Seasonal Reversal of Flood-Tide Dominant Sediment Transport in a Small Oregon Estuary

Seasonal reversal of flood-tide dominant sediment transport in a small Oregon estuary

SAM BOGGS, JR. Department of Geology, University of Oregon, Eugene, Oregon 97403 CHARLES A. JONES1 Department of Geology, Chadron State College, Chadron, Nebraska 69337

ABSTRACT bathymetry of the estuary was mapped, and salinity, temperature, and current velocity were measured. The Sixes River in southwestern Oregon has a summer discharge of only about 2 m3/sec. During these low-discharge conditions, a PHYSIOGRAPHY AND HYDROGRAPHY flood-dominated system of bottom tidal currents develops in the estuary and a deltalike sill, as much as 1.5 m in height, builds across The Sixes River (Fig. 1) is almost 50 km long. The average gra- the mouth of the estuary by upstream progradation. Flood-tide dient of the upper half of the stream is 9 m/km; of the lower half it currents move across this sill at velocities of as much as 90 cm/sec is 0.7 m/km. The estuary is about 3 km long (maximum incursion 15 cm above the bottom, but the velocity of ebb-tide currents usu- of salt water) and ranges in width from about 210 m at the lower ally does not exceed about 40 cm/sec. end to less than 45 m at the upper end (Fig. 2). The gradient within Dispersal patterns of dyed sediment injected at the river mouth the estuary is less than 0.1 m/km. The lowermost part of the es- during low river discharge show that flood-tide currents transport tuary, about 150 m in length, is a narrow channel approximately sand across the sill and up the estuary as far as 0.8 km (about one- 15 m wide. The position of this channel and the angle at which it fourth the length of the estuary) in a single flood-tide phase. During enters the sea shift with the seasons. In summer the channel is ebb tide, the sill impedes movement of salt water along the estuary commonly long and sinuous and empties toward the south. In bottom, producing a sharply stratified two-layer water system. Al- winter the channel is short and straight and is usually oriented per- though tracer experiments show that some fine sand is removed pendicularly to the coast line; however, heavy winds and seas in from the estuary during the ebb phase, primary sedimentary struc- late winter have been observed to displace it to the north. tures and the mineral composition of the sand indicate that Discharge changes significantly from summer to winter. Peak flood-tide dominance of the bottom tidal currents causes a net gain discharge commonly occurs following intense rainstorms from De- of marine sediment in the estuary while the sill is in place. cember to February, although occasional high discharge also oc- River discharge after winter storms may increase to more than curs in November and March. The lowest discharge is from July to 400 m3/sec, and large quantities of detritus, including gravel, are September. Monthly averages of mean daily discharge for the Sixes transported downstream into and through the estuary. High river River at Sixes, Oregon, for the period 1967 to 1970 range from a discharge also causes erosion of the sill, greatly reducing the low of 0.65 m3/sec to a high of almost 112 m3/sec (U.S. Geol. Survey sediment-trapping capacity of the estuary. The finer fluvial detritus, Water Resources Div., 1968, 1969, 1970). The gauging station is together with fine marine sediment deposited during the summer, is approximately 8 km upstream from the mouth of the estuary, and swept from the estuary, leaving it floored largely by gravel. Thus, the hydraulic sediment-trapping mechanisms observed in the estuary of the Sixes River appear to be effective only on a seasonal basis under present hydrologic conditions. Key words: sedimenta- tion, sediment transport, fluorescent tracers, bed forms, heavy minerals, estuaries.

INTRODUCTION

The estuaries of the small high-gradient streams of southwestern Oregon are typically short, narrow, and shallow, and average stream discharge is low. However, the streams attain high velocities during periods of peak discharge in winter, and at such times they Figure 1. Index transport significant quantities of sand and gravel. In order to map showing loca- evaluate the pattern and magnitude of sediment transport in a tion of Sixes River estuary on the southern small estuary, we studied the estuary of the Sixes River. The river is Oregon coast. easily accessible, its hydrologic characteristics have not been exces- sively modified by man, and it is typical of the small coastal streams of southern Oregon and northern California. The study focused on the transport of marine sediment within the estuary and the ability of the estuary to trap and retain sediment from both fluvial and marine sources. Sediment movement was investigated by use of fluorescent tracers (Teleki, 1966; Ingle, 1966; Crickmore, 1967; Kennedy and Kouba, 1970) and by study of sedimentary structures and heavy mineral assemblages. The

'Present address: Bendix Field Engineering Corp., First National Life Building, Suite 104, Austin, Texas 78701.

Geological Society of America Bulletin, v. 87, p. 419-426, 13 figs., March 1976, Doc. no. 60311.

