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 
United States 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 

Abstract .............................. 1 

Introduction and acknowledgments ......... 1 

Description of Bolinas Lagoon ............. 2 

Possible sources of sediment in 

Bolinas Lagoon ...................... .3 

Studies being conducted by the 

U.S. Geological Survey ................ .3 

Fresh-water inflow .................... .3 

Fluvial sediment transport to the lagoon ... .4 

Sediment transport through the tidal inlet .. .4 

Tidal measurement of June 22, 1967 ..... 5 

Tidal measurement of 

October 24-25, 1967 .............. 5 

Page 

Studies being conducted by the 

U.S. Geological Survey--Continued 

Littoral drift .......................... 6 

Circulation in Bolinas Lagoon ............ 6 

Bottom topography .................... 7 

Sediment movement .................... 8 

Grain-size parameters and 

specific weight of lagoonal sediments .... 8 

Mineralogy of Bolinas Lagoon sediments, 

by Edward J. Helley ................. 9 

Summary ............................ 12 

Selected references ..................... 13 

ILLUSTRATIONS 

Page 

Figures 1-5. Maps showing: 

1. Location of data sites and area of Bolinas Lagoon ......................... 1 7 

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 

The U.S. Geological Survey is investigating 

sedimentary and hydrologic conditions in Bolinas 

Lagoon, a 1,100-acre lagoon 15 miles northwest of San 

Fran cisco. The program began in May 196 7 and will 

continue into 1970. Only the study results analyzed 

before June 1968 are summarized in the report. 

Two series of measurements of suspended-sediment 

load and water discharge in the lagoon inlet showed that 

much of the suspended sediment is sand and that the 

average velocity was as much as 4.7 feet per second. 

Littoral drift near the inlet was generally toward the 

inlet, whereas farther from the inlet the pattern is 

irregular. Circulation velocities in the lagoon decrease 

rapidly away from the inlet, but probably remain high 

enough to erode bottom sediment along the channels. In 

most of the lagoon median size of bottom sediment was 

fine sand. Sediment was derived chiefly from Monterey 

Shale. 

INTRODUCTION AND 

ACKNOWLEDGMENTS 

In May 1967 the U.S. Geological Survey 

began an investigation of the hydrology and 

sedimentation in Bolinas Lagoon in cooperation 

with the Bolinas Harbor District and Marin 

County. The lagoon, a potential small craft 

harbor, seems to be filling with sediment. The 

intended purposes of the investigation are to 

determine the rate of sedimentation, to describe 

the sedimentation processes, and to define the 

sources and movement of sediment in the 

lagoon. This report summarizes the results of 

1 

studies conducted from May 1967 through May 

1968 and describes studies now in progress. A 

more detailed description of the studies may be 

found in Ritter ( 1969a and 1969b ). 

The investigation is divided into seven studies, 

each of which is related to sedimentation and 

hydrology. The separate studies are designed to 

provide information on (1) the fresl'·Water 

inflow into the lagoon, (2) the quantity of 

fluvial sediment transported into the lago'1n, (3) 

the quantity of sediment transported into and 

out of the lagoon through the tidal inlet, (4) the 

direction of littoral drift near the inlet, (5) the 

circulation of water within the lagoon, ( 6) the 

bottom topography, and (7) the sediment 

movement within the lagoon. The data and 

conclusions of each study will be correlatr.d with 

those of other studies conducted l'Y the 

Geological Survey and other investigato:t"s. The 

data also are used, whenever possn~•~, to 

interpret past and future conditions in the 

lagoon. 

The investigation by the Geological Survey is 

conducted under the general supervisior of R. 

Stanley Lord, district chief in charge of 

water-resources investigations in Califorria, and 

Loren E. Young, chief of the Menlo Park 

subdistrict office. The manuscript benefited 

substantially from the criticism of Jos~ph R. 

Curray and D.W. Hubbell. The assistance of 

Winchell Smith and William M. Brown III also is 

gratefully appreciated. A special acknowledgment 
is pJven to Archie A. Goldsberry, who patiently 

and cheerfully helped with many phases of the 

investigation. 

DESCRIPTION OF BOLINAS LAGOON 

Bolinas Lagoon (fig. 1 ), bordered by the 

communities of Bolinas and Stinson Beach, is 

about 15 miles northwest of San Francisco. The 

lagoon has a triangular shape with maximum 

dimensions of about 1 by 3 miles. Most of the 

lagoon is tidal flat. At mean lower low water, 

about 70 percent of the lagoon area is exposed 

to the air and at mean higher high water only 

two islands remain above water. The larger 

island, Kent Island, covers about 100 acres; the 

other, near the mouth of McKinnan Gulch, is 

insignificant in size. 

Bolinas Lagoon is protected from the ocean 

by a long sandy spit now known as Seadrift, the 

site of several luxurious homes. Despite the 

recent development of Seadrift for home sites, 

much of it is topped with sand dunes partly 

covered by vegetation. Seadrift Lagoon and its 

northern adjacent neck of land (fig. 1) were 

created by dredging and fill operations 

undertaken in the early 1960's and were the 

only major manmade revisions in the character 

of Bolinas Lagoon in recent years. 

The tidal inlet to the lagoon is between the 

western extremity of Seadrift and a high cliff, 

which is part of the town of Bolinas. The inlet is 

bounded by sandy beaches; however, during 

winter storms the western beach often is entirely 

eroded away, leaving bedrock abutting the high 

cliff. During periods of calm the beach is 

reestablished. The middle of the inlet usually is 

lined with cobbles and bedrock. To pictorially 

record changes in the configuration of the inlet, 

the Geological Survey has mounted a time-lapse 

camera on a cliff overlooking the inlet. The 

camera has taken photographs during daylight at 

intervals of every hour or half hour since 

January 1968. 

Just outside the inlet is a shallow 

crescent-shaped sandbar, which constantly 

2 

changes position. During extreme low water the 

bar is exposed, creating a navigational hazard. 

