Prepared in cooperation with the Bolinas Harbor District and Marin County A Summary of Preliminary Studies of Sedimentation and Hydrology in Bolinas Lagoon, Marin County, California

A Summary of Preliminary Studies of Sedimentation and Hydrology in Bolinas Lagoon, Marin County, California

By J. R. Ritter

G E 0 l 0 G I C A l S U R V E Y C I R C U l A R 627

Prepared in cooperation with the Bolinas Harbor District and Marin County

Washington 1970 Department of the Interior WALTER J. HICKEL, Secretary

Geological Survey William T. Pecora, Diredor

Free on application to the U.S. Geological Survey, Washington, D.C. 20242 CONTENTS

Page Page Abstract ...... 1 Studies being conducted by the Introduction and acknowledgments ...... 1 U.S. Geological Survey--Continued Description of Bolinas Lagoon ...... 2 Littoral drift ...... 6 Possible sources of sediment in Circulation in Bolinas Lagoon ...... 6 Bolinas Lagoon ...... 3 Bottom topography ...... 7 Studies being conducted by the Sediment movement ...... 8 U.S. Geological Survey ...... 3 Grain-size parameters and Fresh-water inflow ...... 3 specific weight of lagoonal sediments ....8 Fluvial sediment transport to the lagoon ... .4 Mineralogy of Bolinas Lagoon sediments, Sediment transport through the tidal inlet .. .4 by Edward J. Helley ...... 9 Tidal measurement of June 22, 1967 .....5 Summary ...... 12 Tidal measurement of Selected references ...... 13 October 24-25, 1967 ...... 5

ILLUSTRATIONS

Page Figures 1-5. Maps showing: 1. Location of data sites and area of Bolinas Lagoon ...... 17 2. Generalized geology of area around Bolinas Lagoon ...... ·.... 18 3. Development of Bolinas Lagoon, 1858 to 1950 ...... 19 4. Circulation in Bolinas Lagoon, September 6, 1967. A, Floodtide. B, Ebbtide ....20 5. Distribution of Monterey Shale fragments, Bolinas Lagoon, 1967 ...... 22

TABLES

Page Table 1. Wentworth scale of particle sizes ...... 9 2. Description of sorting of sediments ...... 9

III

A Summary of Preliminary Studies of Sedimentation and Hydrology in Bolinas Lagoon, Marin County, California

By J. R. Ritter

ABSTRACT studies conducted from May 1967 through May 1968 and describes studies now in progress. A The U.S. Geological Survey is investigating more detailed description of the studies may be sedimentary and hydrologic conditions in Bolinas found in Ritter ( 1969a and 1969b ). Lagoon, a 1,100-acre lagoon 15 miles northwest of . The program began in May 196 7 and will The investigation is divided into seven studies, continue into 1970. Only the study results analyzed each of which is related to sedimentation and before June 1968 are summarized in the report. hydrology. The separate studies are designed to Two series of measurements of suspended-sediment provide information on (1) the fresl'·Water load and water discharge in the lagoon inlet showed that inflow into the lagoon, (2) the quantity of much of the suspended sediment is sand and that the fluvial sediment transported into the lago'1n, (3) average velocity was as much as 4.7 feet per second. the quantity of sediment transported into and Littoral drift near the inlet was generally toward the out of the lagoon through the tidal inlet, (4) the inlet, whereas farther from the inlet the pattern is direction of littoral drift near the inlet, (5) the irregular. Circulation velocities in the lagoon decrease circulation of water within the lagoon, ( 6) the rapidly away from the inlet, but probably remain high bottom topography, and (7) the sediment enough to erode bottom sediment along the channels. In movement within the lagoon. The data and most of the lagoon median size of bottom sediment was conclusions of each study will be correlatr.d with fine sand. Sediment was derived chiefly from Monterey those of other studies conducted l'Y the Shale. Geological Survey and other investigato:t"s. The INTRODUCTION AND data also are used, whenever possn~•~, to ACKNOWLEDGMENTS interpret past and future conditions in the lagoon. In May 1967 the U.S. Geological Survey began an investigation of the hydrology and The investigation by the Geological Survey is sedimentation in Bolinas Lagoon in cooperation conducted under the general supervisior of R. with the Bolinas Harbor District and Marin Stanley Lord, district chief in charge of County. The lagoon, a potential small craft water-resources investigations in Califorria, and harbor, seems to be filling with sediment. The Loren E. Young, chief of the Menlo Park intended purposes of the investigation are to subdistrict office. The manuscript benefited determine the rate of sedimentation, to describe substantially from the criticism of Jos~ph R. the sedimentation processes, and to define the Curray and D.W. Hubbell. The assistance of sources and movement of sediment in the Winchell Smith and William M. Brown III also is lagoon. This report summarizes the results of gratefully appreciated. A special acknowledgment

