list of extant exotic catfishes in British Columbia only included introduced Black Bullhead (Ameiurus melas) and Brown Bullhead (Ameiurus nebulosus). We report that a single Yellow Bullhead (Ameiurus natalis) was collected from Silvermere Lake in the Lower Fraser River drainage. This represents the first record of the Yellow Bullhead in western Canada, and its introduction likely was accidental with a shipment of Largemouth Bass (Micropterus salmoides) rather than dispersal from Washington. Warm, eutrophic, weedy habitat in the Fraser Delta provides ample habitat for Yellow Bullheads and other exotic fishes. A Blue-eyed Panaque (Panaque suttonorum), a loricariid catfish found in 1995 in Shawnigan Lake, Vancouver Island, probably represents a single, illegally released aquarium fish, as does a large Silver Pacu (Piaractus cf. P. brachypomus), which was found in Green Lake on Vancouver Island in 2004.

The Architecture of Prograding Sandy-Gravel Beach Ridges Formed During the Last Holocene Highstand: Southwestern British Columbia, Canada

This study was conducted to establish the sedimentary architecture of a beach-ridge system that developed in response to rising and then stabilizing sea level in a macrotidal setting. The beach ridges document a shift from transgressive to highstand depositional conditions. A beach-ridge complex is being deposited on the Boundary Bay tidal flats that flank the Fraser River delta in southwestern British Columbia. The delta onlapped an offshore island, during the Holocene, and partially incorporated it into its delta plain; sand and gravel eroded from this island is the sediment source for the beach ridges. Four depositional–geomorphic environments are identified: beach ridges, lagoon and swales, washover fans, and tidal flats. The beach ridges prograded over the tidal flats of the Fraser delta when sea level stabilized and the late-Holocene highstand was established. The very earliest beach ridges were deposited about 5200 to 4800 years BP, but most were formed after 2250 years BP with the establishment of the Holocene highstand. Beach-ridge progradation rates varied between 1.6 m a −1 to 0.48 m a −1 . Ground penetrating radar (GPR) was used to provide continuous two- and three-dimensional images of the facies in order to delineate the depositional architecture of the sandy-gravel beach ridges. We supplemented the customary use of well logs and cores for ground truthing GPR lines with cone penetration testing (CPT), which provided a detailed suite of logs invaluable for correlating radar reflections with subtle lithologic variations unidentifiable in core. A synthesis of geomorphic environments, lithologic logs, and radar facies is used to construct a depositional model of the beach-ridge complex.

Musqueam reference grammar

The Musqueam peoples' territory includes much of the Fraser Delta and the city of Vancouver. Halkomelem, one of the twenty-three languages that belong to the Salish Family, is spoken in three distinct forms: Upriver, by the Sto:lo' of the Fraser Valley; Downriver, of which Musqueam is the only surviving representative; and Island, spoken by the Nanaimo and Cowichan of Vancouver Island. Suttles, an anthropologist, worked with elders, eliciting traditional stories, personal narratives, and ethnographic accounts. The grammar covers phonology, morphology, and syntax, illustrated by numerous sentences selected for their cultural relevance that provide insight into traditional practices, social relations, and sense of humour. It is perhaps the fullest account of any Salish language.

Use of length-based models to estimate biological parameters and conduct yield analyses for male Dungeness crab (Cancer magister)

Length-based models were developed for the male Dungeness crab (Cancer magister) population on the Fraser delta near Vancouver, British Columbia. The models incorporate the probability of moulting, moult increments, natural mortality during moulting and non-moulting periods, direct fishing mortality, and handling mortality that occurs when sublegal-sized crabs are caught and released. The models were used to investigate how long-term yield might be affected by the combination of handling mortality and an intensive fishery. The models were calibrated to survey data, and key biological parameters were estimated. The probability of moulting is near one for male crabs in the 130- to 150-mm carapace width range and decreases as crabs get larger. There is a 70.1% probability a crab will survive the 1-month period beginning with a moult. The non-moulting natural mortality rate is 0.97 year–1. When handling mortality is incorporated into the model, yield per recruit increases with the exploitation rate until it reaches approximately 94%. F0.1 is equivalent to 70%. An approach was developed to calculate the threshold ratio of discarded to retained crabs beyond which fishing would reduce the long-term yield.

