Sedimentation in the Fraser River and its estuary, southwestern British Columbia (Canada)

Sedimentation in the Fraser River and its estuary, southwestern British Columbia (Canada)

Estuarine and Coastal Marine ,Science (x98o) xo, 609-633 Sedimentation in the Fraser River and its Estuary, Southwestern British Columbia (Canada) J...

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Estuarine and Coastal Marine ,Science (x98o) xo, 609-633

Sedimentation in the Fraser River and its Estuary, Southwestern British Columbia (Canada)

John D. M i l l i m a n b Geological ,Survey of Canada, Vancouver, British Columbia, Canada Received z6 December z978 and in revised form x5 June I979

Keywords: estuaries; rivers; suspended seasonal variations; British Columbia coast

sediments;

sand transport;

The Fraser River, the largest river (in terms of both water and sediment discharge) reaching the west coast of Canada, is a sand-dominated river in which most sediment transport occurs during freshet in late spring and early summer. More than half the sediment discharged during this 2-3 month period is sand. Throughout the rest of the year, the river is characterized by lower flow and low suspended sediment concentrations (primarily silt and clay); net offshore transport during these months is slight, and nearbottom transport appears to be landward. The dominance of sand transport in the Fraser results in an estuarine depositional regime quite different from most mud-dominated rivers and estuaries. Although most sediment in the river is carried in suspension, about 40% of the sand (zo% of the total load) settles from suspension in the upper estuary and most of the rest settles prior to reaching the lower estuary. In a natural situation, much of the river sand probably would continue moving seaward as bed load, as suggested by the prevalence of migrating sand waves in the middle estuary during freshet. Longshore drift of this sand has built tidal fiats that now dominate the nearshore environment. Dredging of river channels removes an appreciable part of the total annual sand load. Jetties across intertidal fiats and at the river mouth have interrupted longshore transport and increased resuspension of sand in the outer estuary by channelizing flow. All of these factors should combine in shifting tidal fiats and adjacent shoreline from their natural state.

Introduction T h e Fraser River, the largest river reaching the west coast of Canada and one of the largest undammed rivers on the North American continent, is more than x2oo km long and drains an area in southern British Columbia in excess of z3o ooo km 2 (Figure x). I n many respects the Fraser is a mountain-dominated river: during the first xoo km of its length, the Fraser drops nearly r km in elevation, then levels out to moderate gradients throughout the next 900 km (upstream of Hope, B.C.), passing through a series of canyons and gorges. T h e bulk of the runoff within the basin occms in the upper and middle Fraser (east of Hope, B.C.) "Contribution No. 4268 from the Woods Hole Oceanographic Institution bPresent address: Woods Hole Oceanographic Institution, "~VoodsHole, MA 02543, U.S.A.

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Figure x. The Fraser River, showing the location and drainage basin (upper right), the lower Fraser (center), and the adjoining estuary (upper left). Stations are numbered as follows: x=Port Mann; 2 =Dens Island; 3=Steveston; 4----Elbow; 5=Sand Heads; 6=2"7 krn offshore. (Table x). The lower Fraser, extending from Hope to the sea receives additional flow from small rivers draining the Coastal Range (Table I). This section of the river is more mature, being bordered by extensive alluvial flood plains (7oo km 2) that comprise some of the richest farming land in North America. To prevent flooding of this land, a system of dikes has been constructed along approximately 9o% of the river bank downstream from Hope. ~,lost of the sediment deposited by the Eraser since Holocene deglaciation (assumed to be 9ooo years B.P.; J. E. Armstrong, oral communication) has accumulated as a subaerial and subaqueous delta, 975 km2 in area, with an average thickness of xxo m (Mathews & Shepard,

i

61 x

Sedimentation in the Fraser River, Canada

TABLE x. Flow and suspended sediment concentrations in middle and lower Fraser and major tributary rivers. Inability to quantify flow at Port Mann stems from the uncertain tidal influence upon river currents. Data from various x,Vater Survey of Canada publications Maximum Minimum Mean Mean mean mean suspended Annual flow monthly flow monthly flow matter 003 m ~s -1) (to' m ~s -a) (Io a m a s -a) (part/to') Fraser at Hope Harrison River Chilliwack River Fraser at Mission Stave River Pitt River Fraser at Port Mann

2"7 0"4 o'x 3"5 o'x o't ?

7"0 x.o 0"2 8"9 o'2 o'z ?

0"8 0.2 0"04 x'3 o't o'oz ?

21o (-4-42) 7 (4-7) 31 (4-17) 186 (4-43) 2 32 (4-3) x35 (4-39)

1962 ) . Extensive sandy tidal flats characterize the narrow shelf between the shore face and the edge of the delta front (Luternauer & Murray, 1973 ; J. L. Luternauer, in prep.) (Figure 2). Some Fraser sediment has escaped the delta proper and spread throughout the southern Strait of Georgia: several basins in the southern Strait contain Holocene sediments more than 5° m thick, most of which presumably were derived from the Fraser (Tiffin, 1969; Pharo, I972 ). In recent years, the Fraser River has become increasingly important for shipping as well as the site of rapid urbanization and industrialization in southwestern British Columbia. Because of shoaling within the river, more than 2 million tons of material must be removed annually from the navigation channels in order to allow the passage of large ships. Moreover, jetties have been constructed from the shoreline (at Steveston) to minimize shoaling at the river mouth (Figure 2). By channelizing river flow, these jetties have effectively extended the Fraser River 7 km across the narrow shelf to Sand Heads. Although understanding the flow and sedimentology of the Fraser River and its estuary is critical both for scientific reasons and for sound economic development of this river, the Fraser is practically unstudied by geologists, the only published works being on deltaic and offshore deposits (Johnson, 1921 ; Mathews & Shepard, 1962; Luternauer & Murray, I973; Pharo, 1972 ). Most of the detailed information concerning the flow and sedimentary patterns comes from unpublished engineering studies, many of them documenting the feasibility of various river structures, and subsequently tucked away in office files and libraries throughout southwestern British Columbia. Some of these references have been listed in an annotated bibliography by Church & Wahlgren (1974) and in a status report by Hoos & Packman (1974). The first half of this paper describes river flow and sedimentation within the Fraser, summarized largely from suspended and bed load data gathered by the Sediment Survey Section of the Water Survey of Canada. The second half of the report deals with estuarine conditions, based on data from a series of cruises made by the Geological Survey of Canada, and whose methods of sampling and analysis are described in a following section.