1 is pJven to Archie A. Goldsberry, who patiently changes position. During extreme low water the and cheerfully helped with many phases of the bar is exposed, creating a navigational hazard. investigation. Except for Seadrift, Bolinas Lagoon is DESCRIPTION OF BOLINAS LAGOON encircled by steep hills that rise to altitudes of more than 1,500 feet on the east and about 200 Bolinas Lagoon (fig. 1), bordered by the feet on the west. Most of the virgin growth on communities of Bolinas and Stinson Beach, is the hills east of the lagoon was logged during the about 15 miles northwest of San Francisco. The 1850's lagoon has a triangular shape with maximum dimensions of about 1 by 3 miles. Most of the The drainage area tributary to Bolinas Lagoon lagoon is tidal flat. At mean lower low water, is 16.7 square miles and has maximum about 70 percent of the lagoon area is exposed dimensions of about 3 by 9 miler- The largest to the air and at mean higher high water only tributary, , drains about half two islands remain above water. The larger the basin and is the only tributary to enter the island, , covers about 100 acres; the lagoon from the west. The extent of its delta, at other, near the mouth of McKinnan Gulch, is least 1,500 feet into the lagoon, indicates that insignificant in size. Pine Gulch Creek transports more sediment to the lagoon than any other tributar:.r. The several Bolinas Lagoon is protected from the ocean eastern tributaries are short and steep and have by a long sandy spit now known as Seadrift, the drainage basins of less than 1 square mile. They site of several luxurious homes. Despite the have small deltas, but possibly their average recent development of Seadrift for home sites, annual contribution of sediment pr-r square mile much of it is topped with sand dunes partly of drainage area is high because of their steep covered by vegetation. Seadrift Lagoon and its gradient and their sparsely vegetated drainage northern adjacent neck of land (fig. 1) were basins. created by dredging and fill operations undertaken in the early 1960's and were the Bolinas Lagoon straddles the San Andreas only major manmade revisions in the character fault. During the 1906 earthquakr-, the relative of Bolinas Lagoon in recent years. movement along the fault was as rr.~1ch as 1 foot vertically and 13.5 feet horizortally in the The tidal inlet to the lagoon is between the Bolinas Lagoon basin (Gilbert, 19('~, p. 67, 70, western extremity of Seadrift and a high cliff, 72). The displacement must have changed the which is part of the town of Bolinas. The inlet is regimen of at least some of the streams and bounded by sandy beaches; however, during probably increased erosion because of stream winter storms the western beach often is entirely readjustment. eroded away, leaving bedrock abutting the high cliff. During periods of calm the beach is The rock west of the lagoon cc 11sists chiefly reestablished. The middle of the inlet usually is of the Monterey Shale of Miocene age and the lined with cobbles and bedrock. To pictorially Merced Formation of Pliocene an1 Pleistocene record changes in the configuration of the inlet, age; east of the lagoon the rock consists of the the Geological Survey has mounted a time-lapse Franciscan Formation of Cretaceous and camera on a cliff overlooking the inlet. The Jurassic age. Sedimentary deposits of camera has taken photographs during daylight at Quaternary age occur throughout the drainage intervals of every hour or half hour since area (fig. 2). January 1968. Precipitation at Bolinas Lagoon is 30 to 40 Just outside the inlet is a shallow inches per year (Rantz, 1968), m'lst of which crescent-shaped sandbar, which constantly falls from November through Apr:l. Almost no

2 rain occurs during the summer months; however, quantity of sediment transported deJ: ends on fog is more common during June and July than such things as size of drainage area, erodibility during other months. of soils, geology, intensity of rainfall, land use, and vegetation. Although the tributaries to Temperatures range from about 40° F to Bolinas Lagoon have small drainage areas, the 90° F, and the climate is generally mild. The lagoon itself is not large, so that the sediment prevailing winds are from the northwest, but contribution by the tributaries may substantially gales from the southeast have reached help to fill the lagoon. velocities greater than 75 miles per hour. An onshore breeze also often occurs. Wind transports sediment to Bolinas Lagoon when it blows from the south across the POSSIBLE SOURCES OF SEDIMENT unconsolidated sand deposits of Seadrift. The IN BOLINAS LAGOON quantity of sand moved by wind may be considerable. Lagoons are temporary features along a coast. A generalized history of a lagoon may begin Comparison of older maps of Bolinaf Lagoon with a river mouth drowned by a rising sea level. with more recent ones shows that the lagoon has Littoral drift creates a spit or barrier bar across been filling with sediment for at least 100 years. the mouth so that its access to the ocean is Figure 3 shows maps of Bolinas Lago-:>n from restricted to a narrow inlet. The lagoon begins to 1858 to 1950. Although the accuracy in fill with sediment and becomes a tidal flat with mapping has improved, large-scale features channels carrying much of the tidal water. The would not be omitted from the older maps. In filling continues until the lagoon becomes a salt the maps prior to 1900, no delta exists at the marsh as vegetation grows on the rising tidal mouth of Pine Gulch Creek, and Kent Island is flats. Eventually, the lagoon becomes a flat plain absent. However, early charts show parts of that may be eroded by waves lapping against its Kent Island as above lower low wrter but seaward edge. The life of a lagoon depends on probably below mean sea level. Attempts to the rate at which sediment is entrapped in the estimate erosion and deposition will be made in lagoon. later studies by comparing maps and charts published during the past 100 years. Bolinas Lagoon, the seaward end of a rift valley created by the , is now STUDIES BEING CONDUCTED BY THE probably in the stage of a tidal flat that is U.S. GEOLOGICAL SURVEY becoming a salt marsh. All seven studies being conducted by the U.S. There are many possible sources of sediments Geological Survey are now in progress, and some being deposited in lagoons. The three major of the data have been analyzed. In the following transporters of sediment to a lagoon usually are sections the methods used in each study and littoral drift, upland streams, and wind. At early results are described. Bolinas Lagoon the cliff between the inlet and Duxbury Point (fig. 3) is eroding at the rate of FRESH-WATER INFLOW about 2.3 feet per year (A. J. Galloway, oral Since May 1967 the flows of three tributaries commun., 1967). The sediment supplied by the to Bolinas Lagoon have been recorded. At Pine erosion of the cliff may be carried as littoral Gulch Creek and Morses Gulch (fig. 1) gaging drift to the inlet and into the lagoon. The sand stations, equipped with Fisher-Porter digital of Seadrift may also be carried to the lagoon as recorders, were constructed. The water level or littoral drift. stage of the two streams is expressed as a gage height and is punched on a tape every 15 Streams tributary to Bolinas Lagoon transport minutes. The tape is read later by a computer sediment to it, especially during storms. The 3 that determines, for each day, the average gage expression of the concentration of suspended height and other parameters. Discharges of the sediment in the stream at the time the sample -streams are measured at least monthly during was collected. The average concentration, low water and more frequently during high expressed in milligrams per liter, is multiplied by water to define a stage-discharge relation. From the water discharge and by a urits conversion the stage-discharge relation and the continuous constant to obtain the suspended-sediment load, records of stage, daily, monthly, and yearly expressed as tons per day. Daily, monthly, and discharges are computed for each stream. As of yearly loads are computed. June 1968 discharge records for these streams had not yet been computed. Sampling of the three streams began in May 1967. However, as of June 1968 At Audubon Creek a crest-stage gage was suspended-sediment loads had not yet been installed. Only the highest stage of the stream calculated. between visits to this gage is recorded. However, the discharge is measured at least monthly. SEDIMENT TRANPORT THROUGH THf. TIDAL INLET FLUVIAL SEDIMENT TRANSPORT TO THE LAGOON Measurements of the quantity of sediment The quantity of fluvial sediment being moving in and out of the inlet to the lagoon transported to the lagoon by Pine Gulch Creek, during tidal periods were made on June 22, Morses Gulch, and Audubon Creek is being 1967, October 24-25, 1967, and May 16-17, calculated. Because the quantity of sediment 1968. The results of the first two sets of moved by streams is usually related to the water measurements are described in tJjs report. The discharge, the data on water discharge at the results of the third have not yet been analyzed. three gaging stations are used in conjunction with data from sediment sampling to determine A moving-boat technique, in vrhich the boat the sediment load. moved at a rate of about 10 fpm (feet per minute) along a cable stretched across the inlet, Samples of suspenqed sediment, that was used to measure water flovr at the inlet. sediment which is carried in suspension in water, Two current meters, which were suspended from are collected by an observer at least daily during the boat and were adjusted for d'ange in depth, high runoff and less frequently during low measured the velocity of the current runoff at Pine Gulch Creek. At Morses Gulch continuously at 0.2 and 0.8 of the depth. and Audubon Creek samples are collected Direction of flow (sensed by a current-orienting monthly except during storm periods, when compass), depth of water, ftation, meter sampling may be more frequent. revolutions, and time were continuously registered on a panel of counters aboard the The suspended-sediment samples are collected boat. The panel was photographed about every throughout the entire depth by a technique 30 seconds by an automatically triggered called depth integration. Thus, the sample is 35 -mm (millimeters) camera. Tho. difference in representative of an entire column of water. the total count of meter revolutions per 30 Because the concentration of suspended seconds is used to determine the average velocity sediment may vary across a stream, samples are for the increment of width traversed. Then by take across a given section of the stream. The multiplying the average depth of the increment samples, mixtures of water and suspended by its width, the area of the increment is sediment, are sent to a laboratory. First they are obtained. The area multiplied by the velocity weighed. Then the suspended sediment is corrected for the angle of the current to the filtered off, dried, and weighed. The ratio of the measuring section is the flow for the increment. weight of the suspended sedhnent to the weight The total flow is the sum of the flows of the of the sample converted to a volume is an increments. The above calculatior~ are made by

