Eel Margin Research Articles

Transgressive deposits along the actively deforming Eel River Margin, Northern California

New high-resolution CHIRP seismic data acquired along the Eel River margin, northern California, reveal that stratal architecture and sediment thickness of the Holocene transgressive deposits are, in large part, controlled by tectonic deformation and sediment supply. A thick (>20m) transgressive deposit is observed across the Eel margin, a forearc basin that is undergoing active folding perpendicular to the coastline at rates of mm/yr. The transgressive deposits on the Eel margin exhibit marked variations in thickness alongshore; being thickest in the Eel River Syncline and thinnest over the Eureka Anticline. The divergent character of the infill in the syncline suggests that deposition is syntectonic. Fault displacement and structural relief observed along the transgressive surface are consistent with deformation rates measured onshore. The transgressive surface is offset ~0.5m across the Eureka Anticline suggesting deformation has been active since ~10ka. Two distinct acoustic units have been identified within the transgressive systems tract: a basal deposit that infills relief on the transgressive surface and an upper onlapping unit. The basal deposit infills lows along the outer shelf with a maximum thickness of 10m and appears to be controlled by the early sea-level rise (21–7ka) of the last deglaciation. It is separated from the overlying acoustically well-laminated unit by a pronounced surface of onlap. Moving shoreward along the inner shelf (<60m water depth) the transgressive sequence thins and becomes acoustically transparent, which suggests that the finer-grained material is bypassing the inner shelf and being sequestered on the middle to outer shelf. It is here, on the inner shelf where tectonically induced accommodation exhibits the greatest control on sediment thickness. Thus, tectonics played a greater role when sea-level rise slowed after 7ka to rates comparable to or slower than tectonic rates (~3mm/yr). On the middle to outer shelf offshore of the Eel River, there is evidence for progradation and highstand deposition.

Partitioning of organic matter in continental margin sediments among density fractions

Hydrodynamic processes sort and redistribute organic matter (OM) and minerals on continental margins. Density fractionations were conducted on sediments from diverse margins (Mexico margin, Gulf of Mexico, Mississippi River delta, Eel River margin) to investigate the nature, provenance and age of OM among density fractions. Mass, elemental (C and N), lignin, and surface area distributions, as well as stable carbon and radiocarbon isotopic compositions were measured. The lowest density fractions (< 1.6 g cm − 3 ) contained the highest organic carbon (OC) (up to 45%) and lignin concentrations (up to 8 mg g − 1 ) due to abundant woody debris, whereas high density fractions (> 2.5 g cm − 3 ) were OC-poor (%OC < 0.5) mineral material. Most sediment mass was found in the mesodensity fractions (1.6 to 2.5 g cm − 3 ) that contained the highest proportion of OC (up to ~ 75%) for each sediment. Stable carbon isotope compositions (δ 13C − 25.5‰ to − 22.9‰) show terrigenous OC as a significant component of density isolates from the river-dominated sediments (Gulf of Mexico, Mississippi River, and Eel margin), whereas the Mexico margin, least influenced by riverine input, was dominated by autochthonous marine OC (δ 13C ~ − 21.5‰). Radiocarbon compositions of density fractions indicate significant pre-aged OC (Δ 14C as low as − 900‰) in river-influenced sediments but not on the Mexico margin (Δ 14C > − 200‰). Ratios of vanillic acid to vanillin (Ad/Al)v among lignin oxidation products increase with increasing particle density suggesting variable lignin sources or selective degradation of lignin among the different density fractions.

Reconstruction of sediment flux from the Changjiang (Yangtze River) to the sea since the 1860s

