Suspended sediment dynamics in Blue Fjord, western Prince William Sound, Alaska

Estuarine and coastal Marine Science

(1978)

7, 1-16

Suspended Sediment Dynamics in Blue Fjord, Western Prince William Sound, Alaskaa

Charles M. Hoskin, David C. Burrell and Gary R. Freitag Institute of Marine Science, University U.S.A. Received 83dy

of Alaska, Fairbanks, Alaska 99701,

I976 and in revised form 4 August I977

Keywords : sedimentation glacial sediments;

rates; sediment transport; suspended sediments ; sediment traps; fjords; light transmission; Alaska Coast

Glacier meltwater and suspended sediment discharge in Blue Fjord occurs over a brief s-month period in summer. Suspended sediment concentrations in the meltwater stream reach 300 mg 1 -I, and this sediment forms a surface turbid plume at the fjord head. Suspended sediment concentrations in the surface plume range from 200 mg 1 -i at the head to a few mg 1 -i 5 km away at the mouth. Turbidity does not seem to be related to density structure of the water column. Suspended sediment sinks through the water column with most sediment settling at slack low water. Sediment trap measurements show an April sediment flux of 1.5 mg dry sediment cm -* day -l at the head and 0.75 mg cm -a day --t at the mouth (mostly diatom frustules). September trap measurements yield a sediment flux of 53 mg cm --I day -i at the head and 2 mg cm --B day-l at the mouth (mostly detrital inorganic silicates in the mud size range). Bottom sediment in the fjord basin is mostly mud, with an admixture of sand at the fjord head. Grainsize modes decrease from an average of 46 grn at the head to 8 gm 2 km away; no trend is discernable for sediments in the outermost 4 km of the fjord basin. Mud accumulates in the fjord at the rate of about IOO mm/meltwater year at the head, IO mm year --I in mid-fjord, and 4 mm year -l in the 190 m basin inside the sill at the fjord mouth.

Introduction Coastal embayments

of Port Nellie

Juan in western Prince William

Sound represent

an

evolutionary series. One end-member is Derickson Bay, dominated by icebergs from Nellie Juan Glacier. Four km east is Blue Fjord in which Ultramarine Glacier no longer has a tidewater

terminus

and glacier meltwater

and suspended sediment discharge into the marine

environment as surface plumes. Nine km further east is McClure Bay, which has no glacier. The crumbling West Gable cannery at Port Nellie Juan attests to the previous richness of clear-running salmon streams at the head of McClure bay. The purpose of this paper is to report on the sources, dispersal pathways, depositional sites, and rates of sediment accumulation for Blue Fjord, which is part-way along this series. “Institute of Marine Science Contribution

No.

339

I 0302-3524/78/0501-0001

$02.00/0

@ 1978 Academic Press Inc. (London)

Ltd.

2

C. M. Hoskin, D. C. Burrell &f G. R. Freitag

Setting Blue Fjord is located in the Chugach National Forest, western Prince William I 16 km southeast of Anchorage. Blue Fjord is 6.9 km long, 0.48 km wide 1.6 km wide at mid-length, and 0.97 km wide at the mouth (Figure I). Local Jur-Cretaceous graywacke of the Valdez Group (Case et al., 1966), mostly spruce forest. The head of Blue Fjord is an alder-covered outwash fan of Ultramarine

Sound, about at the head, rock is the covered with Glacier.

60031’

60°29

60”28

60”27’

60°26

0 0'

60925’

I

I

I

Figure I. Index map, bathymetry,

cm

0.92

n.mile.s0.5

and location of sediment sampling stations.