Except for Seadrift, Bolinas Lagoon is 

encircled by steep hills that rise to altitudes of 

more than 1 ,500 feet on the east and about 200 

feet on the west. Most of the virgin growth on 

the hills east of the lagoon was logged during the 

1850's 

The drainage area tributary to Bolinas Lagoon 

is 16.7 square miles and has maximum 

dimensions of about 3 by 9 miler- The largest 

tributary, Pine Gulch Creek, drains about half 

the basin and is the only tributary to enter the 

lagoon from the west. The extent of its delta, at 

least 1 ,500 feet into the lagoon, indicates that 

Pine Gulch Creek transports more sediment to 

the lagoon than any other tributar:.r. The several 

eastern tributaries are short and steep and have 

drainage basins of less than 1 square mile. They 

have small deltas, but possibly their average 

annual contribution of sediment pr-r square mile 

of drainage area is high because of their steep 

gradient and their sparsely vegetated drainage 

basins. 

Bolinas Lagoon straddles the San Andreas 

fault. During the 1906 earthquakr-, the relative 

movement along the fault was as rr.~1ch as 1 foot 

vertically and 13.5 feet horizortally in the 

Bolinas Lagoon basin (Gilbert, 19('~, p. 67, 70, 

72). The displacement must have changed the 

regimen of at least some of the streams and 

probably increased erosion because of stream 

readjustment. 

The rock west of the lagoon cc 11sists chiefly 

of the Monterey Shale of Miocene age and the 

Merced Formation of Pliocene an1 Pleistocene 

age; east of the lagoon the rock consists of the 

Franciscan Formation of Cretaceous and 

Jurassic age. Sedimentary deposits of 

Quaternary age occur throughout the drainage 

area (fig. 2). 

Precipitation at Bolinas Lagoon is 30 to 40 

inches per year (Rantz, 1968), m'lst of which 

falls from November through Apr:l. Almost no 
rain occurs during the summer months; however, 

fog is more common during June and July than 

during other months. 

Temperatures range from about 40° F to 

90° F, and the climate is generally mild. The 

prevailing winds are from the northwest, but 

gales from the southeast have reached 

velocities greater than 75 miles per hour. An 

onshore breeze also often occurs. 

POSSIBLE SOURCES OF SEDIMENT 

IN BOLINAS LAGOON 

Lagoons are temporary features along a coast. 

A generalized history of a lagoon may begin 

with a river mouth drowned by a rising sea level. 

Littoral drift creates a spit or barrier bar across 

the mouth so that its access to the ocean is 

restricted to a narrow inlet. The lagoon begins to 

fill with sediment and becomes a tidal flat with 

channels carrying much of the tidal water. The 

filling continues until the lagoon becomes a salt 

marsh as vegetation grows on the rising tidal 

flats. Eventually, the lagoon becomes a flat plain 

that may be eroded by waves lapping against its 

seaward edge. The life of a lagoon depends on 

the rate at which sediment is entrapped in the 

lagoon. 

Bolinas Lagoon, the seaward end of a rift 

valley created by the San Andreas fault, is now 

probably in the stage of a tidal flat that is 

becoming a salt marsh. 

There are many possible sources of sediments 

being deposited in lagoons. The three major 

transporters of sediment to a lagoon usually are 

littoral drift, upland streams, and wind. At 

Bolinas Lagoon the cliff between the inlet and 

Duxbury Point (fig. 3) is eroding at the rate of 

about 2.3 feet per year (A. J. Galloway, oral 

commun., 1967). The sediment supplied by the 

erosion of the cliff may be carried as littoral 

drift to the inlet and into the lagoon. The sand 

of Seadrift may also be carried to the lagoon as 

littoral drift. 

Streams tributary to Bolinas Lagoon transport 

sediment to it, especially during storms. The 

3 

quantity of sediment transported deJ: ends on 

such things as size of drainage area, erodibility 

of soils, geology, intensity of rainfall, land use, 

and vegetation. Although the tributaries to 

Bolinas Lagoon have small drainage areas, the 

lagoon itself is not large, so that the sediment 

contribution by the tributaries may substantially 

help to fill the lagoon. 

Wind transports sediment to Bolinas Lagoon 

when it blows from the south across the 

unconsolidated sand deposits of Seadrift. The 

quantity of sand moved by wind may be 

considerable. 

Comparison of older maps of Bolinaf Lagoon 

with more recent ones shows that the lagoon has 

been filling with sediment for at least 100 years. 

Figure 3 shows maps of Bolinas Lago-:>n from 

1858 to 1950. Although the accuracy in 

mapping has improved, large-scale features 

would not be omitted from the older maps. In 

the maps prior to 1900, no delta exists at the 

mouth of Pine Gulch Creek, and Kent Island is 

absent. However, early charts show parts of 

Kent Island as above lower low wrter but 

probably below mean sea level. Attempts to 

estimate erosion and deposition will be made in 

later studies by comparing maps and charts 

published during the past 100 years. 

STUDIES BEING CONDUCTED BY THE 

U.S. GEOLOGICAL SURVEY 

All seven studies being conducted by the U.S. 

Geological Survey are now in progress, and some 

of the data have been analyzed. In the following 

sections the methods used in each study and 

early results are described. 

FRESH-WATER INFLOW 

Since May 1967 the flows of three tributaries 

to Bolinas Lagoon have been recorded. At Pine 

Gulch Creek and Morses Gulch (fig. 1) gaging 

stations, equipped with Fisher-Porter digital 

recorders, were constructed. The water level or 

stage of the two streams is expressed as a gage 

height and is punched on a tape every 15 

minutes. The tape is read later by a computer 
that determines, for each day, the average gage 

height and other parameters. Discharges of the 

-streams are measured at least monthly during 

low water and more frequently during high 

water to define a stage-discharge relation. From 

the stage-discharge relation and the continuous 

records of stage, daily, monthly, and yearly 

discharges are computed for each stream. As of 

June 1968 discharge records for these streams 

had not yet been computed. 

At Audubon Creek a crest-stage gage was 

installed. Only the highest stage of the stream 

between visits to this gage is recorded. However, 

the discharge is measured at least monthly. 

FLUVIAL SEDIMENT TRANSPORT TO THE LAGOON 

The quantity of fluvial sediment being 

transported to the lagoon by Pine Gulch Creek, 

Morses Gulch, and Audubon Creek is being 

calculated. Because the quantity of sediment 

moved by streams is usually related to the water 

discharge, the data on water discharge at the 

three gaging stations are used in conjunction 

with data from sediment sampling to determine 

the sediment load. 