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Figure 2. Topographic map of the lower part of the Sixes River estuary (mapped with plane table and alidade). one tributary joins the river below the station; thus, discharge into several deep depressions, the floor of the estuary is 3.0 to 4.5 m the ocean is probably somewhat greater than indicated by these below sea level. Salt water flows upstream during high tide as far as figures. 3 km above the mouth of the estuary. Much of this water returns to The maximum tidal height in the vicinity of the Sixes River is 2.7 the sea as the tide ebbs; however, salt water remains in the depres- m in winter and 2.6 m in summer; minimum tidal height is —0.8 m sions and as a thin bottom layer in the lower part of the estuary (U.S. Dept. of Commerce, 1974). The tidal range is 1.3 to 3.6 m in behind the sill (Reimers, 1973). When the sill is eroded by high winter and 1.6 to 3.4 m in summer. Because of a sill as much as 1.5 river discharge in late autumn or winter, salt-water incursion is in- m in height that partly to totally blocks the mouth of the estuary in hibited, and the salt-water wedge extends only a very short distance summer, tidal fluctuations in the estuary upstream from the sill are into the estuary, except during periods when occasional low river usually less than 1.5 m (Reimers, 1973). Flood-tide velocities of discharge allows temporary invasion of salt water along the bot- almost 85 cm/sec were measured at 15 cm above the stream bed tom. When river discharge is high, the incoming salt water mixes during summer conditions. Ebb-tide velocities in summer com- rapidly with fresh water and is then expelled from the estuary dur- monly do not exceed about 40 cm/sec near the bed in the upper part ing the ebb-tide cycle. of the outflow channel, but winter velocities are much higher (may exceed 200 cm/sec at the water surface). FLUORESCENT TRACER ANALYSIS The sand sill mentioned above forms and persists only during summer when river discharge is low, prograding upstream in del- Summer Experiments talike fashion throughout the summer. During some periods of particularly low river discharge and unusually high tides and high seas, Tracer Injection. Three fluorescent tracer experiments were the landward movement of sand into the outflow channel occurs so carried out during low discharge conditions of the river (summers rapidly that the sill grows into a bar that builds up several metres of 1969, 1971, 1974) to determine the distance of transport of above high-tide level and blocks the mouth of the estuary. Drainage sand-size sediment by flood-tide and ebb-tide currents. In the first of the estuary then takes place by seepage through the bar. With two experiments, approximately 150 to 200 kg of natural sand (30 rising river level in winter or increased discharge during autumn percent quartz, 65 percent other light minerals and rock fragments, freshets, the bar is eroded, and the sand is swept to sea. 5 percent heavy minerals and rock fragments) dyed green and 100 When the sill is in place, the Sixes estuary is stratified into a two- to 150 kg of a heavy sand concentrate (mainly magnetite) dyed layer system (type A estuary of Pritchard, 1955) during ebb tide. pink were used as tracers. Mean grain size of the natural sand was Reimers (1973) indicated that this interface is sharply defined by as much as 10°C difference in temperature and a 25°/oo difference in 1 A table of mean daily discharge for the Sixes River (Appendix Table 1) is available 1 salinity (Fig. 3, Table 1, and Appendix Table l ). Average water as supplementary material 76-6 from Documents Secretary, Geological Society of depth in the estuary during summer is less than 1.5 m; however, in America, 3300 Penrose Place, Boulder, Colorado 80301.

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TABLE 1. VERTICAL SALINITY PROFILES

Date Time Depth Salinity Notes (m) (°/oo)

May 27, 1965 1002 0.0 0.6 At high tide (1.7 m) 0.9 20.3 1.8 32.1 2.9 34.1 June 17, 1965 1025 0.0 4.2 Near low tide (—0.4 m) 0.6 4.5 1.4 34.8 2.9 34.8 June 18, 1965 1030 0.0 4.4 Near low tide (—0.3 m) 0.6 5.8 0.9 8.5 0.5 1.0 1.1 33.7 DISTANCE FROM OCEAN (Km) 2.3 34.6

Note: Profiles were measured 0.7 km above mouth of Sixes River estuary (P. E. Reimers, personal commun.).