River flow in the Fraser During 60 years of measurement, the mean discharge of the Fraser (as measured at Hope) has been 27oo m 3 s -x (Table 1). Most of the disc.harge comes from melting snow and as a result, discharge from late fall through early spring is generally less than 15oo m 3 s -1, while during spring freshet (May through mid-July) flowaveragesmorethan4ooom3s-l(Figure3).

6xz

J. D. Milliman

Upon closer inspection, river flows vary considerably in timing, magnitude and duration of the spring freshet (Figure 3). To a large extent, these events depend upon the level of snow fall during the previous winter, as well as the speed and timing of spring melt. Peak freshet flows, for example, have ranged between 5000 and z5 ooo m a s -1 (Water Survey of Canada, i974). Freshet also tends to peak slightly earlier at Hope than at either Mission or

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Figure z. Bathymetric chart of the Fraser River delta and delta front, depths in meters. Stippled areas indicat~ tidal flats. Note North Jetty extending across the entire narrow shelf to Sands Heads. Chart modified from one developed by J. L. Luternauer (GSC).

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Port Mann (Figure 3), indicative of the later melt of snow fields feeding streams draining into the lower Fraser. Flow in the lower Fraser increases by approximately 2o% relative to Hope, the result of Harrison, Sumas-Chilliwaek, Stave, and Pitt river influx. These rivers drain lakes, have low suspended sediment concentrations (2o-3o mg l-X; Table x), and subsequently contribute relatively little to the Fraser sediment load. From Mission to the river mouth, tidal influence becomes marked, particularly seaward of Port Mann-New Westminster. However, saline intrusion occurs considerably seaward of Port ~'Iann. Tides at Port Mann greatly affect river discharge even during spring freshet. At New Westminster (location shown in Figure 6) the river bifurcates into the North Arm (carrying approximately 12% of the total flow) and the Main Arm, carrying the remaining

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Figure 4- Comparison of average monthly discharge and suspended loads at Mission,

1966-t97x. Fall and winter values generally show low discharge and suspended load; spring has high discharge and load, and summer has high discharge, but lower levels of suspended load.

Fraser River sediment

A major source of Fraser sediment is Pleistocene glacial deposits from interior B.C. (e.g., Pharo, I972 ), as well as the erosion of more indurated rocks. Bank erosion also accounts for some sediment contribution to the lower reaches of the river (Morton, I949; Simmons & Buchanan, z955) (see below). The Fraser transports between xz and 3° million tons of sediment annually (Figure 4)Approximately 80% of the sediment discharge occurs during freshet months (Figure 3; Table 2). Early freshet flows carry greater concentrations than comparable flows in subsequent months (Figure 4), presumably reflecting the greater availability of erodable sediment during early spring (e.g. Nordin & Beverage, x965). Closer inspection of this simple picture, however, shows one far more complex: in any one year, as many as six pulses of sedimentladen water may pass Hope prior to the main freshet (Figure 3)" Some of these influxes

Sedimentation in the Fraser River, Canada

6z5

TABLE2. Freshet and annual suspended discharges at Hope, Mission and Port Mann, together with the suspended sand carried during freshets; data supplied by the Water Survey of Canada. Note that freshet totals and freshet sand remain relatively constant relative to annual and freshet totals (columns 3, 5, and 6)

(z)

Year Hope

Mission

Port Mann

(z)

Freshet Annual Observed suspended suspended freshet discharge discharge interval ( x Io ° tons)( x xo' tons)

x967 z M a y t5 July t968 z 5 A p r 3 r July z969 x5 A p r x5 July z97o x M a y 3 ° June I97z x5 A p r 30 June z972 z M a y 15 July z967 z M a y z5 July z968 x5 A p r 3x July z969 x M a y z5 July x97o x M a y x5 July z97x x5 A p r 30 June x97z x l~,iayz5 July z967 z M a y x5 July z968 z M a y z5 July x969 z M a y z5 July z97o z M a y x5 July I97x x M a y z5 July x972 x M a y z5 July

(3)

(4)

Freshet total/ Annual total Freshet ( x zoo) sand (%) ( X zo' tons)

(5)

(6)

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Freshet sand/ Annual total ( x zoo) (%)

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reach Mission, w h i l e others do not, p r e s u m a b l y d e p o s i t i n g along the w a y and b e i n g res u s p e n d e d d u r i n g s u b s e q u e n t high flow (e.g. F i g u r e 5). ° *Although these pre-freshet influxes of sediment appear impressive in terms of concentration, the generally low level of river flow during this period results in relatively low sediment loads. T h e cause of these influxes is not clear, but may be related to the discharge of sediment-laden tributaries into Fraser River Canyon during early spring (lan Stewart, oral communication).

