Mouths Of Main Rivers Research Articles

Численное моделирование распространения пассивных примесей в Белом море

It is hardly possible to make experiments on transport of potentially dangerous tracers in a sea. Therefore numerical simulation is successfully used to assess the risks and consequences of various events like oil spills, flux of toxicants with river runoff after industrial accidents or phytoplankton in case of harmful algae bloom. As the White Sea is a transport hub and a strategically important location for extraction of natural resources, so it is important to know the most likely scenarios of pollutants behavior. The JASMINE numerical model, based on the FEMAO (Finite-Element Model of the Arctic Ocean) simulates evolution of sea state, including three-dimensional currents, temperature and salinity, and sea ice, and is able to calculate tracer advection, so giving answers to a number of questions related to how pollution evolves in the White Sea. We describe numerical experiments to study advection of floating tracers in the White Sea when the initial distribution is delta-like and located in the mouths of main rivers. Also, we calculate the time of almost complete removal of an initially homogeneous concentration of a passive tracer. Spatial resolution is 3 km, vertical grid is 5 m down to the depth level of 150 m (10 m for depths more than 150 m). The time step is 360 s. NOAA/NCEP atmospheric reanalysis is used for atmospheric forcing. Five main rivers are taken into account with the monthly-mean runoff taken from the multi-year observation data. We have estimated the time needed to remove most of the matter from the sea; it depends on morphometric characteristics of the bays and is the highest for the Onezhskiy bay. Matter exchange between the bays is low, only Dvinskiy-Onezhskiy-Mezenskiy transport is present, as we would expect from the geostrophic current pattern.

Organic-walled dinoflagellate cyst assemblages in surface sediments of the Ría de Vigo (Atlantic margin of NW Iberia) in relation to environmental gradients

Organic-walled dinoflagellate cyst assemblages were analysed in 31 surface sediment samples from the Ría de Vigo (NW Iberia) to obtain supporting evidence for the interpretation of past environmental signals from sediment cores. Complementarily, the totals of pollen (and pollen from riparian taxa), foraminiferal linings and freshwater algae were also considered to test their value as (palaeo)environmental indicators in fluvio-marine sedimentary environments. Abundances of foraminiferal linings gradually increased towards the ría's mouth, which supports its use as a proxy to infer the degree of marine influence in the sediment record. The ratio of dinoflagellate cysts to pollen and spores (D/P) increased with distance to the main river's mouth (and water depth) in the inner (<7 m water depth) and the outer (>20 m) parts of the ría. Total pollen concentrations decreased with water depth in the outer part but did not show any clear trend in the rest of the ría. No clear pattern in the distributions of pollen from riparian plants and spores from freshwater microalgae was detected. Multivariate analyses (clustering and RDA) performed on dinoflagellate cyst records and environmental data reflected a marked inshore-offshore distribution pattern mainly controlled by a fluvio-marine environmental gradient. Increasing abundances of cysts of Gymnodinium species and heterotrophic cysts as well as higher cyst diversity characterised the deeper environments of the outer ría. These observations suggest a relationship with increased shelf influence and are compatible with the heterotroph upwelling signal described in previous works. A very different cyst association with a predominance of autotrophic cysts characterised the intermediate and inner parts of ría, where exceptionally high cyst abundances of Lingulodinium polyedra (~56–99% and ~ 200–114,000 cysts.g−1) were reported. Abundances of cysts of L. polyedra positively correlated with shallower and higher river-influenced environments characterised by higher sea-surface temperature. Our results agree with previous studies and reinforce the value of this species as an indicator of warmer and stratified conditions. Moreover, cysts of L. polyedra positively correlated with winter sea-surface nitrate contents, which is compatible with the nutrient enrichment signal (natural or anthropogenic) that has already been described in other deep and stratified estuarine environments and is consistent with historical cyst records obtained in the Ría de Vigo. However, further research is needed to disentangle the nutrient enrichment signal from the influence of other environmental factors.