4 computer from data tabulated from the film. The average concentration of suspended Measurements of flow took from 9 to 25 sediment during flood tide ranged from ? 4 to 98 minutes. The measured flow was assumed to mg/1 (milligrams per liter) and only frmn 34 to equal the flow at midpoint of the time interval 49 mg/1 during ebbtide. The concentrrtion of of measurement. suspended sediment finer than sand (<0.0<:2 mm) remained almost constant throughout the cross While flow was being measured, section of the channel during each measurement. suspended-sediment samples were being Any significant increase in concentration, collected on the same boat. Depth-integrated especially near the banks, was attributat 1e to an samples were collected at five to seven stations increase in the quantity of sand carried in during each measurement. On another boat, also suspension. Thus, the major immediate sources in the inlet, point samples of sediment were of most of the transported sediment ~·~re the collected from various depths, and samples of beaches adjacent to the channel. sediment moving along the bottom of the inlet were collected with an Arnhem sampler so that TIDAL MEASUREMENT OF OCfOBER 24-25,1967 bedload could be calculated. The temperature of the water in the inlet was also measured. The second tidal measurement at the inlet was made on October 24-25, 1967, for an entire TIDAL MEASUREMENT OF JUNE 22,1967 tidal day of two floodtides and two el:lbtides. The measured average velocity reached On June 22, 1967, the first measurement of maximums of 2.7 and 3.2 fps for the flood tides the sediment load for a tidal period was made at and of 4.7 and 1.1 fps for the ebbtides. The the inlet. The tide there is a mixed, maximum average velocity during the major predominantly semidiurnal type. Water flow was ebbtide was thus about 1.5 times that of measured during flood tide with a range of stage flood tide. of about 4 feet and during an ebbtide with a range of only 1 .3 feet. Seventeen flow The maximum measured flow was 5,280 cfs measurements were made. for the ebbtide from higher high water (+2.14 feet) to lower low water ( -2.37 feet). For the The maximum average velocity during minor ebb tide from lower high water (+0.85 floodtide was about 3.5 fps (feet per second), foot) to higher low water (+0.68 foot) the and during ebbtide, about 2.4 fps. The maximum flow was only 1,440 cfs. About 2,330 maximum measured flow was 5,810 cfs (cubic acre-feet of water flowed out of the lagoon feet per second) during floodtide and 3,720 cfs during the earlier ebbtide, which lasted 9 hours, during ebbtide. During the 6-hour period of and only 180 acre-feet during the later one, floodtide, 2,000 acre-feet of water flowed in which lasted 3 hours. through the inlet and during the 4 hours of ebbtide, 770 acre-feet flowed out. The maximum measured flows fcT both floodtides were almost equal; 4,180 cfs for a The quantity of suspended sediment period from higher low water (+0.93 foot) to transported into the lagoon was greater than the higher high water (+2.14 feet) and 4, 230 cfs for quantity that left because ( 1) the duration of that from lower low water (-2.37 feet) to lower the floodtide was longer than that of the high water ( +0.85 foot). The volume cf water ebbtide, (2) the quantity of incoming water was that flowed into the lagoon dur:ng the much greater than the outgoing, and (3) the flood tides also was almost equal, 1,340 and average concentrations of suspended sediment 1,330 acre-feet, even though the later floodtide were higher during floodtide. About 152 tons of lasted an hour longer than the earlier. suspended sediment entered the lagoon and a bout 36 tons left; presumably, 116 tons The highest suspended-sediment load was remained in the lagoon. transported during the major ebbtide. About 5 495 tons of suspended sediment was transported From the few observations it VJas concluded then, whereas each floodtide transported about that the direction of littoral drift, as determined 85 tons and the minor ebbtide transported only from the direction of the longsho·e currents, is 5 tons. Thus, there was a net removal of about not constant because of the changing directions 330 tons of sediment from the lagoon during the of the winds and breaking waves. Near the inlet 25 hours. the circulation is generally toward the mouth of the lagoon and is associated with the water The average concentration of suspended entering and leaving the lagoon. sediment ranged from 18 to 572 mg/1 during the major ebbtide and only from 5 to 26 mg/1 Littoral drift was also studied by tracing the during the minor ebbtide. In contrast to the movement of fluorescent san~~ that were differences in the magnitude of the ranges of deposited on the beaches. The dyP-d grains were average concentration during the ebbtides, the spread on the beach near Stinson Beach on July ranges of average concentration of suspended 23 and on the beach near Bolina~ on July 24, sediment during the floodtides were almost 1968. Results of the tracing have not yet been equal. Average concentration ranged from 11 to completed. 112 mg/1 for the earlier floodtide and from 9 to 125 mg/1 for the later floodtide. The major CIRCULATION IN BOLINAS LAGOON ebbtide thus had a maximum average concentration almost 5 times that of the The circulation of water in Bolinas Lagoon maximum during a floodtide. also was studied by following the movement of dye. The bright red dye, rhodamine B, was The beach along the western end of the spit dropped from a helicopter into nany parts of was the chief contributor of suspended sediment the lagoon on September 6, 1967. The dye was on October 24-25, 1967. The highest usually dropped so that spots of dye formed a concentrations of supended sediment line perpendicular to the channel in the lagoon. consistently were near the spit. Concentrations Three series of dye drops were made ( 10: 15 at the middle of the inlet and near the west bank a.m., 1:05 p.m., and 3:15p.m.) ~nd 17 sets of were lower than those near the east bank color aerial photographs were t~ \ren from an throughout the sampling period. As on June 22 airplane at an altitude of 6,0~0 feet. By the concentration of suspended sediment fmer comparing the location of the center of the than sand (<0.062 mm) remained nearly uniform spots of dye in each sucressive set of across the inlet, but the concentration of sand photographs, the movement of the dye was fluctuated greatly with the highest measured. concentration occurring near the east bank. When the circulation study began, most of the LITTORAL DRIFT flow was confined to the main channels and remained so until the rising tide covered most of Littoral drift is defined as the movement of the lagoon. As water entered Bolinas Lagoon, material in the littoral zone (the zone between some of it moved into Bolinas Channel. The high and low tides) under the influence of waves greater part, however, moved eastward between and currents (Beach Erosion Board, 1961, p. Kent Island and Seadrift where. the surface Al7). By placing dye and floats in the surf zone, velocities were greater than 100 fpm, the highest the direction of the longshore current, which is measured, but by the time the flo'" had reached indicative of the direction of littoral drift, can the vicinity of the eastern shore, it" velocity had be determined. The direction of the longshore decreased substantially (fig. 4.A). Near the currents near the inlet to Bolinas Lagoon was eastern shore the water divided; some flowed noted in May, September, and December 1967. southeast and some, northwest. Rhodamine B dye and brightly colored rubber balls were used.