Summary The Changjiang (Yangtze River) has been effectively gauged since the 1950s and demonstrates the transformation of a river system due to intensified human activities in its drainage basin over the past 50 yr. However, the 50-yr measurements of water and sediment are inadequate to show the long-term trend of sediment flux from the river to the sea or to capture the transition from natural to human dominance over the sediment flux. In this study we used the existing water discharge and sediment load records (1950s–2005) at the Hankou gauging station, together with water discharge recorded since 1865 at the same station, to reconstruct the changes of sediment flux to the sea since the 1860s. We established rating curves between stream discharge and suspended sediment concentration from the recent 50-yr data sets, which show that human disturbances have had a substantial impact on rating parameters. The commissioning of dams and undertaking of soil-conservation works have decreased sediment supply, leading to a decrease in the rating coefficient a of the rating curve equation C s = aQ b . The decreases in suspended sediment concentration have increased the erosive power of the river, and hence increased the rating exponent b . In particular, the commissioning of the Three Gorges Reservoir in 2003 resulted in a further increase of b , and channel scour in the middle and lower reaches has increased sediment flux to the sea to a level higher than sediment supply from the upper reaches. Our results suggest that the rating curves derived from 1954 to 1968 data are appropriate for estimating sediment loads for the period from 1865 to 1953, since both were periods of minimal human disturbance. This approach provides a time series of sediment loads from 1865 to 2005 at Hankou gauging station, which yields a time series of sediment flux from the Changjiang to the sea over the past 140 yr. The estimated mean annual sediment flux to the sea between 1865 and 1968 was ∼488 Mt/yr, a comparable result to the previously published estimate from Milliman and Syvitski [Milliman, J.D., Syvitski, J.P.M., 1992. Geomorphic/tectonic control of sediment discharge to the ocean: the importance of small mountainous rivers. Journal of Geology 100, 525–544] and to that from an equation proposed by Syvitski and Morehead [Syvitski, J.P.M, Morehead, M.D., 1999. Estimating river-sediment discharge to the ocean: application to the Eel margin, northern California. Marine Geology 154, 13–28]. The long-term variation of annual sediment flux from the Changjiang to the sea shows a transition from a river system mostly dominated by nature (the monsoon-dominated period, 1865–1950s) to one strongly affected by human activities (the human-impacted period, 1950s–present).

River sediment flux and shelf sediment accumulation rates on the Pacific Northwest margin

To test the generality of insight obtained from recent STRATAFORM studies of northern California's Eel margin, river sediment sources and continental shelf sinks were examined on the Pacific Northwest margin from 38° to 44.5°N. River discharge and sediment concentration data acquired by the US Geological Survey were used to update long-term annual suspended-sediment loads for 17 rivers that range in basin area from 635 to ∼22,000 km 2. Resulting annual load estimates vary over 3 orders of magnitude (0.065–18×10 9 kg/yr), with major suspended-sediment fluxes supplied by, in decreasing order, the Eel, Klamath/Trinity, Mad, Rogue, Umpqua and Russian rivers. Down-core profiles of excess 210Pb and 137Cs were used to estimate sediment accumulation rates (SARs) at prescribed depths (70 and 110 m) and distances (0–40-km north and south along-shelf) from each of the major rivers. SARs were found to vary much less than the river flux estimates, and are mostly in the range of 1.5 to 6 mm/yr. Most significantly, shelf SARs on the other Pacific Northwest margins are only slightly less than those observed on the Eel shelf, implying that much higher proportions of riverine sediment are retained on those shelves. Likely reasons that the Eel dispersal system exhibits greater off-shelf transport are (1) the narrower and steeper shelf geometry, and (2) the existence of a newly documented cross-isobath sediment transport mechanism that involves wave-modulated fluid mud flows. Testing whether the fluid mud flows are a consequence of the Eel River basin's high sediment yield, and are thus unique to the Eel, or are caused by intense wave energy during discharge events, and hence are operative on many other margins, awaits future bottom-boundary layer measurements.

Sediment deposition in a modern submarine canyon: Eel Canyon, northern California

Previous studies on the Eel margin have shown that a substantial amount of terrigenous sediment may be rapidly transported and deposited seaward of the shelf break during the Eel River flooding season. Eel Canyon, located ∼10 km seaward of the Eel River mouth, has been investigated to determine its role in seaward escape of sediment over seasonal timescales. In order to characterize seasonal deposition, box cores were collected in upper Eel Canyon (<900 m water depth) during periods of high and low sediment discharge from the Eel River (winter and summer, respectively). Cores were collected at the beginning and end of three typical flooding seasons (January 1998–April 2000), to assess the regularity of sediment escape to the Eel Canyon. Results showed that sediment deposition was temporally variable over the course of a year, with thick sediment deposits (up to 19 cm) being formed during the flooding season. These deposits were identified as having detectable 7Be activity, high percentage of clay and distinct physical sedimentary structures. Sediment signatures indicated sediment was composed primarily of recently derived fluvial sediment that was rapidly accreted on the seabed (on the order of days to months from when it was discharged from the river). The 3-year time series showed that sediment reached the canyon on an annual basis and extreme flooding events from the Eel River were not a necessary condition for sediment escape. Fluvial sediment was deposited in most areas of the upper canyon, following each winter flooding season, with the thickest deposits consistently observed in upper channel thalwegs of the canyon. Preferential deposition in thalwegs, along with preservation of physical structures, suggests that gravity-driven sediment flows may be important transport mechanisms in these areas. The spatial distribution of deposit thicknesses for cores collected in April 2000 was used in conjunction with a detailed swath map to construct a seasonal sediment budget for the upper Eel Canyon alone. It was found that 12.0%±3.5% of the Eel River sediment discharge can be accounted for in the upper Eel Canyon, suggesting that the canyon can act as a significant sink of Eel River sediment over seasonal timescales.

