Effects of River Discharge and Sediment Load on Sediment Plume Behaviors in a Coastal Region: the Yukon River, Alaska and the Bering Sea

hydrology

Article Effects of River Discharge and Sediment Load on Sediment Plume Behaviors in a Coastal Region: The Yukon River, Alaska and the Bering Sea

Kazuhisa A. Chikita 1,*, Tomoyuki Wada 2, Isao Kudo 3, Sei-Ichi Saitoh 1 and Mitsuhiro Toratani 4

1 Arctic Research Center, Hokkaido University, Sapporo 0010021, Japan; [email protected] 2 Earth System Science, Inc., Tokyo 1600022, Japan; [email protected] 3 Faculty of Fisheries Sciences, Hokkaido University, Hakodate 0418611, Japan; ikudo@ﬁsh.hokudai.ac.jp 4 Department of Optical and Imaging Science and Technology, Tokai University, Hiratsuka 2591292, Japan; [email protected] * Correspondence: [email protected]; Tel.:+81-11-706-2764

Abstract: In the Bering Sea around and off the Yukon River delta, surface sediment plumes are markedly formed by glacier-melt and rainfall sediment runoffs of the Yukon River, Alaska, in June– September. The discharge and sediment load time series of the Yukon River were obtained at the lowest gauging station of US Geological Survey in June 2006–September 2010. Meanwhile, by coastal observations on boat, it was found out that the river plume plunges at a boundary between turbid plume water and clean marine water at the Yukon River sediment load of more than ca. 2500 kg/s. Grain size analysis with changing salinity (‰) for the river sediment indicated that the suspended sediment becomes coarse at 2 to 5‰ by flocculation. Hence, the plume’s plunging probably occurred Citation: Chikita, K.A.; Wada, T.; by the flocculation of the Yukon suspended sediment in the brackish zone upstream of the plunging Kudo, I.; Saitoh, S.-I.; Toratani, M. boundary, where the differential settling from the flocculation is considered to have induced the Effects of River Discharge and turbid water intrusion into the bottom layer. Sediment Load on Sediment Plume Behaviors in a Coastal Region: The Keywords: Yukon River plume; river discharge; sediment load; flocculation; plunging; underflow Yukon River, Alaska and the Bering Sea. Hydrology 2021, 8, 45. https:// doi.org/10.3390/hydrology8010045 1. Introduction Academic Editor: Carmelina Costanzo One of actions of river discharge and sediment load into the ocean includes the sedimentation by energy dispersal in the estuarine area and through subsequent formation and Received: 17 February 2021 advection of sediment plume in coastal and offshore regions. The sedimentary processes Accepted: 9 March 2021 are often accompanied by the flocculation of suspended sediment, which affects nutrient Published: 12 March 2021 and organic matters’ cycles connected to the food chain or ecosystem in the ocean [1]. Behaviors of river-suspended sediment can also be controlled by how the river water mixes Publisher’s Note: MDPI stays neutral with ocean water under shearing, where the flocculation again plays the important role on with regard to jurisdictional claims in slow or rapid deposition of nutrient and organisms [2]. Hetland and Hsu [3] proposed a published maps and institutional affil- conceptual model of sedimentation in the estuarine, near-field plume, and far-field plume iations. associated with the flocculation. However, there are a few quantitative descriptions of the flocculation effect on sediment dispersion and deposition [2,4]. Meanwhile, dynamic behaviors of the whole river plumes, including buoyant jets or bulges of small scale, have been explored by field observations, satellite image analysis, and numerical simulations, for Copyright: © 2021 by the authors. example, for Mackenzie River plumes [5], Yellow River sediment plumes [6,7], Pearl River Licensee MDPI, Basel, Switzerland. plumes [8], Columbia River plumes [9], small-mouth Kelvin number plumes [10], and This article is an open access article Amazon River plumes [11,12]. With respect to the ecosystem in coastal regions, there are distributed under the terms and some remote-sensing studies of river sediment plume dynamics from the spatio-temporal conditions of the Creative Commons variations of suspended sediment concentration (SSC; mg/L) [13,14]. Dean et al. [15] Attribution (CC BY) license (https:// pointed out that satellite image analyses can be utilized for studies of the Yukon River creativecommons.org/licenses/by/ plume behaviors. By using the river discharge time series from 2002–2014, Pitarch et al. [16] 4.0/).

Hydrology 2021, 8, 45. https://doi.org/10.3390/hydrology8010045 https://www.mdpi.com/journal/hydrology Hydrology 2021, 8, x FOR PEER REVIEW 2 of 13