6 Velocities in the southeastern part of the Perhaps the greatest significance of the lagoon were less than 50 fpm, even though rnuch circulation study is its applicability to the study of the water was confined to channels. Before of movement of sediment. Acco~ding to the water had flowed very far into this part, its Hjulstrom (1935), velocities of 100, SC, and 10 velocity had been reduced to even less than 10 fpm are capable of transporting particles having fpm. diameters as large as 7 mm, 3.5 mm, and 0.7 mm, respectively. Although Hjulstrom's The major flow was northwestward in a relations of velocity to movement of particles of channel along the east shore of the lagoon .. The different sizes are oversimplifications of surface velocity there was 50-100 fpm, but complex phenomena and although other when the water reached the upper basin, the flow investigators, such as Sundborg ( 1956), have decelerated rapidly to less than 10 fpm. shown different relations, Hjulstrom's relations may be used as a basis for suggesting areas of Dye dropped in Bolinas Channel did not get erosion and deposition in the lagoon. into the upper basin, but moved northwtrd into several small channels near the delta of Pine The average particle size in and around the Gulch Creek. Therefore, the water that entered lagoon is about 0.2 mm. The Hjulstrom relations the upper basin must have been mostly from indicate that sediment of this size can be between Kent Island and the east side of Bolinas transported at prevailing velocities almost Lagoon. anywhere in the lagoon except perhaps in the upper basin and in the southeastern part of the The floodtide-circulation studies lasted from lagoon. If the velocities are reduced to r~ro for a 10: 15 a.m. to about 1 :20 p.m. The circulation short time at slack tide and most of the reversal was first noted in the upper basin about transported sediment is deposited, then a 30 minutes before it was noted in many other velocity of about 30 fpm must be reached parts of the lagoon. Most of the first observed (Hjulstrom, 1935) to erode a particle of 0.2 mm reversals were in the western part of the lagoon. from the bed of the lagoon. Hjulstrom's relation Evidently, at the time that the first indications of velocity to particle size shows that the upper of ebbtide were observed, the water was still basin and the southeastern part of the lagoon are advaincing along the east side of the lagoon, but depositional areas, whereas most of the other already was retreating along the west side: and areas are erosional during average tides. Another moving out through Bolinas Channel. area that must be depositional, althC'llgh dye movement was not measured in its vicinity, is During the ebbtide period of observation, the the tidal marsh behind Kent Island; because of velocities of the water were less than those its very nature, it would not be conducive to during floodtide (fig. 4B) because most of the high velocities. outward flow was a sheet flow and not the channel flow observed during much of the BOTTOM TOPOGRAPHY floodtide. Also, a larger volume of water per A hydrographic survey was made to unit time moved into the lagoon during flood tide determine the topography of the bottom of than moved out during ebbtide. The velocities at Bolinas Lagoon and to provide a base for ebbtide were higher at constrictions such as that comparison with past and future surveys. In near the inlet and at Bolinas Channel. August and September 1967, profile; of the Unfortunately, the period of observation did not bottom were surveyed along 26 ranges to include a time when most of the ebbtide flow document the location and configuration of was confined to channels, a condition which channels. Erosion and deposition ,in tl'~ lagoon probably occurred later with resultant higher can be determined by future surveys on these velocities. ranges.

