150 YEARS OF TIDES ON THE WESTERN COAST:

THE LONGEST SERIES OF TIDAL OBSERVATIONS IN THE AMERICAS

CAPTAIN ALBERT E. THEBERGE, JR. , NOAA (RET.)

NOAA Central Library

U.S. DEPARTMENT OF COMMERCEDonald L. Evans, Secretary

National Oceanic and Atmospheric AdministrationVice Admiral Conrad C. Lautenbacher, Jr., U.S. Navy (Ret.)

Undersecretary of Commerce for Oceans and Atmosphere and NOAA Administrator

National Ocean ServiceDr. Richard W. Spinrad, Assistant Administrator for NOAA Oceans and Coasts

Center for Operational Oceanographic Products and ServicesMichael Szabados, Director

Abstract The same year as the acquisition of California by the United States Government, the United States CoastSurvey had been authorized to begin surveying the coast of Oregon Territory from the northern borderof Alta California to the Puget Sound area of Washington Territory. Discovery of gold in 1848 and thesubsequent gold rush added urgency to the requirement to chart our western coast including California.Small crews of Coast Surveyors headed west, either around Cape Horn or to Chagres, Panama, thenceoverland to Panama City, and then by ship to California. Accurate astronomic latitudes and longitudesof prominent points and landmarks on the Coast were first determined, magnetic declinations at thesepoints observed, thence topographic surveys of critical areas including suggested locations forlighthouses were conducted, and finally reconnaissance hydrographic surveys based primarily onastronomic positioning and dead reckoning were run. Once this critical first work was accomplished,local triangulation schemes tied into the established astronomic positions were observed, and thentopographic and offshore hydrographic surveys controlled by the triangulation were conducted. Afteryears of work, an arc of triangulation parallel to the coast tied together all of the local triangulationnetworks and a coherent set of charts of our western seaboard based on a common geodetic datum wereproduced. In the early years, a final step prior to conducting hydrographic surveys was the installationof a tide staff in the local area of a survey so as to be able to determine the stage of tide during the timeof survey work and relate the observed soundings to an arbitrary zero point. This arbitrary zero pointcould be the mean tide level, mean high water, mean low water, or, as in the case of the western shoresof North America, mean lower low water. As technology improved with the development of self-recording instruments and a network of permanent gauges was installed, many serendipitousgeophysical and oceanographic discoveries were made over and above the primary mission of providingsafe passage for mariners. Remarkably, the self-recording gauge installed at Fort Point at the entrance tothe Golden Gate and its successors have subsequently survived storms, earthquakes, the potential forhuman error and intervention, and produced the longest running series of tidal observations in theAmericas. Since the early surveys, the study of tides has matured and accurate long-term tidepredictions have been developed and coupled with real time water-level and meteorologicalobservations to guide the shipping of America into its ports.

150 Years of Tides

Construction of an early visual Tide Elevation Recorder at Alcatraz Island

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The Colonial PeriodFor two hundred years Spanish mariners had

been skirting the coast of California on the returnleg of the Mexico-to-Manila trade route but hadalways stayed well offshore from the rock-boundcoast of California. However by the late 1700’sother colonial powers had begun casting lustfuleyes upon the western coast of North America. Inresponse, in 1769 the Spanish sent out an overlandexpedition to establish a series of missions alongthe coast of Alta California both to convert thenative Americans and to consolidate their claim tothis territory . Led by DonGaspar de Portola, thee x p e d i t i o n w a ssearching for theharbor described byearlier explorers atMonterey. Notrecognizing the bayor any harbor whenpassing through thearea (in fact there isnot much of a naturalharbor at Monterey),t h e e x p e d i t i o ncontinued to thenorth, crossed overthe coast ranges andcamped near present-day Palo Alto in lateOctober, 1769. Portolasent out a group of scouts toinvestigate the surrounding area. On November 3,1769, Sergeant Jose Francisco Maria de Ortegareturned to camp after a three day-excursion toreport that he had discovered a great estuary andridden up its east side for quite a distance beforeturning back.

Ironically, this great estuary, since named SanFrancisco Bay, had been discovered by a sergeant inthe Spanish colonial army. However, it was anotherfive years before the entrance to the bay would beviewed from the site of the present city of San

Francisco and another year before a first Europeanship would sail through its entrance. This first ship,the San Carlos which was commanded by Frigate-Lieutenant don Juan Manuel de Ayala, entered thebay on August 5th, 1775. Carrying full sail with astiff WNW wind blowing from astern, his little shipstrained to stem the current of an ebb-tide flowingout of the narrow entrance. Ayala estimated thecurrent at 6 knots (perhaps an exaggeration butthree knot currents are common), the first inkling ofthe nature of the tides and tidal currents in San

Francisco Bay. Over the nextmonth and a half, an initial

survey of San Francisco Baywas produced. Althoughfew observations of rangeof tide were made, therewere numerous commentsconcerning tidal currents.

The following yearCaptain Juan Bautista deAnza led an expedition tot h e S a n F r a n c i s c oPeninsula for the purposeof choosing the site of amission and a fort. Thesite chosen for the fort, orpresidio, was just to the

east of a point that henamed Cantil Blanco. The

site for a mission was alsoselected and a first mass celebrated

by Fathers Palou and Cambron on June 29, 1776,marking the founding of both the mission and thecity of San Francisco. The birth of the city pre-dates the birth of our Nation by 5 days.