Samples of suspenqed sediment, that 

sediment which is carried in suspension in water, 

are collected by an observer at least daily during 

high runoff and less frequently during low 

runoff at Pine Gulch Creek. At Morses Gulch 

and Audubon Creek samples are collected 

monthly except during storm periods, when 

sampling may be more frequent. 

The suspended-sediment samples are collected 

throughout the entire depth by a technique 

called depth integration. Thus, the sample is 

representative of an entire column of water. 

Because the concentration of suspended 

sediment may vary across a stream, samples are 

take across a given section of the stream. The 

samples, mixtures of water and suspended 

sediment, are sent to a laboratory. First they are 

weighed. Then the suspended sediment is 

filtered off, dried, and weighed. The ratio of the 

weight of the suspended sedhnent to the weight 

of the sample converted to a volume is an 

4 

expression of the concentration of suspended 

sediment in the stream at the time the sample 

was collected. The average concentration, 

expressed in milligrams per liter, is multiplied by 

the water discharge and by a urits conversion 

constant to obtain the suspended-sediment load, 

expressed as tons per day. Daily, monthly, and 

yearly loads are computed. 

Sampling of the three streams began in May 

1967. However, as of June 1968 

suspended-sediment loads had not yet been 

calculated. 

SEDIMENT TRANPORT THROUGH THf. TIDAL INLET 

Measurements of the quantity of sediment 

moving in and out of the inlet to the lagoon 

during tidal periods were made on June 22, 

1967, October 24-25, 1967, and May 16-17, 

1968. The results of the first two sets of 

measurements are described in tJjs report. The 

results of the third have not yet been analyzed. 

A moving-boat technique, in vrhich the boat 

moved at a rate of about 10 fpm (feet per 

minute) along a cable stretched across the inlet, 

was used to measure water flovr at the inlet. 

Two current meters, which were suspended from 

the boat and were adjusted for d'ange in depth, 

measured the velocity of the current 

continuously at 0.2 and 0.8 of the depth. 

Direction of flow (sensed by a current-orienting 

compass), depth of water, ftation, meter 

revolutions, and time were continuously 

registered on a panel of counters aboard the 

boat. The panel was photographed about every 

30 seconds by an automatically triggered 

35 -mm (millimeters) camera. Tho. difference in 

the total count of meter revolutions per 30 

seconds is used to determine the average velocity 

for the increment of width traversed. Then by 

multiplying the average depth of the increment 

by its width, the area of the increment is 

obtained. The area multiplied by the velocity 

corrected for the angle of the current to the 

measuring section is the flow for the increment. 

The total flow is the sum of the flows of the 

increments. The above calculatior~ are made by 
computer from data tabulated from the film. 

Measurements of flow took from 9 to 25 

minutes. The measured flow was assumed to 

equal the flow at midpoint of the time interval 

of measurement. 

While flow was being measured, 

suspended-sediment samples were being 

collected on the same boat. Depth-integrated 

samples were collected at five to seven stations 

during each measurement. On another boat, also 

in the inlet, point samples of sediment were 

collected from various depths, and samples of 

sediment moving along the bottom of the inlet 

were collected with an Arnhem sampler so that 

bedload could be calculated. The temperature of 

the water in the inlet was also measured. 

TIDAL MEASUREMENT OF JUNE 22,1967 

On June 22, 1967, the first measurement of 

the sediment load for a tidal period was made at 

the inlet. The tide there is a mixed, 

predominantly semidiurnal type. Water flow was 

measured during flood tide with a range of stage 

of about 4 feet and during an ebbtide with a 

range of only 1 .3 feet. Seventeen flow 

measurements were made. 

The maximum average velocity during 

floodtide was about 3.5 fps (feet per second), 

and during ebbtide, about 2.4 fps. The 

maximum measured flow was 5,810 cfs (cubic 

feet per second) during floodtide and 3,720 cfs 

during ebbtide. During the 6-hour period of 

floodtide, 2,000 acre-feet of water flowed in 

through the inlet and during the 4 hours of 

ebbtide, 770 acre-feet flowed out. 

The quantity of suspended sediment 

transported into the lagoon was greater than the 

quantity that left because ( 1) the duration of 

the floodtide was longer than that of the 

ebbtide, (2) the quantity of incoming water was 

much greater than the outgoing, and (3) the 

average concentrations of suspended sediment 

were higher during floodtide. About 152 tons of 

suspended sediment entered the lagoon and 

a bout 36 tons left; presumably, 116 tons 

remained in the lagoon. 

5 

The average concentration of suspended 

sediment during flood tide ranged from ? 4 to 98 

mg/1 (milligrams per liter) and only frmn 34 to 

49 mg/1 during ebbtide. The concentrrtion of 

suspended sediment finer than sand (<0.0<:2 mm) 

remained almost constant throughout the cross 

section of the channel during each measurement. 

Any significant increase in concentration, 

especially near the banks, was attributat 1e to an 

increase in the quantity of sand carried in 

suspension. Thus, the major immediate sources 

of most of the transported sediment ~·~re the 

beaches adjacent to the channel. 

TIDAL MEASUREMENT OF OCfOBER 24-25,1967 

The second tidal measurement at the inlet was 

made on October 24-25, 1967, for an entire 

tidal day of two floodtides and two el:lbtides. 

The measured average velocity reached 

maximums of 2.7 and 3.2 fps for the flood tides 

and of 4.7 and 1.1 fps for the ebbtides. The 

maximum average velocity during the major 

ebbtide was thus about 1.5 times that of 

flood tide. 

The maximum measured flow was 5,280 cfs 

for the ebbtide from higher high water (+2.14 

feet) to lower low water ( -2.37 feet). For the 

minor ebb tide from lower high water ( +0.85 

foot) to higher low water (+0.68 foot) the 

maximum flow was only 1 ,440 cfs. About 2,330 

acre-feet of water flowed out of the lagoon 

during the earlier ebbtide, which lasted 9 hours, 

and only 180 acre-feet during the later one, 

which lasted 3 hours. 