7 The ranges were established in lines as parallel are (1) grain-size parameters, (2) mineralogy, and evenly spaced as possible except where and (3) fluorescent particles. Each is suited in its conditions made it necessary to establish own way to various aspects of the study. additional ranges. Short ranges were established Grain-size and mineralogical tracin.~ techniques across the inlet, certain channels, and deltas of have been applied already; application of the tributaries to the lagoon. Bench marks and other fluorescent-particle techniques has just begun. permanent markers were used to mark range GRAIN-SIZE PARAMETERS AND SPECIFIC WEIGHT OF ends wherever possible; elsewhere 6-foot lengths LAGOONAL SEDIMENTS of 3/4-inch steel reinforcing rods were driven into the ground to mark the sites. Samples of bottom sediment were collected with a piston corer to depths of 0.3 and 0.5 Most elevations along the long ranges were foot. Most of the samples were collected in established with an instrument that emits a laser August and September 1967 during the beam of low intensity. The emitted laser beam hydrographic survey. The samples were usually rotates 360° about twice a second in a horizontal split into two parts. Grain-size dist·'ibution was plane. A vertical adjustable unit for receiving ~he determined by sieving one part, and the other laser beam is mounted in a Linker-tape rod. An part was u.sed either for determination of audible signal sounds when the receiver mineralogy or specific weight. intersects the beam, and the point where the beam is received on the rod is a direct reading of Grain-size parameters, such as median grain the elevation of the station. The laser emitter in size and sorting, were calculated as possible this study was left unmanned after its initial statistical measures of depositional environments setup except for occasional checking. The and source areas. The parameters were instrument was set up on a point near the range calculated graphically by the methcds of Inman and the rodman moved along the range. ( 1952, p. 130), using phi units, where Readings were taken at breaks in the slope and at channels. Distances were determined by triangulation.

The laser system was used on the long ranges D being the particle diameter in millimeters because its accuracy was within ±0.1 foot for (Krumbein, 1936, p. 37). For some samples the distances as far as half a mile. For short ranges values used for the calculations of sorting had to and certain parts of long ranges, elevations were be estimated, but the consistency c f the results determined by leveling and distances by stadia. showed that these estimates were probably fairly accurate. The maximum depth measured by the survey was 14.7 feet below mean sea level. Except for Most of the lagoon bottom consists of fine the upper basin the channels were well defined sand as classified by Wentworth (1922) (table in most of the cross sections. The largest channel 1). The finest sediment is in the northwest part passed between Kent Island and Seadrift and of the upper basin. Near the mouth of some extended along the eastern shore of the lagoon streams, the sediment was coarser than the to the upper basin. Bolinas Channel was the only average, indicating that the coarse s~diment was other large channel in the lagoon. carried into the lagoon by streams.

SEDIMENT MOVEMENT Although the methods of Inmar were used, the terms of Folk (1961) were use~ to describe the sorting of the sediment (table 2). Sediments in the lagoon were generally moderately sorted in the central part and tended to be~-;ome poorly . sorted·in the upper basin and southeast end. The Table I.-Wentworth scale of particle sizes MINERALOGY OF BOUNAS LAGOON SEDIMENTS

Diameter By Edward J. Belley Size class Phi units Millimeters The mineralogic character of the ff!diments Boulder______>-8 >256 within Bolinas Lagoon depends on the type, Cobble------­ -8 to -6 256 to 64 distribution, and weathering of rocks which Pebble------­ -6 to -2 64 to 4 underlie the local drainage and coastline. The Granule ------2 to -1 4 to 2 distribution of sediments within the lagoon is Very coarse sand ___ _ -1 to 0 2 to I influenced by their origin and hydrodynamic Coarse sand ______0 to I I to 0.5 behavior. Descriptions of the rocks an1 mineral Medium sand------­ I to 2 0.5 to 0.25 assemblages which underlie local drain~ges have Fine sand------­ 2 to 3 0.25 to 0.125 been made by Gluskoter ( 1962) and Galloway Very fme sand------3 to 4 0.125 to 0.062 ( 1966). Moore (1965) has descr~bed the Silt------4 to 8 0.062 to 0.004 heavy-mineral (specific gravity >2.89) Clay------<8 <0.004 composition of sands from both Stins...,n Beach and .

