Hyperpycnal plume-derived fans in the Santa Barbara Channel, California



[1] Hyperpycnal gravity currents rapidly transport sediment across shore from rivers to the continental shelf and deep sea. Although these geophysical processes are important sediment dispersal mechanisms, few distinct geomorphic features on the continental shelf can be attributed to hyperpycnal flows. Here we provide evidence of large depositional features derived from hyperpycnal plumes on the continental shelf of the northern Santa Barbara Channel, California, from the combination of new sonar, lidar, and seismic reflection data. These data reveal lobate fans directly offshore of the mouths of several watersheds known to produce hyperpycnal concentrations of suspended sediment. The fans occur on an upwardly concave section of the shelf where slopes decrease from 0.04 to 0.01, and the location of these fans is consistent with wave- and auto-suspending sediment gravity current theories. Thus, we provide the first documentation that the morphology of sediment deposits on the continental shelf can be dictated by river-generated hyperpycnal flows.

1 Introduction

[2] There is a strong interest in the rates and implications of sediment output from small, mountainous rivers, because these river systems provide substantial sediment and geochemical inputs to the world's oceans [Milliman and Farnsworth, 2011]. Across-shore sediment gravity flows are common dispersal mechanisms from these small, steep rivers, whether these flows are generated by direct hyperpycnal plunging of river outflow [Mulder and Syvitski, 1995; Warrick and Milliman, 2003; Hicks et al., 2004; Carter et al., 2012; Liu et al., 2012] or by resuspension of recently discharged sediment during energetic coastal conditions [Wright and Friedrichs, 2006; Traykovski et al., 2007; Hsu et al., 2009].

[3] Although increasing evidence for the presence or even dominance of gravity-flow processes in the exchange of sediment across the continental shelf has been found [e.g., Trowbridge and Kineke, 1994; Wright and Friedrichs, 2006; Warrick et al., 2008; Parsons et al., 2009], debate continues about the effects of these flows on marine sediment depositional architecture and geomorphology [e.g., Mulder et al., 2003; Piper and Normark, 2009; Lamb and Mohrig, 2009]. In fact, few modern examples of unique marine geomorphic features from plunging river plumes have been found. For example, broad, smooth midcontinental shelf mud belts are associated with both wave-driven gravity current settings, such as the Eel River, California [Sommerfield and Wheatcroft, 2007], and settings that do not support gravity currents, such as the Columbia River [Nittrouer and Sternberg, 1981]. Some hyperpycnal rivers enter submarine canyons where they can generate violent gravity flows that both build and erode levees and submarine fans [Mulder et al., 2001; Carter et al., 2012; Liu et al., 2012]. However, no simple relations between the type of canyon turbidity-current initiation processes and deposition morphology have been documented [Piper and Normark, 2009], and most submarine canyons have several different processes that supply sediment, making it difficult to decipher the type of initiation event purely from depositional architecture [Piper and Normark, 2001; Normark, et al., 2002; Xu et al., 2010; Carter et al., 2012]. Furthermore, sediment deposits from hyperpycnal plumes may be short-lived because of subsequent erosion and sediment transport [Milliman et al., 2007]. That said, reassessments of some ancient sedimentary fluvio-deltaic systems suggest that many event beds may be attributed to hyperpycnal flows [e.g., Myrow and Southard, 1996; Plink-Bjorklund and Steel, 2004; Lamb et al., 2008].

[4] Submarine morphology influenced by plunging river plumes—where it occurs—should be influenced by the sediment supply rates, the shape and slope of the seafloor offshore of the river mouth, and the wave and current climate. Consistent with this hypothesis, the influence of bathymetry on gravity-current dynamics has been suggested for freshwater reservoirs with hyperpycnal river supply [e.g., Pratson et al., 2008; Olariu et al., 2012] and also for ephemeral river mouth sand deltas of hyperpycnal coastal rivers of California [Warrick and Barnard, 2012].

[5] New marine geophysical data presented here from the Santa Barbara Channel, California, extend the understanding of the morphological implications of hyperpycnal river discharge by revealing a series of submarine fans directly offshore of steep coastal watersheds (Figure 1). Six fans are highlighted here, each originating from watersheds with drainage areas <25 km2, total vertical relief of ~900 m, and Pleistocene rock uplift rates of ~2 mm/yr [Duvall et al., 2004]. Sediment output from these watersheds is dominated by infrequent high discharge during winter storms when suspended-sediment concentrations are dominated by silt and clay grain-size fractions and commonly tens to hundreds of grams per liter [Warrick and Mertes, 2009], which are adequately high to cause hyperpycnal plunging at these river mouths [Warrick and Milliman, 2003].