The sandy inner shelf as a repository for muddy sediment: an example from Northern California

To understand the sedimentary history of a siliciclastic, tectonically active inner-shelf environment, 50 vibracores were collected in water depths ranging from 20 to 55 m on the Eel margin, Northern California. Sediments exhibit changes in grain size along- and across-shelf, as well as vertically. The mean grain size of the surface sand fraction is 2.2–3.6 φ 1 1 φ=− log 2( diameter( mm)/1 mm) (217–82.5 μm), with 7–40% of the sediment finer than 4 φ (64 μm). The along-shelf distribution of sand is controlled predominantly by proximity to the sediment source and by the prevailing oceanic conditions (e.g., waves and currents). Fine sand dominates within ∼10 km of the Eel River mouth, which is consistent with sediment settling velocities out of the Eel River plume. Coarser sediment is found farther (>10 km) north of the Eel River mouth, and may have a source other than the modern Eel River. Cores tend to demonstrate upward fining (except cores collected at ∼30 m) and other vertical grain-size variations, which preserve the record of floods on the Eel margin. Muddy layers, which are assumed to record large floods, are found interbedded with sand. Two types of muddy layers are observed. “Distinct” mud layers have >70% mud and are identifiable through visual observation and a decrease in bulk density. “Diffuse” mud layers contain only 5–15% mud due to reworking caused by wave activity during or after deposition. Diffuse mud layers are not visually obvious, and poor sorting increases their bulk density. Recent flood layers have detectable 210Pb and 137Cs, and the deepest detectable 137Cs is 120 cm below the surface. Assuming that the mud layers result from major floods (e.g., 1964), and using 210Pb and 137Cs radioisotopes to establish chronological control, the accumulation rate of sand on the inner shelf since 1964 ranges from 1.3 to 3.3 cm/yr. Approximately 6–13% of the fine-grained sediment discharged by the Eel River over the past 36 yr is accumulating interspersed with the inner-shelf sand. An additional ∼1% of the fine-grained sediment discharged since 1964 may be accounted for as distinct flood layers (containing >70% mud) interbedded with the inner-shelf sand. In all, 40–50% of the sand discharged by the Eel River, both in suspension and as bedload, can be accounted for on the inner shelf. An additional 2–5% of the sand accumulates in the mid-shelf mud deposit. Transport farther seaward (e.g., Eel canyon), landward (beaches or Humboldt Bay), or along shelf should account for the remainder of the sand. Thus, the inner-shelf region plays a significant role in controlling the fate of sand and mud supplied from terrestrial sources.

Controls of tectonics and sediment source locations on along-strike variations in transgressive deposits on the northern California margin

We identify two surfaces in the shallow subsurface on the Eel River margin offshore northern California, a lowstand erosion surface, likely formed during the last glacial maximum, and an overlying surface likely formed during the most recent transgression of the shoreline. The lowstand erosion surface, which extends from the inner shelf to near the shelfbreak and from the Eel River to Trinidad Head (∼80 km), truncates underlying strata on the shelf. Above the surface, inferred transgressive coastal and estuarine sedimentary units separate it from the transgressive surface on the shelf. Early in the transgression, Eel River sediment was likely both transported down the Eel Canyon and dispersed on the slope, allowing transgressive coastal sediment from the smaller Mad River to accumulate in a recognizable deposit on the shelf. The location of coastal Mad River sediment accumulation was controlled by the location of the paleo-Mad River. Throughout the remainder of the transgression, dispersed sediment from the Eel River accumulated an average of 20 m of onlapping shelf deposits. The distribution and thickness of these transgressive marine units was strongly modified by northwest–southeast trending folds. Thick sediment packages accumulated over structural lows in the lowstand surface. The thinnest sediment accumulations (0–10 m) were deposited over structural highs along faults and uplifting anticlines. The Eel margin, an active margin with steep, high sediment-load streams, has developed a thick transgressive systems tract. On this margin sediment accumulates as rapidly as the processes of uplift and downwarp locally create and destroy accommodation space. Sequence stratigraphic models of tectonically active margins should account for variations in accommodation space along margins as well as across them.