The maximum measured flows fcT both 

floodtides were almost equal; 4,180 cfs for a 

period from higher low water (+0.93 foot) to 

higher high water (+2.14 feet) and 4, 230 cfs for 

that from lower low water (-2.37 feet) to lower 

high water ( +0.85 foot). The volume cf water 

that flowed into the lagoon dur:ng the 

flood tides also was almost equal, 1,340 and 

1,330 acre-feet, even though the later floodtide 

lasted an hour longer than the earlier. 

The highest suspended-sediment load was 

transported during the major ebbtide. About 
495 tons of suspended sediment was transported 

then, whereas each floodtide transported about 

85 tons and the minor ebbtide transported only 

5 tons. Thus, there was a net removal of about 

330 tons of sediment from the lagoon during the 

25 hours. 

The average concentration of suspended 

sediment ranged from 18 to 572 mg/1 during the 

major ebbtide and only from 5 to 26 mg/1 

during the minor ebbtide. In contrast to the 

differences in the magnitude of the ranges of 

average concentration during the ebbtides, the 

ranges of average concentration of suspended 

sediment during the floodtides were almost 

equal. Average concentration ranged from 11 to 

112 mg/1 for the earlier floodtide and from 9 to 

125 mg/1 for the later floodtide. The major 

ebbtide thus had a maximum average 

concentration almost 5 times that of the 

maximum during a floodtide. 

The beach along the western end of the spit 

was the chief contributor of suspended sediment 

on October 24-25, 1967. The highest 

concentrations of supended sediment 

consistently were near the spit. Concentrations 

at the middle of the inlet and near the west bank 

were lower than those near the east bank 

throughout the sampling period. As on June 22 

the concentration of suspended sediment fmer 

than sand (<0.062 mm) remained nearly uniform 

across the inlet, but the concentration of sand 

fluctuated greatly with the highest 

concentration occurring near the east bank. 

LITTORAL DRIFT 

Littoral drift is defined as the movement of 

material in the littoral zone (the zone between 

high and low tides) under the influence of waves 

and currents (Beach Erosion Board, 1961, p. 

Al7). By placing dye and floats in the surf zone, 

the direction of the longshore current, which is 

indicative of the direction of littoral drift, can 

be determined. The direction of the longshore 

currents near the inlet to Bolinas Lagoon was 

noted in May, September, and December 1967. 

Rhodamine B dye and brightly colored rubber 

balls were used. 

6 

From the few observations it VJas concluded 

that the direction of littoral drift, as determined 

from the direction of the longsho·e currents, is 

not constant because of the changing directions 

of the winds and breaking waves. Near the inlet 

the circulation is generally toward the mouth of 

the lagoon and is associated with the water 

entering and leaving the lagoon. 

Littoral drift was also studied by tracing the 

movement of fluorescent san~~ that were 

deposited on the beaches. The dyP-d grains were 

spread on the beach near Stinson Beach on July 

23 and on the beach near Bolina~ on July 24, 

1968. Results of the tracing have not yet been 

completed. 

CIRCULATION IN BOLINAS LAGOON 

The circulation of water in Bolinas Lagoon 

also was studied by following the movement of 

dye. The bright red dye, rhodamine B, was 

dropped from a helicopter into nany parts of 

the lagoon on September 6, 1967. The dye was 

usually dropped so that spots of dye formed a 

line perpendicular to the channel in the lagoon. 

Three series of dye drops were made ( 1 0: 15 

a.m., 1:05 p.m., and 3:15p.m.) ~nd 17 sets of 

color aerial photographs were t~ \ren from an 

airplane at an altitude of 6,0~0 feet. By 

comparing the location of the center of the 

spots of dye in each sucressive set of 

photographs, the movement of the dye was 

measured. 

When the circulation study began, most of the 

flow was confined to the main channels and 

remained so until the rising tide covered most of 

the lagoon. As water entered Bolinas Lagoon, 

some of it moved into Bolinas Channel. The 

greater part, however, moved eastward between 

Kent Island and Seadrift where. the surface 

velocities were greater than 100 fpm, the highest 

measured, but by the time the flo'" had reached 

the vicinity of the eastern shore, it" velocity had 

decreased substantially (fig. 4.A). Near the 

eastern shore the water divided; some flowed 

southeast and some, northwest. 
Velocities in the southeastern part of the 

lagoon were less than 50 fpm, even though rnuch 

of the water was confined to channels. Before 

the water had flowed very far into this part, its 

velocity had been reduced to even less than 10 

fpm. 

The major flow was northwestward in a 

channel along the east shore of the lagoon .. The 

surface velocity there was 50-100 fpm, but 

when the water reached the upper basin, the flow 

decelerated rapidly to less than 10 fpm. 

Dye dropped in Bolinas Channel did not get 

into the upper basin, but moved northwtrd into 

several small channels near the delta of Pine 

Gulch Creek. Therefore, the water that entered 

the upper basin must have been mostly from 

between Kent Island and the east side of Bolinas 

Lagoon. 

The floodtide-circulation studies lasted from 

10: 15 a.m. to about 1 :20 p.m. The circulation 

reversal was first noted in the upper basin about 

30 minutes before it was noted in many other 

parts of the lagoon. Most of the first observed 

reversals were in the western part of the lagoon. 

Evidently, at the time that the first indications 

of ebbtide were observed, the water was still 

advaincing along the east side of the lagoon, but 

already was retreating along the west side: and 

moving out through Bolinas Channel. 

During the ebbtide period of observation, the 

velocities of the water were less than those 

during floodtide (fig. 4B) because most of the 

outward flow was a sheet flow and not the 

channel flow observed during much of the 

floodtide. Also, a larger volume of water per 

unit time moved into the lagoon during flood tide 

than moved out during ebbtide. The velocities at 

ebbtide were higher at constrictions such as that 

near the inlet and at Bolinas Channel. 

Unfortunately, the period of observation did not 

include a time when most of the ebbtide flow 

was confined to channels, a condition which 

probably occurred later with resultant higher 

velocities. 