From city to sea: Spatiotemporal dynamics of floating macrolitter in the Tiber River

Rivers are undoubtedly the main pathway of waste dispersed in the environment that from land reaches oceans and seas increasing the amount of marine litter. Major cities are a great source of riverine litter as large urbanization can originate pressure on the integrated waste management resulting in litter entering the rivers. Within this study, we aim to investigate the dynamic of floating riverine macrolitter (items >2.5 cm) in the city of Rome before it reaches the sea by assessing the composition, amount, and seasonal trends of litter transported from the urban centre to the main river mouth of Tiber River. Visual surveys for a whole year (March 2021–February 2022) were conducted from two bridges, Scienza Bridge (in the city) and Scafa Bridge (at the main river mouth) and followed JRC/RIMMEL protocol for riverine litter monitoring. Overall, similar litter composition was observed from the city centre to the mouth with a prevalence of plastic material, mainly related to fragmentation process (i.e. plastic pieces) and single use items, mainly in food and beverage sectors. An extrapolated annual loading of 4 × 105 items/year was estimated at the main mouth of Tiber River. The litter flux seems to be influenced by the seasonal variability and hydrometeorological parameters. The frequency of size classes decreases with increasing size in both sites, and more than half of the recorded items were below 10 cm. Specific categories belonging to “other plastics” have been reported related to anti-Covid-19 behaviour such as face masks and beverage sector, e.g. bottle lids and rings. The main colour of plastics was white, suggesting weathering process of floating riverine litter. This study contributes to increasing knowledge of the origin, composition and spatiotemporal dynamics of riverine floating litter from the city and entering the sea.

Structural and functional response of coastal macrofaunal community to terrigenous input from the Po River (northern Adriatic Sea)

To assess the influence of river-derived material on the benthic ecosystem in the Po River prodelta area, we investigated the structural and functional features of macrofaunal invertebrates in 14 sites. Biodiversity, species composition, response and effect traits of the community were determined and related to the main sediment physical-chemical and nutritional features (biopolymeric carbon, BPC; protein to carbohydrate ratio, Prt/Cho ratio; phaeopigment to chlorophyll-a ratio, Phaeo/Chl-a), as well as contaminants. We found significant spatial differences in community structure and its functional expression between northern and southern stations, and between nearshore and offshore ones. The shallower stations located nearby the main river mouth displayed higher densities, diversity and functional redundancy compared to southern stations. The main discriminating factors for the community were the higher percentage of sand and Prt/Cho nearby the river mouth, and fine grain-size and high Phaeo/Chl-a at the southernmost and deeper stations. Contaminant concentrations did not seem to severely affect the macrofaunal community. Changes in community structure were due to the dominance of few species, which occurrence varied according to grain-size and organic matter quality that in turn were linked to the different magnitude of depositional loads in the investigated area.Focusing on response traits, we observed a clear influence of high loads nearby the main river mouth, through the dominance of sessile and tube-builder trait-categories at stations with a higher sand content. In contrast, we detected a major expression of burrowers and interface crawlers, and swimmers at southern and principally offshore muddy stations. Effect traits revealed that in the northern part of the prodelta prevailed suspension, surface deposit feeders and conveyor invertebrates that are, respectively, fundamental players in the benthic-pelagic coupling and elemental cycling within benthos. In the southern part, subsurface deposit feeder and biodiffuser organisms dominated. The latter are able to rework huge amounts of sediments also in the deepest layers, amplifying the elemental cycling and decomposition. In highly dynamic systems, as delta areas, the presence of invertebrates able to exploit different resources through bioturbation activities may enhance the functions of riverine costal ecosystems.

A Quantitative Evaluation of Hyperpycnal Flow Occurrence in a Temperate Coastal Zone: The Example of the Salerno Gulf (Southern Italy)

The inner continental shelf is regarded as a repository of hyperpycnal flow (HF) deposits the analysis of which may contribute to hydrogeological risk assessment in coastal areas. In line with the source to sink paradigm, we examined the dynamics of the coastal watersheds facing the Salerno Gulf (Southern Tyrrhenian Sea) in generating hyperpycnal flows and investigated the shallow marine sediment record to verify their possible occurrence in the recent past. Thus, the morphometric properties (hypsometric integral, hypsometric skewness, hypsometric kurtosis, density skewness and density kurtosis) of the watersheds together with the potential rivers’ discharge and sediment concentration, calculated by applying altitude- and extent -based experimental relations, allowed to detect the rivers that were prone to producing HFs. In the shallow marine environment record of the last 2 kyr, anomalous sedimentation, possibly linked to HF events, was identified by comparing the sand-mud ratio (S/M) down-core —at three sites off the main river mouths — to the expected S/M calculated by applying the relation governing the present-day distribution of sand at the seabed in the Salerno Gulf. A return period of major HF events ≤ 0.1 kyr can be inferred for rivers which fall into the category “dirty rivers”. In these cases, the watersheds have a hypsometric index ranging between 0.2 and 0.3, coastal plains not exceeding 30% of the entire catchment area and a maximum topographic height ≥1000 m. A return period of about 0.3 kyr has been inferred for the “moderately dirty rivers”. In these other cases, about 50% of the watersheds develop into a low gradient coastal plain and have a hypsometric index ranging between 0.09 and 0.2. The observations on land and offshore have been complemented to reach a more comprehensive vision of the coastal area dynamics. The method here proposed corroborates the effectiveness of the source to sink approach and is applicable to analogous sediment records in temperate continental shelves which encompass the last 3 kyr, a time interval in which the oscillations of relative sea level can be overlooked.