The Spanish charts of this era were inaccurateand it was not until the winter of 1826-27 that theEnglish surveyor Captain Frederick Beecheyarrived in San Francisco and produced the firstaccurate chart of the bay. It was he who designatedthe prominent point, earlier named Cantil Blanco,on the south side of the entrance Fort Point.

Entrance to the Golden Gate

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Over the next twenty years United Statesinterest in this area grew. Exploring expeditionscame overland; American maritime traders headedaround Cape Horn and traded for cattle hides andsea otter furs. Some stayed to increase theAmerican presence in the region. Ironically, thegateway to the San Francisco Bay was baptizedChrysopylae, or “Golden Gate” by overlandexplorer John Charles Fremont in 1846 because hefelt the wide entrance to the Bay would beadvantageous for commerce. “To this gate I gavethe name of Chrysopylae, or GOLDEN GATE; forthe same reasons that the harbor of Byzantium(Constantinople afterwards) was called Chrysocerasor GOLDEN HORN.” This same year Commodore

Robert F. Stockton took possession of UpperCalifornia for the United States on July 7 and in1848 the Treaty of Guadalupe-Hidalgo was signedceding California and much of the AmericanSouthwest to the United States.

Prior to signing of this treaty, a world-changing event occurred on January 24, 1848, atSutter’s Mill in northern California. James

Marshall discovered gold. With the announcementby President James Polk on December 5, 1848, toCongress that, "Recent discoveries render itprobable that these mines are more extensive andvaluable than was anticipated," the rush was on.The Golden Gate became the port of entry forthousands of miners headed to the gold fields and,virtually overnight, San Francisco became ametropolis. Clipper ship captains headed toCalifornia with children’s school atlases or copiesof old maps produced by Spanish explorers, GeorgeVancouver, Charles Wilkes of the United StatesExploring Expedition, or those of a few othermariners and surveyors who had given the coast acursory reconnaissance years before. The best of

these charts were generallyinaccurate with prominentpoints being in error by asmuch as fifteen miles andv i r t ua l l y no dep thsrecorded outside of majorharbors. Although thechannel islands wereknown, the orientation andlocation of virtually all ofthe islands were eithersignificantly in error or noteven shown on the variousexplorers’ charts.

Early CoastSurvey Work on

the WesternCoast

Because of immigration to the Oregoncountry, the Coast Survey had been making plans tosurvey the coast of Oregon Territory as early as1846. In 1848, Congress authorized this work andthe Coast Survey sent its first crews to the WestCoast in 1849. Unfortunately, the gold rush was on;labor, transportation, and costs of supplies

Early surveying along the Pacific Coast

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skyrocketed with an accompanying stoppage offield operations. One crew, under Assistant JamesWilliams, was sent for the land operations andanother, under Lieutenant William P. McArthur,USN, for the offshore hydrographic surveyingoperations. The Coast Survey Schooner EWINGarrived in San Francisco after fighting its wayaround Cape Horn after a seven-month trip onAugust 1, 1849. The EWING was a topsailschooner 91 feet in length. For a variety of reasonsincluding desertions and a mutiny, the EWING also

was stymiedin 1849 andretired witht h e l a n dcrew to theH a w a i i a nIslands forthe winter of1849-50 toobtain newc r e wm e m b e r sa n d t oresupply atc h e a p e rrates.

Becauseof the above

frustrations, Alexander Dallas Bache, Superinten-dent of the Coast Survey, decided that a crew ofyoung energetic men with “reputation to make” anda desire to overcome all hardships should be sent tothe West Coast in 1850. This group of four menwas led by George Davidson, who would becomethe leader of the West Coast scientific communityover the next half century. James Lawson, A. M.Harrison, and John Rockwell comprised the remain-der. Davidson, Lawson, and Rockwell sailed fromthe East Coast on May 5 on the steamer PHILA-DELPHIA for Panama. They landed at Chagres,hired native Indians for traveling by canoe to thehead of the Chagres River, and then joined a muletrain to go the rest of the way to the city of Panama.On May 30 they embarked on the Pacific Mail

Steamship TENNESSEE and arrived in SanFrancisco on June 20. After a few weeks spentestablishing a base of operations, they proceeded toPoint Conception, landing at El Coxo in mid-July.

In Lawson’s words, “Pt. Conception is one ofthe most notable points on the California coast, andits accurate position was particularly desirable, as itmarked, in fact is the key to, the Northern entranceto the Santa Barbara Channel.” Harrison, the chieftopographer of this group, joined them during thePoint Conception work. By the end of Septemberan accurate latitude and longitude of PointConception had been obtained by preciseastronomic means, its magnetic declinationdetermined, a site for a lighthouse selected, and atopographic survey of the area about the selectedlocation conducted. The labor involved with thiswas quite difficult involving the carrying of largeheavy instruments from El Coxo to PointConception and a 300-pound instrument stand.