7 

Perhaps the greatest significance of the 

circulation study is its applicability to the study 

of movement of sediment. Acco~ding to 

Hjulstrom (1935), velocities of 100, SC, and 10 

fpm are capable of transporting particles having 

diameters as large as 7 mm, 3.5 mm, and 0.7 

mm, respectively. Although Hjulstrom's 

relations of velocity to movement of particles of 

different sizes are oversimplifications of 

complex phenomena and although other 

investigators, such as Sundborg ( 1956), have 

shown different relations, Hjulstrom's relations 

may be used as a basis for suggesting areas of 

erosion and deposition in the lagoon. 

The average particle size in and around the 

lagoon is about 0.2 mm. The Hjulstrom relations 

indicate that sediment of this size can be 

transported at prevailing velocities almost 

anywhere in the lagoon except perhaps in the 

upper basin and in the southeastern part of the 

lagoon. If the velocities are reduced to r~ro for a 

short time at slack tide and most of the 

transported sediment is deposited, then a 

velocity of about 30 fpm must be reached 

(Hjulstrom, 1935) to erode a particle of 0.2 mm 

from the bed of the lagoon. Hjulstrom's relation 

of velocity to particle size shows that the upper 

basin and the southeastern part of the lagoon are 

depositional areas, whereas most of the other 

areas are erosional during average tides. Another 

area that must be depositional, althC'llgh dye 

movement was not measured in its vicinity, is 

the tidal marsh behind Kent Island; because of 

its very nature, it would not be conducive to 

high velocities. 

BOTTOM TOPOGRAPHY 

A hydrographic survey was made to 

determine the topography of the bottom of 

Bolinas Lagoon and to provide a base for 

comparison with past and future surveys. In 

August and September 1967, profile; of the 

bottom were surveyed along 26 ranges to 

document the location and configuration of 

channels. Erosion and deposition ,in tl'~ lagoon 

can be determined by future surveys on these 

ranges. 
The ranges were established in lines as parallel 

and evenly spaced as possible except where 

conditions made it necessary to establish 

additional ranges. Short ranges were established 

across the inlet, certain channels, and deltas of 

tributaries to the lagoon. Bench marks and other 

permanent markers were used to mark range 

ends wherever possible; elsewhere 6-foot lengths 

of 3/4-inch steel reinforcing rods were driven 

into the ground to mark the sites. 

Most elevations along the long ranges were 

established with an instrument that emits a laser 

beam of low intensity. The emitted laser beam 

rotates 360° about twice a second in a horizontal 

plane. A vertical adjustable unit for receiving ~he 

laser beam is mounted in a Linker-tape rod. An 

audible signal sounds when the receiver 

intersects the beam, and the point where the 

beam is received on the rod is a direct reading of 

the elevation of the station. The laser emitter in 

this study was left unmanned after its initial 

setup except for occasional checking. The 

instrument was set up on a point near the range 

and the rodman moved along the range. 

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 

because its accuracy was within ±0.1 foot for 

distances as far as half a mile. For short ranges 

and certain parts of long ranges, elevations were 

determined by leveling and distances by stadia. 

The maximum depth measured by the survey 

was 14.7 feet below mean sea level. Except for 

the upper basin the channels were well defined 

in most of the cross sections. The largest channel 

passed between Kent Island and Seadrift and 

extended along the eastern shore of the lagoon 

to the upper basin. Bolinas Channel was the only 

other large channel in the lagoon. 

SEDIMENT MOVEMENT 

are (1) grain-size parameters, (2) mineralogy, 

and (3) fluorescent particles. Each is suited in its 

own way to various aspects of the study. 

Grain-size and mineralogical tracin.~ techniques 

have been applied already; application of the 

fluorescent-particle techniques has just begun. 

GRAIN-SIZE PARAMETERS AND SPECIFIC WEIGHT OF 

LAGOONAL SEDIMENTS 

Samples of bottom sediment were collected 

with a piston corer to depths of 0.3 and 0.5 

foot. Most of the samples were collected in 

August and September 1967 during the 

hydrographic survey. The samples were usually 

split into two parts. Grain-size dist·'ibution was 

determined by sieving one part, and the other 

part was u.sed either for determination of 

mineralogy or specific weight. 

Grain-size parameters, such as median grain 

size and sorting, were calculated as possible 

statistical measures of depositional environments 

and source areas. The parameters were 

calculated graphically by the methcds of Inman 

( 1952, p. 130), using phi units, where 

D being the particle diameter in millimeters 

(Krumbein, 1936, p. 37). For some samples the 

values used for the calculations of sorting had to 

be estimated, but the consistency c f the results 

showed that these estimates were probably fairly 

accurate. 

Most of the lagoon bottom consists of fine 

sand as classified by Wentworth (1922) (table 

1). The finest sediment is in the northwest part 

of the upper basin. Near the mouth of some 

streams, the sediment was coarser than the 

average, indicating that the coarse s~diment was 

carried into the lagoon by streams. 

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 

Diameter 

Size class 

Phi units Millimeters 

Boulder _____________ _ 

Cobble---------------

Pebble---------------

Granule -------------- Very coarse sand ___ _ 

Coarse sand _________ _ 

Medium sand--------

Fine sand------------

Very fme sand------

Silt-------------------

Clay------------------

>-8 

-8 to -6 

-6 to -2 

-2 to -1 

-1 to 0 

0 to I 

I to 2 

2 to 3 

3 to 4 

4 to 8 

<8 

>256 

256 to 64 

64 to 4 

4 to 2 

2 to I 

I to 0.5 

0.5 to 0.25 

0.25 to 0.125 

0.125 to 0.062 

0.062 to 0.004 

<0.004 

Table 2.-Description of sorting of sediments 

Graphic standard deviatio 

(Inman, 1952) 

<0.35~ 

0.35 to 0.50~ 

0.50 to 1.0~ 

1.00 to 2.0~ 

2.00 to 4.0~ 

>4.00~ 

Sorting values 

(Folk, 1961) 

Very well sorted 

Well sorted 

Moderately sorted 

Poorly sorted 

Very poorly sorted 

Extremely poorly 

sorted 

poor sorting possibly indicates two or more 

sources and perhaps reflects the influence of 

sediments fluvially transported to a marine 

environment. 

