Lake Merced: A Geomorphic History

Lake Merced, in the

southwest corner of the City

of San Francisco, is a young

coastal lagoon-turned-

freshwater lake. It is a surface

exposure of the Westside

Basin groundwater aquifer

that lies under the region. The

lake itself is actually divided

into three separate lakes

commonly known as North,

South, and Impound Lakes,

with the eastern portion of

North Lake is often referred to

as East Lake. Today, the

surface of the lake sits at

approximately 18 feet above

mean sea level (Camp,

Dresser, & McKee, 1999), and

is primarily fed by a series of

underground springs, which

draw water solely from the

aquifer.

In the last 150 years,

significant alterations of the landscape have changed the morphology and hydrologic stability of

the lake. Urban encroachment, and the diversion of surface stream flow away from the lake have

severely reduced the influx of nutrients and sediments into the lake. Furthermore, pumping of

the Westside Basin aquifer for irrigation and municipal uses has created an overdraft situation,

which is reflected by the gradually declining water levels in Lake Merced. Today’s estimates

indicate that if current rates of groundwater extraction are maintained, Lake Merced could wind

Figure 1. Lake Merced in the southwest corner of San Francisco is surrounded on the north and east by urban development, and by private golf courses to the south. It is a surface exposure of the Westside Basin aquifer, which stretches southwest from Golden Gate Park to the San Francisco Airport. Map by David Freitag.

2

up as a dry mudflat as soon as 2009. If groundwater reserves drop too low, the very real risk of

salt-water intrusion into the aquifer would render it permanently unfit for its present use as

drinking water for the 250,000 people who currently rely on it. It is in the best interest of the

greater Lake Merced community and the users of this aquifer to insure that this condition is not

realized.

This paper has two purposes: First, to explore the geologic development of Lake Merced

as a coastal lagoon and identify historical sources of stream sedimentation. Second, the

discussion of how the urbanization of the surrounding landscape has altered these sources, the

morphology of the lake and proposals for restoration and future management of the resource. An

understanding of the history, geology, hydrology, current uses and abuses is essential to

developing a preliminary model for the eventual treatment, restoration and maintenance of water

quality of Lake Merced.

Methodology

The study begins with a review of the complex geology of the Lake Merced region.

Analysis of US Geological Survey reports will reveal a glimpse of what the area might have

looked like prior to the formation of the lake. The first section includes descriptions of the

complex stratigraphy that underlies the region. Theories for the formation of Lake Merced are

presented in conjunction with those of coastal lagoon formations throughout the world.

In the second section, a series of aerial photographs of the eastern portion of Lake

Merced are presented to show how human occupancy has disrupted the annual streams and

catchment basins which once flowed into Lake Merced. A series of historic maps from the

Spring Valley Water Company archives is shown to establish dates of significant, human-

induced morphological change. Rainfall and pumping records are examined to help establish

balanced rates of consumption. Through analysis, we can determine the location of groundwater

recharge zones that once fed the Merced aquifer. This information should help to produce a

model of specific sites where aboveground flow may be restored, thereby creating new recharge

zones for the dying lake and its aquifer.

3

Geologic History

Lake Merced sits atop layers of young,

poorly consolidated sediments just a few miles

north of the active San Andreas Rift Zone. The

underlying geology consists of a tectonic melange

of northward-dipping strata. Difficult to date, the

layers of bedrock and overlying, sedimentary

deposits in the Lake Merced region are usually

youngest closest to the surface. The dipping

layers tend to maintain their chronological

stratigraphy, leveling out near the lake itself.

Three main categories of rock comprise the layers

of the region. The underlying bedrock consists of

a tightly packed mixture of volcanics known as

the Franciscan Assemblage, overlain in most

areas by the relatively unconsolidated sediments

of the Merced and Colma Formations. These

layers are known to have been deposited as a

result of eustatic cycles of coastline transgression

and regression throughout the interglacial periods

of the Pleistocene age (Clifton & Hunter, 1988).

The oldest rocks in the assemblage are thought to be no more than 1.5 million years old.