of the Yukon River plume behaviors. By using the river discharge time series from 2002– 2014, Pitarch et al. [16] related satellite-derived SSC of the Tiber River plume to the magnitude of the river discharge. However, the correlation between the discharge and SSC is very low at R = 0.5, because there is a lack of continuous data of river sediment load such as those of the river discharge and investigations of sediment depositional processes offshore from the river mouth. Hydrology 2021, 8, 45 In this study, considering a response of suspended sediment2 to of 12 a salinity change, behaviors of a river sediment plume in a coastal region are investigated by using time series of daily mean river discharge and sediment load in 2006–2010, which are here ex- related satellite-derived SSC of the Tiber River plume to the magnitude of the river dis- emplifiedcharge. However, by the the Yukon correlation River, between Alaska, the dischargeand the andBering SSC Sea. is very low at R = 0.5, because there is a lack of continuous data of river sediment load such as those of the 2.river Study discharge Area and investigations of sediment depositional processes offshore from the river mouth. In this study, considering a response of suspended sediment to a salinity change, 5 2 behaviorsMost of of a riverthe Yukon sediment River plume drainage in a coastal basin region (area, are investigated 8.55 × 10 bykm using) belongs time to the subarctic regionseries of south daily meanof the river Arctic discharge Circle and (66°33’ sediment N) loadand in is 2006–2010, occupied which by 74.8% are here forest with discon- tinuousexempliﬁed permafrost by the Yukon and River, 1.1% Alaska, glacierized and the Bering area Sea. in Alaska Range, Wrangell Mts., St. Elias Mts.,2. Study etc. Area [17–20]. There are three USGS (U.S. Geological Survey) gauging stations along the mainMost ofchannel the Yukon of Riverthe Yukon drainage River. basin (area,Site PLS 8.55 ×is 10the5 km lowest2) belongs gauging to the sub-station at ca. 170 km ◦ 0 upstreamarctic region of south sites of ALK the Arctic and Circlesite EMK, (66 33 baseN) and vill isages occupied for the by 74.8%coastal forest observations with (Figure 1). discontinuous permafrost and 1.1% glacierized area in Alaska Range, Wrangell Mts., St. TheElias Yukon Mts., etc. River [17–20 ].delta There is are radially three USGS formed (U.S. Geologicalwith branching Survey) gauging the Yukon stations River channel into somealong thesmaller main channelriver channels. of the Yukon Site River. EMK Site is PLS located is the lowestat ca. gauging18 km upstream station at ca. of the delta front. The170 kmeastern upstream shelf of sitesregion ALK of and the site Bering EMK, base Sea villages (2.29 for× 10 the6 coastalkm2 in observations area) is 50–70 m deep on (Figure1). The Yukon River delta is radially formed with branching the Yukon River average.channel into The some region smaller at river a distance channels. of Site less EMK than is located 170 atkm ca. off 18 kmthe upstream delta front of the is very shallow at lessdelta than front. 30 The m eastern in depth, shelf regionwhich of is the due Bering to Seasedimentation (2.29 × 106 km 2fromin area) sediment is 50–70 m load of the Yukon Riverdeep on and average. the surrounding The region at arivers. distance Behaviors of less than of 170 the km Yukon off the delta sediment front is plume very and the conse- shallow at less than 30 m in depth, which is due to sedimentation from sediment load quentof the Yukondispersion River and of thethe surrounding plume-suspended rivers. Behaviors sediment of the are Yukon controlled sediment plumeby the northern move- mentand the of consequent the “Alaskan dispersion Coastal of the Water” plume-suspended [15,21], which sediment is areconnected controlled to by the the bottom distribu- northern movement of the “Alaskan Coastal Water” [15,21], which is connected to the tionbottom of distributionclay minerals of clay on minerals the Alaskan-Chukchi on the Alaskan-Chukchi margin margin [22]. [22 ].

Figure 1. Location of ten observation sites off the Yukon delta front. The label “PLS” shows the Figurelocation of1. the Location lowest USGS of (U.S.ten Geologicalobservation Survey) sites gauging off stationthe Yukon in the Yukon delta River. front. The label “PLS” shows the location of the lowest USGS (U.S. Geological Survey) gauging station in the Yukon River.

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With respect to the discharge and sediment load time series of the Yukon River, Chikita et al. [20] quantiﬁed the contribution of runoffs from the glacierized regions or the non-glacial regions to the total Yukon discharge and sediment load by the tank model, one of the runoff models. As a result, it was revealed that the contribution in the 1.1% glacierized area occupies 21.7–37.5%, and that most of the Yukon River suspended sediment originates from the glacial bedrock erosion in the glacierized regions and the resuspension in the river channels.

3. Methods 3.1. Field Observations In order to obtain time series of suspended sediment load for the Yukon River, water turbidity was monitored at 1 h intervals at site PLS from June 2006–September 2010 by fixing a self-recording turbidimeter of infrared-ray back scattering type (model ATU3-8M, Alec Electronics, Inc.; range, 0–20,000 ppm; accuracy, ±2% of measured value) near the riverbank (Figure1; Chikita et al. [ 19]). The turbidimeter had a wiper to keep the sensor window clean, which worked just before every hourly measurement. Thus, the dirt of the sensor window by organisms was prevented. Each of the hourly turbidity values was stored as an average of ten values measured instantaneously at 1 sec interval. The water turbidity (ppm) from the turbidimeter was converted into suspended sediment concentration (SSC: mg/L) by using the relationship between the turbidity, x, and the SSC, y, from the water sampled simultaneously at mid-channel (y = 5.621x0.6762; R2 = 0.793, p < 0.01)(Figure2a) . The regression curve is close to the one-to-one correspondence between turbidity and SSC. Water temperature was simultaneously measured every hour at site PLS by fixing a temperature logger, TidbiT v2 (Onset computer, Inc., Bourne, MA, USA; accuracy, ±0.2 ◦C), to the turbidimeter. In order to know the representativeness of SSC and water temperature by the one-point method, during their monitoring, fluctua- tions of SSC and water temperature across the river channel were assessed by lowering a TTD (turbidity-temperature-depth) profiler from a boat [23]. As a result, differences between recorded SSC and water temperature and cross-sectionally averaged SSC and water temperature were less than 50 mg/L and less than 0.2 ◦C, respectively, being in- dependent of their magnitude. The small differences between the two SSCs and the two water temperatures are due to the slow settling of suspended sediment (more than 90% silt and clay) and the complete turbulence of riverflow, respectively. Daily mean discharge data at site PLS were downloaded from the National Water Information System on the USGS web site (URL: http://waterdata.usgs.gov/nwis/dv/?site_no=15565447&agency_ cd=USGS&referred_module=sw (accessed on 11 February 2021)). The sediment load, L (kg/s), was calculated by multiplying SSC, C (g/L in this calculation), by discharge, Q (m3/s). The electric conductivity at 25 ◦C (EC25) and salinity of the Yukon River water were also obtained at site PLS using a portable EC meter (Type CM-14P, TOA Corporation; accuracy, ±0.1 mS/m) for sampled water. A relation between turbidity, x, and SSC, y, was obtained also for the sediment plume off the delta front. The regression curve was given by y = 1.597x0.8119 (R2 = 0.932, p < 0.01; Figure2b), indicating that, in the region of brackish to oceanic waters, SSC is ca. 30% smaller than turbidity. This is probably due to an optical feature of marine water different from that of river water. Hydrology 2021, 8, x FOR PEER REVIEW 4 of 13 Hydrology 2021, 8, 45 4 of 12