The purpose of this study is to der~ribe the Table 2.-Description of sorting of sediments mineralogy of modern lagoonal sedimr'"lts in an Graphic standard deviatio Sorting values attempt to trace the source of these ff!diments (Inman, 1952) (Folk, 1961) by means of mineralogical comparison with surrounding rocks. <0.35~ Very well sorted 0.35 to 0.50~ Well sorted The geologic map shown in figure ;. is taken 0.50 to 1.0~ Moderately sorted from an unpublished map by A.J. Galloway 1.00 to 2.0~ Poorly sorted (written commun., 1969), and the minrralogy of 2.00 to 4.0~ Very poorly sorted each formation is described in order of >4.00~ Extremely poorly decreasing age. The chemical formulas for these sorted minerals are taken from Deer, Howie, and Zussman ( 1966). poor sorting possibly indicates two or more Franciscan Formation (from Gluskoter, 1962): sources and perhaps reflects the influence of sediments fluvially transported to a marine Sandstones (arkosic wackes): environment. Primary minerals: 1. Quartz-Si02 The dry specific weight of the bottom sediment ranged from 53 to 102 pounds per 2. Plagioclase feldspar-NaAISi3 Os to cubic foot and averaged 79 poinds per cubic foot. The heaviest deposits were at the inlet; the CaAI2Si20s (here Ano to An2o) lightest were in the upper basin. Specific 3. Orthoclase feldspar-low te:-npera­ weights, in general, became progressively lighter ture monoclinic KAISi3 Os as the distance from the inlet increased.

4. Volcanic and sedimentary ro~k The specific gravity of the bottom sediments fragments ranged from 2.37 to 2.68 and averaged 2.55, which is about that of quartz and feldspar, 3 5. Iron ores-magnetite (Fe+2 Fe+ 2 minerals common to sedimentary deposits. 04 ), hematite (Fe2 03)

9 Franciscan Formation (from Gluskoter, 1962)­ Franciscan Formation (from Gluskcter, 1962)­ Continued Continued Sandstones (arkosic wackes)-Continued Spilites-Continued Primary minerals-Continued 10. Analcite-Na(AlSi2 06 )·P2 0 6. Chert-compact variety of silica 11. Clinozoisite (Si02) composed of minute crystals 12. Orthoclase feldspar of quartz with submicroscopic 2 pores, usually stratified 13. Pumpellyite-Ca4 (Mg,Fe + ) (Al,Fe+3)s O(OHh (Si2 01· h 7. Biotite-K2 (Mg,Fe+2)6_4(Fe+ 3, (Si04 h ·2H2 0 Al,Ti)o-2( Si6 -sA12-3 02o) 14. Laumontite-Ca(Al2Si4C12)·4H20 (OH,F)4 Monterey Shale 8. Muscovite-K2Al4(Si6Al202o) Shale and silty sandstones (OH,F)4 1. Pelitic rock fragments 9. Chlorite-(Mg,Al,Fe) 12 [ (Si,Al)s 02o] (0Hh6 2. Quartz Secondary minerals: 3. Plagioclase 1. Quartz 4. Chert 2. Clinozoisite-Ca2Al·Al2 O·OH(Si2 5. Biotite 07 )(Si04) 6. Hornblende-(Na,K)o-1Ca2 3. Axinite-(Ca,Mn,Fe+2h Al2 B03 (Mg,Fe+2 ,Fe+3 ,Al)s (Si6-1Al2-1 (S4012)0H 022)(0H,Fh 4. Orthoclase feldspar 7. Karinthine-variety of he mblende 5. Calcite-CaC03 8. Glauconite-(K,Na,Ca) 1.2:-2.0 (Fe+3,Al,Fe+2 ,Mg) . (Si _ . Spilites: 40 7 76 Al1-o.40 2o)(OH)4 ·nH2 0 1. Plagioclase feldspar (albite)­ Merced Formation NaA1Si30s Quartz 2. Augite-(Ca,Na,Mg,Fe +2 ,Mn ,Fe+3, 1. Al,Tih [ (Si,Alh 06 ] 2. Plagioclase feldspar 3. Chlorite 3. Orthoclase feldspar 4. Ilmenite-FeTi03 4. Biotite 5. Magnetite 5. Hornblende 6. Pyrite-FeS2 6. Muscovite 7. Apatite-Cas (P04 h (OH,F ,Cl) 7. Chert 8. Calcite 8. Pelitic rock fragments 9. Quartz