The Franciscan Formation

The Franciscan assemblage is a tectonic mixture of older volcanic and sedimentary rocks

that underlies the poorly consolidated surface deposits of the Lake Merced region. These rocks

are part of a “central belt” of the coast ranges, thought to have uplifted to the surface about 175

million years ago as a result of the subduction of the Fallon plate beneath the North American

plate (Clifton & Hunter, 1988). Rocks from this period can be seen distinctly throughout the

higher peaks of central San Francisco as dark, golden-colored, serpentine outcroppings, and

along the northwestern edge of Lake Merced. These formations may have provided the initial

structural control that prevented mouth of the Lake Merced valley from becoming excessively

Figure 2. Distribution of rock formations in the vicinity of Lake Merced. The lake sits atop the poorly consolidated sandstones of the Colma formation (Clifton & Hunter, 1988).

Lake Merced

4

wide (Konigsmark, 1998). Sedimentary graywacke sandstone and siltstone make up about 90%

of the bedrock underlying the lake basin. It is deposited in a depression above the San Bruno

Fault scarp, which is thought to run directly under the lake parallel to the San Andreas Fault.

The relatively young sediments of the San Bruno terrain are relatively high in potassium-rich

feldspar deposits. Fragments of other metamorphics, cherts, and quartzes are also found in these

deposits. The remaining 10% of the Franciscan bedrock consists mostly of deposits of green

volcanic basalts, which are less resistant to weathering than the softer sedimentary layers they

are mixed with.

The Merced Formation

Overlying the Franciscan bedrock are two relatively young layers of loosely consolidated

sediments known as the Merced and Colma formations. These oldest layers of these deposits are

thought to date no further back than the first glacial episode of the Pleistocene era, known as the

Nebraskan glacial age (Konigsmark, 1998). The depositional history of these formations is tied

directly to a unique interplay between tectonic movement along the San Andreas Fault system,

and the transgressive / regressive dynamics of shoreline

movement due to repeating interglacial cycles.

About 65 million years ago, near the end of the

Cretaceous period, the Franciscan subduction zone became

inactive, and parts of the assemblage began to uplift. As parts of

the coastal ranges were forced upwards, other areas began to

subside. What is now the region of Lake Merced, along the San

Andreas Fault, subsided below sea level, forming a valley and

sediment catchment basin. Surface streams along the western

edge of the peninsula began to erode the Franciscan rocks and

deposit them in this small coastal valley. The basins eventually

filled with deposits of sandstone, shale, siltstone, and

conglomerate. The locations of these basins fluctuated

dramatically along the fault system. Some of the micro-valleys

became uplifts, of which several re-warped into valleys. A thick

layer of volcanic ash exists in the Merced formation beneath the

Fort Funston viewing platform, possibly as a result of the violent

Figure 3. Cross-sectional stratigraphy of the Merced formation, showing relative age, elevation, and fossil record (Clifton & Hunter, 1988).

5

eruption of Lassen Peak around 400,000 years ago. This ash was deposited at a time when this

part of the Merced formation was uplifted above sea level (Konigsmark, 1998).

Throughout the Pleistocene, the Pacific shoreline off the coast of San Francisco

fluctuated dramatically. As glacial episodes began, sea level would drop dramatically, extending

the shoreline tens of miles westward of its present position. During these times, the San

Francisco Bay become a dry valley, and the Sacramento and San Joaquin Rivers would meander

out the mouth of the Golden Gate, depositing their loads of Sierran sediments in deltas and sand

dunes far to the west of the present shore. As the tidal action of the nearshore regressed from its

present position, marine terrace steps were left high and dry, as much as twenty to thirty miles

inland from the retreated tidal zones. During the most recent Wisconsin glaciation of the

Holocene, as sea level fell, the land was gradually uplifted north of the San Andreas Fault.

About 10,000 years ago, as the glaciers again receded, sea level rose to its present position, and

tidal, wavecutting action began to erode the barrier cliffs of the Merced formation west of Lake

Merced at Fort Funston (Konigsmark, 1998).

It is interesting to note that parts of the Merced formation were deposited directly in the

basin of the San Andreas Fault. As the Pacific plate slid northward, half of the Merced

formation moved northwest of the Golden Gate. This created the Bolinas Headlands in Marin

County, which are identical in age to those found beneath the viewing platform at Fort Funston,

and present identical bands of deposition (Konigsmark, 1998).