box attached to a 4-m metal pipe with which the sediment was scooped from the bottom to a depth of about 1 cm. The box could be turned into an upright position with a long steel wire, and by careful retrieval, very little of the fine sediment was lost. The capacity of the box was 1.5 to 2.0 kg, dry weight. The purpose of the third experiment was to find out if sand-size sediment was transported out of the estuary over a sill. Samples were therefore collected from the outflow channel below the sill and from the beach near the mouth of the outflow channel. About 4 hr were allowed after release of the tracer before sampling began. Current Measurements. On the day of the first tracer experiment, current velocities were measured in the upper end of the outflow channel using a Price current meter mounted on a wading rod. Measurements conducted at approximately one-half hour in- 0.5 1.0 1.5 tervals over a 24-hr period were taken vertically throughout the DISTANCE FROM OCEAN (Km) water column on a 15-cm spacing in the deepest part of the channel Figure 3. Salinity and temperature data from Sixes River estuary, Au- that we could safely reach by wading. However, we were fre- gust 1969 (P. E. Reimers, personal commun.). quently forced toward the edge of the channel during the rapid rush of incoming water; thus, the velocities recorded are probably lower than the actual maximum tidal velocities. Rhodamine dye was also 2.00 phi (0.25 mm) with a standard deviation of 0.44 phi, and used to evaluate qualitatively the pattern of current velocities mean size of the heavy concentrate was 2.59 phi (0.17 mm) with a within the estuary. standard deviation of 0.37 phi (statistical measures after Folk, Figure 4 summarizes the results of velocity measurements during 1968). The dyed sand was released into the lower part of the this 24-hr period. Discharge recorded at the gauging station on this outflow channel about 2 hr before peak high tide and about 30 min. date was 0.6 m3/sec; salinity and temperature of water in the es- after tidal currents began to flow into the estuary. Adequate pre- tuary are shown in Figure 3. Meterologic and hydrologic conditions cautions were taken to ensure that dyed grains sank quickly to the were approximately normal for this time of year (no extremes of bottom and that no grains floated on the water surface. wind condition, river discharge, or temperature and salinity of the In the third experiment, approximately 25 kg of dyed heavy sand water). Northeasterly winds produced small ripples on the water and 75 kg of light sand were prewet and placed in bags on the bot- surface; however, wind velocity appeared too low to measurably tom of the estuary about 50 m upstream from the 1.2-m-high sill. affect bottom-current velocities. Figure 4 shows that during ebb About 30 min. before ebb currents began to flow out of the estuary, conditions, downstream currents in the upper part of the outflow the sand was emptied along the bottom where the current velocity channel did not attain velocities greater than 30 cm/sec 15 cm appeared to be strongest. above the channel bed, whereas flood-tide currents attained up- Sampling. In the first two experiments, the objective was to ex- stream velocities at least as high as 84 cm/sec. Current velocities amine movement of sediment during a single flood-tide phase. This were measured on the day preceding (2.1-m tide) and the day fol- required that sampling be delayed long enough to permit maximum lowing (2.3-m tide) the 1971 summer experiment. These data are upstream transport, but yet allow time to complete sampling before also summarized in Figure 4. Maximum flood-tide velocity at 15 reversal of the tidal current. Therefore, only a limited number of cm above the stream bed exceeded 90 cm/sec on the first day and 80 samples could be taken with available manpower and equipment. cm/sec on the last day. Samples were taken along six bank-to-bank traverses and at some Flood-tide velocities were not measured during the 1974 summer isolated points; they were spaced to permit the lower end of the es- experiment, because the experiment involved only transport of sed- tuary to be sampled in the time available. A second set of samples iment by ebb-tide currents. Maximum ebb-current velocity mea- was collected from approximately the same sites ten days after the sured 15 cm above the bottom near the upper end of the outflow first set in order to find out if tracer grains remained in the estuary channel three hours after reversal of tide was 40 cm/sec. Discharge after multiple tidal cycles. data are not available for the 1971 and 1974 summer experiments Samples were collected from a small boat, using a hinged metal owing to discontinuance of the gauging station on the Sixes River.

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1—I—I—I—r~I—r Floats were used to measure current velocity, because water was too deep in the estuary to allow use of a Price current meter. Weighted floats were timed over 30-m courses in various parts of low tide,-0.3m the estuary. The results, shown in Table 2, indicate that surface- current velocities ranged from a maximum of 92 cm/sec in the upper part of the estuary to a maximum of 204 cm/sec in the •4— EBB outflow channel. We were unable to calculate bottom-current velocities from this information; however, comparison of the rate of discharge with that recorded during the first summer experiment high 1ide, 2.0m (39 m3/sec compared to 0.6 m3/sec) suggests bottom-current velocities several orders of magnitude higher than during summer. Aug. 30 Aug. 29 Laboratory Methods

low tide,-0.3m Samples obtained during the tracer experiments were dried, weighed, and sieved to obtain two size fractions: 1.5 to 2.5 phi (0.35 to 0.18 mm) and 2.5 to 3.5 phi (0.18 to 0.09 mm). These size fractions were chosen for analysis because they include the size range of most of the dyed sand grains used in the experiments. Each size fraction was split further to obtain analytical samples of about 5 g, and all of the dyed grains in these splits were identified and high tide, 2.3m counted. The number of tracer grains of each type per gram of sample were then calculated from these data. Fluorescent grains injection of tracer .. were counted using a Syntron Vibratory Parts Feeder and were viewed in ultraviolet light.