10 Sediment sampling within the lagoon was Because fragments of the Monterey Shale are done during August and September 1967 with a easily recognized, they were studied separately, piston coring device, and grab samples were and percentages were calculated from counts of collected along Stinson Beach and Bolinas Cliffs 400 recognizable fragments in each sample. and at the mouths of streams draining into the Figure 5 shows the distribution of samples lagoon. Only the uppermost half a foot of each composed of one-third or less and between core sample was studied. Each sample was split one-third and two-thirds of Monter~y Shale by washing over a 62-micron sieve. The coarse fragments. As expected, the higher fraction of each sample was impregnated with concentrations lie along the western half of the leucite (methyl methacrylate monomer) and lagoon close to the sources. It could not be studied by petrographic techniques; both flat­ ascertained, however, whether the Monterey and universal-stage methods were used. The fine Shale came mainly from the Pine Gulch Creek fractions were analyzed by X-ray diffraction; drainage or the Bolinas Cliffs just outside the only dry bulk patterns were run. A total of 82 lagoon. Certainly both sources supply Monterey samples was studied. Shale to the lagoon. The irregularitir,s in the boundary between samples with one-third and In general, all sediments studied, if lithified, those having more than one-third I ·1 onterey would range in composition from lithic arkosic Shale probably reflect the local sources of wackes to lithic or arkosic arenites1. The wackes Franciscan detritus shed from drainages east of would predominate within the lagoon, whereas the lagoon. outside in the beach areas arenites would Franciscan-derived sediment seems to predominate. The abundance of lithic wackes dominate the eastern one-third of the lagoon, and lithic arenites in the lagoon is not surprising especially at the mouths of streams drrining the since depositional sites lie very close to erosional east side. The western two-thirds seems to be sites. dominated mostly by the Monterey Shale. In fact, the easily recognized shale fragments All minerals described in the surrounding appear to be ubiquitous throughout lagoonal rocks with the exception of axinite, analcite, sediments and nowhere make up less than 10 1au m on tit e, and pumpellyite have been percent of the sediment studied. The sediments recognized in sediments now in the lagoon. In in most areas studied contain 33 percent or addition to minerals described by Gluskoter more Monterey Shale fragments. Hence, the ( 1 9 6 2), glaucophane, sphene, zircon, and Monterey must be considered the dominant serpentine-group minerals have been recognized, source of lagoonal sediment. but only in extremely minute quantities. Individual mineral species are difficult to use With present data it is not possible to state here as tracers because many distinctive species definitely whether the Pine Gulch Creek which are found as discrete grains are also found drainage or the Bolinas Cliffs is the more in rock fragments. This is expected because important source of the Monterey Shale. erosion and deposition take place over short However, an analysis made using the: erosion distances. rates of cliff retreat given by A.J. Gallo'~'ay (oral commun., 1967) suggests that the Bolinas Cliffs might be the more important. Using hi~ average lThe terms"wacke" and "arenite" are used herein as defined by Gilbert (Williams, Turner, and Gilbert, 1954, of 2.3 feet of retreat per year, the Bolinas Cliffs, p. 291-293). Wackes are impure sandstones containing which are approximately 120 feet l'igh and 10 percent or more argillaceous matrix. Arenites are 7,000 feet long, would yield an average annual pure sandstones having less than 10 percent argillaceous volume of about 72,000 cubic yards of matrix. Those wackes or arenites containing more rock sediment. Much of this material is silt and clay fragments than feldspar grains are either lithic wackes or which is transported in suspension into Bolinas arenites, whereas those having more feldspar grains than Bay and does not enter the lagoon. Powever, rock fragments are termed arkosic wackes or arenites. study of sand-size samples taken from all over 11 the lagoon indicates that all the samples contain cfs and 3.2 fps, respectively; during the ebb tides at least 33 percent Monterey Shale. If one the measured maximums occurred from higher assumes that at least 33 percent of the volume high water to lower low water anrl were 5,280 of material eroded from the Bolinas Cliffs is of cfs and 4. 7 fps, respectively. sand size, the annual volume of sand-size material eroded is 24,000 cubic yards. If this Much of the sediment being mC'ved through annual rate of production of sand-size material is the inlet is sand, which probably is eroding from extended for I 00 years, the length of time the sand beaches on the sides of the inlet. during which measurements of cliff retreat have been made, the Bolinas Cliffs could have Observations of released dye and surface supplied 2,400,000 cubic yards of Monterey floats indicated that near the inlet the littoral Shale to Bolinas Lagoon. This volume, spread drift is generally toward the mouth but away evenly over the I , I 00-acre lagoon, would have from the inlet the pattern varies acc')rding to the caused 0.45 yard of vertical aggradation. Since wind and wave conditions. even spreading is unlikely, it is probable that greater amounts of local aggradation have The circulation in Bolinas Lagoon was studied occurred over the last I 00 years. The presence in on September 6, 1967. The circulation pattern, lagoonal sediments of shell fragments of in general, seemed to be counterclockwise with organisms that presently live near the Bolinas the tide first turning from flood to ebb along the Cliffs strengthens the hypothesis that the major western side of the lagoon. The overall velocities source for the Monterey Shale in the lagoon is during floodtide were higher than those during the rapidly eroding cliffs. ebbtide, probably because of diffe~ences in the constriction of flow and the quantities of water SUMMARY involved. Velocities in the central part of Bolinas Lagoon may be adequate to erode and transport Measurement of water flow and sediment load bottom sediment, whereas those in the upper at the lagoon inlet for a I 0-hour tidal period basin and the southeastern part may not be. (flood tide and ebbtide) on June 22, 1967, revealed that in that specific period of time 152 Cross sections of the lagoon wer~ surveyed in tons of suspended sediment was carried into the August and September 1967 to d~termine the lagoon by the floodtide, whereas only 36 tons topography of the lagoon botton, especially was carried out of the lagoon by the ebbtide. with regard to the location and configuration of The maximum measured water flow and the channels. The maximum depth measured maximum average velocity during floodtide were was 14.7 feet below mean sea level. 5,810 cfs and 3.5 fps, respectively; during ebbtide the maximums were 3,720 cfs and 2.4 Grain-size parameters show that the sediment fps. The range in stage of floodtide was about 4 in the central part of the lagoon is generally feet and of ebbtide, about 1.3 feet. coarser and better sorted than that in the upper basin and in the southeastern _rart, perhaps On October 24-25, 1967, measurements of because of hydrodynamic diffe.rences. The water flow and suspended-sediment load at the specific weight of the bottom seditr~nt averaged inlet for a tidal day showed that about 500 tons 79 pounds per cubic foot; the specific gravity of suspended sediment was transported out of averaged 2.55. the lagoon during the ebbtides and 170 tons was transported into the lagoon during the The influences of different sources of the floodtides. The maximum water discharge and sediment were determined from identification of maximum average velocity measured during the minerals in the lagoon. Sediment derived from floodtides occurred in the period from lower the Franciscan Formation dominate1 the eastern low water to lower high water and were 4,230