Relative to the wages of the times, each of theyoung Coast Surveyors was paid $30.00 per month.A cook they hired in San Francisco was paid$125.00 per month, making more than this wholegroup of skilled engineers. Lawson suggested theweather was better than the storied “Italian skies”for this sojourn at Point Conception but noted thecontinual offshore fog hid the Channel Islands fromview for the first six weeks of their stay. Finally,early in October the work was finished and the crewhired a pack train and headed into Santa Barbara toawait transportation to San Francisco. They “didthe town” while there and met the famous otterhunters George Nidever and Isaac Sparks. Theyhad earlier made friends with Don Luis Carillo, sonof Don Anastasio Carillo, the owner of the PointConception area. They stayed at Don Anastasio’shome while awaiting transportation and had manyconversations with Don Luis. He felt they werenear to transgressing the truth when they describedthe multi-story buildings of the East Coast and therailways, but “morally certain they lied” when theydescribed the wonders of the telegraph.

Little was accomplished in southernCalifornia the following year as the Coast Survey

ALEXANDER DALLAS BACHEThe second Director of the Coast Survey

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concentrated its efforts to the north of SanFrancisco. Although a source of supplies, southernCalifornia was still considered a relative backwaterat this time. However, an astronomic position,magnetic declination, and site for a lighthouse weredetermined at San Diego and a triangulation schemeand topography carried southward to the Mexicanborder from San Diego. The EWING proceedednorth in a first reconnaissance survey from SanFrancisco to the Columbia River entrance andconducted a few surveys at the river entrance aswell.

In 1852, the Coast Survey Steamer ACTIVE,under the command of Navy Lieutenant JamesAlden (one of approximately 800 Navy officerswho served with the Coast Survey in the NineteenthCentury), made a first reconnaissance hydrographicsurvey from San Francisco to San Diego. On thistrip George Davidson was put ashore with hisequipment and acquired astronomic positions at SanLuis Obispo, Santa Barbara, Prisoner's Harbor onSanta Cruz Island, San Pedro, Santa Catalina Island,San Clemente Island, San Nicolas Island, andCuyler's Harbor on San Miguel Island. Thismarked the first time that these islands had beenadequately located. By the end of 1852, most of themajor headlands and points of interest for marinersbetween the California-Mexico border and CapeFlattery, Oregon Territory, had beenaccurately determined by GeorgeDavidson and his assistant John Rockwell.Rudimentary charts of many of theobserved harbors and islands of SouthernCalifornia were produced by the end of1852.

During these first three years ofCoast Survey operations on the westerncoast, a relatively accurate general outlineof the coast was sketched in and manydangerous errors corrected, the geographicpositions of the major headlands andlandmarks determined, magneticdeclinations at strategic points observed,and loca t i ons fo r l i gh thousesrecommended. The detail work of

connecting the various independent astronomicallydetermined locations by triangulation (much morerigid positioning than attainable through astronomicmeans), conducting topographic mapping of theshoreline and offshore hydrographic surveying thatwould be controlled by the triangulation networkwas ready to begin. However, little had been donewith observing tides other than the establishment ofa few tide staffs to measure tides duringhydrographic survey operations such that waterdepths could be reduced to a local plane ofreference. To obtain readings, either a sailorattached to the survey party or a local citizen washired to read the water level on the tide staff eitherevery hour or some fraction of an hour never lessthan every 15 minutes.

Of the four young men who came west in1850, George Davidson (1825-1911) and JamesLawson (1828 – 1893) would remain on the WestCoast for most of the remainder of their lives.Davidson made his home in San Francisco and wasby far the most well-known of the group as he wasconsidered California’s most prominent scientist formany years and had many geographic featuresnamed after him including Mount Davidson in SanFrancisco and Davidson Seamount to the southwestof Monterey. Lawson made his home in Olympia,Washington. A.M. Harrison and John Rockwell

Section of U.S. Coast Survey Chart engraved in 1859

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returned to the East Coast.Rockwell died an earlydeath in 1857 and Harrisonpassed away in 1881.

The TidesT h e r e a r e f e w

backdrops as dramatic as theGolden Gate to observe thenature of our clockworkuniverse. Twice daily thetides rush into the bay on thefloods and twice out on theebbs. Ships plan sailingtimes and arrival times onthese daily risings andfallings. The commercialand naval wharves,seawalls, the great bridges ofthe Bay Area, underwatercommunications cables,pipelines, and other engineering works all havebeen designed and built taking the tides intoconsideration. Fishing trips are planned to coincidewith stages of the tide, recreational beachcombingand tidepooling are planned to coincide withfavorable stages of the tide, surfers plan trips tofavorite breaks based on tide predictions, and eventhose who come to the shore for love, friendshipand renewal can be affected by the action of thetides. How do we predict the stages of the tides forship operations, engineering purposes, commercialand recreational fishing, or other recreational andpersonal activities?

There is a grand symphony that has beenplayed out for billions of years – an orchestration ofmoon, sun, earth, and ocean. There are physicalconsequences to the Earth and Moon revolvingabout a common center of gravity, the Earth-Moonsystem revolving about the sun, the varyingdistances between Earth and Moon and Earth andSun, and the progression of continually changingdeclination of the moon and sun relative to theearth. All of these motions and interactions are

manifested by predictable, but changing,gravitational forces acting upon the atmosphere, theoceans, and the solid earth itself. The most visibleresult of these forces, particularly for those who livealong our coastlines, is the continual changing ofthe level of the sea as the tide rushes in and out.