Figure 2. Relations between turbidity (ppm) and SSC (suspended sediment concentration; mg/L) at (Figurea) site PLS 2. Relations and (b) the coastalbetween region turbidity off the Yukon (ppm) delta and front. SSC (suspended sediment concentration; mg/L) at (a) site PLS and (b) the coastal region off the Yukon delta front. At sites B1 to B10 in Figure1, which were located at less than 30 km from the delta front near siteA relation EMK, profiler between observations turbidity, and water x, and and SSC, sediment y, was samplings obtained were also performed for the by sediment plume boat on 6 September 2009 and 22 June 2010. A TCTD (turbidity-conductivity-temperature-0.8119 2 depth)off the profiler delta (modelfront. ASTD687,The regression JFE Advantech, curve Co.,was Ltd., given Japan) by was y =then 1.597 utilizedx for(R the = 0.932, p < 0.01; verticalFigure measurements2b), indicating at 0.1 that, m pitch. in the The region B5–B8 line of andbrackish B6–B10 to line oceanic correspond waters, to two SSC is ca. 30% longitudinalsmaller than lines turbidity. offshore fromThis the is fan-shapedprobably due Yukon to River an optical delta. feature of marine water different fromWith that respect of river to thewater. offshore extent of the Yukon sediment plume, relatively clear RGB (Red-Green-BlueAt sites B1 to colorB10 model)in Figure composite 1, which images were of MODIS/aqua located at less were than downloaded 30 km from the delta from the NASA oceanic web site (URL: https://oceancolor.gsfc.nasa.gov (accessed on 19front and near 24 February site EMK, 2021)), profiler which were observations selected on and dates water closest and to those sediment of the coastalsamplings were per- observations.formed by boat on 6 September 2009 and 22 June 2010. A TCTD (turbidity-conductivity- temperature-depth) profiler (model ASTD687, JFE Advantech, Co., Ltd., Japan) was then 3.2. Laboratory Experiments utilized for the vertical measurements at 0.1 m pitch. The B5–B8 line and B6–B10 line cor- During the turbidity record, river-suspended sediment and river-channel sediment wererespond sampled to two at site longitudinal PLS, and their lines grain offshore size analyses from werethe fan-shaped conducted by Yukon the photo- River delta. extinctionWith method respect with to a the centrifuge offshore for particlesextent of of the grain Yukon size d ≤ sediment44 µm (micrometers) plume, relatively and clear RGB by(Red-Green-Blue the sieving method color for d model) > 44 µm composite particles [19 ].imag The channeles of MODIS/aqua sediment was awere new onedownloaded from depositedthe NASA in theoceanic glacier-melt web andsite rainfall(URL:season https://ocean of the year.color.gsfc.nasa.gov With respect to the flocculation (accessed on 19 and 24 ofFebruary river-suspended 2021)), sedimentwhich were in the sele coastalcted region,on dates an effectclosest of to salinity those on of grain the coastal size of observations. sediment was explored by suspending the river channel sediment in pure water of 0‰ and NaCl water of 2–35‰. In the grain size analyses, a sediment sample of d ≤ 44 µm in3.2. a quartzLaboratory cell was Experiments repeatedly used with increasing the salinity from 0 to 35‰. Also, in orderDuring to know thethe existence turbidity of cohesive record, sediment river-suspen such asded kaolinite, sediment illite, etc., and clay river-channel particles sediment of d ≤ 4 µm in river-suspended and plume-suspended sediments were mineralogically were sampled at site PLS, and their grain size analyses were conducted by the photo- extinction method with a centrifuge for particles of grain size d ≤ 44 μm (micrometers) and by the sieving method for d > 44 μm particles [19]. The channel sediment was a new one deposited in the glacier-melt and rainfall season of the year. With respect to the flocculation of river-suspended sediment in the coastal region, an effect of salinity on grain size of sediment was explored by suspending the river channel sediment in pure water of 0‰ and NaCl water of 2–35‰. In the grain size analyses, a sediment sample of d ≤ 44 μm in a quartz cell was repeatedly used with increasing the salinity from 0 to 35‰. Also, in order to know the existence of cohesive sediment such as kaolinite, illite, etc., clay particles

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of d ≤ 4 μm in river-suspended and plume-suspended sediments were mineralogically identified by the X-ray diffraction (XRD) method. In the XRD method, a peak at a certain identiﬁed by the X-ray diffraction (XRD) method. In the XRD method, a peak at a certain 2θ value (θ: Bragg angle) indicates the existence of one to three minerals specified. 2θ value (θ: Bragg angle) indicates the existence of one to three minerals speciﬁed.