The Colma Formation

The Colma formation is the youngest of all the rock sequences in the Lake Merced

region. It sits atop the Merced formation at a slight angular nonconformity, due to a slight uplift

of the Merced formation prior to the deposition of Colma sediments. The Colma and Merced

formations are comprised of almost identical materials, the layers of which would be virtually

indistinguishable if it weren’t for this slight unconformity. Both are comprised of poorly

consolidated sediments that were deposited in the nearshore environment. Interestingly, the

sediments of both the Colma and Merced formations contain a mixture of marine and non-marine

sediments. The sediment sequencing distinctly reflects the coastal cycles of transgression and

regression that occurred throughout the Pleistocene glacial epochs. As glaciers advanced,

streams flowed through these coastal valleys, depositing sediments eroded from the surrounding

Franciscan bedrock. As the glaciers receded, sea levels rose, flooding these valleys with deposits

6

of sandy marine sediments and fossils. The fossil record of both the Merced and Colma

formations show imbedded footprints of mammals beneath younger layers of marine

invertebrates (Geo/Resources, 1993).

Above the Colma formation is evidence of extensive coastal sand dunes that were blown

inland from the shoreline. Most of the sand deposited on the beaches west of Lake Merced was

carried down from the Sierra Nevada by the Sacramento River (Konigsmark, 1998). The sand is

composed of rounded grains of quartz and feldspar, the main components of the Sierra granitic

intrusion. These sand dunes cover most of the western edge of the San Francisco peninsula, and

stretched as far inland as what is now the financial district. The sand dunes were stabilized by

native scrub vegetation, which can still be seen at Fort Funston, and on the mesa, north of Lake

Merced (Holzman, 2000).

The Formation of Lake Merced

The origin of Lake Merced can be traced back no later than 15,000 years ago, or the

beginning of the Wisconsin glaciation. As these North American glaciers advanced for the last

time, sea levels lowered, exposing two small coastal valleys that drained westward to the sea.

Small streams flowed westward from the central hills of the peninsula, incising the basins that

now are occupied by Lake Merced. One stream flowed northwest from the Westlake region of

Daly City and the other flowed west from the Ingleside district through a deep canyon now

occupied by the SFSU campus. These two major streams eroded the poorly consolidated sands

of the Colma formation, joined together near what is now the San Francisco Zoo, and deposited

the sediments west of the present shoreline. Six historical creek beds have been identified whose

watercourses all converge in the Lake Merced basin. According to geologist Neil Fahy, “The

poorly consolidated nature of the sediments offered little resistance to the erosive power of the

streams in creating their valleys.” Thus, we can conclude that the combined effects of the

tectonic uplift of the Merced terrace and fluvial incision were responsible for creating the

structural control of the Lake Merced basin (Fahy, 1974).

The Wisconsin glaciation lasted for about 5000 years. At the end of this epoch, the

glaciers receded, releasing huge volumes of water back into the oceans causing sea levels

surrounding San Francisco to rise to their present height. The advancing seas inundated the

extended coastal valleys occupied by the streams, creating a lagoon. San Francisco Bay was

7

flooded once again, creating the familiar shoreline of the Bay Area we recognize today. A

process of coastal deposition was the final stage in the formation of Lake Merced.

According to Fahy, the rise in sea level caused Point Reyes to become a large peninsular

projection, which deflected the southward-moving California Current away from the Golden

Gate. This created an eddy in the Gulf of the Farallones, which created an inner coastal current

moving northward up the peninsula, past the entrance of the newly formed Lake Merced lagoon.

As tidal action began to erode the sandy sea cliffs of the Merced formation along the San Mateo

Coast, the sediments were transported northward along the peninsula by this current. At the

mouth of the lagoon, the freshwater outflow from the lagoon reduced the competence of the

offshore current, resulting in the deposition of sand at the southern end of the lagoon’s outlet to

the sea. A barrier spit grew northward, evolving into a barrier beach that grew high enough to

prohibit the coastal creeks from flowing to the sea. Apparently, the competence of the outwash

streams was not high enough to keep this sand spit from completely enclosing the lagoon. Many

other lagoons along the California coast were formed in this manner at this time, though not all

were permanently closed from the sea. Barrier beaches which enclose the UCSB lagoon, the

Goleta Slough, Morro Bay, and Humboldt Bay were also formed at this time (Donetz, 1999,

Fahy, 1974). This theory of the formation of Lake Merced is consistent with explanations of

formations of similar systems worldwide (Kjerfve, 1994). Many of the barrier islands that

enclose lagoon systems along the southeast coast of the United States were formed during this

same period of interglacial activity (Barnes, 1980).