The

Suspendedsedimentdynamics, WesternPrince William Sound

glacier snout lies about 1-5 km up the valley, and is the main source of freshwater input to the fjord. The intertidal portion of the outwash fan terminates at its seaward edge in a slope of 17” to the fjord floor. The rocky west wall is near-vertical whereas the east fjord wall is more gently sloping. The fjord floor slopes continuously seaward from the toe of the slope at the fjord head (65 fm, 119 m) to a flat basin (104 fm, 190 m) inside the sill; this basin occupies about 10% of the fjord floor area. There is a sill at the mouth, cresting at 115 m (63 fm) below sea level, with 75 m (41 fm) of relief. Outside the sill, the bottom falls away steeply to 512 m (280 fm) and flatter areas 5.5 km to the north. There are some rough areas in the mid- and inner parts of Blue Fjord with relief of about 25 m (13 fm). There is a large slump at the head, and at least one smaller slump along the east wall near the mouth; these are interpreted to result from the 1964 Alaska earthquake (epicenter 80 km to the northeast, Plafker, 1971). The contour of zero uplift parallels the east shore of Blue Fjord, with sinking of 0.3 m (from displaced Fucu..r)measured near the mouth (Plafker, 1971). Wind and wave energy appears to be relatively unimportant. Measurements during the period 13-19 September 1973 from a 6 m tower erected on the outwash fan show winds blow from the southwest 71 y. of the time at an average velocity of 2.1 m s-l. Average and maximum range of the tide is 2.95 and 5.5 m, respectively, at Culross Bay, 24 km north of Blue Fjord (U.S. Department of Commerce, 1975). The average annual rainfall is 4’1 m, there are an average of 150 wet days/year, and the average annual snowfall is 5.1 m (Johnson & Hartman, 1964).

Methods Streams Determinations of suspended sediment concentration were made by passing approximately 0.25-l water samples through tared 0.4 pm Nuclepore filters. Stream discharge measurements were made with a Price current meter and a depth-integrating suspended sediment sampler (USDH 48 1548). Suspendedsediment in the water column A Martek XMS transmissometer and DMS depth sensor were used to trace suspended sediment distribution through the water column. Output of the XMS was recorded on a strip-chart recorder with depth information added at 2-m intervals by a manually actuated event marker. The XMS-DMS sensors were attached to the underside of a Bissett-Berman STD cage; a typical cast was to record transmissivity on the down leg, and STD data on the up leg. Using the strip-chart record, depths were selected for sampling with 5-l Niskin bottles and another cast was made, following the first within 15 min. Suspended sediment concentration at these selected depths was determined by passing 0.25-4.0 1of water through tared 0.4 pm Nuclepore filters, followed by a wash of the filter in distilled water, drying at 37 “C, and reweighing. The volume of water filtered was determined by how much could pass the filter before it became clogged. Precision of this method from IO surface casts at one station 0.2 km off the west meltwater stream mouth was determined to be 35*3--41.1 mg-l, 8=38.5f2.0 mg l- 1. Reproducibility of filtering for IO replicates from one Niskin bottle was f4% for a suspended sediment concentration of 20 mg 1-r. Using these data, transmissivity records were calibrated in terms of suspended sediment concentration in the range 0*2-10.0 mg 1-r. Sediment concentration data reported here are coded (F) for filter and (T) for transmissivity to indicate the method of determination.

4

C. M. Hoskin, D. C. Burrell & G. R. Freitag

Sediment traps We wished to identify those places where suspended sediment became bottom sediment, and to measure the rate of this transfer. As no standard trap design was known to us, we made two experiments to evaluate traps of our own design. Simple plastic cylinders open at the top and closed at the bottom, were attached to a line supported at the surface by a float and held taut by a weight, well off the bottom. A line and anchor attached to this weight secured the trap array from drift. Measurements in Blue Fjord over a four day period showed no difference in sediment capture for cylinders 025 and 0.5 m long. To test effects of trap diameter, wood frames were built to support cylindrical traps 0.25 m long, with diameters of 2.9,26.2,47*0 and 241.0 mm. These frames were attached to moorings as described above. Measurements in Reid Inlet, Glacier Bay National Monument showed that traps of 47.0 mm yielded the most consistent results, and therefore, this size was used in all subsequent measurements. We interpret the smaller amount of sediment captured in the 241.0 mm trap to be due to scour, and the smaller catch in the 2.9 mm trap due to the size of flocculated sediment being larger than this. The amount of sediment caught in traps may seriousl! exceed the amount accruing to the fjord floor. We will evaluate this, and the problem of resuspension in a later section. The proximate source of sediment caught in traps used in lakes and estuaries elsewhere has been resuspended sediment (Davis, 1968; Young, 1971; Gviatt & Nixon, 1975). Hakanson (1976) used sediment traps to show that the rate of sediment accumulation in Lake Ekoln, Sweden, decreased away from the mouth of the River Fyris.