4. Results 4.1. Time Series of Discharge and Sediment Load Figure 33 shows shows temporaltemporal variationsvariations ofof (a)(a) dailydaily meanmean waterwater temperaturetemperature andand SSC,SSC, and (b) discharge discharge and and sediment sediment load load at at site site PL PLSS June June 2006–September 2006–September 2009. 2009. In Inlate late May May or earlyor early June, June, the the first ﬁrst large large peaks peaks of of SSC, SSC, disc discharge,harge, and and sediment sediment load load in in the year werewere produced by snowmelt runoffs justjust after the breakup ofof river ice. Then, water temperature greatly increased at more than 0 ◦°C.C. Thereafter, glacier-melt and rainfall runoffs resultedresulted in some peaks of SSC and sediment load in July–September. WhenWhen glacier-meltglacier-melt sedimentsediment runoffs prevailed in July–August, water temperature decreased by 1–2 ◦°CC after the peak in mid-July. InIn FigureFigure3 3b,b, it it is is seen seen that that the the sediment sediment load load at at more more than than 3000 3000 kg/s kg/soccurs occurs for more 1 month as a whole.whole. The water temperature waswas recordedrecorded atat lessless thanthan 00 ◦°CC forfor aa while in the ice-covered season. This means that the temperature logger was then enclosed by river ice, due to the ice growth.

FigureFigure 3. 3.Temporal Temporal variations variations of of ( a(a)) daily daily mean mean water water temperature temperature and and SSC, SSC, and and ( b(b)) daily daily mean mean dis- chargedischarge and sedimentand sediment load atload site at PLS site in PLS June in 2006–September June 2006–September 2009 (modified 2009 (modified after Chikita after Chikita et al. [19 ]). et al. [19]). 4.2. Observational Results of Plume Behaviors 4.2. ObservationalFigure4 shows Results cross-sections of Plume Behaviors of SSC, water temperature, salinity, and water density σ from the profiler observations on sites B5 to B8 line obtained on 6 September 2009 T Figure 4 shows cross-sections of SSC, water temperature, salinity, and water density (Figure1). Here, water density σT was calculated by σT = ρ(S, T, 0)—1000, where ρ(S, σT from the profiler observations on sites B5 to B8 line obtained on 6 September 2009 T, P) is water density (kg/m3) and a function of salinity S (‰), water temperature T (Figure 1). Here, water density σT was calculated by σT = ρ(S, T, 0)—1000, where ρ(S, T, P) (◦C), and pressure P (= 0 at 1 atm). Here, the lag time of 2 days (equal to the distance, is water density (kg/m3) and a function of salinity S (‰), water temperature T (°C), and 170 km, divided by rough riverflow speed, 1 m/s) was considered as an approximate time pressure P (= 0 at 1 atm). Here, the lag time of 2 days (equal to the distance, 170 km, di- needed to flow downstream from site PLS to the coastal region. The correspondent, daily vided by rough riverflow speed, 1 m/s) was considered as an approximate time needed to mean SSC, discharge, sediment load, and water temperature at site PLS were 391 mg/L, 7.39 × 103 m3/s, 2.89 × 103 kg/s, and 11.7 ◦C on 4 September 2009, respectively.

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flow downstream from site PLS to the coastal region. The correspondent, daily mean SSC, discharge, sediment load, and water temperature at site PLS were 391 mg/L, 7.39 × 103 m3/s, 2.89 × 103 kg/s, and 11.7 °C on 4 September 2009, respectively. A boundary between the plume water and relatively clean marine water was clearly seen near site B9 as shown by the dotted vertical line in Figure 4. The SSC on the boundary, corresponding to a plunging point, was ca. 30 mg/L. Offshore from the boundary, a bottom layer at 40 mg/L or more SSC was observed probably as a downslope bottom current or underflow. This plunging seems to have been preceded on the gentle slope between sites B5 and B4, since the turbid water at SSC of 160–240 mg/L was accumulated near the bottom as a nepheloid layer. The water temperature distribution indicates that vertical mixing due to the entrainment occurred at the plunging point [24–26]. A boundary between the plume and marine water and the correspondent plunging were also observed between site B3 and site B7 (Figure 5). However, the resultant bottom current appears to

Hydrology 2021, 8, 45 have been advanced wholly in the lower layer of more than 27.5‰ salinity, since the6 ofSSC 12 cross-section indicates a gradual SSC decrease offshore, accompanied by the vertical uniformity.

FigureFigure 4.4. LongitudinalLongitudinal cross-sectionscross-sections of SSC, water temperature, salinity, and water density σσTT byby thethe coastalcoastal observationobservation on the the B5–B8 B5–B8 line line on on 6 6Se Septemberptember 2009 2009 (Figure (Figure 1).1). The The dotted dotted line line in inthe the SSCSSC cross-sectioncross-section showsshows aa boundaryboundary betweenbetween turbidturbid plumeplume waterwater andand relativelyrelatively cleanclean oceanocean waterwater observedobserved inin situ.situ.