616

07. D. Milliman

Sediment transport involves both suspension and bed load movement, the latter involving esentially particle-to-bed interactions. In the Fraser River two types of suspension occur, one being continuous (wash load) in which the material remains in suspension throughout all phases of the tidal cycle, and the other being discontinuous, in which the material is suspended only during higher flows (bed-material load). Discontinuous suspensions consist mostly of very coarse silt and sand (Figure 5), and during peak spring flows, can account for an appreciable amount of the total suspended load: an average of 5o% of the freshet suspended sediment transported past Hope is sand, 57% at Mission and 4x% at Port Mann (Table 2, column 5)- lXiore striking, however, are the consistent percentages of total annual discharge represented by the freshet sand. At ~lission, for instance, freshet sand accounted for 44, 45 4 6, 44, 46 and 45% of the annual suspended discharge from 1967 through x97z (Table z, column 6). The consistency of these figures is particularly startling in view of the threefold variation in annual suspended load during this interval. Peak sand concentrations at Hope i

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Figure 5. Daily variation of suspended sand and fine silt+clay (finer than x6 lain) at Hope (upper) and Mission (lower) during spring freshet, I969. Note that sand and mud pulses at Hope in mid-April and early May are represented only by mud at Mission, indicating that the sand had settled out. The high sand levels during late May and early June at Mission, however, suggest that this material was subsequently resuspended and transported downstream.

Sedimentation in the Fraser River, Canada

6x 7

predate those at giission and Port Mann by a month (e.g. Figure 5). Also, the amount of sand in suspension at Port Mann is always less than that at Mission (presumably a function of downstream deposition and perhaps of increased bed load). Only a few rivers (such as the Columbia; Haushild et al., 1973) have been shown to carry such high percentages of sand. The dominance of sand in the Fraser probably is related to a number of factors, such as the close proximity of the mountains (essentially only IOO km from the river mouth), the undammed nature of the entire river and the marked seasonality in river flow. The coarse nature of the Pleistocene glacial deposits that form a major sediment source presumably is also an important factor (e.g. van Andel, 1955). Apparently most of the Fraser sediment leaving the mountains and passing Hope is in suspension; bed load is assumed to be less than 5% of the total load (D. A. Dobson, oral communication). Annual mean concentrations of suspended load at Hope average 21o mg 1-1 (Table I). Although influx of sediment-poor waters farther downstream (from the SumasChilliwack and Harrison Rivers) causes lower average concentrations at ~Iission (Table i), increased water flow results in higher suspended sediment discharge, about 5% higher than at Hope (Figure 4). Presumably this downstream increase in sediment discharge is related to channel and bank erosion (Simmons & Buchanan, 1955). In contrast, the suspended sediment discharge at Port Mann averages 17% less than the levels calculated for Mission (Table 2), presumably reflecting deposition and increased bed load in the lower Fraser. The estuary Tidal effects in the Fraser River estuary are felt east of Port Mann throughout the entire year, even during spring freshet. This strong tidal influence reflects the channelized nature of the estuary, as well as the great tidal range within the adjacent Strait of Georgia. The tides, which are mixed with a strong diurnal component, can exceed 4 m at Sand Heads and Steveston. Tidal range decreases landward and also with increasing river flow. Thus, during winter months, tides at Port Mann can change river level by more than I m, while during spring freshet, ranges may only reach xo to 2o cm. However, even a height difference of IO cm can severely impede or accelerate river flow at Port Mann (see below). This impedance and acceleration results from the storage and discharge of tidally-exchanged water in Pitt lake, upstream of Port Mann. Moreover, because of the time required to fill or empty tidal waters from Pitt lake, high water at Port 1Vlann generally occurs about I h after that at Sand Heads, while low tide shows a delay of 2 to 3 h (Ages & Woollard, 1976). Saline intrusion, which apparently has been enhanced by the deepening of ship channels (Hodgins, 1974), occurs as far landward as Annacis Island (location shown in Figure 6) during periods of low discharge, but does not extend past the lower part of the estuary at freshet. The configuration and extent of the saline layer also depend upon tidal conditions, as will be shown in subsequent discussions. Few data on either ~edimentary or hydrographic characteristics of the Fraser River estuary were available before this study. Perhaps the most comprehensive data were taken during the spring freshet of I975 by the Water Survey of Canada, which show a general seaward decrease in suspended load (primarily from decrease in sand transport) but an increase at Steveston Elbow (Figure 6). The 1975 data also show that sand concentrations at low tide are substantially higher than those at high tide while wash load (silt and clay) concentrations remain remarkably consistent (Figure 6). The downstream decrease in suspended sand indicates deposition and infers increased importance of bed load transport in the lower river. Deposition is indicated by shoaling in the river, requiring dredging of 2-3

SUSPENDED t SI< CLAY

SUSPENDED~" SAND ~ 5

SUSPENDED MATTER

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Fi~,ure 6. Down-river variation in total suspended matter, sand, and silt+clay (expressed in mg 1-1) during xt-x3 June x975, as measured by the Water Survey of Canada. Solid lines (and dots in the bottom diagram) represent measurements taken during lower tidal levels (i.e. higher current velocities), and the dashed lines during higher tidal levels (lower current velocities). Numbers on the individual lines refer to vertical station number across the river at each sample area. Note the general increase of suspended sediment concentration at the Elbow.

moll

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Ox b-* Oo

Sedimentation in the Fraser River, Canada

619

TABLE 3. Dredging efforts on the Fraser River (see footnote). Data courtesy of the Department of Public Works, Vancouver, B.C.