Seasonal hydrography The deep fjord-estuaries of south-central Alaska have characteristic circulations both at the surface and within the basins below sill depth. Because of the overall control on sediment dispersion in the fjords by the hydrography, it is necessary here to describe briefly these patterns in general, and in Blue Fjord in particular. In the general case, water circulation at the surface is normal gravitational entrainment flow during the summer with a (relatively shallow) up-fjord counter current as described by McAlister et al. (1959). In Blue Fjord, however, the maximum freshwater inflow is only a few per cent of the tidal prism and no longitudinal salinity gradients have been observed. Winds tend to be channeled directly up or down the fjord axis, either augmenting or attenuating the surface outflow; the prevailing winds during the winter months are predominantly down-fjord. Freshwater inflow is seasonal, ceasing entirely during the winter months. The pattern of freshwater discharge into the Prince William Sound region appears to be basically unimodal with peak flow in early summer (Carlson et al., 1969). Little quantitative information is available, however. The situation in Blue Fjord is considered further below. The resultant marked seasonal surface density structure, augmented by intense surface cooling during the winter, is illustrated in Figure 2, which is a time series plot for a station located immediately outside the entrance sill of Blue Fjord. The water column approaches homogeneity at middepths during the oceanographic winter months (January-June) but the available evidence for 1972-1973 (Figure 2) does not support massive thermohaline convection as was described in nearby Valdez Arm during the previous (more severe) winter by Muench & Nebert (1973). Water structures deeper within the fjord basins are strongly influenced by events occurring on the contiguous shelf. Royer (1975) has described circulation patterns developed in the Gulf of Alaska and the major features may be briefly reiterated here. During the winter,

Suspended sediment dynamics, Western Prince William Sound

S

surface waters are transported coastwards by the prevailing Aleutian low pressure system and a downwelling condition exists at the margins. These pressure patterns relax during the summer, however, and sub-surface, denser Gulf waters are upwelled onto the shelf and into the outer reaches of the coastal inlets. This general cyclic pattern is clearly evident in Figure 2. Subsequent impact within the fjord basins is largely a function of the depth of the respective barrier sill (Muench & Heggie, in press). As illustrated in Figure 2, the deep density maxima approximately coincide with summer (September) surface minima which are the result of local run-off and insolation, and the minimum density gradient zone is

JMMJJNJMMJ

Figure 2. Sigma-t time series (November 197z-July 1974) for a station located immediately outside the Blue Fjord entrance sill.

(BF-0)

centered around the ISO m depth. This is very close to the Blue Fjord sill depth and large and relatively rapid vertical excursions would be expected, and have been observed (Figure 3). At certain times of the year denser source water may be advected over the sill and retained within the basin but, because of the relationship between sill height and density structure noted above, the effect on the ambient water in Blue Fjord is less marked than in some other Alaskan fjords. The signatures of several such influxes are apparent over the period illustrated by Figure 3 (December rgya-August 1974) although the sampling intervals are too Oh

43 ---

I-----

Figure 3. Longitudinal section Blue Fjord (A) to within Prince late November 1972, and the distance A-B is approximately

6- -

showing temperature (“C) structure from within William Sound (B). The continuous line represents broken line represents early February 1973. The 75 km.

6

C. M. Hoskin, D. C. Burrell &7 G. H. Freitag

coarse to characterize influx rates or residence times. Figure 3 shows a core of cold (<4 “C) water outside the sill in early December 1972. This water appears also to have been previously advected into the inner basin of Blue Fjord and is apparent inside the sill at this date. By late January (1973) the pronounced temperature signature is absent and only a remnant (<4*25 “C) remains in the outer basin at around zoo m. Suspended sediment dynamics Input