Although humankind has observed thesephenomena for thousands of years, it was not untilrelatively recently that we took to studying,attempting to understand, and predicting the tides.Coming from areas adjacent to the MediterraneanSea, both Alexander the Great in 325 B.C. andJulius Caesar in 55 BC almost met disaster becauseof tidal phenomena. Alexander’s fleet almost metits end on the Indus River as the result of tides anda similar occurrence caused Caesar to retreat fromthe shores of England after suffering damage to hisfleet after anchoring in tidal waters. There is acontinuing but unfounded rumor that in 322 BCAristotle committed suicide because he was unableto determine the cause of the tides.

Golden Gate view from the San Francisco tide gauge

6Portable tide gauge installation with tide staff

Some progress was made in understanding thetides in the classical era and even through the DarkAges. The relationship between phases of the moonand tidal range was noted by many observers andeven crude tide tables were produced for a fewareas. However, it was not until 1687 that Sir IsaacNewton developed the concept of tides beingcaused as a result of predictable but varyinggravitational forces resulting from the changingrelative positions of sun and moon relative to theEarth. But, recognizing the cause of tides and beingable to predict them are two different things. Theconfiguration of oceanic basins and local conditionssuch as water depth, configuration and slope ofbottom, and meteorological effects all combined toconfound attempts to understand the nature of tides.This situation was exacerbated by inadequatetechnology to observe tides under a variety ofconditions, lack of accurate solar and lunar tables,lack of accurate time-keeping instruments, and lackof any scientific infrastructure to coordinateobservations from geographically dispersedlocations.

This remained thesituation until the early tomid-Nineteenth Century.By this time moreaccurate sun and moont a b l e s h a d b e e ndeveloped, better time-keeping mechanisms tocoordinate observationshad been invented, andp e r h a p s , m o s timportantly, dedicatedscientists and cadres ofengineers in organizationssuch as the United StatesCoast Survey had begunturning their energy tosolving the problems oftide observations and tidepredictions. Until themid-Nineteenth Centurythe only method for

observing the tide was to place a vertical staff in thewater graduated in some unit of linear measure (feetin the United States Coast Survey) and station anobserver close enough to the tide staff such that hecould read the stand of the tide on the staff everyhour or some increment of an hour and record thesevalues.

Although challenging in confined harborswith easy access to piers and other structures, onopen coasts the observation of tides was difficult ifnot impossible. In the case of tide staffs installed inharbors, a slight variation over fixed-staffobservations was the introduction of a floating staffencased in a stilling well that would move verticallywith the tides. The top of this moving staff wouldbe equipped with rollers that slid up and down inguides attached to a fixed staff graduated in feet anddecimal parts of feet. The observer would read thevalue that was adjacent to the very top of themoving staff. This arrangement of fixed andmoving staff often was enclosed in a small tidehouse not unlike those in use in many locationsalong the United States coastline today. The

advantage of this arrangement was thatthe observer was not exposed to theelements and that it was possible, onaverage, to make much more accurateobservations than was possible from aremotely viewed fixed staff. The tidestaff method, whether fixed or floating,was subject to human error,carelessness, and to some degreesubjectivity. Although there were afew professional tide observers in theCoast Survey, notably GustavusWurdemann who was repeatedlypraised in annual reports for hisaccuracy and devotion to duty, in mostcases the observations were entrustedto members of the hydrographic partyor local citizens who did not alwaysmeet the same high standards asWurdemann.

The problem of varying quality ofobservations was noted in the first

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attempt to obtain accurate tide observations on thewestern coast of the United States. Twocoordinated sets of observations were planned fromstaffs at Rincon Point (famous now for being closeto Pac Bell Park and the site of Barry Bonds’homerun marathon) and Sausalito. Theseobservations were conducted for a little less than amonth in late January through early February ofboth 1852 and 1853. The Rincon Pointobservations, under the direction of LieutenantJames Alden, commanding officer of the CoastSurvey Steamer ACTIVE, were praised for theiraccuracy and attention to detail. Conversely, it wassuggested that the Sausalito observations “were notmade with the same care which appears tocharacterize” the Rincon Point observations.

As a result of the short series of Rincon Pointobservations, Alexander Dallas Bache,Superintendent of the Coast Survey and great-grandson of Benjamin Franklin, was able to drawmany conclusions of importance to the navigationof San Francisco Bay. The first of these was thatthere was a large diurnal inequality between thesuccessive high and low waters of each lunar day(24 hours and 50 minutes for moon to complete arevolution about the Earth). What this meant tonavigators was that an obstruction having three anda half feet of water over it at a first low tide of theday could be awash at the time of the second lowtide of the day. This had great significance both forthe immediate needs of assuring safe passage ofcommerce but also relative to choosing a plane ofreference for soundings for charting purposes.Ultimately, mean lower low water was chosen asthe plane of reference for Coast Survey charts onthe West Coast and Alaska as opposed to mean lowwater as used on the East Coast and Gulf Coast. Ashydrographic surveys were performed for creatingupdated nautical charts of the region, the data fromthe tide gauges were used to correct the soundingsfor stage of tide and refer them to a common datum.Other information obtained from this short series ofobservations were rules for determining times ofhigh and low water relative to the declination of themoon, average range of tides between highest high

water and lowest low water, and probable greatestrange of tides in the Bay.