Tsunami Risk in Japanese Coast and Recent Mitigation Technique using Removable Breakwater

In this paper, the tsunami risk in the west coast of Japan is introduced. Especially, the inundation risk in Osaka city is discussed. Three visor gates are installed at the mouths of main rivers in the city. The gates are mainly installed to prevent the storm surge due to typhoons but they are effective to reduce the tsunami height at the mouth point of river. Though the gates are necessary to prevent the tsunami inundation, the stability against tsunami forces is not enough because the gates were constructed in 1960s. Therefore, the removable breakwater is now under consideration to reduce the tsunami force. The hydraulic experiment was carried out in order to investigate the applicability of the removable breakwater. The reduction rate by the breakwater was derived from the experiment.

Role of depositional dynamics and riverine input in shaping microbial benthic community structure of Po prodelta system (NW Adriatic, Italy)

To understand to what extent depositional dynamics and riverine input contribute to shape the microbial benthic community structure in coastal systems, surface sediments were collected along one nearshore and two land-sea transects in the Po River prodelta, and analyzed for lithology, sediment structures, grain-size, short-lived radioisotopes (7Be and 137Cs) and organic matter (OM) quantity and origin (C, N, stable isotopes). The most recent surface supply (i.e. 5–6 mo = SUP layer) and the underneath, older sub-surface deposits (i.e. >6 mo = SUB layer) were analyzed at six selected sampling sites and checked for possible differences in the microbial community abundance and structure. According to different depositional dynamics, three distinct groups of sites were detected: nearshore muddy and sandy sites, and offshore sites. Sedimentary records reflected the interplay of different elements and/or processes acting in this complex and highly dynamic system. River-derived sediments, as depicted by high contribution of silty particles and recent riverine OM inputs, were confined to nearshore locations in front of the main river mouth. They resulted highly correlated to bacterial distribution and prokaryotic abundances in SUP layer. Nevertheless, in subsurface sediments affected by the delivery of high amount of mud, potential burial phenomena and/or the development of anoxic conditions occurred, leading to a change in the bacterial community structure. We speculated that the decrease of Gamma- and increase of Delta-Proteobacteria in the SUB layer could be related to their sulfur-oxidizing and sulfate-reducing activities, respectively. On the other hand, at offshore sites, the presence of a low, and regular sedimentary input and less intense hydrodynamic conditions favored a more stable microbial structure across surface and subsurface sediments. Four discriminating proxies, such as grain-size representing the system energy, Total N as an indicator of land-derived nutrient inputs, 7Be as a tracer of the freshness of riverine discharge, and stable C isotope as an indicator of the type and age of OM, mainly summarize the complex relationships between sedimentary dynamics and bacterial community structure in the Po River prodelta area.

The spatiotemporal distribution of dissolved carbon in the main stems and their tributaries along the lower reaches of Heilongjiang River Basin, Northeast China.

The Heilongjiang River Basin in the eastern Siberia, one of the largest river basins draining to the North Pacific Ocean, is a border river between China, Mongolia, and Russia. In this study, we examined the spatial and seasonal variability in dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and dissolved total carbon (DTC) concentrations along lower reaches of Heilongjiang River Basin, China. Water samples were collected monthly along the mouths of main rivers (Heilongjiang River, Wusuli River, and Songhua River) and their ten tributary waters for 2 years. The DOC concentrations of waters ranged from 1.74 to 16.64 mg/L, with a mean value of 8.90 ± 0.27 mg/L (n = 165). Notably, mean DIC concentrations were 9.08 ± 0.31 mg/L, accounting for 13.26∼83.27% of DTC. DIC concentrations increased significantly after the Heilongjiang River passed through Northeast China, while DOC concentrations decreased. Over 50% of DIC concentrations were decreased during exports from groundwater to rice fields and from rice fields to ditches. Water dissolved carbon showed large spatial and temporal variations during the 2-year measurement, suggesting that more frequently samplings were required. Carbon (DIC + DOC) loads from the Heilongjiang River to the Sea of Okhotsk were estimated to be 3.26 Tg C/year in this study, accounting for 0.64% of the global water dissolved carbon flux. DIC export contributed an average of 51.84% of the estimated carbon load in the Heilongjiang River, acting as an important carbon component during riverine transport. Our study could provide some guides on agricultural water management and contribute to more accurately estimate global carbon budgets.