Meltwater from Ultramarine Glacier in the Sargent Icefield is the main source of freshwater and sediment input to Blue Fjord. Gaddis (1974) h as shown that meltwater and suspended sediment discharge from Wolverine Glacier occurs during the period June through September. The gauging station for Wolverine Glacier is 35 km west of Blue Fjord at an elevation of 366 m (Guymon, 1974) and affords the closest available data. Water and suspended sediment concentration from Wolverine Glacier appear to correlate with rainfall (Gaddis, 1974). Carlson et al., (1969) show a unimodal discharge pattern for Port Valdez in northeastern Prince William Sound, peaking in early summer. Glacier meltwater streams in Norway have two periods of increased suspended sediment concentration, the first in early June due to large volume glacier meltwater discharge, and a second about the end of July. Curiously, the period of most rainfall occurs in between (0strem et al., 1969). The melt season at lower elevations in Blue Fjord probably is longer, but it is at best 5 months long, with a period of seven months at minimal flow with virtually no sediment input to the fjord basin. Measurements on 22 April 1974 in Blue Fjord show water discharge of 0.15 m3 s-l with an average suspended sediment concentration of 2.0 mg l-r, yielding a sediment discharge of 1073 g h-l. In early June 1974, water discharge visually increased (no measurements) and the average suspended sediment concentration was 103.5 mg 1-l. On 23 September 1975, water discharge was 10.2 m3 s-l with an average suspended sediment concentration of 300 mg l-l, yielding a sediment discharge of II x 10~ g h-r. In contrast, September 1973 suspended sediment concentrations were much lower; 28.9 mg 1-l near the glacier snout, and 23 and 19 mg 1-l in the east and west stream mouths, respectively. September 1975 was rainy, and September 1973 was dry; rainfall data for Blue Fjord are not available. Recording stream gauges and suspended sediment samples should be obtained over the course of several years to indicate water and sediment discharge for a typical year; these data are not available. Meltwater streams from snow banks contain small amounts of suspended sediment, and measurements from eight of these streams on 8 June 1974 show the average suspended sediment concentration to be 0.3 mg l- l. Collapse of snow cornices generates avalanches, and inspection of eroding edges of these snow slides at tidewater in April 1974 showed organic debris from vegetation, but no inorganic particles. Our data are instantaneous measurements, and because of known hour-by-hour fluctuations in sediment discharge of glacial meltwater streams (Rainwater & Guy, 1961; Figure 7, p. 9) we chose units of g h-l. For comparison, I x 10~ g h-r is equivalent to 24 tonnes day-l. Dispersal of suspended sediment. In April,

Blue Fjord waters are clear and the bottom is visible to IO m. Profiles in the upper 80 m, measured along the fjord from head to mouth on 24 April 1975 indicated suspended sediment concentrations in the range 0.2-0.7 mg 1-r (F). The only exception was a turbid layer at about 15 m with a suspended Seasonal variations