12 one-third of the lagoon. This sediment was Krumbein, W.C., 1936, Application of transported to the lagoon by the eastern streams logarithmic moments to size f ..equency tributary to the lagoon. The sediment in the distribution of sediments: Jour. Sed. western two-thirds of Bolinas Lagoon was Petrology, v. 6, p. 35-42. derived chiefly from the Monterey Shale. This Moore, D.B., 1965, Recent coastal s~diments sediment was carried into the lagoon by Pine Double Point to Point San Pedro, California: Gulch Creek and by tidal currents and waves Berkeley, California Univ., Hydraulic that eroded Bolinas Cliff. Engineering Laboratory tech. rept., p. 2-14. Rantz, S.E., 1968, Average precipitation and SELECTED REFERENCES runoff in north coastal California: U.S. Geol. Survey Hydrol. Inv. Atlas HA-298. Beach Erosion Board, 19 61 , Shore protection Ritter, J.R., 1969a, Measurement of water flow planning and design: Washington, U.S. Govt. and sediment load, Bolinas Lagoon, Bolinas, Printing Office, Tech. Rept. 4, 242 p. California: U.S. Geol. Survey Prcf. Paper Deer, W.A., Howie, R.A., and Zussman, Jack, 650-B, p. B189-Bl93. 1966, An introduction to the rock-forming --1969b, Preliminary studies of minerals: New York, John Wiley & Sons, Inc., sedimentation and hydrology in Bolinas 518 p. Lagoon, May 1967 to June 1968: U.S. Geol. Folk, R.L., 1961. Petrology of sedimentary Survey open-file rept., 95 p. rocks: Austin, Texas, Hemphill's, 154 p. Slemmons, D.B., 1962, Determin~tion of Galloway, A.J., 1966, Geology of northern volcanic and plutonic plagioclases using a California: California Div. Mines and Mines three to four axis universal stage: Geol. Soc. Bull. 190, p. 429-440. America Spec. Paper 69, p. 34. Gilbert, G.K., 1908. to Bolinas Sundborg, Ake, 1956, The river Kalr~a1ven, a Lagoon, in Lawson, A.C., chm., and others, study of fluvial processes: Geografiska The California earthquake of April 18, Annaler, v. 38, p. 127-316. 1906-Report of the State Earthquake Turner, F.J., 1951, Observations on twinning of Investigation Commission: Carnegie Inst. plagioclase in metamorphic roc"s: Am. Washington Pub. 87, 3 v., 1 atlas. Mineralogist, v. 36, p. 581-589. Gluskoter, H.J., 1962, Geology of a portion of Wentworth, C.K., 1922, A scale of grade and western Marin County, California: Berkeley, class terms for clastic sediments: Jour. California Univ., Ph. D. thesis, 184 p. Geology, v. 30, p. 377-392. Gorai, Masao, 1951, Petrological studies in Williams, Howell, Turner, F.J., and Gilbert, plagioclase twins: Am. Mineralogist, v. 36, p. C.M., 1954, Petrography, an introduction to 884-901. the study of rocks in thin sections: San Hjulstrom, Filip, 1935, Studies of the Francisco, Calif., W.H. Freeman and Co., 382 morphological activity of rivers as illustrated p. by the River Fyris: Bull. Geol. lnst. Univ. Upsala, v. 25, p. 221-528. Inman, D.L., 1952, Measures for describing the size distribution of sediments: Jour. Sed. Petrology, v. 22, no. 3, p. 125-145.

13

FIGURES l-5

EXPLANATION s Gaging station ® Tide gage

0 1000 2000 3000 fi~ET I II II I I I

PACIFIC OCEAN

Figure 1.-Location of data sites and area of Bolinas Lagoon.

17 122°~0'

EXPLANATION ~

1000 2000 3000 FEET All u ti;~;J and }~ ~ ~ Te:r:~~~~~::its ~ ~ ~~u - ~~ u 1: { 0 ~~~~ ~ }f-.~~z<< ~ f-. ~ i:: i: Merced Formation ~ < < gz ~{ ~ }~~~ ~ Monterey Shale f-. f-. <

~ ~ }.~0·u~~ ------...... - u u Contact ell Dashed where approxi­ mately located

-::::::::::::::::::::::::::::::::::::. 0 & •••• 0 •• 0. 0 •••••• 0 •••••• 0 0. 0 0. 0 •••• 0 0 0. 0 •• 0. 0 •••• 0 •••• • • 0 ••• 0 •• 0. 0 ••••••• • • • • 0 •••••• 0. . 0 . •••••. . . . .

PACIFIC OCEAN ·.·.·~·.·.·.·.··:·:·:-:-:·:-:-:-:·: ...... • • 0 ••••••••••••••• • • • 0 •••• Duxbury Point

1950

..,.,.... .,

0 1 2 MILES I

Figure 3.-Development of Bolinas Lagoon, 1858 to 1950. 1858: From a plat of Rancho Las Baulines surve:yed by R.C. Matthewson, October 1858, under instructions from the U.S. Surveyor General. 1897: From U.S. Geological Survey Tamalpais topographic sheet. 1941: From Corps of Engineers, U.S. Army tactical map. Tamalpais topo­ graphic quadrangle. 1950: From U.S. Geological Survey Mount Tamalpais quadrangle.

19 122.40'

EXPLANATION ITJIJJ Above water at start of study Above- hi1h water ~ Channel • Initial dye spot Current direction and velocity (velocity, in feet N 0 per minute) --+ 37• 55'; > 100 .... S0-100

-+ 10;50

0 1000 2000 3000 4000FEET I 11 11 I I I I 4 122.40'

EXPLANATION E2SJ Above water at start of study

Above- high water ~ Channel • Initial dye spot Current direction and velocity (velocity, in feet N per minute) - -+ 37.55' > 100 ... so -108

~ 10 -so

<- 10

0 1000 2000 3000 4000 FEET ! I I I I J ! ' I ~

Figure 4.-Circulation in Bolinas Lagoon, September 6, 1967. A, Floodtide. B, Ebbtide. EXPLANATION Ptnentap of,...... of •oaterey S..._ ia bottom aectimat ~--·

0 1000 2QOO 3()')() FEET I 11 11 I I

JIIA.CIFIC OCEAN

Figure 5 .-Distribution of Monterey Shale fragments, Bolinas Lagoon, 1967.

*U. S. GOVERNMENT PRINTING OFFICE : 1970 0 - 380-308 22