Per cent dredged from

Before trifurcation x959--7o During construction ,97o-73 Trifurcation completed x973-75

Average annual dredging (x xoe m ~)

Steveston and Seaward

2"3

38

35

27

*'8

38

37

24

2"4

55

*9

25

New

Steveston- %Vestminster New and Westminster Upstream

× xo6 m 3 (2 million tons) annually (Table 3)-* Perhaps the best indication of bed load is the presence of large migrating sand waves, up to 4"5 m high and x5o xn long, that form seaward of Port ~Vlann and move downstream at a rate of up to 75 m day-* during spring freshet (Pretious & Blench, I95I ; Allen, x973). This equates to a daily bed load during peak runoff of approximately I - 2 × xo5 tons. As the freshet wanes, the sand waves become inactive and by autumn generally disappear, suggesting a marked decrease in bed load transport. Estuarine observations

Tidal and seasonal variations in the hydrographic and suspended matter regimes within the estuary were defined by four cruises during x975-76. T h e first cruise in late August, x975, occupied stations in the middle estuary (at Dens Island), in the channel between the two jetties (Steveston Elbow and Sand Heads), and ~'7 km directly seaward of Sand Heads (Figure ,). Water samples were collected in 6-1 van Dorn bottles, and filtered through an in-line system onto preweighed 47 m m Millipore ® filters, having nominal openings of o.45 pm. Temperature and salinity of the water samples were measured using a bucket thermometer and optical refractometer. Observations were made every other hour for 25 h, with the vessel steaming continuously between two stations at 2-h intervals, Total water depth in the estuary ranged from ,o to I2 m; sample depths were at o, 3 and 6 m. T h e seaward station, more than xoo m deep, was sampled at o, 3, 6 and xo m. These same stations, plus additional stations in the upper estuary (Port Mann) and lower estuary (Steveston), were occupied during subsequent cruises in February, April, and ~iay, I976. At the landward three stations, the ship was anchored for 25-h periods, while at aApproximately 25% of the dredging occurs upstream of Port Mann-New West° minster. Historically, the amounts dredged between New Westminster and Steveston.equalled the amount taken from seaward of Steveston. However, construction of river training works at New Westminster in x97o-73 reduced dredging in the middle estuary, but transferred the problem downstream (i.e. increased dredging at Steveston) and did not reduce the total amount of dredging (Table 3). Most of the Fraser dredge spoils have been used for construction in the municipalities bordering the river. In recent years, hok-ever, demand for spoils has surpassed public dredging capacities, thus necessitating private dredging. This increased need may well double the amount of annual dredging by x98o (R. Pierce, x976, oral communication). Such high levels of dredging may therefore cause river channel deepening; more importantly, the lack of sand escaping offshore may affect offshore topography and development.

6zo

ft. D. Millirnan

Elbow, Sand Heads and 2. 7 km offshore, sequential observations were made bi-hourly. Samples also were taken r- 3 m above the bottom; the seaward station was sampled at o, 5, io and ao m depths. Observations made during the February cruise showed that coarser suspended material escaped capture by the van Dorn bottle, the result of decreased turbulence in the bottle when it was lowered through the water column. This problem is particularly severe during high flow when sand represents a major portion of the suspended load. Thtis, during the April and May cruises, a suspended sediment river sampler (Model P-6x) was used to obtain water samples at the three anchor stations. At the three drifting stations in the lower estuary and adjacent Strait, van Dorn bottles were used (a river sampler proving ineffective on a moving ship). Sand, when prominent, was separated from the finer material by a 6z ttm sieve, and both size fractions were collected on Millipore filters. Current velocities were measured at the three anchor stations on April and May cruises. In April, a Hydro-Products Savonius current meter measured velocities r m above the bottom. During late iVIay, a Price current meter, attached to the P-6x sampler, was used, with measurements at the surface, 3 m, 6 m, 9 m, and at several intervals above the bottom. Based on river data discussed above, hydrographic and sedimentologie conditions in the Fraser River estuary can be separated into three general seasons: autumn-early spring (low water/sediment discharge), freshet (high water/sediment discharge), and summer (relatively high water di~charge[low sediment discharge). Observations taken during the r975-76 cruises, therefore, will be discussed within this general framework:

Autumn-early spring River flow during both the February and April cruises averaged about iooo m 3 s-X; as a result of this low discharge, even surface waters at Deas Island were brackish and salinities near the bottom often exceeded 2O~oo(Figures 7 and 8). At Sand Heads, surface salinities seldom fell below 5~oo,and offshore only a thin layer of brackish water overlay the more saline Strait of Georgia waters. While maximum salinity at Sand Heads coincided with high tide, it was 2 h later at Steveston and 3 to 4 h later at Deas Island. This lag in saline intrusion up the estuary, as noted by Hodgins (1974), depends upon advection by near-bottom currents, which can continue flowing landward long after a reversal of surface current flow (see below). During April, near-bottom currents at the three anchored stations clos61yfollowed salinity trends. At Port Mann, surface flow was continually seaward (even during HHT), but nearbottom flow reversed for several hours (Figure 9). In contrast, at both Dens Island and Steveston, surface currents reversed landward briefly during flood tides, while near-bottom flow was primarily landward throughout the entire tidal cycle, thus explaining the predominance of saline waters in bottom waters of the lower estuary. Sediment load during February was extremely low: concentrations were generally 5 to 7 mg I-1 at Port Mann and decreased downstream with increasing salinity (Figure 7). During LLT, however, concentrations at Port Mann increased to 95 mg 1-1 (x m above the bottom), but decreased again at slack water. Similar flow-related fluctuations in suspended matter concentration were noted at Deas Island and Steveston, but were not seen at Elbow or Sand Heads. Because of the low terrigenous content, percentages of organic material were relatively high (an average of I8~o combustible matter). Judging from visual inspection, a large percentage of this material was wood fibre, derived from logging activities on the river. Although river flow in April was similar to that in February, suspended concentrations were significantly higher, reflecting the higher suspended loads carried by pre-freshet flows (Figure 3)" Average suspended concentrations at Port Mann averaged about 3 ° mg 1-1, and m