Suspendedsedimentdynamics, WesternPrince William Sound

7

particle concentration near 1-5 mg l-1 (F).The turbid layer at 15 m was plankton, as determined by inspection of the filters. By June glacier melt was active and surface suspended sediment concentrations were in the range 13-50 mg l-1 (F) for the inner fjord, about 08 km off the mouths of the glacier meltwater stream. At 1.6 km off the stream mouths, surface suspended sediment concentrations were 4 mg 1-l (T), and just inside the sill, surface suspended sediment concentrations were 2.83~6 mg 1-l (F). By September, surface suspended sediment concentrations produce visibly turbid water which moves up and down the fjord with the tides. Programmed water samplers (Isco Model 1391) were used to obtain data for suspended sediment concentrations near the head of Blue Fjord. These programmable, battery-powered samplers were placed in moored rubber rafts, and hourly samples collected. Pump intakes were positioned at 0.3 m and 6.1 m. Decreasing sediment concentrations were found at both depths over the period 18-19 September 1975, but with no clear relation between suspended sediment concentration and the tide. The suspended sediment concentration at 0.3 m (250-100 mg 1-l) was much greater than 6.1 m (so-15 mg l-l), showing that little suspended sediment supplied at the surface settles past 6 m at this place in the fjord. Suspended sediment in the water column. Using the transmissometer at an anchor station 0.2 km off the west mouth of the glacier meltwater stream, we found turbid water at the surface and near-bottom, with clearer water at mid-depths (Figure 4). Examination of the filters used to determine the suspended sediment concentrations showed the bulk of the particles to be inorganic. Transmissivity profiles at a station 0’9 km (Figure 5) and 2 km from the glacier meltwater stream mouths showed the surface water to be turbid, with smoothly-decreasing amounts of suspended sediment at greater depths (to the limit of our cable, about 80 m; maximum depth of the inner fjord was about ISO m). No relationship has been found between turbidity in Blue Fjord water (transmissometer profiles and filtered water samples) and the density structure of the water column (a, from STD data) although this has been reported for the continental shelf and submarine canyons by others (Drake, 1971; Drake & Gorsline, 1973 ; Harlett & Kulm, 1973). The anchor station at the head of Blue Fjord (Figure 4) showed water with the greatest transmissivity (60% m-l) between IO and 15 m. It may be that this relatively clear water represents a tongue of more saline marine water entering on the flood tide. Temperature structure of the water column for the inner fjord shows a strong gradient from 2 to 3 “C at the surface to 12 “C at 4 m, a IO m thick layer with temperatures between II and 12 “C, a smooth decrease from II “C at 15 m to 4 “C at 55 m, with 4 “C water continuing to the bottom. Salinity increased from 26x,, (1ower limit of our STD sensor) at IO m to about 31%~ at 55 m, with a more gradual increase to 32x,, at the bottom (Figure 6). To summarize, fresh, cold, turbid glacier meltwater enters the fjord at the surface and dominates the top few meters. A zone of mixing extends downward to about 55 m, and marine water occupies the rest of the water column. Current speed and direction time-series measurements off the east glacier meltwater stream mouth seem to support the hypothesis of tidal control for the mid-depth low-turbidity layer (Figure 7). Flow directions in the upper 50 m are generally north and towards the fjord mouth (discounting surface measurements influenced by the wind). There are too few measurements in the less turbid water below 50 m to further test this idea; if correct, flows at this depth and below should be to the south on the flood tide. Sites and rates of sediment accumulation Variation of sedimentjlux with seasons.Sediment traps were moored in Blue Fjord for 94 h

8

C. M. Hoskin, D. C. Burrell U G. H. heitag

0 I

Ln I

0I

M I

s I

-~.-

..__-_-.

Suspendedsedimentdynamics, WesternPrince William Sound

9

C. M. Hod%,

IO

D. C. Burrell & G. 1-Z.Freitag

in late April 1975. Inspection of the sediment recovered on 0.4 pm Nuclepore filters showed the sediment to be mostly diatoms. These are interpreted to have settled from suspension rather than to have grown in the traps as the sides of the traps were clean and free of encrusting films of organisms when recovered. Plankton from the 15 m level (transmissometer data) probably contributed most of the sediment caught; there were a few sand grains in both surface and near-bottom traps, possibly transported from beaches to the fjord basin on the surface film. Near-bottom traps of the inner fjord 0.4 km off the mouths of the glacier meltwater stream-almost no flow at this time of year-captured about 1.5 mg dry sediment cm -2 day-l and traps 5.5 km away in the deep basin inside the sill captured 0.75 mg cm-’ day-l. 2 3 III 25 O1

4

26

Temperature PC) 5 6 7 8 9 IO II 12 13 ,,,I,,, I1 Salinity (%.I 27 28 29 30 31 32 3

- 60IT ‘; o 80cl 100 120 14o=J---18 19

20

21 22 23 24 25 Sigma - T Cruise 178 Statton :9

Figure 6. Profiles of salinity, from the head of Blue Fjord.

1007

--

80

i

temperature and water density (or) at a station 0.9 km STD sensors did not provide data shallower than IO m.

Local 1044

F load --~~ time 1215

- ---++Ebb 1320

A?7 t Figure 7. Time series (approximately hourly) measurements of current speed and direction station 0.4 km off the east mouth of the meltwater at the head of Blue Fjord, 25 September 1975.

14-20

15.20

--+ IE

50 40 30 20 IO %Trons/m

i

80 1

of optical transmittance/m with (at solid circles) for an anchor stream from Ultramarine Glacier

= or.

lat 27.0 l6-25Sept.

28.0 16-25Sept.

29.0 30.0

31.0

Figure 8. Sediment flux measured with sediment traps for melt season, Blue Fjord, 25 September 1975.