Assessment on Pollution by Heavy Metals and Arsenic Based on Surficial and Core Sediments in the Cam River Mouth, Haiphong Province, Vietnam

The Cam River mouth (Haiphong Province) is one of the main river mouths of the Red River System, which is one of the most important water resources in Northern Vietnam. Over the past 50 years, the strong socio-economic development in the area has caused a considerable contamination with heavy metals (Co, Cr, Cu, Mn, Ni, Pb, and Zn) and arsenic. In this study, the vertical and horizontal distributions of heavy metals and arsenic in sediments from the Cam River mouth were investigated. In addition, the history, origin, and degree of contamination were assessed. Normalized (with respect to Al) heavy metal and arsenic concentrations in sediment cores and absolute dates obtained from the 137Cs analysis were used to reconstruct the pollution history of the river mouth. As, Cu, Mn, Pb, and Zn concentrations increase rapidly by approximately two times or more from 1954 to 1975, and then remain nearly unchanged from 1975 until 2008, whereas Co, Cr, and Ni concentrations slightly increase from 1954 until 2008. In addition, background values for heavy metals and arsenic have also been determined with regard to the period before 1954. In the study area, Co, Cr, Cu, Ni, Mn, and Zn are evaluated as minorly enriched, whereas As and Pb are classified as moderately enriched. Generally, the anthropogenic activities in the Haiphong harbor and industrial zone locally contribute to the contamination by heavy metals and arsenic in the Cam River mouth.

Historical and recent changes of the Sele River coastal plain (Southern Italy): natural variations and human pressures

As in other alluvial coastal plains in Italy, the Holocene evolution of the Sele River coastal plain was strongly controlled by the events occurring after the Last Glacial Maximum (ca. 19 ky) when, because of progressive sea-level rise, marine transgression affected the outer portion of the present plain. From the Middle Holocene (ca 5.0–6.0 ky BP), the decrease in the sea-level rise led to coastal progradation testified by the building and the seaward migration of a barrier-lagoon system formed by the composite Sterpina–Laura beach-dune ridge system and the back-ridge flat area behind it, characterised by lagoonal to marsh environments. The present paper deals with the reconstruction of the coastal evolution focusing on the last 3,000 years. Geomorphological and geoarchaeological data were used to constrain the evolution of the area in remote times, while the analysis and comparison of historical and recent topographic maps, orthophotographs and aerial photographs allowed to highlight the shoreline trends in recent centuries. The coastline changes that occurred along the Sele River coastal plain during the last 3,000 years highlight two main evolutionary trends in time: firstly a coastal progradation and then a coastal retreatment which has recently been followed by a phase of substantial coastline stability. The coastal progradation (up to 500 m ca.) was favoured by the increased amounts of fluvial sediments delivered to the Sele coast because of the increase in agricultural activity and of periods of climate deterioration (the last being the Little Ice Age). During the last 150 years the Sele coast has been affected by prevailing erosion. From 1870 to 1984 the shoreline progressively retreated, reaching the highest erosion rates around the main river mouths. This shoreline retreatment can mainly be related to human activities (land reclamation, dam building, urbanisation) that reduced the quantity of sediment delivered to the coast. Due to this recent shoreline retreatment, even the historical dune system was locally affected by erosion too. Since 1984 the shoreline has been fairly stable while the Sele mouth zone has experienced erosion.

Biologic impact on the coastal belt of the province of Venice (Italy, Northern Adriatic Sea): preliminary analysis for the characterization of the bathing water profile