Holocene faulting on a tectonic margin: Georgia Basin, British Columbia, Canada

Multibeam bathymetric data collected in the Strait of Georgia, British Columbia, have revealed two areas of seabed disturbance, interpreted to be faults. The easterly fault zone (Fraser Delta Fault) is demarked by a pockmark chain extending along strike of a southwesterly-dipping fault offset 1 km to the northeast and having a throw of over 50 m. The pockmarks occur in a region of high sedimentation, located in the Fraser River prodelta. The eastern strait fault zone (Porlier Pass Fault) occurs within a fold of the Cretaceous Nanaimo Group. Here, a series of thrust faults displace sediments from Cretaceous to Holocene by up to 40 m with over 2 km of surface expression. Based on Holocene stratigraphic displacement in an area of significant sedimentation, these fault zones are considered to be active.

MARINE AND AGRICULTURAL HABITAT PREFERENCES OF DUNLIN WINTERING IN BRITISH COLUMBIA

We examined winter habitat preferences of individual dunlin (Calidris alpina) in the Fraser River delta, British Columbia, Canada, adjacent to agricultural land near an area of dense and increasing human settlement. We used radiotelemetry and compositional analysis to quantify and describe dunlin habitat selection at 2 scales (regional and local) throughout the 24-hr day and daily tidal cycles. We tested for differences between sex and age classes, and among birds captured at different sites. Patterns of habitat preference differed between sexes and among dunlin from different sites in the delta, but we detected no difference between age classes. We ranked habitat types in order of dunlin preference and tested for significant differences among habitat ranks. Dunlin showed a significant preference for tidally influenced marine habitats at both scales and throughout the study area. However, most individuals (>80%) also used terrestrial habitats, usually during high tide and primarily at night. The role of terrestrial habitats in the ecology of Fraser River delta dunlin previously had been underestimated because these habitats are used far more at night than during the day. Regionally, soil-based agricultural crops ranked above other terrestrial habitats, and pasture was the only terrestrial habitat that was ranked highly and preferred at both scales. Pasture vegetation tends to be short, and pasture fields in the Fraser River delta are fertilized heavily and naturally with cattle manure. We recommend that managers promote the maintenance of a mosaic of soil-based agricultural crops—with a particular emphasis on naturally fertilized pastures—for dunlin and other shorebirds wintering in the Fraser delta. Terrestrial habitat fragmentation also should be kept to a minimum, as dunlin preferred large fields, likely in response to predation risk.

1-D and 2-D analyses of weak motion data in Fraser Delta from 1966 Duvall earthquake

The 1996 Duvall earthquake in Washington State triggered ground motion stations in the Fraser Delta, British Columbia, located on varying depths of Holocene and Pleistocene soils to a maximum depth of 800 m and one station on bedrock. Recorded ground motions were used to examine the applicability of 1-D and 2-D site response analyses for amplification studies in the Delta. 1-D response analysis gave a good indication of the period of peak response. Response spectra computed by 1-D analysis did not compare well with recorded spectra except at very deep sites. The use of 2-D analyses to include buried topography generally improved the predictions of site response spectra at the shallow sites. However, for the shallow earth sites, the recorded motions showed a strong spectral response at short periods in the range of 0.1–0.25 s. Neither 1-D nor 2-D analysis predicted this response.

Kidney, liver and bone cadmium content in the western sandpiper in relation to migration.

Cadmium content was measured in kidney, liver and tarsus bones of western sandpipers (Calidris mauri) at a temperate migratory stopover site (Fraser Delta, British Columbia, Canada) and a wintering site (Playa el Agallito, Chitre, Panama) over a two year period. Cadmium content in liver and kidney was age and sex dependent. Adult females generally had lower kidney and liver cadmium than adult males (P < 0.05), but a sex difference was not detected in juveniles. Cadmium increased with age in kidney, liver and to a lesser extent in bone (P < 0.001) with average "steady-state" kidney and liver content being reached within the sandpipers first year. In general, tissue cadmium residues in adult males and females were independent of sampling location although for bone, site-specific differences did occur (P < 0.001). Bone cadmium was lower in females sampled from their wintering grounds as compared to temperate stopover sites suggesting that bone cadmium may be mobilized during periods of feather molt. Comparison of cadmium residues among sandpipers of increasing age suggest that exposure is occurring along the Pacific Coast, at stopover sites as the birds migrate north to Alaska and south to Panama. This study points to the importance of considering the ecology of the species (e.g., in this case migratory behavior) in interpreting trace metal residues.