SALINITY (°/m,o l

$USPIrNOEO M A T T E R

Time (h)

Time (h) O0

(rag/I)

06

18

12

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15! Figure 7. Temporal variation of salinity ( ~ ; left) and suspended matter (mg l-X; right) in the Fraser River estuary, "3-27 February I976. River discharge was about zoos m 3 s - L Port M a n n values were generally low due to sampling errors (see text), and thus are not shown. Vertical axis is depth in meters, horizontal axis is time in hours (dots representing sample polnts). I n this figure and following ones salinSties within shaded areas are less than t ~o; cross-hatch greater than 3o~o. I n suspended matter, hatched areas representvalues less than x mg 1- z cross hatch greater than 6o m g I - L T h e river surface is shown relative to the tidal curve predicted for each locality on the river, thus showing periods of high and low tide. Bottom lies about t - 3 m below the deepest samples.

622

.7. D. MilEman

SUSPENOED MATTER(nag/I)

SALINITY(%0)

Time (h)

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18

Port Fresh

Mann

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0

06







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Figure 8. Temporal variations of salinity (7oo; left) and suspended matter (rag I-*; right) in the Fraser River estuary, 5-9 April x976. River discharge was approximately xooo m ~ s - l • Refer to Figure 7 for further explanation of symbols and configurations used.

Sedimentation in the Fraser River, Canada

6z3

more than 60 mg 1-1 during low tides; values were nearly 4 ° mg 1-1 at Steveston (Figure 8). High concentrations during low tide reflected bottom and resuspension by increased flow (Figure 9). Interestingly, silt and clay concentrations increased slightly just prior to peak flow (best seen at Port IyIann; Figure 9), but then fell with increased discharge (during which sand transport increased). Presumably this phenomenon reflects the initial scour of finer I0(~ ~

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12

[~os

Idond

i

18

,

i

w.,.....

O0 i

06 i

12 /

11: i

(30 l

t. O

Steveslon

Figure 9. Relation of river stage (as indicated by current velocity and salinity) and suspended matter (sand and s i l t + d a y ) in the Fraser, x m above the bottom, during the April Cruise. TOTAL SUSPENDED MATTER (mg/I}

SALINITY (%e3

Time ( h ) 06

Time ( h ) 06

12

Time (hi

12

12

1O •.

A,

2.7 km offshore 5

I0

15

:i,

$0

• 0

IE

SUSPENDED SAND ( m g / I )

\

f

i

/ L0

Figure xo. Temporal variation of salinity (~oo; left), total suspended matter (rag l - t ; center) and suspended sand (mg I - I ; right) at Sand Heads and z'7 km offshore during a fresh flood on zz May z976. River flow was 8ooo m ~ s - ' . See Figure 7 for further reference to symbols and configurations used.

16

624

3e. D. Milliman

bottom material during increased river flow; once stripped of this finer sediment, sand is resuspended during higher flow. While maximum sediment movement at Port ~Iann occurred during low tide, highest concentrations at both Deas Island and Steveston occurred during peak landward flow, during flood tide. VELOCITY

(era/s)

Time ( h } 12

~

06

O0

I

!

~'" "" ~~

5



°

PoEt Moan

/

o



'"

(

.

"

Oeos

tO



°

°

,

~$1and



15

0

Sleveston I0

D

Q

0

15

Figure zz. Temporal variation of current speed (cm s -x) at Port Mann (upper) Deas Island (center) and Steveston (lower) during 25-28 May x976. Vertical scale is depth in meters, horizontal scale is time in hours. Dots represent data points•

SALINITY (%o)

SUSPENDEDMATTER(n'R/I) Time(h) R

Port

Fresh

Monn "E

5

11

O0

O~

........

:

O u

©

Fresh

IMond -.

"

I¢•







°/.



II

Stevnton

Fresh

I~

11

O0

"

Io

Time(h)

~°~"

s"

m

o

12

Elbow

.

s

"~

0

o

Sond Heods

= ~

t

IC

i

2.7 km offshore

s

10



10)t . . . . . . 2C

-

/ ~

Figure z2. Temporal variation in salinity ( ~ ; left) and suspended matter (rag I-Z; right) in the Fraser ttiver estuary during =5-a9 May z976. River discharge was ?ooo m s see. Hatched areas are suspensions less than 8o mg 1-1; crossed-hatched are greater than zooo mg ] -1.