A boundary between the plume water and relatively clean marine water was clearly seen near site B9 as shown by the dotted vertical line in Figure4. The SSC on the boundary, corresponding to a plunging point, was ca. 30 mg/L. Offshore from the boundary, a bottom layer at 40 mg/L or more SSC was observed probably as a downslope bottom current or underﬂow. This plunging seems to have been preceded on the gentle slope between sites B5 and B4, since the turbid water at SSC of 160–240 mg/L was accumulated near the bottom as a nepheloid layer. The water temperature distribution indicates that vertical mixing due to the entrainment occurred at the plunging point [24–26]. A boundary between the plume and marine water and the correspondent plunging were also observed between site B3 and site B7 (Figure5). However, the resultant bottom current appears to have been advanced Hydrology 2021, 8, x FOR PEER REVIEWwholly in the lower layer of more than 27.5‰ salinity, since the SSC cross-section indicates7 of 13

a gradual SSC decrease offshore, accompanied by the vertical uniformity.

Figure 5. Longitudinal cross-sections of SSC, water temperature, salinity, and water density σT by Figure 5. Longitudinal cross-sections of SSC, water temperature, salinity, and water density σT by the coastal observation on the B6–B7 line on 6 September 2009 (Figure1). The dotted line in the the coastal observation on the B6–B7 line on 6 September 2009 (Figure 1). The dotted line in the SSC cross-section shows shows a aboundary boundary between between turbid turbid plume plume water water and and relatively relatively clean clean ocean ocean water water observed in situ. situ.

The traverse cross-section along the B1–B5 line (Figure 6) indicates that there is a zone with relatively high SSC of 140–250 mg/L between sites B6 and B5. This zone corresponds to the mixing zone upstream of the plunging points in Figures 4 and 5 [24], which is here accompanied by the vertical uniformity of SSC, water temperature, and salinity. Then, the SSC and water temperature decrease and the salinity increases gradually toward site B6 from site B5. This reflects the vertical mixing by relatively high river water flux at site B5. The longitudinal center of the sediment plume is, thus, located near site B5 rather than site B6 (Figure 1). If the high SSC zone on the B5–B6 line was produced by the resuspension of bottom sediment by tidal currents, wind-driven current, or littoral currents, then a high SSC zone should appear in the relatively shallow B6–B1 zone. Hence, the plunging is considered to have been raised through the mixing responding to the magnitude of the Yukon River sediment load. Figure 7 shows longitudinal cross-sections of SSC, water temperature, salinity, and σT along the B6–B10 line by the coastal observation on 22 June 2010. The daily mean SSC discharge, sediment load, and water temperature at site PLS were 240 mg/L, 9.12 × 103 m3/s, 2.19 × 103 kg/s, and 14.6 °C on 20 June 2010, respectively. The discharge was 19%

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Figure 5. Longitudinal cross-sections of SSC, water temperature, salinity, and water density σT by the coastal observation on the B6–B7 line on 6 September 2009 (Figure 1). The dotted line in the SSC cross-section shows a boundary between turbid plume water and relatively clean ocean water observed in situ.

The traverse cross-section along the B1–B5 line (Figure 6) indicates that there is a zone Hydrology 2021, 8, 45 7 of 12 with relatively high SSC of 140–250 mg/L between sites B6 and B5. This zone corresponds to the mixing zone upstream of the plunging points in Figures 4 and 5 [24], which is here accompanied by the vertical uniformity of SSC, water temperature, and salinity. Then, the SSC andThe water traverse temperature cross-section decrease along the and B1–B5 the salinity line (Figure increases6) indicates gradually that theretoward is asite zone B6 fromwith relativelysite B5. This high reflects SSC of the 140–250 vertical mg/L mixing between by relatively sites B6 high and B5.river This water zone flux corresponds at site B5. Theto the longitudinal mixing zone center upstream of the of sediment the plunging plume points is, thus, in Figures located4 and near5[ site24 ],B5 which rather is than here siteaccompanied B6 (Figure by 1). the If the vertical high uniformitySSC zone on of SSC,the B5–B6 water line temperature, was produced and salinity. by the resuspen- Then, the sionSSC andof bottom water sediment temperature by tidal decrease currents, and thewind-driven salinity increases current, gradually or littoral towardcurrents, site then B6 afrom high site SSC B5. zone This should reﬂects appear the vertical in the mixingrelatively by shallow relatively B6–B1 high zone. river Hence, water ﬂux the atplunging site B5. isThe considered longitudinal to have center been ofthe raised sediment through plume the mixing is, thus, responding located near to site the B5 magnitude rather than of sitethe YukonB6 (Figure River1). sediment If the high load. SSC zone on the B5–B6 line was produced by the resuspension of bottomFigure sediment 7 shows by longitudinal tidal currents, cross-sections wind-driven of current, SSC, water or littoral temperature, currents, salinity, then a highand σSSCT along zone the should B6–B10 appear line by in the the coastal relatively observation shallow on B6–B1 22 June zone. 2010. Hence, The daily the plunging mean SSC is discharge,considered sediment to have been load, raised and water through temperature the mixing at responding site PLS were to the240 magnitude mg/L, 9.12 of × the103 mYukon3/s, 2.19 River × 10 sediment3 kg/s, and load. 14.6 °C on 20 June 2010, respectively. The discharge was 19%

Figure 6. Traverse cross-sections of SSC, water temperature, salinity, and marine water density σT by the coastal observation on the B1–B5 line on 6 September 2009 (Figure1).