Results and Discussion

high tide, July 14 The results of the tracer experiments are summarized in Figures 5, 2.4m 6, 7, and 8. For the purpose of simplifying illustrations, the data from both the 1.5- to 2.5-phi and 2.5- to 3.5-phi size fractions have high tide, July 12 been combined. Data from the second summer experiment are 2.2m omitted from the figures, because the distribution pattern is similar to that of the first summer experiment. Sand-size sediment moves into or out of the estuary depending upon the physiographic and hydrologic conditions of the estuary. During summer when river discharge is low and a sill is in place across the outlet, sediment is transported upstream across the sill too on each incoming tide. Heavy and light particles in the size range of VELOCITY (CM/SEC) 0.09 to 0.35 mm move up the estuary as far as 0.8 km in a single flood tide (Figs. 5 and 6). Figure 4. Plot of current velocities against time, 1969 and 1971 summer experiments; velocity measured 15 cm above stream bed. Comparison of the tracer-sand distribution maps with the bathymetric map (Fig. 2) leads to the conclusion that flood tides move with greatest velocity through the channel that marks the Weather conditions were approximately normal for this time of deepest part of the estuary. Experiments with Rhodamine B dye year, however, which suggests that discharge was probably 1 to 2 showed that incoming tidal water moved across the sill and sank m3/sec, based on a comparison with previous years. quickly below the fresh surface water upstream from the sill. The tidal currents obviously follow the deep channel throughout the es- Winter Experiment tuary as they flow upstream along the bottom. They retain enough of their initial velocity to transport sand-size particles almost 1 km An additional tracer experiment was conducted in February into the estuary. 1970 to test the ability of the estuary to trap and retain fine sand The 1971 summer experiment produced distribution patterns during conditions of high river discharge following a rainstorm. similar to the first experiment. However, tracer grains were carried Discharge at the gauging station increased from 16 m3/sec two days upstream only about 300 m compared to almost 800 m in the first before the storm to 39 m3/sec on the day of the experiment and then experiment. Sampling procedures and river discharge were similar 3 tapered off to 27 m /sec the following day. This was a moderate in the two experiments; thus, this difference in dispersal distance discharge event, inasmuch as discharges of 90 to 140 m3/sec are oc- casionally recorded (U.S. Geol. Survey Water Resources Div., TABLE 2. VELOCITIES OF SURFACE WATER 1970). IN SIXES RIVER ESTUARY, FEBRUARY 8, 1970 Tracer sand was placed in the estuary approximately 1.3 km upstream from the mouth, with precautions again being taken to en- Location in estuary Time Water depth Velocity sure that no grains floated on the water surface. Injection was (m) (cm/sec) timed to coincide with low tide to ensure maximum velocity of 76-92 downstream currents. Sampling was delayed until the following 1.6 km above mouth 1500-1515 1.2-1.5 day to allow sufficient time for dyed grains to disperse thoroughly 0.4 km above mouth 1540-1550 3.0-3.5 87-99 in the estuary. We hoped to determine by this experiment whether Outflow channel 1615-1630 2.4-2.5 153-204 sand-size sediment carried into the upper end of the estuary passed quickly out to sea or was retained in the estuary for at least a short Note: Velocities were determined at low tide by timing a float over a period of time. 30-m course.

imum velocity of 40 cm/sec 15 cm above bottom in the upper part of the outflow channel. The 1974 experiment proves that some sand transported into the estuary by flood-tide currents can be transported out again during ebb tide. However, the fact that tracer grains remained in the estuary at least 10 days after the initial experiment and that the sill progrades upstream during summer suggests a net gain of sandy and muddy sediment in the estuary while the sill is in place. The velocity of flood-tide currents in the upper part of the outflow channel is commonly about twice that of ebb-tide currents, and net upstream transport of sandy sediment is produced by this • SAMPLE SITE flood-dominant time-velocity asymmetry of tidal currents in the es- I I 0- I GRAINS/GM tuary. Klein (1970) reported similar flood-dominant tidal currents over the steep faces of some intertidal sand bars in Minas Basin, • 1-5 Nova Scotia. Klein found that dispersal of grains from a point • > 5 source was radially elliptical and that maximum distance of transport was in the direction of dominant tidal flow over the bars. The Figure 5. Distribution map of light (natural sand) tracer grains based on two-layered water system that develops during ebb tide as a result number of tracer grains per gram of equivalent-size sediment recovered, of the sill may also be a factor in effecting net upstream gain of 1969 summer experiment. The 1.5- to 2.5-phi and 2.5- to 3.5-phi size fractions are combined in this map. Dashed line indicates water level at low sandy sediment. The more static layer of salt water trapped along tide. bottom behind the sill tends to shield bottom sediment from the transporting action of higher velocity fresh-water currents moving over the top of the salt-water layers, as suggested by Clifton and others (1973). The movement of sediment into the mouths of estuaries has been reported from other coastal areas. Kulm and Byrne (1966) found beach sediments 2.4 km upstream from the mouth of the Yaquina River of the central Oregon coast, and Meade (1969) presented