Although these first tide observationsprovided the mariner with a rough means todetermine the times of high and low water in theBay, it was understood that only a small inroad hadbeen made in understanding the Pacific tides.Fortunately a new technology had been recentlydeveloped. The great instrument-maker JosephSaxton invented a self-registering tide gauge in1851 which ran twenty-four hours per day withminimal human care. This gauge was not the firstself-recording tide gauge but was considered to be amarked improvement over existing instruments. Itconsisted of a float attached by wire to a gearingmechanism. The gearing mechanism drove thelocation of a pencil relative to a rotating drumcovered with a paper tide record sheet. The wholesystem was time synchronized such that the penciltracings of the risings and fallings of the tide in asinusoidal curve could later be scaled for heightsand times of various stages of the tides. This newsystem effected a revolution in both the quantityand quality of tide records acquired by the CoastSurvey. Shortly after the introduction of andpossibly as a result of this new technology, a newtidal division was established in the Washington,D.C., headquarters of the Coast Survey under CountLouis F. de Pourtales, a Swiss immigrant likeFerdinand Hassler (1770-1843), the founder of theCoast Survey. An interesting aspect of this newdivision was that Alexander Dallas Bache hiredMary Thomas as a tides computer, the secondwoman science professional in the FederalGovernment. The first was Maria Mitchell, thegreat astronomer, who had been hired by Bache todo observations for the Coast Survey in the mid-1840’s and then hired by the Nautical AlmanacOffice. Not only was the Coast Survey the pioneersurveying organization on our coastlines, but it wasthe pioneer agency in hiring talented womenscientists and mathematicians to work side-by-sidewith their male counterparts.

In 1853 Superintendent Bache sent adedicated tides party under Army Lieutenant

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William P. Trowbridge to the West Coast with threeof the new self-registering tide gauges. Thesegauges were to be installed at San Francisco andSan Diego in California and atAstoria, Oregon. Trowbridgeaccompanied by Army enlistedman Andrew Cassidy and a fewother observers (apparently allArmy enlisted men), who werechosen for their intelligenceand devotion to duty, traveledto California via steamship toPanama, proceeded over thenearly complete PanamaRailroad to Panama City, andcontinued to San Francisco byPanama steamer, arriving inJuly 1853. They established aself-registering gauge in theNorth Beach area of SanFrancisco and then proceeded

to San Diego where Cassidy was left in charge ofthe newly installed gauge. From there Trowbridgeproceeded to Astoria.

Although a self-registering gauge wasestablished in San Francisco in 1853, it was decidedto move the gauge to Fort Point on the grounds ofthe Presidio in July 1854 as portions of SanFrancisco were in turmoil because of land squatterscausing civil unrest. Amazingly, since that timethis gauge and its successors have produced thelongest running unbroken series of tidalobservations in the Americas. It is probable thatthere is no other geophysical phenomena in theWestern Hemisphere that has a longer continuousrecord.

Although the San Francisco record is thelongest continuous tidal record, it is noted that self-registering tide gauges were established at fourother permanent locations at that time – Governor’sIsland, New York; Old Point Comfort, Virginia;San Diego, California; and Astoria, Oregon.Because of storm, disaster, carelessness, or any of amyriad of other possible causes, the only gauge ableSelf-registering tide gauge "Saxton"

Presidio tide station

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to survive with an unbroken record of observationswas the San Francisco gauge. For one hundred andfifty years observations from this gauge haveassisted the mariners of the world entering andsailing from the ports of the Bay Area as well ashaving helped in the planning and construction ofall the waterfront facilities of this great port.

Serendipitous Science Observation of tides for commercial and naval

shipping interests was and remains the primarypurpose of the San Francisco tide gauge. However,this particular gauge has a record of adding to ourknowledge of the oceans and its relationship to theEarth in general that is without peer. Within sixmonths of the installation of the gauge at Fort Point,a great earthquake occurred on December 23, 1854,near the central coast of Japan raising a series ofgreat tsunamis along portions of the Japanese coast.The tsunamis traveled across the Pacific Ocean andwere recorded as attenuated waves on the self-registering tide gauges along the western coast.These waves were superimposed upon the regulartidal record as a series of sinusoidal squiggles. Thefirst person to recognize the significance of thesesquiggles was Lieutenant Trowbridge who wrote toSuperintendent Bache in early 1855, “There isevery reason to presume that the effect was causedby a sub-marine earthquake.” This was an amazinginsight given that recording seismographs were stilltwenty-five years in the future and that no

earthquake had ever been remotely sensed by anymeans up to this time.

Trowbridge’s insight was validated whenword of a major earthquake occurring on the coastof Japan on December 23 reached SuperintendentBache. Armed with knowledge of the time of theearthquake, its location, times of arrival of thetsunami waves at both San Francisco and San Diego(from the tide gauge records), and times betweencrests of the various waves, Superintendent Bachewas able to estimate the average depth of thePacific Ocean. Bache was familiar with the latestbasic research published by Sir George Biddell Airyand his treatise on Waves and Tides in theEncyclopaedia Metropolitana in 1849. Airy hadmathematically developed theoretical expressionsthat govern the motion of waves in canals ofuniform depth and compiled tables for expressingthe relationships between wave length, wave period,wave velocity and depth of water. Bacheinterpolated the Airy table values using his distanceestimates and the tide gauge measurements for thetheoretical tsunami wave travel lines betweenShimoda, Japan and both San Diego and SanFrancisco. Using two separate estimates for thetimes of the disturbance due to the tsunami on thetide gauge curve at San Francisco, Bache estimatedthe average depth of the Pacific between Shimodaand San Francisco to be 13,380 feet and 15,000feet. For the line between Shimoda and San Diego,the average depth was estimated to be 12,600 feet. Considering that these were estimates of theaverage depth of the Pacific Ocean using indirectmeasurement and theoretical relationships ofwaves for canals of uniform depth, these numbersagree remarkably well with modern publishedvalues based upon modern measurementtechnology. Modern day estimates for the averagedepth for the depth profile from Shimoda to SanFrancisco are 15,504 feet and 15,221 feet fromShimoda to San Diego. Bache published hisestimates at a time when deep sea soundingtechnology was in its infancy, inaccuratesoundings ranging between 30,000 to 50,000 feetwere fairly common, and there was great

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uncertainty concerning the true average depth of theoceans.