Figure7 shows longitudinal cross-sections of SSC, water temperature, salinity, and σT along the B6–B10 line by the coastal observation on 22 June 2010. The daily mean SSC discharge, sediment load, and water temperature at site PLS were 240 mg/L, 9.12 × 103 m3/s, 2.19 × 103 kg/s, and 14.6 ◦C on 20 June 2010, respectively. The discharge was 19% larger than that on 4 September 2009, but sediment load was 24% smaller because SSC was 39% smaller than that on 4 September 2009. Then, a boundary between the plume and marine water was not clearly seen near site B3 or farther. Such a sediment plume’s plunging as in Figures4 and5 is, thus, judged not to have occurred. In fact, the SSC in the cross-section was relatively very small at 10–60 mg/L. The SSC was then vertically uniform, being probably due to turbulent mixing by relatively strong river-induced current from the higher river discharge. Hydrology 2021, 8, x FOR PEER REVIEW 8 of 13

larger than that on 4 September 2009, but sediment load was 24% smaller because SSC was 39% smaller than that on 4 September 2009. Then, a boundary between the plume and marine water was not clearly seen near site B3 or farther. Such a sediment plume’s plunging as in Figures 4 and 5 is, thus, judged not to have occurred. In fact, the SSC in the cross- section was relatively very small at 10–60 mg/L. The SSC was then vertically uniform, being probably due to turbulent mixing by relatively strong river-induced current from the higher river discharge. Hydrology 2021, 8, 45 8 of 12 Figure 6. Traverse cross-sections of SSC, water temperature, salinity, and marine water density σT by the coastal observation on the B1–B5 line on 6 September 2009 (Figure 1).

FigureFigure 7. 7.Longitudinal Longitudinal cross-sections cross-sections of of SSC, SSC, water water temperature, temperature, salinity, salinity, and andσ TσTon on the the B6–B10 B6–B10 line byline the by coastal the coastal observation observatio on 22n on June 22 2010June (Figure2010 (Figure1). 1).

4.3.4.3. ExperimentalExperimental ResultsResults forfor YukonYukon Sediment Sediment FigureFigure8 8shows shows a salinitya salinity effect effect on on ( a )(a cumulative) cumulative grain grain size size distributions distributions of the of the Yukon Yu- channelkon channel sediment sediment and and (b) their(b) their mean mean size size and and standard standard deviation. deviation. When When the the salinity salinity µ increasedincreased from from 2 2 to to 5 5‰,‰, the the suspended suspended sediment sediment became became coarse coarse at at grain grain size sized d < < 16.5 16.5 μmm µ withwith increasingincreasing thethe mean size from from 16.6 16.6 to to 19.0 19.0 μm.m. This This means means that that the the differential differential set- settlingtling from from flocculation flocculation or or coagulation coagulation occurre occurredd at at more more than 2‰ 2‰ salinity forfor thethe YukonYukon suspended sediment of d < 16.5 µm [27]. At salinity of 10‰ or more, the grain size did not suspended sediment of d < 16.5 μm [27]. At salinity of 10‰ or more, the grain size did not change greatly with mean size of 17.9 to 18.7 µm. The standard deviation decreased from change greatly with mean size of 17.9 to 18.7 μm. The standard deviation decreased from 13.6 µm at 0‰ to 12.0 µm at 2‰, and then varied at a small range of 10.8 to 12.5 µm at 13.6 μm at 0‰ to 12.0 μm at 2‰, and then varied at a small range of 10.8 to 12.5 μm at 2– 2–35‰. Hence, the addition of salinity to the suspension tends to increase the uniformity 35‰. Hence, the addition of salinity to the suspension tends to increase the uniformity in in grain size of suspended sediment, reflecting the flocculation. This experiment dealt with grain size of suspended sediment, reflecting the flocculation. This experiment dealt with the behaviors of suspended sediment in still water. If a certain shearing by a flow was the behaviors of suspended sediment in still water. If a certain shearing by a flow was given, the flocculation could be more enhanced [28]. given, the flocculation could be more enhanced [28]. Figure9 shows the mineralogy of Yukon River and plume suspended sediments at less than 4 µm in gain size by the XRD method. Such clay minerals as kaolinite, chlorite, smectite, and illite could easily produce the flocculation of suspended sediment in the brackish region offshore from the Yukon River mouth. The flocculation of clay particles induces the coarseness of the river-suspended sediment, which more rapidly accumulates the sediment near the bottom as an agent for the plunging. In fact, the plume-suspended sediment includes few clay minerals, suggesting their gravitational settling or deposition offshore from the delta front. Especially, kaolinite, smectite, and illite are more cohesive to easily flocculate.

HydrologyHydrology2021 2021,,8 8,, 45x FOR PEER REVIEW 9 of 139 of 12

FigureFigure 8. 8. ChangesChanges in in(a) ( acumulative) cumulative grain grain size size distributions distributions and ( andb) correspondent (b) correspondent mean size mean and size and Hydrology 2021, 8, x FOR PEER REVIEWstandardstandard deviation deviation of ofthe the Yukon Yukon Rive Riverr bank bank sediment sediment at grain at grainsize d size≤ 44 μ dm≤ by44 increasingµm by10increasing of the 13 the salinitysalinity from from 0 0to to 35 35 ‰. ‰.

Figure 9 shows the mineralogy of Yukon River and plume suspended sediments at less than 4 μm in gain size by the XRD method. Such clay minerals as kaolinite, chlorite, smectite, and illite could easily produce the flocculation of suspended sediment in the brackish region offshore from the Yukon River mouth. The flocculation of clay particles induces the coarseness of the river-suspended sediment, which more rapidly accumulates the sediment near the bottom as an agent for the plunging. In fact, the plume-suspended sediment includes few clay minerals, suggesting their gravitational settling or deposition offshore from the delta front. Especially, kaolinite, smectite, and illite are more cohesive to easily flocculate.