Application of an analytical model of critically stratified gravity-driven sediment transport and deposition to observations from the Eel River continental shelf, Northern California

An analytical model of down-slope sediment transport and deposition by wave-supported gravity-driven flows is applied to examine the formation of the mid-shelf flood deposit on the continental margin off of the Eel River in northern California. The model reproduces observed time series of near-bed velocity and deposition following flood events when sufficient fine sediment is available to critically stratify the wave boundary layer. Comparative estimates of down-slope gravity-driven flux and along-shelf sediment delivery suggest that critical stratification will dominate the near-bed dynamics when greater amounts of sediment are delivered to the inner shelf by river floods than can be removed by gravity-driven processes. If insufficient sediment is delivered to cause critical stratification in the wave boundary layer, energetic waves can enhance drag, retard down-slope transport, and limit deposition. Analytic predictions of deposition suggest that the magnitude of wave energy is more important than the magnitude of the flood event in controlling the local thickness of mid-shelf gravity-driven deposition following floods of the Eel River. Higher wave energy increases the capacity for critically stratified gravity flows to transport sediment to the mid-shelf and results in greater gradients in flux and hence deposition. Flood magnitude determines how close to shore the flood deposit begins and how far along-shelf it extends. The bathymetry of the Eel margin plays a critical role controlling wave-supported gravity flows. Analytic predictions indicate that gravity-driven deposition on the mid-shelf begins roughly 7–8 km north of the river mouth. Closer to the river mouth, the seaward increasing mid-shelf slope associated with the concave downward subaqueous delta causes gravity-driven flux divergence, preventing significant mid-shelf deposition by wave-supported gravity flows and favoring sediment bypassing. A seaward decrease in shelf slope in the vicinity of the observed flood depo-center leads to greater flux convergence by gravity-driven flows, and hence greater deposition. Farther north, the supply of sediment diminishes sufficiently to prevent significant gravity-driven deposition. The analytic predictions of mid-shelf mud deposition are spatially and temporally consistent with field observations and provide strong evidence that wave-supported gravity flows control the emplacement and location of the Eel margin flood deposit.

Lower Pleistocene to present structural deformation and sequence stratigraphy of the continental shelf, offshore Eel River Basin, northern California

Seismic stratigraphic sequences and deformational features are mapped beneath the continental shelf of the offshore Eel River Basin of northern California, using a closely spaced, high-resolution multichannel seismic grid. Geometries of sequences and morphologies of bounding unconformities reflect competing tectonic and glacioeustatic influences, producing shifting sedimentation patterns in the offshore basin during the last ∼1 Myr. Estimated timing of unconformity formation correlates generally with a deep-ocean global δ 18O curve, both before and after a ∼500-kyr transition from 41-kyr to 100-kyr dominated cycles. We suggest that glacioeustatic fluctuations are the dominant control on unconformity formation basinwide. However, regional tectonic influences on sequence development are also observed. Folding associated with Gorda–North America Plate convergence ended progressively from south to north between ∼1.0 Ma and ∼500 ka, in response to northward migration of the Mendocino Triple Junction (MTJ). Since ∼500 ka, continued encroachment of the MTJ produces rotation of preexisting structures, uplift of the Table Bluff Anticline (TBA), related periods of channel incision south of the TBA, and generally reduced sediment preservation across the shelf. MTJ-related uplift may also have induced formation of Eel Canyon. Over the past ∼1.0 Myr, a dominant northern sediment source becomes progressively less important; a southern source, probably the paleo-Eel River, has become dominant since ∼750 ka. Seismic unconformities fall into two categories: (1) irregular surfaces of limited mappable extent, interpreted as incision/exposure surfaces forming during relative sea-level lowstands, and (2) smooth, laterally extensive regional unconformities, interpreted as ravinement surfaces that erode lowstand surfaces in all but deeply incised areas during sea-level transgressions. A sequence stratigraphic model for sediment deposition and preservation on the Eel River shelf predicts that preserved sediment on the Eel margin is dominated by fluvially derived highstand silts and muds, deposited by longshore-directed currents and waves. Lowstand sediments are preserved only in fluvial channels infilled during late lowstand/early transgression. Preserved transgressive sediments are likely limited to a thin veneer of well-sorted coarse sediment or shell debris, deposited above the transgressive ravinement. Future deep coring will be required to confirm these predictions.