This paper presents a preliminary study of the water profile with reference to microbiological parameters, required by Directive 2006/07/EC (European Community 2006) concerning the management of bathing water quality, in the coastal belt of the Province of Venice (Italy, Northern Adriatic Sea). A historical database has been implemented with monitoring data for the period 2000-2006 (data on rivers, bathing and marine coastal waters and on the characterization of Wastewater Treatment Plant (WWTP) discharges) from the institutional activity of Veneto Regional Environmental Prevention and Protection Agency (ARPAV). An integrated areal analysis for the microbiological investigation of homogeneous stretches along the coast of the Province of Venice was performed for a preliminary characterization of the bathing water profile considering water quality status and existing pressure sources. ARPAV is the institutional body responsible for environmental monitoring and control activities. Data were produced from monitoring and controls made available by the Regional Environmental Informative System and extracted and elaborated for the period of interest (2000-2006). Sampling and analysis of microbiological parameters were executed following the official Italian methods in accordance with international procedures (APHA et al. 1998). For the purpose of this study, the coast was divided into eight stretches, which were considered to be homogeneous according to their physical and geographical characteristics. An ANOVA statistical assessment has been performed on stretches I, V and VIII. From the integrated areal analysis of microbiological parameters in the homogeneous stretches along the coast of all the investigated matrices, high mean levels of faecal contamination were found in some cases. The most critical situation amongst the stretches evaluated is to be found in stretch VIII-Ca' Roman, Sottomarina and Isola Verde shores (Southern part of the Province). These results can be widely attributed to pressure sources from the Brenta and Adige rivers, rather than to local contributions. Stretches VI and VII, which correspond to the area from Punta Sabbioni (Cavallino shore N-E) to the Pellestrina shore (S-W), present the best conditions for faecal contamination parameters (low pollution levels). These situations can probably be explained given that WWTPs 7 and 8 have two submarine outfalls at a distance of about 4 km from the coastline and therefore a discharge point which is distant from the bathing and marine coastal monitoring stations; there are also no river mouths in these two stretches. Due to lack of data, it was not possible to implement seasonal assessments. This study aimed to develop an integrated areal analysis for biological parameters along the coast over the period 2000-2006 for the identified homogeneous stretches in order to develop a preliminary approach for the characterization of the water profile, as requested by directive 2006/7/EC. From the integrated analysis of the stretches, it is evident that in some cases there are high levels of faecal contamination along the coast which can be caused by river flows which heavily condition the quality of coastal waters as verified with the statistical assessment. From the preliminary analysis, the submarine outfalls seem to be the best solution to guarantee good bathing water quality on the coast. Nevertheless, it must be observed that this solution cannot overlook the possible impact of the discharges on seawaters intended for a specific use, such as mussel farms with reference to coastal hydrodynamics. Therefore, a sound wastewater dispersion analysis on the discharges of the WWTPs for the identification of the interested area and the level of contamination must be performed; a modelling study on pollution dispersion is complementary to this study (Scroccaro et al. 2009). This analysis highlights the critical stretches as being numbers II and, above all, VIII with particular correlation with the main river mouths. Thus, it is evident that the issue of microbiological impact must be studied following a river basin approach according to the influence of river loads on coastal areas.

Characterization of the Brazilian continental shelf adjacent to Rio Grande do Norte state, NE Brazil

This study focuses on the analysis of high-resolution seismic profiles, integrated with sedimentological, echosounder, SRTM and satellite image datasets, of the Brazilian continental shelf adjacent to the Rio Grande do Norte State, NE Brazil. Located in the northeast of Brazil, the State of Rio Grande do Norte is bounded by two main coastal and shelf systems: the eastern coastal-shelf, from the Sagi River to the Touros High, and the northern coastal-shelf, extending from Touros High to Tibau. This shelf represents a modern, highly dynamic mixed carbonate-siliciclastic system characterized by reduced width and shallow depths as compared with other parts of the Brazilian shelf. It has an average width of 40 km, the shelf-break lying at a depth of ~ 60 m. This shelf is subject to the full strength of the westerly South Equatorial current combined with high winds and moderate to high tides and waves. A sharply defined stratigraphic boundary, probably between the Pleistocene and Holocene deposits, is clearly to be observed in the seismic record. Incised-valleys extending from the main river mouths (e.g.the Potengi, Açu, and Apodi) to the shelf break dominate the area investigated and may indicate periods of lower sea level.

Sediment budget of the Rhône delta shoreface since the middle of the 19th century