Figure 6. Distribution map of heavy tracer grains, 1969 summer experiment. The 1.5- to 2.5-phi and 2.5- to 3.5-phi size fractions are combined.

appears to be largely due to differences in configuration of the outflow channels. The channel emptied north into the ocean at the time of the second experiment (south in the first) and formed almost a 90° bend at the point where it connected with the main part of the estuary. Duration of the flood tides and velocity of the Figure 7. Distribution map of light (L) and heavy (H) tracer grains re- flood-tide currents were approximately equal during the two exper- covered in 1974 summer experiment. The 1.5- to 2.5-phi and 2.5- to 3.5-phi iments, but the sharp bend in the 1971 channel appears to have dis- size fractions are combined. sipated part of the energy of the currents, thus reducing their effective velocity as they entered the main part of the estuary. Several samples collected from the estuary ten days after the 1969 experiment contained moderate to abundant amounts of tracer grains, indicating that some sand carried into the estuary by flood-tide currents remains through several ebb-tide episodes. However, samples collected from the beach near the mouth of the estuary on the day following the experiment also contained tracer grains. These grains may have been transported back out of the estuary over the sill, or they could have lagged in the outflow channel below the sill until reversal of the tide caused them to move to the beach. • SAMPLE SITE A third experiment was conducted during the summer of 1974 to obtain additional data on the efficiency of sill-related differences in l~l 0-1 GRAINS/GM flood- and ebb-current velocities as a trapping mechanism for • 1-5

sand-size sediment. Tracer sand was placed in the estuary about 50 • > 5 m upstream from the sill, and sampling was carried out in the outflow channel below the sill and on the beach. The results, sum- Figure 8. Distribution map of heavy tracer grains recovered in 1971! 0 marized in Figure 7, show that some heavy and light tracer grains winter experiment. The 1.5- to 2.5-phi and 2.5- to 3.5-phi size fractions are were transported out of the estuary by ebb currents having a max- combined.

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evidence of landward transport of beach sediment into several es- In winter, the bed of the estuary is covered largely by sandy tuaries of the Atlantic coast. Meade also cited the studies of Ter- gravel (Fig. 10). Some sand and mud may accumulate in small areas windt and others (1963) and Van Straaten (1960) on rivers of of the estuary during occasional periods of low discharge condi- northern Europe and Ottman and Urien (1965, 1966) on the Rio tions, or they may be deposited during slack tide conditions when de la Plata; all of these investigators reported evidence of the current velocities are at a minimum. The mud layers may persist for movement of marine sediment into the mouths of estuaries. a time and inhibit further transport of sand, as observed in the Pis- The winter experiment demonstrated that the estuary is an tol River by Clifton and others (1973). However, the dominance of inefficient trapping mechanism for sand-size sediment moving sea- coarse sediment on the estuary floor in winter reflects the greatly ward during high discharge conditions when a sill is absent. No increased velocity during high discharge conditions and the absence light tracer sand was recovered from the estuary in this experiment, of a sand sill near the mouth of the estuary; the sill appears to play and only one sample from the beach near the mouth of the estuary a major role in trapping sand and mud in the estuary during the contained light tracer grains. Heavy tracer grains were recovered summer. Although the texture of sediment in the deeper part of the from some sample sites immediately downstream from the tracer estuary changes rather markedly from winter to summer, sediment injection point and at the mouth of the estuary and along the beach along the margins of the estuary, above the summer low-water line, (Fig. 8). However, no tracer grains of either type were recovered was observed to change relatively little. Textural patterns of this from within the main part of the estuary. The tracer sand moved sediment are determined largely by winter conditions, and little 1.5 km through the lower part of the estuary in less than 24 hr. change occurs during the low-water conditions in summer. These results suggest that fluvial sand-size detritus delivered to the estuary in winter is transported almost immediately to the littoral Primary Sedimentary Structures zone. The estuary is thus only a seasonal trap for sand. Current ripples in the Sixes River estuary were studied over a NATURAL INDICATORS OF period of two years. They develop only during low discharge condi- SEDIMENT TRANSPORT tions in summer and are restricted to the lowermost part of the estuary and to the outflow channel. We were particularly interested Sediment Textural Patterns in investigating the large dunes with wave lengths as great as 1 to 2 m and wave heights as much as 0.5 m (Fig. 11). The grain-size distribution of surface sediment in the estuary was The dunes in the outflow channel are approximately uniformly examined during both low (summer) and high (winter) discharge spaced, and crests commonly are only slightly curved or irregular. conditions to determine the extent of seasonal changes in sediment They may extend completely across the channel but are generally textures. Textural patterns along the margin of the estuary were best developed in curved portions of the channel where tidal cur- mapped by visually estimating percentages of mud, sand, and rents are deflected against the bank. The dunes form during gravel; similar mapping was carried out within the estuary during flood-tide stage, and slip faces dip upstream. Upstream from the sill diving operations. The percentages of mud, sand, and gravel in this orientation persists during the ebb tide, because the layer of samples collected for tracer analysis were also determined by siev- salt water trapped behind the sill appears to inhibit ing (Jones, 1972). seaward-flowing currents from reaching bottom and eroding the These data are combined to produce Figures 9 and 10, which ripples. A similar phenomenon was observed in the Pistol River es- show the approximate areal grain-size distribution under summer tuary by Clifton and others (1973). Downstream from the sill, the and winter conditions, respectively. Although the estuary is floored upstream orientation of the dunes is preserved only in part during dominantly by sandy gravel throughout the year, during low dis- the ebb-tide stage. charge conditions in summer the surface of this gravel is covered by Initial ebb currents are slow, because brackish water from the es- a layer of sand or muddy sand that is as much as 20 cm thick (Fig. tuary flows seaward over the top of salt water still pulsing slowly 9). The sand is thickest and has the least amount of mud in the into the estuary. With continued drop of the tide, velocity of down- lower part of the estuary near the sill. In other parts of the lower stream currents increases, particularly in the deeper part of the estuary some mud may be mixed with the sand or may form a thin outflow channel, and the currents eventually reach bottom and layer on top of the sand. The mud commonly contains marine or- begin to erode and modify the dunes. By this time, however, falling ganic debris, such as bits of seaweed and jellyfish, and much of it is water level in the channel has either exposed dunes in shallower probably of marine origin. The very low turbidity of the river water parts of the channel, usually along the edges, or has left them in during low discharge conditions suggests little transport of sus- very shallow water of low current velocity. Consequently, dunes in pended fluvial sediment (U.S. Geol. Survey Water Resources Div., some parts of the outflow channel may be preserved with little 1968, 1969, 1970). modification through a complete tidal cycle.