The dry specific weight of the bottom 

sediment ranged from 53 to 102 pounds per 

cubic foot and averaged 79 poinds per cubic 

foot. The heaviest deposits were at the inlet; the 

lightest were in the upper basin. Specific 

weights, in general, became progressively lighter 

as the distance from the inlet increased. 

The specific gravity of the bottom sediments 

ranged from 2.37 to 2.68 and averaged 2.55, 

which is about that of quartz and feldspar, 

minerals common to sedimentary deposits. 

9 

MINERALOGY OF BOUNAS LAGOON SEDIMENTS 

By Edward J. Belley 

The mineralogic character of the ff!diments 

within Bolinas Lagoon depends on the type, 

distribution, and weathering of rocks which 

underlie the local drainage and coastline. The 

distribution of sediments within the lagoon is 

influenced by their origin and hydrodynamic 

behavior. Descriptions of the rocks an1 mineral 

assemblages which underlie local drain~ges have 

been made by Gluskoter ( 1962) and Galloway 

( 1966). Moore (1965) has descr~bed the 

heavy-mineral (specific gravity >2.89) 

composition of sands from both Stins...,n Beach 

and Bolinas Bay. 

The purpose of this study is to der~ribe the 

mineralogy of modern lagoonal sedimr'"lts in an 

attempt to trace the source of these ff!diments 

by means of mineralogical comparison with 

surrounding rocks. 

The geologic map shown in figure ;. is taken 

from an unpublished map by A.J. Galloway 

(written commun., 1969), and the minrralogy of 

each formation is described in order of 

decreasing age. The chemical formulas for these 

minerals are taken from Deer, Howie, and 

Zussman ( 1966). 

Franciscan Formation (from Gluskoter, 1962): 

Sandstones (arkosic wackes): 

Primary minerals: 

1. Quartz-Si02 

2. Plagioclase feldspar-NaAISi3 Os to 

CaAI2Si20s (here Ano to An2o) 

3. Orthoclase feldspar-low te:-nperature monoclinic KAISi3 Os 

4. Volcanic and sedimentary ro~k 

fragments 

5. Iron ores-magnetite (Fe+2 Fe+3

2 

04 ), hematite (Fe2 03) 
Franciscan Formation (from Gluskoter, 1962)-

Continued 

Sandstones (arkosic wackes)-Continued 

Primary minerals-Continued 

6. Chert-compact variety of silica 

(Si02) composed of minute crystals 

of quartz with submicroscopic 

pores, usually stratified 

7. Biotite-K2 (Mg,Fe+2)6_4(Fe+ 3, 

Al,Ti)o-2( Si6 -sA12-3 02o) 

(OH,F)4 

8. Muscovite-K2Al4(Si6Al202o) 

(OH,F)4 

9. Chlorite-(Mg,Al,F e) 12 [ (Si,Al)s 02o] 

(0Hh6 

Secondary minerals: 

1. Quartz 

2. Clinozoisite-Ca2Al·Al2 O·OH(Si2 

07 )(Si04) 

3. Axinite-(Ca,Mn,Fe+2 h Al2 B03 

(S4012)0H 

4. Orthoclase feldspar 

5. Calcite-CaC03 

Spilites: 

1. Plagioclase feldspar (albite)-

NaA1Si30s 

2. Augite-(Ca,Na,Mg,Fe +2 ,Mn ,Fe+3, 

Al,Tih [ (Si,Alh 06 ] 

3. Chlorite 

4. Ilmenite-FeTi03 

5. Magnetite 

6. Pyrite-FeS2 

7. Apatite-Cas (P04 h (OH,F ,Cl) 

8. Calcite 

9. Quartz 

10 

Franciscan Formation (from Gluskcter, 1962)-

Continued 

Spilites-Continued 

10. Analcite-Na(AlSi2 06 )·P2 0 

11. Clinozoisite 

12. Orthoclase feldspar 

13. Pumpellyite-Ca4 (Mg,Fe +2) 

(Al,Fe+3)s O(OHh (Si2 01· h 

(Si04 h ·2H2 0 

14. Laumontite-Ca(Al2Si4C12)·4H20 

Monterey Shale 

Shale and silty sandstones 

1. Pelitic rock fragments 

2. Quartz 

3. Plagioclase 

4. Chert 

5. Biotite 

6. Hornblende-(Na,K)o-1Ca2 

(Mg,Fe+2 ,Fe+3 ,Al)s (Si6-1Al2-1 

022)(0H,Fh 

7. Karinthine-variety of he mblende 

8. Glauconite-(K,Na,Ca) 1.2:-2.0 

(Fe+3 ,Al,Fe+2 ,Mg)4.

0(Si7_7.

6 

Al1-o.40 2o)(OH)4 ·nH2 0 

Merced Formation 

1. Quartz 

2. Plagioclase feldspar 

3. Orthoclase feldspar 

4. Biotite 

5. Hornblende 

6. Muscovite 

7. Chert 

8. Pelitic rock fragments 
Sediment sampling within the lagoon was 

done during August and September 1967 with a 

piston coring device, and grab samples were 

collected along Stinson Beach and Bolinas Cliffs 

and at the mouths of streams draining into the 

lagoon. Only the uppermost half a foot of each 

core sample was studied. Each sample was split 

by washing over a 62-micron sieve. The coarse 

fraction of each sample was impregnated with 

leucite (methyl methacrylate monomer) and 

studied by petrographic techniques; both flatand universal-stage methods were used. The fine 

fractions were analyzed by X-ray diffraction; 

only dry bulk patterns were run. A total of 82 

samples was studied. 

In general, all sediments studied, if lithified, 

would range in composition from lithic arkosic 

wackes to lithic or arkosic arenites1. The wackes 

would predominate within the lagoon, whereas 

outside in the beach areas arenites would 

predominate. The abundance of lithic wackes 

and lithic arenites in the lagoon is not surprising 

since depositional sites lie very close to erosional 

sites. 

All minerals described in the surrounding 

rocks with the exception of axinite, analcite, 

1 au m on tit e, and pumpellyite have been 

recognized in sediments now in the lagoon. In 

addition to minerals described by Gluskoter 

( 1 9 6 2), glaucophane, sphene, zircon, and 

serpentine-group minerals have been recognized, 

but only in extremely minute quantities. 