Studies performed by the US Geological Survey have determined that Lake Merced is

connected to an underground, freshwater aquifer via a system of springs that well up through

fissures beneath the lake (Yates, et al, 1990). After Lake Merced became isolated from the sea,

the salt water evaporated and was replaced by this underground supply of freshwater captured

above an impermeable layer of clay. Over the years, Lake Merced evolved from a brackish

lagoon into a completely freshwater ecosystem. Historical records from the 1774 Anza

expedition reveal that the lake contained freshwater at the time of their encampment (Fahy,

1974). The lake was named by Father Palou on September 24, 1774, as La Laguna Nuestra

Senora de la Merced, for “The Lake of Our Lady of Mercy (Holzman, 2000).”

8

Human Influence

The Ohlone Indians were the first known people to make use of the resources of Lake

Merced. The tule reeds, which still

grow around the lake, were used

extensively by the native fishermen

for housing materials, sleeping mats,

clothing, and hunting decoys.

Although no permanent settlement

was found, a seasonal encampment

was excavated along the northern

edge of the SFSU campus during its

construction (Holzman, 2000). There

is no record that the native people

made any significant alterations to the

natural watershed of Lake Merced.

During the time of Mexican

occupation in the early 1800s, the lands surrounding Lake Merced were granted to the Gallindo

family by the Jose Castro, the Mexican Governor of California. Rancho Laguna de la Merced

was operated as farmland until 1868, when the newly formed Spring Valley Water Company was

granted the land, to be operated as a public water supply for the growing City of San Francisco.

In the late evening of November 23, 1852, a significant earthquake shook the region. The

following morning, it was observed by parties residing in the area that:

“. . . A great channel between the lake and the sea had been opened, through a broad and high sand bank, during the

night, by which the waters [of Lake Merced] had found a way and been discharged (Fahy, 1974).”

Once again, Lake Merced had re-established its tidal connection to the sea. After this

earthquake, a fissure 300+ yards wide was created in the vicinity of what is now the San

Francisco Zoo, and the water level dropped 20 feet. It is assumed that the perennial streams that

formed Lake Merced continued flowing into it from the east and that some of this water made its

way through the system of lakes to the sea, in its natural attempt to achieve dynamic equilibrium.

During the high tidal swells of the winter season, it is likely that occasionally, seawater would

invade the lagoon via the reopened tidal channel. The longshore depositional processes

Figure 4. Lake Merced at the time of the Ohlone Indians. Seasonal encampments were located near the banks of freshwater streams. Drawing by Nancy Morita, 1992.

SFSU

9

responsible for the original closure

of the inlet would probably have

naturally re-established this closure

over time.

However, in 1895, the

Spring Valley Water Company

began significant structural and

morphological changes at Lake

Merced that would change its

environment forever. First, a dam

was constructed across the mouth of

the tidal channel at the north end of

North Lake, artificially advancing

the natural process of lagoon

closure and preventing any further

incursions of salt water. Skyline

Boulevard crosses this area today.

Secondly, North and South Lakes

were permanently separated from

each other by another dam, which

provides the modern auto entrance

to Harding Park.

From this point forward, the lake would be managed as a clean source of fresh water for

the developing city of San Francisco. After the dam was completed, the lake level was raised

and was maintained at a height of 28 feet above sea level (Fahy, 1974). These dams not only

prevented the waters of the Pacific from entering Lake Merced, they also prohibited the

sediments flowing in from the creeks from being discharged into the sea. During the winter

storm seasons, muddy water would wash down these creeks into Lake Merced, causing increased

turbidity, and lowering water quality. As a result, the Spring Valley Water Company created a

system of diversion channels that carried the seasonal runoff from these streams away from Lake

Merced. The three main creeks were diverted into a culvert south of the lake, and through an

outlet tunnel to the sea beneath Fort Funston. This culvert, the Vista Grande Diversion Canal,

Figure 5. Southwestern San Francisco c. 1890, just prior to the development of Lake Merced as a municipal reservoir. The tidal connection to the sea was re-opened by the earthquake of 1852. It was permanently sealed by the Spring Valley Water Company in 1895 (Oakland Museum, 2000).