Figure 9. Sediment texture map of Sixes River estuary, summer 1969. Figure 10. Sediment texture map of Sixes River estuary, winter 1970.

Terwindt (1970) reported that the asymmetry of ripples (30 to -OCEAN ESTUARY 100 cm in height) in tidal channels of southwestern Netherlands may change with the turn of the tidal currents, but not in every case, and the asymmetry of ripples (100 to 200 cm in height) usually does not change with the turn of the tide. Terwindt concluded that if the difference between the maximum ebb velocity and the maximum flood velocity in the lowermost 1/10 of the vertical water reactivation surface column exceeds 0.10 m/sec, the ripples are asymmetric, and the direction of movement of the ripples is then determined by the flow outline of dune during flood tide direction with the highest maximum velocity. He further concluded outline of dune after ebb tide 0.5 that a rapid shift of the ripple asymmetry after the turn of the tide is I not likely and that the asymmetry of the ripple is determined by the foreset beds METERS prevalent tidal current. Ebb currents in some parts of the outflow channel of the Sixes Figure 12. Diagrammatic sketch of dunes, showing approximate loca- estuary do reach sufficient velocity to erode the bottom and modify tion at which box cores (Fig. 13) were taken. the bed forms. Sand is eroded from the crests of dunes and redepo- sited in part on the seaward-facing side. This eventually produces a seaward-oriented slip face, with the result that a group of dunes may retain an upstream asymmetry near the edge of the channel but take on a seaward-facing orientation in the middle of the channel where maximum modification occurs during the ebb tide (Fig. 11). The shape of the dune crests is also changed during this proc- ess; they become S-shaped, being concave upstream near the edges of the outflow channel and concave downstream in the central part of the channel. The internal structure of several sets of dunes was investigated using the box-coring technique described by Bouma (1969, p. 309) Figure 13. Photograph of box cores from large dune (Fig. 12) showing and Clifton and others (1971). The main purpose of the box-core internal structure. Note seaward-dipping foreset beds in upper part of cores study was to investigate the relationship of internal stratification to A and B and reactivation surface (arrow). external morphology of the bed forms, particularly those dunes modified by ebb-tide currents. An example of the results is shown in Figures 12 and 13. Three cores were taken along a line normal to the modified dunes, the dominant direction of sediment transport the crest of a large dune that had been modified during ebb tide to during a tidal cycle, when dunes are forming, is upstream into the have a seaward-dipping slip face (Fig. 12). Core A, collected near estuary. These observations thus suggest that during low discharge the crest of the dune, shows a few seaward-dipping foresets at the conditions in summer, a net gain of sediment in the estuary is top, which truncate upstream-dipping foresets in the lower part of achieved during each tidal cycle; this corroborates the results of the the core along a sharp boundary (Fig. 13 A) that resembles the reac- tracer studies and other observations reported in preceding sec- tivation surfaces described by Collinson (1970) and Klein (1970) tions. and the diastems of Boersma (1969). Core B (Fig. 13B) also shows a weakly defined reactivation surface and a few foresets that dip in Heavy Minerals a seaward direction; however, the foresets in core C (Fig. 13C), taken near the upstream end of the stoss side of the dune, all dip in Boggs (1969) noted evidence of flood-tide dominance of sedi- an upstream direction. ment transport in the Sixes estuary based on the heavy-mineral Several dunes exhibited seaward-dipping foreset beds near the composition of the sediment. Samples from the upper part of the crest, as in the example above; however, most of the foresets in all estuary, the lower estuary, and the beach show distinct differences of the dunes dipped in an upstream direction. The internal struc- in the relative abundance of certain heavy minerals, as indicated in tures thus indicate that despite the seaward-oriented slip faces of Table 3. The ratio of total amphibole to total clinopyroxenfe increases slightly in the lower part of the estuary and increases markedly on the beach. Epidote, garnet, and hypersthene likewise increase significantly in abundance in the lower part of the estuary and on the beach. Marked differences in appearance of certain heavy minerals from the lower estuary and beach, as compared to the same minerals in upstream samples, were also observed. Some of the green hornblende from the lower estuary and beach is very highly colored and almost opaque, but no such hornblende was found in samples from the remainder of the Sixes River. Hypersthene in the lower estuary and beach sand is also generally more elongate and euhedral, and pleochroism is much stronger. Further, heavy minerals from the lower estuary are moderately well rounded, similar to those of the beach, whereas most heavy mineral grains in the upper estuary and other parts of the Sixes River are angular to subangular. These differences in relative abundance and physical properties indicate that the heavy-mineral suite in the lower part of the estuary has been affected by mixing with heavy minerals from a source outside the drainage basin of the Sixes River. This provides Figure 11. Dunes in outflow channel of Sixes River estuary, looking up- additional evidence of the importance of the flood-tide-dominated stream at low tide. bottom-current system in establishing sediment-dispersal patterns.