20-25Sept.

IIIIIIIIIIIllllllllllllllllllllllllllllllllllllllll

60°26,0’N

- - -

12

C. M. Hoskin, D. C. Burrell & G. R. Frcitag

Identical traps were moored in outer Blue Fjord in late September 1975 for 215 h, and for IZI h in the inner fjord (Figure 8), and for still shorter periods near the mouths of the glacier meltwater streams. Near-bottom traps moored at the toe of the slope at the fjord head yielded 53 mg dry sediment cmV2 day-l, mid-fjord traps caught 5.5 mg cmm2 day-’ and traps in the basin adjacent to the sill caught z mg crnm2 day-i. The amount of sediment caught in the near-bottom trap for the inner fjord in September is 34%~ that for April, indicating the magnitude of the seasonal supply of sediment particles. Source of sediment. Data in Figure 8 indicate most sediment is supplied to surface water at the head of Blue Fjord. Sediment caught in traps at 0.3 m in the mouths of the glacier meltwater stream is consistently greater in amount than in traps at 6.1 m. From head-tomouth in the fjord basin, more sediment is caught in near-bottom traps than in traps suspended at I m, but not dramatically so. This observation suggests that bottom flows, slumps and resuspension by the tides were not important suppliers of sediment for these times of measurement. The sedimentation rate (as measured by traps) along the axis of the fjord decreased exponentially away from the sediment source at the head of the fjord. A comparison of surface and near-bottom traps for September 1975 (Figure 8) shows nearly equal capture rates, indicating that sediment falls through the water column. illthough not satisfactorily resolved, it appears that little inorganic sediment enters the fjord basin from outside the sill; if importation was significant, a reversal in the gradient of sediment capture rates would have been found. Tidal effects. Sediment captured in traps in Reid Inlet, Glacier Bay, was seen to have alternate dark and light layers. These traps were in operation 5 days, and there were five pairs of sediment layers visible through the transparent walls of the trap. This suggested a relationship between sediment supply and the tides. To explore this, an Isco automatic water sampler was deployed to recover suspended sediment in the glacier meltwater stream above the fork and tidal influence, and about IO min flow time upstream from the stream mouths. Two other Isco samplers were placed in a rubber raft moored in the east mouth of the glacier meltwater stream. A wooden frame was used to position the pump intakes at 0.3 m. One sampler was used to monitor the suspended sediment concentration at 0.3 m The other Isco sampler in the raft had the pump intake connected to the bottom of a 47.0 x 250 mm sediment trap. All Isco samplers were programmed to take hourly samples. Results of this sampling showed that there was a diurnal variation in the suspended sediment concentration of the glacier meltwater stream, although this was modified by episodes of rain during the sampling period. Measurements of water and sediment discharge are not available for this time. Suspended sediment concentration at the location of the sediment trap varied between 25 and 250 mg l-r, with an average value of about 140 mg 1-l. The hourly increments of sediment caught in the trap varied between 25 and 160 mg h-l; with peak rates of sediment capture corresponding fairly well with times of low tide. The sum of the hourly increments of sediment caught and pumped from this trap was 1.960 g, yielding a sediment flux of 4.7 mg dry sediment cm -2 h-l (corrected for sediment from turbid water pumped to flush the trap at each sampling time). An ordinary 47.0 x 250 mm sediment trap was also positioned at 0.3 m at this place and for this sampling period. The sediment flux yielded by the ordinary trap was 6.2 mg cmm2 h-l, a reasonably close match to the incremental trap. The variability of sediment flux at this place was determined to be about *20% several days earlier. Bottom sediments There are four areas where sediment

has accumulated

in Blue Fjord;

the outwash fan at

Suspended sediment dynamics, Western Prince William Sound

I3

the fjord head (9 samples), beaches along the eastern side (7 samples; there is only one beach on the western side), the fjord basin (26 samples), and the sill (2 samples). No samples were collected for steep slopes of the fjord walls. The following data are for sediments at or near the water-sediment interface (see Figure I for locations). Outwashfan. Eight of these samples are gravel (grainsize names after Folk, 1954) and one is silt. The most common particle size in the gravels is near 16 mm (5 samples) followed by coarse silt of 53 l.trn (4 samples). Beaches. Six samples are gravel, one is mud. No recurrent grainsize mode (the most abundant