10

reduced the size of Lake Merced’s watershed from a historic size of 6,320 acres to a mere 650

acres of land immediately surrounding the lake (Camp, Dresser, & McKee, 1999).

Urbanization of Lake Merced

From1935 to1955, the lands

that lay primarily to the east of Lake

Merced experienced rapid

urbanization. Virtually all of this

growth occurred immediately above

the lands that served as Lake

Merced’s groundwater recharge

zones. Over 90% of the land above

the historic watershed of Lake

Merced was either paved, or densely

filled with housing and commercial

structures. Virtually all of the

historic creeks that once fed Lake

Merced were diverted underground

into storm drains that emptied into the Vista Grande Diversion Canal. A primary factor that

allowed for this period of explosive growth on the eastern shores of the lake can be attributed to

the completion of the Hetch Hetchy aqueduct system in 1938. At that time, the Spring Valley

Water Company sold its rights to the land surrounding Lake Merced back to the city of San

Francisco, to be managed by the Public Utilities Commission as an emergency potable water

supply. Spring Valley had already subdivided and sold much of its land east of the lake to

private developers. Once water was available from Hetch Hetchy and efficient public

transportation routes were in place, the region experienced remarkable rates of growth (Westfall,

1999).

One of the most significant morphological changes to occur at Lake Merced during this

time was the development of the new campus at San Francisco State. Historically, the

easternmost arm of Lake Merced extended 300+ yards to the east of its present location, beneath

what is now the lower parking structure on the northwestern part of the SFSU campus. Above

this point, a perennial stream flowed down through a canyon, which had incised 75 feet deep into

Figure 6. The 100-year old Vista Grande diversion canal carries runoff from the Westlake District of Daly City along the south side of John Muir Drive, to an outlet tunnel to the sea. Historically, this water would empty into Lake Merced. The return of treated runoff to the lake would help to restore a portion of the lake’s historic watershed. Photo by David Freitag.

11

the marine terrace on its way down

to the lake. This canyon was

dramatically filled in, creating three

level, buildable surface areas for the

new campus. The Cox Stadium

exists above what was once the

deepest part of this canyon. In

1948, another 10 acres of the

eastern lake was filled in, in order

to straighten the course of Lake

Merced Boulevard. Today, the

SFSU tennis courts occupy this

reclaimed land about 31 feet above

sea level (Westfall, 1999). All of

the surface streams were diverted into underground culverts, which flow south to the Vista

Grande diversion channel. During this time period, virtually all of the farm and grazing lands

that surrounded Lake Merced on the north, east, and southeast were converted to urban

residential and commercial land uses. By 1955, the lake environment appeared much as it does

to this day.

Figure 7. Area of the SFSU campus in 1935. Lake Merced Blvd. bisects the easternmost arm of the lake. To the east, 19th Avenue beheads the top of the canyon. Housing develops in the Ingleside District on property subdivided and sold by the Spring Valley Water Co. (Westfall, 1999).

Harding Park G

olf Course

East Lake Merced 19

th A

ve

SFSU

lake canyon

stream

Figure 8. 1943. The canyon is filled to make way for the new campus facilities. The creek is culverted. Development begins at Parkmerced above the lake’s primary groundwater recharge zone (Westfall, 1999).

Figure 9. 1955. Reorientation of Lake Merced Blvd. reclaims 10 acres of wetlands and replaces it with ball fields and tennis courts. All runoff east of here is directed away from the lake. Stonestown property is developed (Westfall, 1999).

ING

LESIDE

12

Historic Reservoir Use

From the period of 1875-1935, the Spring Valley Water Company maintained fairly

consistent water levels at Lake Merced. Examination of their historical archives shows that they

were pumping less water from the table annually than was flowing into it via groundwater

recharge. Delivery of water pumped from the Lake Merced area grew from approximately 295

mg/y (million gallons per year) in 1877, to 1,588 mg/y in 1887, to 1,774 mg/y in 1902 (Matuk &

Salcedo, 2000).