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TABLE 3. AVERAGE PERCENT COMPOSITION OF SELECTED phases of the field work, in particular, H. E. Clifton of the U.S. NONOPAQUE HEAVY MINERALS FROM THE SIXES RIVER Geological Survey and P. E. Reimers of the Oregon Fish Commis- ESTUARY AND ADJACENT BEACH sion. L. R. Kittleman, I. N. McCave, and G. de V. Klein reviewed the manuscript and made many helpful suggestions. Total Total A/CPX Epidote Garnet Hyper- amphibole clino- ratio sthene pyroxene REFERENCES CITED

Upper estuary 27 52 0.5 6 3 0.7 Boersma, J. R., 1969, Internal structure of some tidal megaripples on a Lower estuary 27 42 0.6 11 7 2 shoal in the Westerschelde Estuary, the Netherlands: Geologie en Mi- jnbouw, v. 48, p. 409-414. Beach 35 24 1.5 17 10 5 Boggs, Sam, Jr., 1969, Distribution of heavy minerals in the Sixes River, Note: Minerals are in the 0.177- to 0.125-mm fraction of sediment. After Curry County, Oregon: Ore Bin, v. 31, p. 133-150. Boggs (1969). Bouma, A. H., 1969, Methods for the study of sedimentary structures: New York, John Wiley & Sons, 458 p. Clifton, H. E., Hunter, R. E., and Phillips, R. L., 1971, Depositional struc- CONCLUSIONS tures and processes in the non-barred high-energy near-shore: Jour. Sed. Petrology, v. 41, p. 651-760. The dispersal of sediment in the estuary of the Sixes River is Clifton, H. E., Phillips, R. L., and Hunter, R. E., 1973, Depositional struc- strongly affected by seasonal changes in physiography and hydrog- tures and processes in the mouths of small coastal streams, southwest- raphy of the estuary. ern Oregon, in Coates, D. R., ed., Coastal geomorphology: State Univ. A sand sill builds across the mouth of the estuary in summer New York Pubs, in Geomorphology, p. 115—140. when river discharge is low. This sill accentuates the flood-tide Collinson, J. D., 1970, Bedforms of the Tana River, Norway: Geog. An- naler, v. 52, p. 31—56. dominance of the tidal currents, as well as creates a sharply Crickmore, M. J., 1967, Measurement of sand transport in rivers with spe- stratified two-layer water system during ebb tide. cial reference to tracer methods: Sedimentology, v. 8, p. 175-228. Tracer data show that when a sill is in place, sand-size marine Folk, R. L., 1968, Petrology of sedimentary rocks: Austin, Texas, Hem- sediment is transported approximately 1 km into the estuary in a phills, 170 p. single flood-tide episode. Some sand moves from the estuary back Ingle, J. C., Jr., 1966, The movement of beach sand: An analysis using to the beach during ebb tide; however, grains remaining in the es- fluorescent grains, in Developments in sedimentology, Vol. 5: New tuary ten days after injection of the tracer show that part of the sed- York, Elsevier, 221 p. iment deposited by flood tides remains through multiple tidal cy- Jones, C. A., 1972, A study of sediment transport and dispersal in the Sixes cles, thus effecting a net gain of sediment in the estuary. This gain River estuary, Oregon, utilizing fluorescent tracers [Ph.D. thesis]: Eugene, Univ. Oregon, 126 p. appears to be due to the combined effects of the flood-dominated Kennedy, V. C., and Kouba, D. L., 1970, Fluorescent sand as a tracer of system of bottom tidal currents and the presence of a somewhat fluvial sediment: U.S. Geol. Survey Prof. Paper 562-E, 13 p. static bottom layer of salt water that is trapped behind the sill dur- Klein, G. de V., 1970, Depositional and dispersal dynamics of intertidal ing ebb tide. sand bars: Jour. Sed. Petrology, v. 40, p. 1095-1127. Dunes develop during flood tide in the outflow channel and in Kulm, L. D., and Byrne, J. V., 1966, Sedimentary response to hydrography the lower estuary under summer conditions; initially they have slip in an Oregon estuary: Marine