In situ acoustic and laboratory ultrasonic sound speed and attenuation measured in heterogeneous soft seabed sediments: Eel River shelf, California

We compared in situ and laboratory velocity and attenuation values measured in seafloor sediments from the shallow water delta of the Eel River, California. This region receives a substantial volume of fluvial sediment that is discharged annually onto the shelf. Additionally, a high input of fluvial sediments during storms generates flood deposits that are characterized by thin beds of variable grain-sizes between the 40- and 90-m isobaths. The main objectives of this study were (1) to investigate signatures of seafloor processes on geoacoustic and physical properties, and (2) to evaluate differences between geoacoustic parameters measured in situ at acoustic (7.5 kHz) and in the laboratory at ultrasonic (400 kHz) frequencies. The in situ acoustic measurements were conducted between 60 and 100 m of water depth. Wet-bulk density and porosity profiles were obtained to 1.15 m below seafloor (m bsf) using gravity cores of the mostly cohesive fine-grained sediments across- and along-shelf. Physical and geoacoustic properties from six selected sites obtained on the Eel margin revealed the following. (1) Sound speed and wet-bulk density strongly correlated in most cases. (2) Sediment compaction with depth generally led to increased sound speed and density, while porosity and in situ attenuation values decreased. (3) Sound speed was higher in coarser- than in finer-grained sediments, on a maximum average by 80 m s −1. (4) In coarse-grained sediments sound speed was higher in the laboratory (1560 m s −1) than in situ (1520 m s −1). In contrast, average ultrasonic and in situ sound speed in fine-grained sediments showed only little differences (both approximately 1480 m s −1). (5) Greater attenuation was commonly measured in the laboratory (0.4 and 0.8 dB m −1 kHz −1) than in situ (0.02 and 0.65 dB m −1 kHz −1), and remained almost constant below 0.4 m bsf. We attributed discrepancies between laboratory ultrasonic and in situ acoustic measurements to a frequency dependence of velocity and attenuation. In addition, laboratory attenuation was most likely enhanced due to scattering of sound waves at heterogeneities that were on the scale of ultrasonic wavelengths. In contrast, high in situ attenuation values were linked to stratigraphic scattering at thin-bed layers that form along with flood deposits.

The link between abrupt climate change and basin stratigraphy: a numerical approach

To use basin stratigraphy for studying past climate change, it is important to understand the influence of evolving boundary conditions (river discharge and sediment flux, initial bathymetry, sea level, subsidence) and the complex interplay of the redistribution processes (plumes, turbidity currents, debris flows). To provide understanding of this complexity, we have employed source to sink numerical models to evaluate which process dominates the observed variability in a sedimentary record of two coastal Pacific basins, Knight Inlet in British Columbia and the Eel Margin of northern California. During the last glacial period, the Eel River supplied comparatively more sediment with a less variable flux to the ocean, while today the river is dominated by episodic events. Model results show this change in the variability of sediment flux to be as important to the deposit character as is the change in the volume of sediment supply. Due to the complex interaction of flooding events and ocean storm events, the more episodic flood deposits of recent times are less well preserved than the flood deposits associated with an ice-age climate. In Knight Inlet, the evolving boundary conditions (rapidly prograding coastline, secondary transport by gravity flows from sediment failures) are a strong influence on the sedimentary record. The delta and gravity flow deposits punctuate the sedimentary record formed by hemipelagic sedimentation from river plumes. Missing time intervals due to sediment failures can take away the advantage of the otherwise amplified lithologic record of discharge events, given the enclosed nature of the fjord basin.