Figure 1.

Submarine fans on the continental shelf of northern Santa Barbara Channel, California. (a) The Santa Barbara Channel of central California. Box shows location mapped in Figure 1b. (b) Bathymetry and topography along the coastal margin of six creeks draining the central Santa Ynez Mountains. Six submarine fans occur immediately offshore of these creeks between 25 and 70 m water depth, and each is labeled (fans a–f).

2 High-Resolution Bathymetry and Geophysics

[6] During summer 2008, high-resolution bathymetry and acoustic backscatter of the northern Santa Barbara Channel, California, were collected by the U.S. Geological Survey using 234.5 kHz phase-differencing side-scan sonar aboard the R/V Parke Snavely [Dartnell et al., 2010]. Vertical precision of these sonar data was range-dependent, approximately 0.1 m at 57 m water depth and 0.2 m at 114 m.

[7] Nearshore bathymetric data were also provided by a 2009 bathymetric-lidar survey conducted by the U.S. Army Corps of Engineers and Fugro Pelagos using the airborne SHOALS-1000 T system. These data extended to ~25 m water depth and were used to complete the digital elevation model between the inshore extent of the sonar bathymetry and the shoreline.

[8] A seismic reflection survey was conducted over a limited portion of the Refugio Fan (Figure 1b, fan “c”) in September 2012 aboard the R/V Connell, which is operated by the University of California, Santa Barbara, California. An EdgeTech 216S Chirp System with a frequency sweep of 2.0–15.0 kHz and a recording length of 20 ms was used for this survey, which provided a vertical resolution of ~20 cm, assuming an acoustic velocity of 1500 m/s.

3 Submarine Fans

[9] Six distinct lobate fans were observed between 25 and 70 m water depth of the northern Santa Barbara Channel (Figure 1b). The smallest fans—located offshore of Arroyo Quemada and Venadito Creek—consist of single lobes detached from the shoreline and are centered at 40–50 m water depth (Figure 1b, fans “a” and “d”). The two largest fans are located offshore of Tajiguas and Refugio Creeks (Figure 1b, fans “b” and “c”). At least 10 distinct lobes can be observed in the Refugio fan, and all of these lobes originate from a massive depositional feature on the inner shelf (Figure 2a). Intermediate-sized fans occur offshore of Los Flores and El Capitan Creeks (Figure 1b, fans “e” and “f”), the latter of which has lobes that radiate outward from its deltaic headland. Acoustic backscatter intensity data of these fans reveal that all have an intermediate intensity between lower values of adjacent muddy shelf sediments and higher values of rock outcrops, which may imply measurable sand fractions within these deposits [see Dartnell et al., 2010].

Figure 2.

(a) Shaded relief map of the ten primary lobes and locations of seismic profiles of the Refugio fan (fan c in Figure 1b). Lobes are identified with yellow arrows, and profile locations are shown with red lines. (b) Three Chirp seismic profiles of the seafloor within and near the Refugio fan. Subsurface unconformity highlighted with green arrows. Note that horizontal scales differ.

[10] Alongshore bathymetric profiles through the two largest fans show that these features have 2–3 m of relief (Figure 3). These fans also transition from a rounded massive morphology on the inner shelf to a more complex, multilobate morphology at depth (Figure 3). Seismic reflection profiles through the largest fan show a prominent reflector 0.5–4 m below the seafloor (Figure 2b). In many locations, a parallel set of dipping reflectors is imaged below the prominent reflector, which we interpret as an angular unconformity (Figure 2b, see profile c), consistent with interpretations of nearby seismic reflection surveys summarized by Draut et al. [2009]. The acoustic character of sediment above the prominent reflector is transparent to chaotic in character with only one discontinuous coherent internal reflector, which suggests massive sediment deposits. The thickness of this massive upper package appears to vary in the fan lobes, although there was reduced and incomplete acoustic penetration in many of the thickest portions of the fans (Figure 2b).

Figure 3.

Alongshore bathymetric profiles through fans b and c (Tajiguas and Refugio). Profiles extracted along straight lines connecting isobaths from flat sections outside of the fans. Shaded areas highlight seafloor that is elevated (dark shading) above or (light shading) below the mean bathymetry of the profile endpoints. The areas shown with dark shading were used to estimate fan volumes shown in Figure 4c.