particle size) is discernable. bask. All 26 samples have grain-size distributions that place them in the mud range; three are silts, six are clay, and 17 are mud. The single most abundant particle size is coarse silt of 53 l.trn, with the next smallest abundant particles clustering in the range 4’7-9.3 l.un. From head-to-mouth there is an overall decrease in the mean grainsize modes, At the head, the mean grainsize mode is 46 l.trn, 0.4 km seaward the mean is 21 pm, 0.9 km further seaward it is 9 pm, and in another 0.65 km seaward (1.9 km from the fjord head) the mean grainsize mode is 8 l.tm. Geographic distribution of grainsize modes in the outer 4 km of the fjord basin is more variable, and a simple gradient is not obvious. Fjord

Sill. Two pipe dredge samples from the sill crest are mud, with grainsize modes at 5.5 and ~2.8 l.trn. These sill crest sediments contain slightly more gravel and sand than sediment inside the fjord basin. Mud budgets The necessary data are too incomplete to construct a rigorous budget for one meltwater year, but the influence of the meltwater stream can be shown, and attention drawn to exchange across the sill. It is necessary to assume that the sediment trap data can be applied to large areas in the fjord basin, that insignificant amounts of sediment accumulate on the steeplysloping fjord walls, and that resuspension of sediment by the tides is unimportant. Gravel and sand, supplied as bedload from the glacier meltwater stream are excluded here. Current flow velocity and direction profiles were measured on the sill crest at times midway between tide stages in an attempt to evaluate exchange of water with Prince William Sound. These data showed that flow was out of Blue Fjord from the surface down to about 91 m (total volume out was 9800 ms s-l) and flow direction was in from 91 to IIS m for a total volume in of 200 ms s-l, midway between ebb and flood tide. At mid-flood tide, flow in to Blue Fjord was found between 27 and 71 m (total volume in was 2600 ma s-1, with flow direction of the remainder of the water column out, for a total volume out of IO 400 ms s-l. As volume of flow at mid-ebb and mid-flood was IO ooo and 13 ooo ms s-l, respectively, and freshwater flow from the glacier meltwater stream was only IO m3 s-1, the flow volumes are obviously not time-symmetrical with respect to the tides. Thus timeseries measurements are required for rigorous analysis of flow. The volume of flow from snow-melt streams and other surface runoff was not measured, but probably does not make up the volume difference for flow in and flow out of nearly 3000 m3 s-l. Using data for suspended sediment concentration distribution within the water column of the basin inside the sill (M.-L. Lee, personal communication), calculations show 24 x 106 g h-l suspended sediment transported on the ebb flow of 21 September 1975. The ultimate