Over the next 150 years the San Francisco tidegauge recorded many of the great tsunamigenicevents of the Pacific Ocean. It even recordedtsunami waves from the great Krakatau explosionof August 26, 1883, a few hours after the event andthe Coast and Geodetic Survey published notice ofan extraordinary event prior to any notice of thedetails or location of the disaster were known. Thegauge has also survived many major events in itsvicinity including the Hayward earthquake of 1868which did major damage to the East Bay and to landfill areas in San Francisco, the great earthquake of1906, and the 1989 Loma Prieta earthquake.

It may seem strange, but elevationsthroughout the United States and North Americahave been determined relative to mean sea level asdetermined at Coast and Geodetic Survey tidestations. Attempts to determine elevations of pointsinland from coastal tide stations began as early as1857 when a line of levels was run up the HudsonRiver between tidal bench marks in New York Cityand Albany, New York. Bench marks, usuallydistinct monuments in the form of concretecylinders with brass monuments on top that are setin the ground, or in the early years of tidalobservations marks etched on permanent rocksurfaces, are established at all tide stations in orderto assure that there has been no change of positionof a tide staff between the water surface and theland surface. After a series of tidal observationshave been made, local mean sea level can bedetermined at a gauge location and the elevationabove sea level of the bench marks in the generalarea can be determined. It was not until 1904 thatthe first trans-continental line of levels connectingthe tide gauge at Seattle, Washington with the oneat Sandy Hook, New Jersey was completed. Overthe next twenty years there were a number ofadditional connections made between Atlantic andPacific gauges. In 1929 the Sea Level Datum of1929 was introduced by the Coast and GeodeticSurvey which incorporated data from twenty-onetide stations in the United States and five in Canada.

This datum was the basis of elevation determinationfor all government mapping and for the planningand design of all major engineering projects in theUnited States. Prior to this time there was nostandard means of determining elevations in theUnited States and the establishment of this datumbegan with the tidal observations of the CoastSurvey. Since 1929 there have been two majorreadjustments of the vertical geodetic datum 1(seefootnote).

An issue related to the determination ofmean sea level as an elevation datum is the conceptof changing sea level. The San Francisco tidegauge is the longest continuous record of sea levelchange in existence in the western hemisphere.Whether sea level is increasing, decreasing, orremaining static is of major importance to peopleliving in coastal regions. The determination ofchanging sea level is a difficult issue. Because oftectonic forces, subsidence caused by withdrawal ofsubsurface fluids or mineral material from coastalareas, isostatic adjustment or rebound of land areaspreviously covered by glaciers, or a combination ofthese effects, coastal lands can be rising relative tothe sea, sinking relative to the sea, or remainingstatic. However, after taking into account theseperturbing forces, most of the last century hasshown a steady rise in sea level as determined bytidal records augmented over the last decade bysatellite altimetry. Tidal records show rise rates ofapproximately 2 mm per year over the last centurywhile satellite altimetry is showing even higherrates of rising sea levels (Note: the satellitealtimetry record is only 10 years long, so several

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This Sea Level Datum of 1929 was re-named as National GeodeticVertical Datum of 1929 (NGVD29) and superseded by North AmericanVertical Datum of 1988 (NAVD88) so that geodetic datums could bede-coupled from mean sea level observations at tide gauges. There isno consistent vertical relationship between NGVD29, NAVD88 andmean sea level around the coast. The long-term tide gauge records showus that trends in relative mean sea level are highly variable around thecoast due to varying rates of vertical land movement and using themtogether as baseline geodetic datum un-ravels over time. Modern tidegauges, precisely tied to the new geodetic networks and GPS referenceframes, are helping to distinguish regional sea level trends from globalsea level rise due to climate change and from vertical land movement.

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Variations in Annual Mean Sea Level at San Francisco: 1856 - 2002

more years of record are required to establish atrend). These rates of sea level rise have manyramifications for human occupation of coastalareas. If sea level continues rising at present rates,engineering works will have to be rebuilt ormodified; less areawill be available forhuman habitation;wetland habitats willbe drowned and lost;low-lying islands willbe inundated; andmany areas immunetoday from stormsurges caused bycoastal storms will inthe future be subjectto the walls of waterthat accompany majorwind events

Extreme high water events during periods ofEl Nino are clearly seen in the San Franciscohistorical tide record. El Nino events generallyoccur every 3 to 7 years in the Pacific Basin andare caused by the interaction between unusuallywarm sea surface currents and high sea levelsgenerated in the tropical Pacific drifting eastwardand colliding with lower temperatures in theEastern Pacific. According to historical records, themost severe El Nino events have occurred in the20th Century, and most recently during the period1997-1998.