Geology, v. 4, p. 85-118. faces that dip upstream. Those dunes within the estuary are little Meade, R. H., 1969, Landward transport of bottom sediments in estuaries affected by ebb-tide currents, because ebb currents are inhibited of the Atlantic coastal plain: Jour. Sed. Petrology, v. 39, p. 222-234. from eroding the dunes by the presence of the more static bottom Ottman, François, and Urien, C. M., 1965, Observaciones preliminares sobre la distribución de los sedimentos en la zona externa del Río de la layer of salt water. Dunes in some parts of the outflow channel are Plata: Acad. Brasileira Ciênc. Anais, v. 37, suppl., p. 283-288. modified during ebb tide, which results in seaward-dipping slip 1966, Sur quelques problèmes sédimentologiques dans le Rio de la faces. However, box cores reveal that internal foresets, except Plata: Rev. Géographie Phys. et Géologie Dynam. (2), v. 8, pt. 3, p. those near the crests of the modified dunes, dip mainly in an up- 209-224. stream direction, suggesting net upstream sediment transport. Pritchard, D. W., 1955, Estuarine circulation patterns: Am. Soc. Civil En- Some heavy minerals in sand from the lower part of the estuary gineers Proc., v. 81, separate no. 717, 11 p. show evidence of derivation from a source outside the drainage Reimers, P. E., 1973, The length of residence of juvenile Chinook salmon in basin of the Sixes River. These minerals must have been trans- Sixes River, Oregon: Oregon Fish Commission Research Repts., v. 4, ported into the estuary by flood tides. no. 2, 43 p. Teleki, P. G., 1966, Fluorescent sand tracers: Jour. Sed. Petrology, v. 36, p. Available evidence thus indicates a dominance of upstream sed- 468-485. iment transport in summer which is related to the presence of a sill Terwindt, J.H.J., 1970, Observation on submerged sand ripples with that forms in the outflow channel. However, fluvial discharge in- heights ranging from 30 to 200 cm occurring in tidal channels of S.W. creases several orders of magnitude during winter storms, bringing Netherlands: Geologie en Mijnbouw, v. 49, p. 489-501. about erosion and destruction of the sill. During periods of high Terwindt, J.H.J., Dejong, J. D., and Van Der Wilk, E., 1963, Sediment river discharge, flood-tide currents are ineffective in moving marine movement and sediment properties in the tidal area of the Lower sediment more than a few metres into the estuary, and strong Rhine (Rotterdam waterway): Koninkl. Nederlands Geol. Mijnbouw downstream currents sweep sand-size fluvial sediment through the Genoot. Verh., Geol. Ser., v. 21, p. 243—258. estuary and out to sea. These currents also remove all or most of U.S. Department of Commerce, 1974, Official 1974 tide table for Astoria and vicinity: Astoria, Oregon, Astorian-Budget Pub. Co. the fine sediment deposited during summer conditions; thus, little U.S. Geological Survey Water Resources Division, 1968, 1969, 1970, sustained accumulation of fine sediment takes place in the estuary, Water resources data for Oregon, parts 1 and 2 (surface water rec- except as interstitial filling of gravel deposits. ords). Van Straaten, L.M.J. U., 1960, Transport and composition of sediments, in ACKNOWLEDGMENTS Symposium Ems-Estuarium (Nordsee): Koninkl. Nederlands Geol. Mijnbouw Genoot. Verh., Geol. Ser., v. 19, p. 279-292. Funds for this research were provided in part by the Marine

Geology Branch of the U.S. Geological Survey, the University of MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER 30, 1974 Oregon Department of Geology, and the Research Institute of REVISED MANUSCRIPT RECEIVED APRIL 18, 1975 Chadron State College. A number of persons assisted in various MANUSCRIPT ACCEPTED APRIL 30, 1975

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