Individual mineral species are difficult to use 

here as tracers because many distinctive species 

which are found as discrete grains are also found 

in rock fragments. This is expected because 

erosion and deposition take place over short 

distances. 

lThe terms"wacke" and "arenite" are used herein as 

defined by Gilbert (Williams, Turner, and Gilbert, 1954, 

p. 291-293). Wackes are impure sandstones containing 

10 percent or more argillaceous matrix. Arenites are 

pure sandstones having less than 10 percent argillaceous 

matrix. Those wackes or arenites containing more rock 

fragments than feldspar grains are either lithic wackes or 

arenites, whereas those having more feldspar grains than 

rock fragments are termed arkosic wackes or arenites. 

11 

Because fragments of the Monterey Shale are 

easily recognized, they were studied separately, 

and percentages were calculated from counts of 

400 recognizable fragments in each sample. 

Figure 5 shows the distribution of samples 

composed of one-third or less and between 

one-third and two-thirds of Monter~y Shale 

fragments. As expected, the higher 

concentrations lie along the western half of the 

lagoon close to the sources. It could not be 

ascertained, however, whether the Monterey 

Shale came mainly from the Pine Gulch Creek 

drainage or the Bolinas Cliffs just outside the 

lagoon. Certainly both sources supply Monterey 

Shale to the lagoon. The irregularitir,s in the 

boundary between samples with one-third and 

those having more than one-third I ·1 onterey 

Shale probably reflect the local sources of 

Franciscan detritus shed from drainages east of 

the lagoon. 

Franciscan-derived sediment seems to 

dominate the eastern one-third of the lagoon, 

especially at the mouths of streams drrining the 

east side. The western two-thirds seems to be 

dominated mostly by the Monterey Shale. In 

fact, the easily recognized shale fragments 

appear to be ubiquitous throughout lagoonal 

sediments and nowhere make up less than 10 

percent of the sediment studied. The sediments 

in most areas studied contain 33 percent or 

more Monterey Shale fragments. Hence, the 

Monterey must be considered the dominant 

source of lagoonal sediment. 

With present data it is not possible to state 

definitely whether the Pine Gulch Creek 

drainage or the Bolinas Cliffs is the more 

important source of the Monterey Shale. 

However, an analysis made using the: erosion 

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 

of 2.3 feet of retreat per year, the Bolinas Cliffs, 

which are approximately 120 feet l'igh and 

7,000 feet long, would yield an average annual 

volume of about 72,000 cubic yards of 

sediment. Much of this material is silt and clay 

which is transported in suspension into Bolinas 

Bay and does not enter the lagoon. Powever, 

study of sand-size samples taken from all over 
the lagoon indicates that all the samples contain 

at least 33 percent Monterey Shale. If one 

assumes that at least 33 percent of the volume 

of material eroded from the Bolinas Cliffs is of 

sand size, the annual volume of sand-size 

material eroded is 24,000 cubic yards. If this 

annual rate of production of sand-size material is 

extended for I 00 years, the length of time 

during which measurements of cliff retreat have 

been made, the Bolinas Cliffs could have 

supplied 2,400,000 cubic yards of Monterey 

Shale to Bolinas Lagoon. This volume, spread 

evenly over the I , I 00-acre lagoon, would have 

caused 0.45 yard of vertical aggradation. Since 

even spreading is unlikely, it is probable that 

greater amounts of local aggradation have 

occurred over the last I 00 years. The presence in 

lagoonal sediments of shell fragments of 

organisms that presently live near the Bolinas 

Cliffs strengthens the hypothesis that the major 

source for the Monterey Shale in the lagoon is 

the rapidly eroding cliffs. 

SUMMARY 

Measurement of water flow and sediment load 

at the lagoon inlet for a I 0-hour tidal period 

(flood tide and ebbtide) on June 22, 1967, 

revealed that in that specific period of time 152 

tons of suspended sediment was carried into the 

lagoon by the floodtide, whereas only 36 tons 

was carried out of the lagoon by the ebbtide. 

The maximum measured water flow and 

maximum average velocity during floodtide were 

5,810 cfs and 3.5 fps, respectively; during 

ebbtide the maximums were 3,720 cfs and 2.4 

fps. The range in stage of floodtide was about 4 

feet and of ebbtide, about 1.3 feet. 

On October 24-25, 1967, measurements of 

water flow and suspended-sediment load at the 

inlet for a tidal day showed that about 500 tons 

of suspended sediment was transported out of 

the lagoon during the ebbtides and 170 tons was 

transported into the lagoon during the 

floodtides. The maximum water discharge and 

maximum average velocity measured during the 

floodtides occurred in the period from lower 

low water to lower high water and were 4,230 

12 

cfs and 3.2 fps, respectively; during the ebb tides 

the measured maximums occurred from higher 

high water to lower low water anrl were 5,280 

cfs and 4. 7 fps, respectively. 

Much of the sediment being mC'ved through 

the inlet is sand, which probably is eroding from 

the sand beaches on the sides of the inlet. 

Observations of released dye and surface 

floats indicated that near the inlet the littoral 

drift is generally toward the mouth but away 

from the inlet the pattern varies acc')rding to the 

wind and wave conditions. 

The circulation in Bolinas Lagoon was studied 

on September 6, 1967. The circulation pattern, 

in general, seemed to be counterclockwise with 

the tide first turning from flood to ebb along the 

western side of the lagoon. The overall velocities 

during floodtide were higher than those during 

ebbtide, probably because of diffe~ences in the 

constriction of flow and the quantities of water 

involved. Velocities in the central part of Bolinas 

Lagoon may be adequate to erode and transport 

bottom sediment, whereas those in the upper 

basin and the southeastern part may not be. 

Cross sections of the lagoon wer~ surveyed in 

August and September 1967 to d~termine the 

topography of the lagoon botton, especially 

with regard to the location and configuration of 

the channels. The maximum depth measured 

was 14.7 feet below mean sea level. 

Grain-size parameters show that the sediment 

in the central part of the lagoon is generally 

coarser and better sorted than that in the upper 

basin and in the southeastern _rart, perhaps 

because of hydrodynamic diffe.rences. The 

specific weight of the bottom seditr~nt averaged 

79 pounds per cubic foot; the specific gravity 

averaged 2.55. 