6z6

if. D. Milliman

Spring freshet Because of warm weather during early spring in 1976, Fraser discharge peaked in early May and then fell later in the month. Peak flow conditions, unfortunately, were measured only during 8 h of sampling on xz May when river flow at Mission was 8000 m 3 s -x (D. A. Dobson, oral communication). Spring tides were nearly 4 m in range, thus accentuating flow conditions in the estuary. At L L T , the entire water column at Sand Heads was fresh, and offshore surface values were as low as 3~oo (Figure 1o). Tidal influence was even more obvious in sediment discharge. At L L T , total suspended matter at Sand Heads exceeded 60o mg 1-: throughout most of the water column and was greater than 18oo mg 1-1 near the bottom, the highest concentration measured during the 1975-76 cruises. Most of the L L T load was sand, with near-bottom concentrations approaching 1ooo mg 1-1 (Figure io). During L H T , however, sand concentrations fell to less than I m g 1-t throughout most of the water column, the suspended material being predominantly wash load (silt and clay). Offshore suspensions were very high, more than 400 mg I - t at times. These high values within the fresh water plume clearly reflect sufficient vertical turbulence to maintain high levels of suspension. The estuary was sampled more completely in late May when river flow was slightly lower, about 7ooo m s s -x. As expected, velocities far exceeded those measured in pre-freshet conditions: at Port Mann surface currents fell below zoo cm s -1 only at high tide (Figure 1:). Velocities decreased with increasing depth, but still exceeded too c m s -1 at most times. Similar tidal and vertical variations in currents were noted at Dens Island and Steveston, but velocities were markedly reduced compared to Port Mann, partly because of bifurcation of river flow below Port Mann, and partly because of the downstream widening of the river At Steveston, surface and near-bottom currents were nearly identical, maximum velocities occurring at intermediate depths (3 to 9 m). This uniform current velocity throughout the water column suggests effective vertical mixing. Because of the high discharge, surface waters as far downstream as the Elbow remained fresh throughout the tidal cycle (although near bottom salinities ranged from 5 to ao~oo) (Figure lZ). The upper 5 m of water z. 7 km seaward of Sand Heads were generally brackish, and surface values occasionally less than 1~oo (Figure xz). Suspended concentrations were lower than those measured in mid-May, ranging from 8o to xao mg 1-1 except at ebb flow, when surface concentrations increased to more than 13o mg 1-1 and bottom values (at Port Mann) to I4oo mg 1-a. Concentrations at Dens Island and Steveston decreased (relative to Port Mann), particularly in near-bottom waters at L L T (Figure In). However, during low tide at Elbow and Sand Heads, near-bottom concentrations increased to more than 800 mg 1-1, indicating that bottom sediment was resuspended and transported seaward. Although concentrations offshore were not as high as those earlier measured in the month, values in excess of 1oo mg 1-1 were measured at the surface just after low tide. As seen during the April cruise, the high suspended loads during maximum discharge reflect high levels of ~and in suspension. At Port Mann, for instance, near bottom sand concentrations increased from less than ioo mg 1-1 (at H H T ) to as high as 14oo mg 1-1 at L L T , while silt and clay concentrations remained relatively constant (Figure 13). Similar tidal variations in sand concentrations were notcd downstream.

Summer (post-freshet) Post-freshet flow in summer has relatively high discharge, but low suspended loads (Figure 4). Unfortunately, only the cruise documenting this period did not occupy stations at Port Mann or Steveston. Moreover, neither near-bottom water samples nor current measure-

627

Sedimentation in the Fraser River, Canada

12 I

O0 I

I

'

12 •

t

l

O0

I

I

t

I

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l

I

I

I

I

I

I

I

`

I

'

12 t

'

'

I

H

• 1400 • 1200 • 1000

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-6oo ~ -40(1

-200 .

-0

O0

. . . .

12

,

O0

,

,

12

,

, 7

Figure x3. Variation of river stage (expressed by current velocity and salinity) and suspended sand and silt+day, r m above the bottom, Fraser River, 25-29 May x976• ments were taken. Nevertheless, the data do provide some indication of summertime conditions in the Fraser River estuary. River flow during late August (I975) was just under 3o00 m a s - l ; although no nearbottom observations were taken, river water at Dens Island was completely fresh, while downstream it was brackish (Figure x4). Salinities at Steveston Elbow reached a maximum just after high tide, while minimum surface salinities at Sand Heads and at the offshore station both occurred several hours after low tide. Suspended matter concentrations averaged between zo and 3 ° mg 1-1 in the river water, except during low tide, when concentrations exceeded 60 mg 1-1. Undoubtedly these high concentrations represented ambient bottom material stirred up by increased flow during low-tide discharge. In contrast, during high tide salt-water intrusion, concentrations at Sand Heads and Elbow often were less than 5 and xo mg 1-x respectively. Only during low tide did the suspended matter offshore exceed zo mg 1-1, and at to m water depth concentrations during high tide were less than x mg 1-1 (Figure z4). Discussion and summary In terms of both river flow.and sediment transport, the Fraser River can be viewed as having two distinct regimes when conditions are radically different. x. Throughout most of the year, from mid-summer through early spring, river flow is relatively low (generally less than 3ooo m s s -1) and the small suspended sediment load is dominated by silt and clay-size materials. Although reliable measurements are lacking, bed load during these 9-xo months appears to be insignificant. Field observations during the summer, winter, and early spring show that the estuary is partially mixed, with saline bottom waters extending at least to Deas Island (in the middle estuary) at high tide. Throughout much of this period, even the surface waters in the lower

6z8

ft. D . Milliman

SALINITY (%0)

SUSPENDED MATTER ( r a g / I ] Time Ih) 12

OtOll lllond

b---. ~. o

I Time Ih}

12

18

O0

i

18

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06

Elbow

~..l',

.\.-

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l 0 ~0

2O

Send Heodl

01

5

2.7 km offshore 10 10

IS-

Figure z4. Temporal variation of salinity (~oo;left) and suspended matter (mg l-a; right) in the Fraser River estuary, z9-zx August x975. River discharge was 3000 m s s -t. Refer to Figure 9 for explanation of symbols and configurationsused. estuary are slightly brackish (Figure z5). Suspended matter concentrations within the estuary during all the~e months are generally less than 5° mg 1-1, and during high tide often less than z 0 mg 1-1 (Figure zS). Current and suspended matter measurements (April x976) allow a rough calculation of the magnitude and direction of near-bottom (z m above the bottom) suspended sediment transport within the estuary during periods of low river discharge. Near-bottom transport at Port Mann is primarily seaward (landward transport being limited to several hours at high tide); suspended sand transport occurs only during peak flows at L L T (Table 4; Figure z6). At both Dens Island and Steveston, net near-bottom transport is landward, with seaward

10

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WINTER



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l

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25 -29,1976



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Et

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ol



5 - 9 , 1976

19-21,1975

April

DI

August

ST

EARLY SPRING

E

SH

HHT



°

LLT





SUMMER Figure z5. High and low tide variations in salinity and suspended matter on the Fraser River throughout the year. In each set of diagrams, HHT salinities (~oo; upper) and suspended matter concentrations (mg l-Z; lower) are shown in the left, LLT in the right, for Port Mann (PM), Deas Island (DI), Steveston (ST), Elbow (EL), Sand Heads (SH) and 2"7 km offshore. Dots represent sample stations. February conditions represent winter conditions; April, early spring; May, spring freshet; and August, summer.