Biological alteration of physically structured flood deposits on the Eel margin, northern California

Sediment profile and surface images collected in December 1995 were used to examine sediments from the mid-continental shelf off northern California. The Eel margin, as the area is known, has been subjected to episodic flood deposition associated with high discharge events of the nearby Eel and Mad Rivers. Sediments in the deposit region consisted of stratified silty clays which were highly bioturbated. Three regions were defined within water depths of 28–83 m, near the S transect of the STRATAFORM study, based upon image data and information from other studies. They were inshore shelf sands, a transitional region where sands transported by storms alternate with flood beds, and mid-shelf flood deposits. Infaunal bioturbational features were measured along a cross-shelf transect. The number and type of biogenic features changed with changing sedimentary region. The degree of bioturbation was highest in the deposit region where burrows, surface unconsolidated layer, active and relict feeding voids, and animals were more prevalent. The uppermost 5–10 mm in the deposit region were highly porous due to an abundance of 0.3 mm diameter polychaete burrows. In the transitional zone, there was evidence for both shallow and deep infaunal activity, but also apparently less utilization of the storm sand layer. Sediment laminae and the positions of bioturbational features suggest that the infaunal community has adapted to the combined influence of deposition and storm transport in the transitional region. Whereas the deeper infauna appear to be exploiting the resources within the flood deposit sediments, they in turn appear to support surface-dwelling fauna. A hypothetical model is proposed to explain the organism–sediment interactions for this area.

River-plume sedimentation modeling for sequence stratigraphy: application to the Eel margin, northern California

Sediment deposition in the coastal zone from riverine input is of key importance in understanding the construction of sediment sequences through time. A turbulent-jet model is used to simulate sediment input to the continental margin from a riverine source. Turbulent-jet models track the momentum, which is the first-order force carrying most of the sediment flowing into the ocean. The Froude number at the river mouth remains supercritical during floods of the Eel River, justifying the use of a jet model for the initial dispersion of the sediment-laden plume. Sediment is deposited from the plume prior to the centerline Froude number decreasing to its critical value. Empirically derived first-order removal-rate constants are used for each grain size in an advection–diffusion equation to predict the areal extent of sediment deposition. One strength of using a jet model is its ability to simulate deposits on a daily time-step over intermediate time scales (thousands of years) using a limited number of input parameters. A number of simulations of the Eel River plume are shown for conditions of the present day and 18,000 yr BP, during the last glacial maximum. The model results for modern conditions show that a majority of the deposition occurs within 20 km north of the river mouth and is typically confined to within 10 km of the coastline, corresponding well with the shelf sands on the Eel margin. However, the initial mud deposit from a 1995 flood was farther offshore and northward. Additional advection of the mud fraction must occur after it leaves the surficial plume and prior to its initial deposition. The model runs of 18,000 yr BP (with 100 m lower sea level, wetter and colder climate, and the river mouth assumed to be at the canyon) show that the plume is able to transport sand, silt and clay out of the canyon and onto the continental slope.

Detailed investigation of continental shelf morphology using a high-resolution swath sonar survey: the Eel margin, northern California

The Eel shelf, northern California, lies within an active compressional tectonic margin subject to abundant terrigenous sediment input from the Eel River. A recent high-resolution swath sonar survey provides us with the opportunity to investigate seafloor morphology and acoustic backscatter patterns within this dynamic region. Our analysis of the statistical character of bathymetry demonstrates a clear separation into large- and small-scale morphologies at a ∼3–10 km scale, with smaller-scale morphology heavily damped relative to large-scale morphology. The Eel shelf bathymetry is subtle, but several structures can be readily discerned in a residual bathymetry formed by removal of the downslope gradient. Some shelf structures are evidently related to depositional processes (as evidenced by correlation with 100-yr sediment accumulation rates), whereas others appear related to tectonic processes (as evidenced by correlation with subsurface synclines and anticlines). The sidescan structure of the shelf is dominated by the low backscatter over the Eel and Mad River subaqueous deltas, evidently associated with the sand-to-mud transition. However, contrary to usual correlations between backscatter and grain size, in this situation higher backscatter is associated with the muddy sediments. In addition, we observe a series of shore-perpendicular striations, or `ribbons', spaced ∼0.2–1.0 km apart, which extend northward from the Eel River subaqueous delta and lie at or near the sand-to-mud transition. Some aspects of ribbon morphology suggest that they may be associated with down-slope flows.