In 1940, the San Francisco Recreation & Park Department was given management

jurisdiction over the lands immediately

surrounding the lake. From 1940-1980, the

lake was successfully managed as one of

the most popular and healthy urban trout

fisheries in the nation. Hundreds of people

could be seen lining the banks of Lake

Merced on the weekends for fishing and

recreation purposes (Plummer, 2001).

Analysis of Groundwater Models

Lake levels would vary seasonally,

a natural response to changes in

precipitation. However, in the early 1980s,

the level of the lake began a slow, steady

decline. Following period of heavy

precipitation during the winter of 1982-83, it was observed that the lake level did not respond

with an appropriate net increase. In fact, the level of the lake responded with only a fractional

increase, suggesting a potential instability. As a result, in 1988, the San Francisco Water

Department commissioned its first study by the US Geological Survey of the Westside Basin

Aquifer. The study showed an average net loss of water, most likely the result of significant

pumping from the aquifer in the vicinity of Lake Merced (Yates, et al, 1990).

The 1988 USGS groundwater study estimated the amount of movement of water between

Lake Merced and the surrounding shallow aquifer system. It employed Darcy’s Law, which

states “ground-water flow is proportional to hydraulic conductivity, cross-sectional flow, and

Figure 10. Lake Merced as it appears today. Over the last century, diversions, filling, and urbanization have reduced the size of the watershed from a historic 6350 acres to just 650 acres of land immediately surrounding the lake. Drawing by Nancy Morita, 1992.

13

water-level gradient” (Yates, et al, 1990).

Inflow was measured using flow tubes at

well sites around the lake. Outflow was

measured taking into account factors of

evaporation, pumping records and surface

outflow. No natural surface outflow has

been recorded since 1895, when the North

Lake dam was completed.

Over the last 10 years, at least 3

different groundwater modeling projects have been conducted at Lake Merced. None have been

able to accurately predict the movement of water between the lake and the shallow, underground

aquifer. However, the initial 1988 study performed by the USGS is the most comprehensive.

Examination of the data from all the models reveals a complex network to transport groundwater

between Lake Merced and the shallow aquifer. It appears as if the sections of the lake are

actually the surface exposure of a shallow aquifer, separated by a layer of impermeable clay from

a larger, deeper aquifer (Matuk & Salcedo, 2000).

A later USGS report of 1990 presents that Lake Merced contained about 1.2 billion

gallons of surface water as of 1988 (Yates, et al, 1990). Recent reports from the water

subcommittee of the Lake Merced Task Force reveal that there

are over 140 wells of varying depths drilled into the Westside

aquifer. Of these, less than 1/2 are listed as active, with around

one third equipped with flow meters. Many older wells that

irrigate portions of the Harding Park Golf Course and Olympic

Club are drilled to depths of less than 100’. At that depth, these

tap into the shallow aquifer, which is directly connected to Lake

Merced (LMTF, 2001).

Today, 4 different municipalities draw inexpensive

groundwater from the Westside Basin for their municipal drinking

water. Sixteen cemeteries and 4 golf courses also pump water for

irrigation. According to the reports of the water committee, an

estimated 9 million gallons/day is being extracted from the

Figure 11. Cross-section of the Westside Basin aquifer beneath Lake Merced (Matuk & Salcedo, 2000).

Figure 12. Old SF municipal irrigation wells at Harding Park. Photo by David Freitag.

14

Westside Basin. Of this, about 4 million gallons/day is

used for drinking water by the cities of Daly City, San

Bruno and Colma. The 1990 USGS hydrologic model

shows that in 1988, return flow to the Lake Merced area

of the aquifer via rainfall, runoff and leaking water pipes

averaged about 371 million gallons/year (Yates, et al,

1990). It is also assumed that due to the porous nature of

the overlying sands of the Merced and Colma

formations, a significant portion of the water used for

irrigation makes its way back down into the water table.

However, this data shows that, at present extraction rates,

the aquifer is providing the peninsula with over three

billion gallons of water annually, an equivalent of three

times the 1988 volume of Lake Merced, while only a

fraction is being returned via groundwater recharge

(Yates, et al, 1990).

The 1990 USGS report projects that if

groundwater pumping for irrigation of the 4 golf courses in the immediate area of Lake Merced

was stopped, the water level would rise approximately 4 to 5 feet. Clearly, the combined effect

of these dramatic rates of pumping for irrigation has placed excessive demands on the capacity

of the aquifer to maintain equilibrium (Yates, et al, 1990).