FigureFigure 9. 9.MineMineralogyralogy of suspended of suspended sediment sediment at grain at grainsize d size≤ 4 μ dm≤ by4 theµm X-ray by the diffraction X-ray diffraction method. method.The sediments The sediments were sampledwere sampled at (a )at site (a) PLSsite PLS and and (b) ( siteb) site B9 B9 in in September September 2009. 2009. θ:: Bragg angle (◦). angle (°).

5. Discussion River-induced plumes released from a river mouth are dynamically dispersed by littoral, tidal, and wind-driven currents, and thereby control the ecosystem and sedimentation in coastal and offshore regions. However, the observational results in Figures 4–7 indicate that, in the estuarine and near-field plume regions, the magnitude of river discharge and sediment load could control the level of mixing between river water and ocean water and the subsequent dynamic behaviors of river-induced plume. The Yukon River has only water density σT= −0.133 at SSC of 391 mg/L, water temperature of 11.7 °C, and EC25 of 27 mS/m (salinity, 0.127‰) at site PLS on 4 September 2009. Thus, the plunging into the lower ocean water as in Figures 4 and 5 cannot occur under pycnal condition between the river water and ocean water. Otherwise, the differential settling from the flocculation of suspended sediment probably produced the nepheloid layer near the bottom in the mixing zone, followed by the underflow. The flocculation shown in Figure 8 could be enhanced by the shearing in the mixing zone and the existence of cohesive clay minerals such as kaolinite, smectite, and illite (Figure 9). The plunging and subsequent underflow in Figures 4 and 5 appear to have been produced on the gentle slope between sites B5 and B4 or sites B6 and B3. By the comparison between Figures 4 and 6, it is seen that the plunging may occur at the Yukon River sediment load of more than ca. 2500 kg/s at site PLS. The sediment load of more than 2500 kg/s Hydrology 2021, 8, 45 10 of 12

5. Discussion River-induced plumes released from a river mouth are dynamically dispersed by littoral, tidal, and wind-driven currents, and thereby control the ecosystem and sedimentation in coastal and offshore regions. However, the observational results in Figures4–7 indicate that, in the estuarine and near-field plume regions, the magnitude of river discharge and sediment load could control the level of mixing between river water and ocean water and the subsequent dynamic behaviors of river-induced plume. The Yukon River has only ◦ water density σT = −0.133 at SSC of 391 mg/L, water temperature of 11.7 C, and EC25 of 27 mS/m (salinity, 0.127‰) at site PLS on 4 September 2009. Thus, the plunging into the lower ocean water as in Figures4 and5 cannot occur under pycnal condition between the river water and ocean water. Otherwise, the differential settling from the flocculation of suspended sediment probably produced the nepheloid layer near the bottom in the mixing zone, followed by the underflow. The flocculation shown in Figure8 could be enhanced by the shearing in the mixing zone and the existence of cohesive clay minerals such as kaolinite, smectite, and illite (Figure9). The plunging and subsequent underflow in Figures4 and5 appear to have been produced on the gentle slope between sites B5 and B4 or sites B6 and B3. By the comparison between Figures4 and6, it is seen that the plunging may occur at the Yukon River sediment load of more than ca. 2500 kg/s at site PLS. The sediment load of more than 2500 kg/s can be recorded in July–September (Figure3), when the suspended sediment contains more fine-grained particles supplied by the glacier-melt and rainfall runoffs [20]. A two-dimensional hydrodynamic condition of the underflow in Figure4 is judged by using the densimetric Froude number Fd and Reynolds number Re in the following:

1/2 Fd = U/(g’Hp) , (1)

Re = UHp/ν, (2)

where U is the underflow’s velocity (m/s), Hp is the water depth (m) at the plunging 2 point, g’ = g (ρ–ρ0)/ρ0 (g: acceleration due to gravity in m/s , ρ: underflow’s density in 3 3 2 kg/m , ρ0: ambient water density in kg/m ), and ν is the kinematic viscosity (m /s). 3 3 From the σT distribution in Figure4, ρ = 1025 kg/m , ρ0 = 1020 kg/m , Hp= 7.5 m, and ν = 1.32×10−6 m2/s for salinity at 25‰ and water temperature at 10.6 ◦C gave 4 5 Fd = 0.017–0.17 and Re = 5.7 × 10 –5.7 × 10 in Equations (1) and (2). Then, U = 0.01–0.1 m/s. Similarly, for the underflow downstream of the plume boundary in Figure5, 3 3 −6 2 ρ = 1022 kg/m , ρ0 = 1015 kg/m ,Hp= 4.5 m, and ν = 1.33 × 10 m /s for 27‰ salinity ◦ 4 5 and 10.6 C water temperature, which gave Fd =0.018–0.18 and Re = 3.4 × 10 –3.4 × 10 at U = 0.01–0.1 m/s. Thus, the underflow may be subcritical and turbulent, indicating the low entrainment from the ambient water above. This indicates that, in spite of the small scale, the underflow could flow down in a relatively long travel distance. When the river discharge increased and the sediment load decreased, the mixing zone was extended offshore as in Figure7, where the plunging was not observed. As far as seeing the RGB satellite images of the Yukon sediment plume (Figure 10) on the dates closest to those of the coastal observations, the extent of the surface plume appears to be much larger on 4 September 2009. The Yukon River surface plume could be wholly advected northwestward by the “Alaskan Coastal Water” [21] (Figure 10). In Figures4 and5, there was no turbid surface layer in the location of the plunging and subsequent underflow formation. Thus, the liftoff of the sediment plume into the surface layer is considered to have occurred in more northeastern regions, where the surface sediment plume is extended northward (Figure 10a). Hydrology 2021, 8, x FOR PEER REVIEW 11 of 13