As shown in the plot above, the effect of the1983 El Nino is clearly pronounced and is the eventof record in the monthly and annual mean sealevels. By analyzing the Interannual to Decadalvariations in sea level, especially from a longbaseline record like San Francisco, it=s now possibleto better understand the El Nino SouthernOscillation phenomenon and help predict futureevents.

Sea level records from the San Franciscogauge are indispensable for conducting climatechange research, investigations of global warmingand predicting El Nino events and the impacts of

sea level change on coastal communities. Analysisof localized sea level trends also provide insight andbetter understanding of regional tectonic changesand accompanying seismic activity. SanFrancisco's 150 years of sea level record adds a

w e a l t h o finformation to thek n o w l e d g e o fglobal cl imatec h a n g e a n drelative rates ofsea level rise. Infact , the SanFrancisco recordindicates that thepositive sea levelt r e n d ( 1 . 4 1mm/year) has nota l w a y s b e e nuniform over time

and experienced a downward trend between 1875and 1913. This sea level anomaly has also beennoted in the historical records of comparable longterm sea level records world wide.

Establishing the RecordTo obtain a continuous record of the tides in

San Francisco since 1854 has been a monument tohuman perseverance and ingenuity coupled withimprovements in the technology of water levelmeasurement. The self- registering tide gauge,established at Fort Point in June 1854, wasaccompanied by the establishment of bench marks,which are permanently fixed vertical referencepoints; in this case, the bench mark was a cross cutin the face of a large stone near the wharf where thegauge was installed. Leveling surveys wereconducted on a yearly basis between the benchmark and tide gauge to insure that stability of thegauge was maintained. Tide observers made dailyvisits to the gauge to make tide staff readings andcheck gauge time. Problems with wharf settlementwere soon discovered, but were corrected bystabilizing the area around the wharf with rock and

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stone fill. In 1877, the wharf came under disrepairand was abandoned. A new gauge site was chosenacross the Golden Gate at Sausalito. Meticulouscare was taken to preserve the Fort Point data seriesthrough the simultaneous operation of the FortPoint gauge and Sausalito gauge coupled with thetransfer of tide staff elevations by a water crossingtechnique. This involved limiting the refractioneffect during survey leveling operations bysimultaneously observing targets at bench marksites at Fort Point and on the north shore at LimePoint, a little more than a mile distant across theGolden Gate, and then transferring elevationsthrough repeated leveling and eventually to benchmarks at the new Sausalito gauge on GovernmentWharf at the Fort Baker military reservation. In1881, the Sausalito wharf began to deteriorate andthe gauge was moved to a more stable site; thenin1897, it was finally decided to move the gaugeback across the San Francisco Bay to the Presidioarea, which was just east of the original Fort Pointsite and in the approximate vicinity of the presentday gauge.

This era in the history of the San Franciscotide gauge was also marked by the association oftwo major Bay Area authors with the work ofNOAA ancestor agencies. It is little-known, butboth John Muir and Jack London were associatedwith the Coast and Geodetic Survey and the UnitedStates Fisheries Commission respectively. Muir

worked through the Sierra Nevada and the GreatBasin as a guide and artist for the Coast Surveyduring reconnaissance work for the 39th ParallelSurvey which was the first great survey lineconnecting the Atlantic and Pacific coasts of theUnited States. This work was done with CoastSurvey Assistant Augustus Rodgers, brother of thenaval hero Rear Admiral John Rodgers. Muir’sonly published work on this part of his wildernesslife was “Snowstorm on Mount Shasta” althoughdiaries of his Great Basin experiences are still inexistence. Jack London worked for and against theFisheries Commission in his youth as both afisheries enforcement agent and oyster pirate onSan Francisco Bay and as a deck hand on a sealingschooner in the Bering Sea. His San Francisco Bayexperience is recounted in “The Raid on the OysterPirates,” a delightful tale in which London, on theright side of the law, captures a group of oysterpirates by stealing their boats and using the risingtide to force their surrender and arrest them. “TheSea Wolf”, the dark tale of seal poacher WolfLarsen, draws on his sealing schooner experiences.

Standard Tide Gauge

The Harris-Fischer Tide prediction machine or "Old Brass-Brains."

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Tide gauge technology evolved little from1854 until the early 1960’s. The processing of tidalrecords also changed little and was very laborintensive during these years as it required manualscaling of tidal heights and time from the pen-recorded sinusoidal record. The scaledobservations were entered into record books forfurther processing. Tide predictions were computedmanually in the early years of the Coast SurveyTidal Division. However, in 1867 a new method ofcomputing tides, termed the harmonic method, wasintroduced by Sir William Thomson of GreatBritain. This method, although showing promise ofgreater accuracy in tide predictions, was impracticalbecause it was extremely labor intensive. Thomsonsolved this by inventing a tide prediction machinethat summed ten constituents of the harmonicanalysis equations in 1876. The values of theconstituents were obtained manually from theobserved high and low waters. William Ferrel ofthe Coast and Geodetic Survey designed a secondcomputing instrument of this type in 1880 whichwas operational by 1884. Reducing harmonicanalysis to a series of gears, pulleys, and levers, theFerrel machine computed times and heights of tidalmaxima and minima using nineteen constituents.The harmonic method of tide prediction has beenused by NOAA since that time. A second tideprediction machine was built by the Coast andGeodetic Survey and became operational in 1912.This machine, formally known as the Harris-FischerTide Prediction Machine, or “Old Brass-Brains” asit came to be affectionately termed, summed 37constituents and was used until the advent of digitalcomputers in the early 1960’s. This machine has along and honorable history and was used not only tocompute domestic tide predictions, but during theSecond World War computed tide predictions forworld-wide for use by our naval and amphibiousforces. Obtaining the values of the harmonicconstituents for input into the tide predictionmachine was also accomplished manually up to thetime of computers using a series of forms andstencils on the tabulated hourly heights of the tidegauge record.