Political Climate

Since 1993, the Friends of Lake Merced, a non-profit organization, has been working

with community groups and local governments to address the growing concerns about Lake

Merced. In 1999, the Lake Merced Task Force was formed as an advisory commission to the

San Francisco Public Utilities Commission, manager of the waters of Lake Merced. The water

subcommittee of the Task Force recently updated the hydrologic model, confirming the

projections of previous models. The Task Force passed a resolution recommending that the San

Francisco and Daly City Public Utilities Commissions issue a moratorium on the development of

any new wells in the immediate vicinity of Lake Merced. Furthermore, the Task Force is

encouraging well operators to begin working with the Daly City Municipal Water District to

Table 1. Water budget for two sampled sections of the Westside Basin aquifer, water year 1988. Values are in acre/feet per year. Study conducted by US Geological Survey (Yates, et al, 1990).

15

implement the use of reclaimed water for all non-potable purposes in the Westside Basin area.

These policies have spawned new rounds of political debate between golf course operators and

the Friends of Lake Merced.

Golf course managers claim over 100 years of legal use of their wells, which is true.

They are against the higher costs of buying tertiary treated water from municipal sources. The

San Francisco and Daly City water districts are unwilling to bear the cost to upgrade municipal

water treatment plants for irrigation projects at the private golf courses. Interestingly, San Mateo

and San Francisco Counties are the only two urbanized counties in all of California that do not

have reclaimed water irrigation programs in place (Plummer, et al, 2001).

In January 2001, California Trout, another non-profit, filed a petition with the State

Water Resources Control Board and Department of Health against the 16 cemeteries and 4 golf

courses that pump groundwater for irrigation. The petition charges that even though they claim

legal right to use the water, it is illegal to jeopardize the public water supply. CalTrout hopes to

avoid a protracted legal battle by settling out of court.

Most parties understand that changes need to be made much sooner than later, or the

lake’s shallow aquifer faces potential exhaustion by as early as 2009 (Plummer, et al, 2001).

Proposals for the Future

The results of a decade of

groundwater modeling projects

and hydrologic assessments all

point to the fact that Lake Merced

is deteriorating in quality and

gradually drying up. It is also

commonly accepted that the

Westside Basin aquifer is being

heavily over drafted in the

vicinity of Lake Merced with too

little recharge from the now

urbanized watershed. Surface runoff that does make it into the lake is heavily laden with toxic

sediments. Petroleum-based compounds from surface streets along with chemicals and fertilizers

applied to the golf courses and cemeteries also run directly into the lake. The lake’s decreased

Figure 13. Chemical spreading equipment and storage shed at Harding Park maintenance yard. Photo by David Freitag.

16

volume concentrates these pollutants, thereby endangering aquatic organisms and jeopardizing

the lake’s status as an emergency supply of public drinking water.

It is essential that the groundwater overdraft be

addressed to maintain long-term stability of the

environment. Alternatives must be explored and

implemented to reduce the pollutant runoff into the lake

such as outfitting storm drains terminating in the lake with

chemical-free filtration systems.

Water from the Vista Grande Diversion Canal

could be treated and then directed back into the lake. This

would return a portion of the original surface runoff to the

lake, thereby re-incorporating a significant portion of the

lake’s historic watershed. Ideally, the SFPUC and the

Lake Merced Task Force could begin working with the

community to explore ways of reintroducing surface water

back into the lake’s groundwater recharge zone. One way to achieve this would be to encourage

private homeowners to limit the amount of impervious concrete surfaces around their homes.

The installation of greenways in the medians of wide residential streets would also facilitate

movement of rainfall back into the ground.

Additionally, the cities of San Francisco and Daly City must work together to implement

the use of recycled water. All fresh water remaining in the ground should be designated for

municipal drinking water only. Antiquated legal claims to this groundwater for the irrigation of

private golf courses and cemeteries is an outdated practice that will endanger the health of the

public water supply. The cities have a responsibility to work with these heavy consumers to

implement conversion to recycled water as soon as possible. Those with private wells can

voluntarily waive their rights and phase in the use of treated wastewater for irrigation, or face the

potential of their municipalities enacting laws restricting those rights. Albeit reluctantly, the

entire community must help to absorb these costs, rather than face the risk of salt-water intrusion

and thus total loss of the aquifer.