can be recorded in July–September (Figure 3), when the suspended sediment contains more fine-grained particles supplied by the glacier-melt and rainfall runoffs [20]. A two-dimensional hydrodynamic condition of the underflow in Figure 4 is judged by using the densimetric Froude number Fd and Reynolds number Re in the following:

Fd = U/(g’Hp)1/2, (1)

Re = UHp/ν, (2)

where U is the underflow’s velocity (m/s), Hp is the water depth (m) at the plunging point, g’ = g (ρ–ρ0)/ρ0 (g: acceleration due to gravity in m/s2, ρ: underflow’s density in kg/m3, ρ0: ambient water density in kg/m3), and ν is the kinematic viscosity (m2/s). From the σT distribution in Figure 4, ρ = 1025 kg/m3, ρ0 = 1020 kg/m3, Hp= 7.5 m, and ν = 1.32× 10-6 m2/s for salinity at 25‰ and water temperature at 10.6 °C gave Fd = 0.017–0.17 and Re = 5.7 × 104– 5.7 × 105 in Equations (1) and (2). Then, U = 0.01–0.1 m/s. Similarly, for the underflow downstream of the plume boundary in Figure 5, ρ = 1022 kg/m3, ρ0 = 1015 kg/m3, Hp= 4.5 m, and ν = 1.33 × 10−6 m2/s for 27‰ salinity and 10.6 °C water temperature, which gave Fd =0.018–0.18 and Re = 3.4 × 104–3.4 × 105 at U = 0.01–0.1 m/s. Thus, the underflow may be subcritical and turbulent, indicating the low entrainment from the ambient water above. This indicates that, in spite of the small scale, the underflow could flow down in a relatively long travel distance. When the river discharge increased and the sediment load decreased, the mixing zone was extended offshore as in Figure 7, where the plunging was not observed. As far Hydrology 2021, 8, 45 as seeing the RGB satellite images of the Yukon sediment plume (Figure 10) on the dates11 of 12 closest to those of the coastal observations, the extent of the surface plume appears to be much larger on 4 September 2009.

FigureFigure 10. RGB 10. (Red-Green-BlueRGB (Red-Green-Blue color model) color comp model)osite images composite of MODIS/Aqua images of MODIS/Aquaon (a) 4 Septem- on (a) ber 20094 September and (b) 22 2009 Juneand 2010. (b The) 22 red June rectangles 2010. The show red rectanglesthe area of showthe coastal the area observations of the coastal on 6 observa- Septembertions on2009 6 Septemberand 22 June 2009 2010. and The 22 Junelabel 2010.“PLS” The shows label the “PLS” location shows of thethe locationUSGS gauging of the USGS station gauging at the stationPilot Station at the village. Pilot Station village.

The6. Conclusions Yukon River surface plume could be wholly advected northwestward by the “Alaskan CoastalBy the observations Water” [21] of(Figure Yukon 10). River In sedimentFigures 4 plumesand 5, there in the was coastal no regionturbid ofsurface the Bering layer Sea,in the it location was found of the out plunging that, at Yukon and subsequent River sediment underflow load offormation. more than Thus, ca. 2500the liftoff kg/s, the of theplume sediment plunges plume at into a boundary the surface of the layer plume is considered and ocean to water. have Theoccurred production in more of north- the bottom easterncurrent regions, or underflowwhere the surface from the sediment plunging plume is probably is extended due tonorthward the differential (Figure settling 10a). from the flocculation of river-suspended sediment. Both the suspended sediment concentration (SSC) and sediment load of the Yukon River were relatively high in the glacier-melt and rainfall runoffs of July–September. In such runoffs, suspended sediment becomes relatively fine, including clay minerals such as kaolinite, smectite, chlorite, and illite to easily make flocs in the brackish zone. Hence, temporal variations of glacier-melt and rainfall sediment runoffs of the Yukon River could change behaviors of the sediment plume in the coastal region. It was experimentally demonstrated that the flocculation of suspended sediment occurs at 2‰ or more salinity. This flocculation may produce the underflow in the deeper zone by accumulating the flocculated sediment on the onshore gentle slope. It should be needed to experimentally and theoretically clarify the mechanism of the underflow’s production from the flocculation.

Author Contributions: K.A.C., T.W. and I.K. participated in all the filed surveys to set and manage field instruments; K.A.C. wrote the paper; S.-I.S. conceived and designed the Yukon River—Bering Sea research project; M.T. analyzed many images of MODIS/Aqua. All authors have read and agreed to the published version of the manuscript. Funding: This study was financially supported by the Japan Aerospace Exploration Agency (JAXA) as a research in the joint project of IARC/JAXA. Acknowledgments: We appreciate the official support of Emeritus S. Akasofu, L. Hinzman and Y. Kim, the International Arctic Research Center (IARC), the University of Alaska at Fairbanks (UAF) and the welcome data supply of the U.S. Geological Survey. Conflicts of Interest: The authors declare no conflict of interest.

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