Regional variability of slope stability: application to the Eel margin, California

Relative values of downslope driving forces and sediment resisting forces determine the locations of submarine slope failures. Both of these vary regionally, and their impact can be addressed when the data are organized in a Geographic Information System (GIS). The study area on the continental margin near the Eel River provides an excellent opportunity to apply GIS spatial analysis techniques for evaluation of slope stability. In this area, swath bathymetric mapping shows seafloor morphology and distribution of slope steepness in fine detail, and sediment analysis of over 70 box cores delineates the variability of sediment density near the seafloor surface. Based on the results of ten geotechnical studies of submarine study areas, we developed an algorithm that relates surface sediment density to the shear strength appropriate to the type of cyclic loading produced by an earthquake. Strength and stress normalization procedures provide results that are conceptually independent of subbottom depth. Results at depth are rigorously applicable if sediment lithology does not vary significantly and consolidation state can be estimated. Otherwise, the method applies only to shallow-seated slope failure. Regional density, slope, and level of anticipated seismic shaking information were combined in a GIS framework to yield a map that illustrates the relative stability of slopes in the face of seismically induced failure. When a measure of predicted relative slope stability is draped on an oblique view of swath bathymetry, a variation in this slope stability is observed on an otherwise smooth slope along the mid-slope region north of a plunging anticline. The section of slope containing diffuse, pockmarked gullies has a lower measure of stability than a separate section containing gullies that have sharper boundaries and somewhat steeper sides. Such an association suggests that our slope-stability analysis relates to the stability of the gully sides. The remainder of the study area shows few obvious indications of slope instability except for a feature that has become known as the `Humboldt Slide,' but it is too deep-seated to be amenable to the slope-stability-prediction techniques presented herein. In general, few slope failures have been mapped in the Eel margin study area despite the high level of seismicity, the relatively high rates of sediment accumulation, and the extent of gas charging observed by others.

Spatial variability in sedimentary processes on the Eel continental slope

The northern California continental margin receives episodically large quantities of sediment from the Eel River, which seasonally discharges to the shelf. Regional circulation broadcasts this sediment widely, not only affecting sedimentation on the shelf, but on the slope as well. A suite of 60 cores has been collected to examine the processes of sediment accumulation and sediment delivery to the Eel continental slope in waters <800 m deep. Spatial surveys of accumulation rate (0.2–1.3 g cm −2 year −1) and surficial sediment grain size (4–8.5 φ) demonstrate that sediment is accumulating throughout the Eel margin, reflecting a rapid and widespread redistribution of the fluvial discharge. Downcore grain-size profiles of heterogeneous sediments suggest that hemipelagic and other episodic sediment-delivery processes may be important for the slope in areas proximal to the river. In more distal areas, downcore profiles reveal relatively homogeneous sediments, delivered dominantly by hemipelagic processes. A budget for fine-grained sediment shows that this upper portion of the slope contains, at a maximum, 20% of the river's annual discharge; a combined budget for the shelf and upper slope suggests that at least 60% of the annual sediment load is not accounted for in these areas. The Eel continental slope, because of the large terrigenous sediment input, the narrow width of the adjacent shelf, and its energetic environment, is a good analogue for slopes during transitional stands of sea level and for modern, tectonically active settings. Processes typically associated with these geological settings (i.e., supply of sediment to the slope, and downslope mass movement) are actively occurring.

Estimating river-sediment discharge to the ocean: application to the Eel margin, northern California

HYDROTREND, a synthetic river-discharge numerical model, can simulate the natural variability (daily, annually and longer) in the flux of water and sediment delivered to the coastal ocean. The numerical model is designed to make discharge predictions based on climate, even though field measurements of river flow are not available. Provided that appropriate assumptions are made regarding past climate, the model can predict how a river may have behaved in the geological past. Sediment load is determined from a river's rating coefficients. When applied to the flood-dominated Eel River, northern California, HYDROTREND captures the intermonthly and interannual variability of discharge and loads over a century or more. HYDROTREND is applied to the ice-age paleo-Eel River when sea level was lower and climate much cooler and wetter. The paleo-Eel River at 18,000 year BP would see its hinterland runoff increase, but not the magnitude of the peak discharges. The post-dicted sediment load of the paleo-Eel River was 25% greater than today's, although basin precipitation was 45% greater, and the hinterland area 4% greater (through sea-level lowering). A method is provided to obtain sediment-discharge rating coefficients for unmonitored modern rivers and paleo-rivers. The relationship Q s= αρg 1/2 H 3/2 A 1/2 provides for an estimate of a river's sediment load Q s, to within a factor of 3 over 7 orders of magnitude, from knowledge of basin relief H, and basin area A. Rating coefficients are determined from this estimate of Q s and the power-law rating curve, Q s= aQ b+1 where Q is discharge.