Comparing Shear-Wave Velocity Profiles from MASW with Borehole Measurements in Unconsolidated Sediments, Fraser River Delta, B.C., Canada

Recent field tests illustrate the accuracy and consistency of estimating near-surface shear (S) wave velocities calculated using Multi-channel Analysis of Surface Wave (MASW). To evaluate the technique in a variety of near-surface conditions and through a wide range of velocities, MASW-derived S-wave velocity profiles (S-wave velocity vs. depth) were statistically compared to S-wave velocity profiles measured in seven boreholes in the unconsolidated sediments of the Fraser River delta, near Vancouver, B.C., Canada. An overall difference of approximately 15 percent was observed between these two uniquely determined sets of S-wave velocities from the seven well locations. A blind test of the standalone accuracy of MASW was conducted at an eighth well. For this blind test, S-wave velocity measurements made in and interpreted from the borehole were not made available during MASW data processing. Differences between S-wave velocities using MASW from those measured in the blind test borehole averaged nine percent. No systematic differences between these results were observed in data from any of the eight test sites. The MASW method provided reliable S-wave velocity profiles from [Formula: see text] below the ground surface at some sites in the Fraser delta.

Recent geological evolution and human impact: Fraser Delta, Canada

Abstract Throughout the Holocene, the river dominated Fraser Delta on the Pacific coast of Canada has prograded by continuous channel switching and avulsion into a deep (&gt;300 m) basin. However, at the beginning of the 20th century the delta was modified to provide a navigable channel and port facilities for the city of Vancouver. Now most of the sand brought down by the river (35% of the sediment load) is removed from the system by dredging. The remaining fine-grained sediment is transported in a plume past the intertidal estuary within the distributary channels then deflected northwards by the dominant flood tidal flow into the basin. Two causeways to the south of the main channel and one to the north that cross the intertidal zone to the delta foreslope act as barriers to the dominant northward sediment transport causing estuarine and localized seabed erosion. An eroded distributary channel failure complex has been exposed on the delta foreslope, off the southern causeways, by flood tidal flows that scour the seabed and form northward migrating subaqueous dunes, further increasing the delta slope. This, combined with slow sea-level rise and seismicity, intensifies the risk of further erosion and instability of the delta, particularly along the subaqueous delta front and the intertidal estuaries.

Seismic site response in the greater Vancouver, British Columbia, area: spectral ratios from moderate earthquakes

Three-component, digital recordings of two recent moderate earthquakes provide valuable new insight into the response to seismic shaking in the greater Vancouver area, particularly on the Fraser River delta. The 1996 M = 5.1 Duvall, Washington, earthquake (180 km southeast of Vancouver) triggered strong-motion seismographs at seven sites and the 1997 M = 4.3 Georgia Strait earthquake (37 km west of Vancouver) triggered instruments at 13 sites in the greater Vancouver area. The latter data set is especially important because it contains the first three-component recordings made on bedrock in greater Vancouver. Both data sets represent weak ground motion, with peak horizontal accelerations of 0.5-1.5% gravity (g) for the Duvall earthquake, and 0.2-2.4% g for the Georgia Strait earthquake. Using the method of spectral ratios, we estimate the site response for each of the strong-motion instrument soil sites. On the Fraser River delta amplification is observed over a relatively narrow frequency range of 1.5-4 Hz (0.25-0.67 s period), with peak amplification of 4-10 (relative to competent bedrock) for the thick soil delta centre sites, and about 7-11 for the delta edge sites. Relative to firm soil, the peak amplification ranges from 2 to 5 for the thick soil delta centre sites, and 2 to 6 for the delta edge sites. At higher frequencies, little or no amplification, and in many cases slight attenuation, is observed.Key words: seismic site response, Fraser delta, earthquakes.

Sedimentation rates and patterns on a deep-water delta (Fraser Delta, Canada); integration of high-resolution seismic stratigraphy, core lithofacies, and 137 Cs fallout stratigraphy

ABSTRACT The Fraser Delta is a Holocene feature with complex surficial geology that has built into the deep (>300 m) water of the Strait of Georgia on Canada's west coast. We used high-resolution marine geophysical data, core lithofacies, and fallout stratigraphy to examine modern sedimentation rates on the delta slope and prodelta. Modern sedimentation rates, measured by the peak and onset of 137Cs in cores, are in excess of 13 cm yr-1 near the mouth of the main fluvial distributary but drop to less than 3 cm yr-1 farther than 4 km offshore. Asymmetry to the pattern of sedimentation rates reflects both the prevailing oceanographic circulation and human influence. The map of modern sedimentation provides a synoptic view of this deltaic system that is of value to understanding older deltaic systems as well as the modern offshore environments on this populous and heavily used delta. In attempting to estimate the annual sediment discharge of the river from our sedimentation-rate map, we were able to account for slightly less than 40% of the measured river discharge. Our inability to adequately characterize the deposition and resedimentation of the sand fraction is probably responsible for most of the discrepancy.