This study analyses long-term (1841–1872–1895–1974) changes in the Grand Rhône prodeltaic lobe to (1) quantify the accumulation, (2) determine the evolution of relict prodeltaic lobes and (3) establish long-term relationships between river sediment discharge, the shoreface and the continental shelf. Our results show a reduction of the sedimentation of the prodeltaic lobe at the main river mouth, since 150 yr by a factor of 3.7 (12.63 to 3.41 × 10 6 m 3 yr − 1 ). At the minor mouth of the river, erosion dominates and speeds up during the same period (− 0.53 to − 1.34 × 10 6 m 3 yr − 1 ). These changes are found to directly result from the river sediment input decrease related to the natural decreasing of the frequency of major floods (end of the Little Ice Age), the reforestation in the catchment area, the dam construction and the dredging activities (since the 1950s). Our results indicate that while there is large sediment accumulation in the area around a growing prodeltaic lobe, there is also a reduced contribution of the river sediments to the non-adjacent beaches of the mouth. Following a shift in the river channel and mouth, the relict prodeltaic lobes (Petit Rhône–St Férreol, Bras de Fer and Pégoulier) are reworked by waves and their sediments contribute partially to the growth of the spits (Espiguette, Beauduc and Gracieuse). This suggests that there is a “time-shift” between the input of river sediment to the sea and the build up of a beach. The chronic erosion of the coastline is likely to continue in the future since (1) a river shift is not possible, because the river channels are controlled by dykes and human intervention, (2) the decrease of river sediment input and (3) the relict prodeltaic lobes constitute sedimentary reservoirs that are gradually being used up.

Late Holocene development of a tide-induced elongate delta, the Solo delta, East Java

The Solo River, East Java, is a typical monsoonal river. The discharge and transport of appreciable amounts of sediment is mainly restricted to the wet season (December–March) and has resulted in the rapid outgrowth of an extensive delta system. The Solo delta is characterized as a highly mud-dominated and rapidly prograding single-finger delta, a specimen of a high-constructive, elongate delta type. The presence of firmly built natural levees, more or less ‘channelizing’ river outflow, the role of flow inertia and the rapid deposition of sediment in front of the main river mouth explain the very strong longitudinal growth of the delta and determine its long, rather straight and narrow shape. The presence of these pronounced natural levees is clearly a natural response to river outflow conditions, since maximum river output is observed at high tide. This phenomenon results from the interference of tidal components in the waters surrounding the Solo delta. The tidal model, a conceptual model, assumes that the tidal motion is capable of introducing a temporary increase in deltaic river slope during high tide by creating a local difference in the high water levels North and East of the delta. The lateral dispersal of sediment is limited and mainly depends on the transport in distributary channels. Natural crevasses are rather scarce and do not have a stable character.

The geology and geochemistry of the surficial sediments off Thraki, northern Greece

A geological and geochemical investigation has been carried out on the surficial sediments of the Samothraki Plateau, an area of some 4600 km 2 off Thraki, northern Greece. Most of the surface sediments are sands or silty sands but finer-grained sediment occurs near the mouths of the main rivers and seawards of the shelf break. The sediments are a mixture of several phases, modern river run-off, reworked older sediments and biogenic material. Modern sedimentation is only important near the main river mouths and in nearshore areas where coastal erosion is taking place. The silty-sands and sands which cover much of the mid-shelf are relict sediments although their fine-grained component may have been brought in by recent current action. The Fe and Al contents of the sediments closely reflect the amounts of very very fine sand or silt and clay-sized material present. The distribution of Ti, Cr and Zr largely follows that of Fe and Al. Although significant enrichments occur, major deposits of Ti-, Cr- or Zr-bearing placers do not seem likely in surface sediments in this area on the basis of the present survey.

Sedimentation bordering the Rio Balsas delta and canyons, western Mexico

Western Mexico's second largest river apparently built its delta across the narrow continental shelf so that the river pours sediment directly onto the continental slope. As in other cases of shelf encroachment, the slope is characterized by numerous valleys, of which one with canyon-sized dimensions can be traced seaward to the Mid-America Trench. Evidence for turbidity currents was established by current-meter records and observations from scuba and vehicle diving. A dense grid of accurately located sounding and seismic lines shows the nature of deposition seaward of this delta, but in a zone off both sides of the main river mouth, seismic profiles of the slope suggest the presence of basement rock. However, it seems somewhat more likely that, despite some rock outcrops where erosion has exceeded deposition in Necesidad Canyon, the slopes are underlain mostly by an appreciable thickness of sediments that have been homogenized by extensive slumping which has destroyed any internal reflectors. Farther seaward from this zone, where slopes are gentler, there is a broad area in which good reflectors show clear evidence of natural levees, cross-bedding, valley fills, and other signs of slope progradation. Despite all the evidence of progradation of the slope beyond the delta, we have clear indications that much sediment is being bypassed along the two main canyons of the area. Turbidity currents partly initiated by submarine landslides are eroding locally the steep canyon heads and transporting a large amount of sediment toward the Mid-America Trench. These flows are powerful enough to move coarse sediment and coarse plant debris, and to leave large flow ripples along their broad, flat floors.