The influences of different sources of the 

sediment were determined from identification of 

minerals in the lagoon. Sediment derived from 

the Franciscan Formation dominate1 the eastern 
one-third of the lagoon. This sediment was 

transported to the lagoon by the eastern streams 

tributary to the lagoon. The sediment in the 

western two-thirds of Bolinas Lagoon was 

derived chiefly from the Monterey Shale. This 

sediment was carried into the lagoon by Pine 

Gulch Creek and by tidal currents and waves 

that eroded Bolinas Cliff. 

SELECTED REFERENCES 

Beach Erosion Board, 19 61 , Shore protection 

planning and design: Washington, U.S. Govt. 

Printing Office, Tech. Rept. 4, 242 p. 

Deer, W.A., Howie, R.A., and Zussman, Jack, 

1966, An introduction to the rock-forming 

minerals: New York, John Wiley & Sons, Inc., 

518 p. 

Folk, R.L., 1961. Petrology of sedimentary 

rocks: Austin, Texas, Hemphill's, 154 p. 

Galloway, A.J., 1966, Geology of northern 

California: California Div. Mines and Mines 

Bull. 190, p. 429-440. 

Gilbert, G.K., 1908. Tomales Bay to Bolinas 

Lagoon, in Lawson, A.C., chm., and others, 

The California earthquake of April 18, 

1906-Report of the State Earthquake 

Investigation Commission: Carnegie Inst. 

Washington Pub. 87, 3 v., 1 atlas. 

Gluskoter, H.J., 1962, Geology of a portion of 

western Marin County, California: Berkeley, 

California Univ., Ph. D. thesis, 184 p. 

Gorai, Masao, 1951, Petrological studies in 

plagioclase twins: Am. Mineralogist, v. 36, p. 

884-901. 

Hjulstrom, Filip, 1935, Studies of the 

morphological activity of rivers as illustrated 

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 

Krumbein, W.C., 1936, Application of 

logarithmic moments to size f .. equency 

distribution of sediments: Jour. Sed. 

Petrology, v. 6, p. 35-42. 

Moore, D.B., 1965, Recent coastal s~diments 

Double Point to Point San Pedro, California: 

Berkeley, California Univ., Hydraulic 

Engineering Laboratory tech. rept., p. 2-14. 

Rantz, S.E., 1968, Average precipitation and 

runoff in north coastal California: U.S. Geol. 

Survey Hydrol. Inv. Atlas HA-298. 

Ritter, J.R., 1969a, Measurement of water flow 

and sediment load, Bolinas Lagoon, Bolinas, 

California: U.S. Geol. Survey Prcf. Paper 

650-B, p. B189-Bl93. 

--1969b, Preliminary studies of 

sedimentation and hydrology in Bolinas 

Lagoon, May 1967 to June 1968: U.S. Geol. 

Survey open-file rept., 95 p. 

Slemmons, D.B., 1962, Determin~tion of 

volcanic and plutonic plagioclases using a 

three to four axis universal stage: Geol. Soc. 

America Spec. Paper 69, p. 34. 

Sundborg, Ake, 1956, The river Kalr~a1ven, a 

study of fluvial processes: Geografiska 

Annaler, v. 38, p. 127-316. 

Turner, F.J., 1951, Observations on twinning of 

plagioclase in metamorphic roc"s: Am. 

Mineralogist, v. 36, p. 581-589. 

Wentworth, C.K., 1922, A scale of grade and 

class terms for clastic sediments: Jour. 

Geology, v. 30, p. 377-392. 

Williams, Howell, Turner, F.J., and Gilbert, 

C.M., 1954, Petrography, an introduction to 

the study of rocks in thin sections: San 

Francisco, Calif., W.H. Freeman and Co., 382 

p. 

FIGURES l-5 

PACIFIC OCEAN 

EXPLANATION 

s 

Gaging station 

® 

Tide gage 

0 1000 I II II I 2000 I 

3000 fi~ET 

I 

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

17 
1--' 

00 

122°~0' 

1000 2000 3000 FEET 

PACIFIC OCEAN 

EXPLANATION ~ 

All u ti;~;J and }~ 

~ ~~u ~ Te:r:~~~~~::its -~ ~ ~~ 

u 1: ~ { ~ }f-. ~ 0 f-. ~ ~~~ ~ ~~z<< 

i:: i: Merced Formation ~ < < gz 

~{ ~ }~~~ ~ Monterey Shale f-. f-. < 

~ ~ ~ ~ }.~0· <z<CI.l Francisca~ Formation ~ < ~ g ::>u~~ -------- ...... - u u Contact ell 

Dashed where approximately 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 ••••• 

. . . . . . . 

·.·.·~·.·.·.·.··:·:·:-:-:·:-:-:-:·: ............ 

• • 0 ••••••••••••••• 

• • • 0 •••• 
0 

I 1 

Duxbury 

Point 

2 MILES 

1950 

..,.,. ... ., 

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 topographic quadrangle. 1950: From U.S. Geological Survey Mount Tamalpais quadrangle. 

19 
N 

0 

37• 55'; 

122.40' 

0 1000 2000 3000 4000FEET I 11 11 I I I I 

EXPLANATION 

ITJIJJ 

Above water 

at start of study 

Above 

-

hi1h water 

~ 

Channel 

• 

Initial dye spot 

Current direction 

and velocity 

(velocity, in feet 

per minute) 

--+ 

> 100 

.... S0-100 

-+ 

10;50 

4 
N 

- 37.55' 

122.40' 

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

EXPLANATION 

E2SJ 

Above water 

at start of study 

Above 

-

high water 

~ 

Channel 

• 

Initial dye spot 

Current direction 

and velocity 

(velocity, in feet 

per minute) 

-+ 

> 100 

... so -108 

~ 

10 -so 

< -10 

Figure 4.-Circulation in Bolinas Lagoon, September 6, 1967. A, Floodtide. B, Ebbtide. 

~ 
JIIA.CIFIC OCEAN 

EXPLANATION 

Ptnentap of,....... of •oaterey S..._ 

ia bottom aectimat 

~--· 

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

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

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

22