• May

February 24-28, 1976

J\

DI

LLT

IO ,,Q

3

2"

630

J. D. Milllman

transport only during LLT. A somewhat similar situation has been noted in the Columbia River estuary during periods of reduced discharge (Hubbell et al., x975). Because flow in the surface waters of the lower estuary is primarily seaward, the net transit of suspended sediment integrated throughout the water column of the Fraser may be seaward. Nevertheless, sand transported past Port Mann must accumulate in the river channel (and/or continue downstream movement as bed load) since it does not reach Dens Island in suspension. 2. During freshet flow in spring and early summer, the Fraser River estuary is essentially fresh, except at Sand Heads and Elbow during high tide, when a prominent salt wedge

FRESHET MAY25-28, 1976 (RIVERDISCHARGE~7000m3/sec) 0

4

HOURS (RELATIVE,]

8

{2

16

20 u

24

12.0 EARLYSPRING APRIL 5-8, {976 (RIVERDISCHARGE~IO00m3/sec)

IO.O

i

PORTMANN

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HOURS (RELATIVE) 0

4

8

~2

t6

20

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~

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~ "~ O ! f ~ ~ O.

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STEVESTON g ~

2.0

O.! r 0

-O.It -0.;3

STEVESTON ,f~ ~ - -

:

: :

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Figure z6. Calculated sediment transport (kg per c m ~ of cross-sectional area per minute) ! m above the bottom, d u r i n g 5-8 April (leh) and 25-28 M a y (right) x976. T r a n s p o r t was calculated b y multiplying the suspended sediment load (sand and s i l t + c l a y ) b y the current velocity z m above the bottom. See Figures I x and z 5 for graphical representations o f the data. Note that the vertical scale for M a y is an order of magnitude greater than for April.

63x

Sedimentation in the Fraser River, Canada

develops (Figure z5). Eighty percent of the annual suspended sediment is transported during freshet, about half of which is sand (in contrast to non-freshet conditions when silt and clay predominate). Bed load transport also can be significant, as suggested in the following calculations: Current and suspended sediment measurements made during the late May (I976) freshet show a near-bottom sediment flux about 3 ° times higher than that calculated for pre-freshet flow (Figure x6, Table 4). Furthermore, no landward transport was noted at any station. The daily flux of suspended sand, however, decreased seaward from 393 ° kg cm- z cross-sectional area at Port Mann to 680 kg cm-2 at Steveston (Table 4). Presumably, this loss of suspended sand reflects deposition and]or transfer to bed load. Not surprisingly, the river channel between Port Mann and Dens Island has maximum sand wave development during spring freshet (Pretious & Blench, i951 ). Although no current measurements were taken either at Elbow or Sand Heads, near-bottom suspended sand concentrations during ebb flows in May were significantly higher than at Steveston (Figure x7), indicating that sediment is resuspended and subsequently transported out of the estuary. This condition contrasts strongly with April, when little indication of a turbidity maximum was noted (Figure x7). Resuspension would explain the significant amount of suspended sand noted in the surface offshore waters (2. 7 km off Sand Heads) during both May cruises (e.g. Figure x2). TABLE4- Calculated sediment fluxes (kg cm -2 day-t), x-meter above the bottom at Port Mann, Dens Island and Steveston, early April and late May, x976 Pre-Freshet (5-8 April t976) Downstream Upstream Port iMann Silt and clay Sand Deas Island Silt and clay Sand Steveston Silt and clay Sand

98 54

xt

x3 --

29 5

4 __

z7

Freshet (25-28 May I976) Net

Downstream Upstream 700

+x4x

--

Net

+4630

3930

__

--2 x

66o x3to

--

+ x970

--23

7xo 680

---

+ x59°

A final word should be mentioned about the impact of modern civilization upon Fraser River sedimentation. Prior to the 2oth century, an appreciable amount of the freshet sediment presumably was deposited on the subaerial portion of the delta by flooding river waters. Of the material escaping the estuary, much was transported by longshore currents and accumulated on tidal fiats that dominate the narrow shelf between the shoreline and the delta front (J. L. Luternauer, personal; communication). T h e remainder of the riverine material presumably escaped offshore. Construction of dikes along the river banks has effectively ended flood-related deposition on the low-lying Fraser River valleyland. Active dredging of the river channels (2 million torts per year and probably more than 3 million tons by 198o) has removed much of the sand that could have reached the sea. The jetties between Steveston and Sand Heads undoubtedly have helped channelize river flow, thus increasing resuspension in the outer estuary and facilitating offshore transport. Finally, building breakwaters and jetties across the tidal fiats (Figure 2) has decreased longshore transport of sediment that does escape onto the narrow

63z

ft. D. 2~Iilliman

40--

i

i'

30

X

i

I

i

I

! 0

20 APRIL

D

t0

0

E d §

600

l

l

f

.

t

I

=

0

O - I % o at

X •

O-t%. O-t%,=

III • n

2 - 3 % o at surf 2 - 3 % o at 3m 2 - 3 % + at 6 m

A

4-5%o

®

500

I !