In Jan-uary 1976, adigital paperpunch recordert e r m e d a na n a l o g t od i g i t a lr e c o r d e r( A D R )replaced thev e n e r a b l ep e n c i l a n dd r u mr e c o r d i n gm e c h a n i s m .T h i si n s t r u m e n trecorded theheight of thetide at set time

intervals, usually every six minutes, by punchingholes indicating the observed times and float heighton an aluminum-backed paper tape. As the paperpunch tapes could be machine-read, this systemsped up the processing of tide records but the gaugeitself still relied on a float/wire water level sensorand gearing mechanism on the recorder thatsynchronized time and rate of advance of the paperrecord. A tide observer was also required tomaintain the gauges on correct time and to makedaily tide staff readings.

This instrument had a life-span ofapproximately one human generation as beginningin 1985, the National Ocean Service embarked on amajor upgrade of what had become termed theNational Water Level Observation Network. Thenetwork of old float/wire systems was replaced bythe Next Generation Water Level MeasurementSystem (NGWLMS) which consisted of an airacoustic water level sensor coupled with anelectronic data acquisition system. These systemshave numerous advantages including the directleveling of the water level sensor to localbenchmarks (tide observers and tide staffs are nolonger required), electronic data storage, a backup

The ADR Gauge, a mechanical “punch” recorder.

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pressure water level sensor with its own datalogger, and ancillary sensor capability such as waterand air temperature, wind speed and direction andbarometric pressure. The acoustic sensor capabilityallowed much more accurate water level readingsbut what really set this system apart from the earliersystems was the ability to transmit data to a centralfacility via telephone line or via NOAA’sGeostationary Operational Environmental Satellite(GOES) data collection system for near real-timedata analysis, processing, and distribution. Bycomparison, prior to introduction of this system,tidal data rolls were removed monthly from theADR gauges and mailed to the National OceanService for processing. The NGWLMS systemreplaced the ADR gauge at San Francisco inJanuary 1996.The modern day ports in SanFrancisco Bay region continue to play a vital role inthe nation’s economy. Approximately 95% offoreign trade in and out of the U.S. is by ship andevery U.S. citizen, not just those living along thecoast, relies upon the nation’s ports for energydelivery, exports, transportation, and cost effectiveconsumer goods. The new water levelmeasurement gauges have also been integrated intothe NOAA Physical Oceanographic Real-TimeSystem (PORTS), that has been introduced intomany major United States harbors including SanFrancisco Bay. This system measures real-time

water levels, currents, and meteorologicalphenomena such as winds and visibility and makesthese data immediately available to the local userfor operational decision-making.

These decisions include when to load or off-load more cargo, when the best time to maketransits, when there is enough clearance to go undera bridge, or when to sail or not to sail with oragainst the currents and tides. This information iscritically important considering that there is anaverage of 261 deep-draft vessels entering SanFrancisco Bay each month and that there areapproximately 85,000 registered pleasure boatsusing approximately 100 yacht clubs in the Baysystem.

SummaryA U.S. Coast Survey tide gauge was installed

at San Francisco on June 30, 1854 and will soonhave produced the continuous recording of waterlevel for 150 years. The tide station, now operatedby NOAA, is part of a network of 175 long-termtidal and Great Lakes water level stations that havebeen established throughout the continental UnitedStates, Alaska, Hawaii, Pacific Island territories,Puerto Rico and the Virgin Islands.

The historical record from the tide station atSan Francisco transcends the maritime history ofthe San Francisco Bay, from the days when clipperships relied upon tide predictions provided by thestation to navigate the dynamic waters of theGolden Gate, to the modern day mariner thatobtains real-time water levels so that the huge shipand crane barge operators can tell if they haveenough depth in the channels and enough clearanceunder the bridges.

The record from the station continues to beused to update national nautical chart and shorelinereference datums. The data record itself containsthe signatures of important maritime events thathave affected human populations and the Californiaculture over time, from the traces of Pacific OceanTsunamis, to high tides from storm surges, to highsea levels due to El Nino, and to the long termThe Next Generation Water Level Measurement System

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record of sea level rise since the turn of the century.It is one of the longest continuous records of sealevel in the world and has been used by thescientific community in research for estimatingglobal sea level rise.

Today, the San Francisco tide gauge plays acentral role in the San Francisco Bay PhysicalOceanographic Real-Time System (PORTS) whichsupports safe, cost efficient navigation and providesshipping interests with accurate real-time tide,current and meteorological data and is an importantcomponent of the NOAA Tsunami WarningSystem. It continues to provide information criticalto maintaining and improving economic prosperityfor California and for maintaining and monitoringport activities important for Homeland Security.And finally, the data from the gauge are used toprovide water level and reference datuminformation needed for the increasing numberhabitat and marsh restoration programs in the bayregion.

Crane barge with just enough bridge clearance during transit