4 Discussion and Conclusions

[11] Bathymetric and seismic reflection data of the northern Santa Barbara Channel continental shelf reveal several fan-like depositional features that have thicknesses of 2–4 m, cover areas of several square kilometers, and are immediately offshore of the outlets of the region's steep watersheds. To evaluate the origin of these fans, it is important to consider the shelf setting upon which they have been deposited, the physical geometry of the fans, and the geophysical processes of sediment gravity currents (Figure 4).

Figure 4.

Shelf setting and depositional information for the two largest fans of the northern Santa Barbara Channel study area. (a, b) Mean bathymetry and slope profiles for the continental shelf adjacent to the fans. (c) Histograms of the sediment volumes above the mean shelf elevation for every 2 m increment of shelf depth. The total volume of each fan is also provided. (d) Gravity current potential (GCP) for the range of wave orbital velocities (minimum, mean, and maximum labeled; dark shading is mean ± s.d.) expected during river discharge events. Vertical bar around GCP = 1 shows the approximate threshold for gravity currents assuming a range in CD of 0.003–0.006.

[12] Mean bathymetric across-shelf profiles for the seafloor immediately adjacent to the two largest fans are upwardly concave over the area for which fan deposition was observed (Figure 4a). Bathymetric evidence of deposition starts at approximately 25 m water depth—immediately downslope of the maxima in shelf slope of over 0.04—and is centered where the slope decreases from 0.04 to 0.01 (Figure 4b).

[13] The volumes of sediment in each fan can be estimated from the combination of the bathymetric and geophysical data. Unfortunately, the line spacing of the seismic reflection survey and incomplete imaging of the basal reflector within the fans do not allow for a complete isopach analysis at this time [cf. Figure 2B]. However, using the seismic reflection profiles, the height of the fans above the adjacent seafloor (i.e., Figure 3, dark shading) provides first-order volume estimates of 1.5 and 1.7 million m3 for the two largest fans (Figure 4c). The majority of this sediment occurs in the lowermost portion of the fans beneath 50 m water depth, where the fans have multiple lobes (Figure 4c). The massive, single-lobe portion of the fans occurs on the innermost shelf above approximately 35 m water depth and represents less than 10% of the total estimated sediment volume (Figure 4c). Although the fan sizes vary, they can be summarized as being 2–4 m thick, 500–2000 m wide in the alongshelf direction (with individual lobes scaling ~100 m wide) and 1000–3000 m long across the shelf.

[14] A number of other observations are relevant to the origin of these fans. First, no submarine landslide or slump scars were observed upslope of these fans or within the region (Figure 1b) [cf. Draut et al., 2009]. Second, the fans only occur directly offshore of the mouths of creeks, which suggests limited alongshore transport from either a positively buoyant plume [cf. Wright and Friedrichs, 2006] or from the Coriolis force [cf. Myrow and Southard, 1996]. Third, the continuous, flat angular unconformity under the entire study area suggests that these fans could not have been preserved during the most recent sea level transgression during the early Holocene [cf. Draut et al., 2009].

[15] These observations suggest that the fans were generated by sediment-discharge events from the watersheds of the Santa Ynez Mountains, which is consistent with the high watershed sediment yields and hyperpycnal-generating suspended sediment concentrations (>40 g/L) measured during exceptional discharge in these creeks [Warrick and Mertes, 2009]. If sediment gravity currents generated these fans, then these fans should be consistent with gravity current process theory. We test this hypothesis by applying the theory of Wright et al. [2001] to determine whether sediment gravity currents could extend to the locations observed and the theory of Lamb and Mohrig [2009] to determine if the inshore location of the fans is related to plume plunging patterns.

[16] The theory of Wright et al. [2001] suggests that gravity currents from river-derived fine-grained sediment respond to shelf slope, nearbed currents, and buoyancy from the suspended sediment to obtain Chezy-like force balances, and these theories have been applied successfully to numerous settings [e.g., Wright and Friedrichs, 2006; Friedrichs and Scully, 2007]. We suggest that this theory is applicable to our study site owing to the dominance of silt and clay in suspended sediment samples of this region's creeks during high flows (e.g., silt and clay fraction of samples = 85 ± 11% (± s.d.) by mass; suspended concentrations = 10–232 g/L; n = 21; after Warrick [2002]). Below we also address the potential effects of grain-size variations on these results.