I

.,

i

I |

surf

at 3 m at 6 m

at surf

Im above bottom P" 4 0 0

(aLl sat[~lt[es)

MAY

300 ® ®

200

x x

100



o

o

o

o 0

!

PORT MANN

I

1

, !

DEAS' STEVESTON ELBOW ISLAND

!

SAND HEADS

II ,-

!

2.7 km OFFSHORE

Figure z7. Average suspended sediment concentrations at various depths and salinities along the Fraser River estuary, 5-8 April (upper) and 25-29 May (lower) z976. In April, the values decrease from Port Mann to Steveston and then remain constant to seaward; in contrast, during May, values increased downstream of Steveston, suggesting resuspension within an active turbidity maximum. Note that vertical scale for May is zo times that for April.

shelf. Combined, these actions should decrease the influx of sand to nearshore areas, and result in non-accumulation (perhaps erosion) on some tidal flats (and perhaps shorelines) (Luternauer & Murray, I973). Projected increases in dredging of river chazmels and further jetty/breakwater construction can only hasten this shift from natural state.

Acknowledgements This study was undertaken during z975-76 , when I was employed by the Geological Survey of Canada. Many people helped with various parts of the program. Much of my under-

Sedimentation in the Fraser River, Canada

633

standing of the Fraser River and its estuary come from discussions with various colleagues in Vancouver, particularly D. A. Dobson (Water Survey of Canada), Gall Ashley and W. H. Mathews (University of British Columbia), and J. L. Luternauer, J. E. Armstrong and D. L. Tiffin (Geological Survey of Canada). In addition, I am particularly grateful to D. A. Dobson for supplying unpublished data, without which the conclusions in this paper would have been difficult, if not impossible, to reach. Shipboard and analytical help of Lesley Simpson and Davis Swan are particularly appreciated, along with the assistance of J. L. Luternauer, G. Ashley, R. Gardner, G. Seid, M. Waskett-Meyers, R. Linden, J. B. Southard, and R. G. Jackson, III. Luternauer, Mathews, I. N. McCave, M. G. Fitzgerald, R. H. Meade, T. J. Conomos and D. G. Aubrey offered valuable suggestions on the manuscript, for which I thank them. References Ages, A. & Woollard, A. x976 The tides in the Fraser estuary. Pacific Marine Science (Canada) Rept. 75-5, ioo pp. Allen, J. R. L. x973 Phase differences between bad configuration and flow in natural environments, and their geological relevance. Sedimentology ~o~ 3a3-329. Church, M. & ~,Vahlgran, R. I974 Reference materials on sedimentation and geomorphology of the lower Fraser River. Dept. Geogr., Univ. British Columbia, xz5 pp. (mimeogr.). Haushild, ~,V. L., Stevens, H. H., Jr., Nelson, J. L. & Dempster, G. R., Jr. x973 Radionuclides in transport in the CoIumbia River from Pasco to Vancouver, x~Vashington, U.S. GeoI. Survey. Prof. Paper 433-N. Hodgins, D. O. x974 Salinity intrusion in the Fraser River, British Columbia. Dept. Civil Eng., Univ. British Columbia, PhD Thesis, I47 pp. Hoos, L. M. & Packman, G. A. x974 The Fraser River estuary--status of environmental knowledge to x974. Environ. Canada, Special Estuary Series No. x, 5x8 pp. Hubbell, D. ~V., Glenn, J. L. & Stevens, H. H., Jr. x97x Studies of sediment transport in the Columbia River estuary. Proe. Tech. Conf. on Estuaries of the Pacific Northwest. Eng. Exp. Station (Oregon State Univ.) Circular No. 42, x9o--226. Johnson, W. A. t9at Sedimentation of the Fraser River delta. Geol. Survey Canada, Memoir x25, 46 pp. Luternauer, J. L. & Murray, J. W. x973 Sedimentation on the western delta-front of the Fraser River, British Columbia. Canadian ffournal of Earth Sciences Io, 164z-x663. Mathews, ~V. H. & Shepard, F. P. x96z Sedimentation of Fraser River delta, British Columbia. Bulletin of the American Association of Petroleum Geology 46, x4x6-t438. Morton, K. W. t949 Fraser River system, Province of British Columbia: History o£1mprovements, x87 x to x948. Can. Dept. Publ. Works, Vancouver, 66 pp. Nordin, C. F., Jr. & Beverage, J. P. x965 Sediment transport in the Rio Grande, New Mexico. U.S. Geological Survey Prof. Paper 46a-F. Pharo, C. H. I972 Sediments of the central and southern Strait of Georgia, British Columbia. Dept. Geol., Univ. British Columbia, Ph.D. Thesis, 29o pp. Pretious, E. S. & Blench, T. x95x Final report on special observations of bed movement in lower Fraser River and Ladner Reach during x95o freshet. Natural Resources Council Canada Rept. xa pp. Simmons, G. E. & Buchanan, J. x955 A preliminary report on bank erosion on the lower Fraser River, British Columbia. Dept. Lands and Forests, Water Rights Branch, Water Res. Invest. Rept. ~78, 53 pp. Tiffin, D. L. x969 Continuous seismlc reflection profiling in the Strait of Georgia, British Columbia. Inst. Oceanogr. and Dept. Geophys., Univ. British Columbia, PhD Thesis, x66 pp. van Andel, Tj. H. x955 Sediments of the Rhone Delta. II. Sources and deposition of heavy minerals. Koninklijk Nederland. Geologic en Mijnbouw. Geol. Serie x5, 5x5-543. Water Su~'ey of Canada t974 Historical Streamflow Summary, British Columbia to z973. Inland "~Vaters Directorate, Ottawa, 694 Pp.