C. M. Hoskin, D. C. Burrell &f G. R. Freitag

fate of that sediment is unknown; some probably settles from suspension as tides transport turbid water in and out of Blue Fjord, and some is exported to Prince \Villiam Sound as seen in conventional aerial photographs and LANDSAT imagery. April 1975. Eased on the distribution of sediment trap moorings, the fjord basin was divided into an outer area of 4.9 x 10~ m2 (890;; of the total) and an inner area of 0.59 x 10~ m2. Extrapolating measurements from the sediment traps gave the flux for each area, and summing these gave the total sediment accumulation for the fjord basin. For April 1975 this was 1.85x106g h-l; input from the glacier meltwater stream was 1.07 x 10~ g h-l, and inorganic particle exchange across the sill was unknown, but believed to be small. The inner fjord accumulated 17y:~ of the fjord basin total. Glacier meltwater stream input was 0.06~~ of the amount accumulated, reinforcing the importance of the biogenic component (diatom frustules) in Spring, prior to beginning of glacier melting. September 1975. With better distribution of sediment traps, the fjord basin was divided into three areas. Total sediment accumulation was 20.88 x 10~ g h-l, input from the glacier meltwater stream was I I x 10~ g h-l, and exchange across the sill was unknown. The inner I I o/0of the fjord total area received 63 o/Oof the sediment accumulated, which emphasizes supply from the glacier meltwater stream. The outer 44”/ of the total fjord area received only 100/o of the total sediment accumulated. Measured glacier meltwater stream input was 53% of the total sediment accumulated. Many more measurements for the glacier meltwater stream discharge and sediment flux are required before meaningful evaluation can be made of the apparent discrepancy between stream input and total fjord basin accumulation. Relationship betwee? layering and rate of sediment deposition. Sedimentary structures were not obvious in any of the cores from Blue Fjord basin, except core 016, which is a 2-m thick graded bed with zs-mm pebbles (mud free) at the bottom, grading upwards to sandy mud at 15 cm below the top of the core. There is an apparent interruption in the trend of increasing mud content between 65-90 cm. We believe this graded bed was deposited by a turbidity current generated by a slump during the 1964 Alaska earthquake. If so, the top of this graded bed provides a dated horizon, useful as an independent check on sedimentation rates extrapolated from sediment trap measurements. Gravity cores taken at the toe of the steep slope near the head of Blue Fjord (60” 26.2’ N Lat.) in April 1974 yielded about 900 mm of mud overlying a sand layer, and, although longer cores were visible inside the core liner they consistently separated at this sand layer when lifted out of the water. A piston core (016) taken at the same station as the gravity core recovered 2100 mm of sediment, with clean gravel at the bottom, and a sand layer near the top. These core recoveries were duplicated on several successive casts. We interpret this to mean that in 1974, mud was present at the water-sediment interface and downwards for about 900 mm to a sand layer. The piston corer, being much heavier and difficult to adjust so as to just catch the water-sediment interface, in fact penetrated this surface layer of mud before the winch could be stopped. When stopped, the winch line restrained the piston at about 900 mm below the water-sediment interface; the piston corer thus recovered the 2-m thick graded bed beneath the overlying mud which was recovered by the gravity corer. If this interpretation is correct, and if the top of the graded bed is the top of the 1964 slump event, then the overlying 900 mm of mud represents IO years of sediment accumulation, or an average of 90 mm year-l. Sediment traps gave 12 mm of wet sediment in 215 h for September 1975, equivalent to 1.34 mm day-l at the head of Blue Fjord. It is possible to extrapolate these daily sedi-

Suspended sediment dynamics, Western Prince William Sound

IS

mentation rates for a meltwater year. Gaddis (1974) found that for Wolverine Glacier, about 20% of the total suspended sediment discharged in a meltwater year occurred in September. Therefore, 1.34 mm day-l x 30 days=qo*2 mm (or one fifth of the total) x 5= 201 mm/meltwater year, which is about 2.23 x that predicted from the core data. As this sediment is mud, considerable compaction due to gravity will have occurred. Assuming 50% of the deposited thickness is water which will be lost to compaction (no data are available for this place), the accumulated mud thickness per meltwater year would be about IOO mm, not far from the go mm year-l predicted. Converting sediment weight per unit area caught in sediment traps per 24 h to sediment thickness (corrected for 50% compaction) gives the result that the inner fjord basin may accumulate IOO mm of mud per meltwater year, 10 mm year-l would accrue to the mid-fjord region, and 4 mm year-1 for the basin inside the sill. The gradient of sediment Aux appears to affect the abundance and diversity of the April macrobenthos in Blue Fjord. The bulk of the macrobenthos is composed of deposit-feeding polychaetes and pelecypods (details available in Hoskin, in press). From head to sill crest, biomass increased from 2 to 52 g m- a blotted wet weight; diversity increased from Ig to 45 species; number of individuals increased from 440 to 1610 m2; and abundance of suspension feeders increased from 6 to 20% as the sediment flux decreased. A similar relationship between sediment flux and the macrobenthos has been found for Queen Inlet, a physiographically similar fjord in Glacier Bay National Monument (Hoskin et al., 1976).

Acknowledgements This work was supported in part by NSF grant 4or5gX and contract No. AT(45-I)-2229 from the Energy Resource and Development Administration. We thank the officers and crew of R/V Acona for their skilful seamanship, and Dolly Dieter, Valerie Williamson, Ray Hadley, Munling Lee and Meng-Lein Lee for their cheerful assistance.

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