Figure 14. Storm drains managed by CalTrans drain untreated runoff directly into the west end of Lake Merced. The drains used to discharge eight feet below the water line. Photo by David Freitag.

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Today, Lake Merced can be seen as the “canary in the coal mine” for the health of the

entire Westside Basin aquifer. If nothing is done to reverse the effects of a century of human-

induced morphologic change, the region will be faced with the loss of one of its most precious

resources.

Figure 15. Looking west form the Sunset Bridge, a lone kayaker and shorebird share the calm waters of North Lake. The future of Lake Merced’s existence remains uncertain. Photo by David Freitag.

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References

Barnes, R.S.K., 1980. Coastal Lagoons: The Natural History of a Neglected Habitat. New York: Cambridge University Press. Camp Dresser & McKee. 1999. Lake Merced Water Sanitary Survey, November 1999

Report. San Francisco Public Utilities Commission. Clifton, H. E., Hunter, R.E., and Gardner, J.V. 1988. "Analysis of Eustatic, Tectonic, and

Sedimentologic Influences on Transgressive and Regressive Cycles in the Late Cenozoic Merced Formation." In:New Perspectives in Basin Analysis. Paola, C., and Kleinspehn, K.L., eds. New York: Springer-Verlag, pp. 109-128.

Donetz, Anthony, 1999. “UCSB Campus Lagoon Website.” Accessed: 4.20.2001. http://www.engineering.ucsb.edu/~donetz/lagoon.html Fahy, Neil E. 1974. "Origin of Lake Merced." California Geology: August 1974. Geo/Resource Consultants, Inc.,1993. Lake Merced Water Resources Planning Study.

San Francisco Water Department in association with Montgomery/Watson, Jones and Stokes Asso., Inc. Public Affairs Management. San Francisco, CA GRC Project No. 1756-00.

Holzman, Barbara, ed., 2000. “The Biogeography of Lake Merced.” Accessed:

2.24.2001. http://bss.sfsu.edu/envstudies/lakemerced/LakeMerced(frames)/ Kjerfve, Bjorn, ed., 1994. Coastal Lagoon Processes. New York: Elsevier Science B.V. Konigsmark, Ted, 1998. Geologic Trips: San Francisco and the Bay Area. Gualala,

California: GeoPress. Lake Merced Task Force, 2000. “Report to the Lake Merced Task Force by the Water

Subcommittee, Sept. 28, 2000.” San Francisco: Lake Merced Task Force. Matuk, Vivian and Salcedo, Nicholas, 2000. “Lake Merced Hydrology and Water

Quality.” The Biogeography of Lake Merced. (website) Accessed: 4.20.01. http://bss.sfsu.edu/envstudies/lakemerced/LakeMerced(frames)/

Oakland Museum of California. Guide to San Francisco Bay Area Creeks: The Oakland Museum of California Creek and Watershed Information Source. Accessed: 5.10.01.

http://www.museumca.org/creeks/SFTopoCreeks.html Pethick, John, 1984. An Introduction to Coastal Geomorphology. Baltimore: Edward Arnold. Plummer, John, Gary Seput, and David Dawdy, 2001. “Lake Merced in Trouble.” San

Francisco Chronicle, March 5, 2001, p. A23.

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Spring Valley Water Company Archives. An online collection of the U.C. Berkeley Bancroft Library. Accessed: 2.2.2001 http://www.lib.berkeley.edu/Eart/digital/springvalley.html

Swift, Donald and Harold Palmer, eds., 1978. Coastal Sedimentation. Stroudsberg, PA:

Dowden, Hutchinson, & Ross, Inc. Westfall, John, 1999. San Francisco State University: An Air-Photo History of the Lake

Merced Campus. San Francisco State Department of Geography. Yates, Eugene B, Scott N. and Lisa Horowitz-McCann, 1990. “Geohydrology, Water

Quality and Water Budgets of Golden Gate Park and the Lake Merced Area in the Western Part of San Francisco, California.” U.S. Geological Society, Water Resources Investigation Report 90-4080. Prepared in Cooperation with the S.F. Water Department. Sacramento, CA.