[17] One implication of this theory is that gravity currents cannot be sustained below a minimum shelf slope (θmin), which is defined by:

display math(1)

[18] where Ricr is the critical bulk Richardson number of the gravity current, equivalent to ~0.25, CD is the seafloor drag coefficient, equivalent to 0.003–0.006, |u| is the velocity scale relevant to shear and friction on the gravity current, and ug is the speed of the gravity current. Without the influence of waves and currents, |u| equals ug and equation (1) reduces to the Chezy formulation in which sinθmin is 0.01–0.02 m/m [cf. Wright et al., 2001]. With significant waves and currents (i.e., |u| > ug), sediment suspension increases, thereby allowing for transport over seafloor with slopes less than the values reported above.

[19] Here we use this theory with the combination of wave and current observations during river discharge events to evaluate the gravity current potential (GCP), which is defined to be the ratio of actual seafloor slope (θactual) to θmin as computed from equation (1) (see full description in Supplementary Information). We use the general application that |u| is dominated by wave orbital velocities (uw) and use linear wave theory and wave statistics from Kniskern et al. [2011] to evaluate the depth dependence of GCP. Using this framework, GCP greater than one suggests gravity currents would be supported and bypass sediment, and GCP less than one suggests that gravity currents would extinguish and deposit sediment.

[20] Computed values of GCP for the wide range of wave conditions experienced during river discharge events for the study area are shown in Figure 4d. GCP values are highest and well above one inshore of the sediment fans, and the inshore-most deposition of the fans coincides with sharp decreases in GCP to values ~1 (Figure 4d). The multilobate sections of the fans, where the majority of sediment has been deposited, occur where mean GCP values approach and fall below 1 for all wave conditions (Figure 4d). Thus, the fans are located where fine-grained sediment gravity currents are expected to extinguish and deposit sediment.

[21] While the application of this theory is based on the assumption that the gravity currents are dominated by silt and clay, which is consistent with creek discharge conditions as noted above, we acknowledge that grain size has a first-order effect on gravity current speeds and runout distances [e.g., Dade et al., 1994; Sequeiros et al., 2008]. Thus, if the source grain size was dominated by sand and/or gravel (e.g., from a supply of slumped littoral sediment), then our predictions of GCP (Figure 4d) will greatly overestimate the extent of gravity flows.

[22] Another consideration for fans with hyperpycnal-discharge origin is that the inshore initiation of fan sedimentation may be related to the depth at which the plume fully plunges (h), which according to Lamb and Mohrig [2009] occurs when:

display math(2)

[23] where q is the discharge per unit width, and Δρ is the difference in density between the flow and the ambient ocean water (ρa). Using values relevant to the study area (q ~ 4 m2/s; Δρ ~ 40 kg/m3; ρa ~ 1025 kg/m3) [cf. Warrick and Mertes, 2009], h is found to be ~5 m, which is much shallower than the fans. This suggests that the location of the plume plunge point does not influence initial sedimentation, largely because the gravity currents would be efficiently transported downslope to depths of 30–70 m, as shown in Figure 4d.

[24] Combined, these observations suggest that sediment discharge events from the Santa Ynez Mountain watersheds can cause gravity flows of sediment that deposit kilometers offshore in distinct, lobate fans. Because these fans radiate directly from the terminal points of the watersheds, it is unlikely that the sediment pathways were initially dominated by hypopycnal (or positively buoyant) plumes, like the better-studied Eel River system, because hypopycnal plumes rapidly advect sediment in the alongshore direction [cf. Warrick et al., 2008; Parsons et al., 2009]. The rapid formation of sediment gravity currents is a defining characteristic of hyperpycnal river discharge conditions [e.g., Parsons et al., 2009], and this finding is consistent with the high river suspended sediment concentrations and marine sediment gravity flow conditions observed in this region [Warrick and Milliman, 2003; Warrick et al., 2008]. Although additional sampling will be needed to better understand the sedimentary characteristics of these fans, our observations provide the first physical evidence that recent river-generated hyperpycnal flows can influence the depositional morphology of the continental shelf.


[25] This work was supported by assistance from the USGS Coastal and Marine Geology Program, the California Ocean Protection Council, the California Coastal Conservancy, and the University of California, Santa Barbara. We thank A. Draut, S. Johnson, P. Hart, and K. Straub for useful comments and discussions.

[26] The Editor thanks Kyle Straub and an anonymous reviewer for their assistance in evaluating this paper.