Elkhorn Slough Tidal Wetland Plan
9/7/05
Evolution of Elkhorn Slough and Associated Wetlands
20,000 years before present (ybp) to 1880 A.D.
Andrea Woolfolk, Elkhorn Slough
National Estuarine Research Reserve
Elkhorn Slough is part of a larger regional estuarine network, whose evolution has been closely linked to
changes in sea level. During the last glacial period, sea level was approximately 120 m lower than it is today,
and a freshwater river flowed through Elkhorn Valley to Monterey Bay. About 18,000 years ago, the earth’s
glaciers began to melt, and sea level rose rapidly. Eventually, ocean water entered the mouths of Elkhorn and
Moro Cojo Sloughs and converted these former streambeds into high-energy tidal inlets, which were then
colonized by oysters and clams. As sea level rise slowed 7,000 or 8,000 years ago, tidal energy decreased,
sediment began to accumulate in local tidal wetlands, and mudflats and salt marshes developed. As sediment
deposition continued, mudflats became narrower and salt marshes expanded.
Over the last 5,000 years, estuarine environments have existed in Elkhorn, Moro Cojo, Bennett,
Tembladero, and McClusky Sloughs as well as the lower stretches of the Salinas and Pajaro Rivers. Like most
estuaries, these wetlands received variable amounts of tidal and freshwater influence over time. Fluctuations
were based on the shifting Salinas river mouth, the location of freshwater springs, seasonal rain inputs, and sand
bars that formed on the shore. Local geologists also hypothesize that the Pajaro River played an important and
changing role in Elkhorn Slough’s environment over time, although published data are not available. Despite
these variations, the lower ends of these wetlands appear to have been dominated for thousands of years by
tidally influenced salt marshes and mudflats along main channels and tidal creeks; while their upper margins
have harbored brackish or freshwater marshes. These habitats, in turn, have supported a rich mix of plants,
invertebrates, fishes, birds, and perhaps, marine mammals.
This table is a summary of known, published information on Elkhorn Slough’s history. Because the
historical record is incomplete, this timeline undoubtedly simplifies the true dynamic nature of local geological
and hydrological processes. Nonetheless, these sources can give us a glimpse into Elkhorn Slough’s evolution
over time.
Evolution of Elkhorn Slough and Associated Wetlands 20,000 years before present (ybp) to 1880 A.D., Elkhorn Slough Tidal
Wetland Plan, 9/7/05
1
Time
20,000 ybp
Event
Last glacial maximum. Ice covers large land areas in northern latitudes, and sea levels
stands 120-130 m lower than today.
Monterey’s shoreline is 5-8 km west of present location, except where the Monterey
submarine canyon approaches much closer to current shoreline and the Elkhorn
channel. Elkhorn is a freshwater river. Moro Cojo also a freshwater system.
Source
Lambeck and Chappell 2001
Lambeck at al 2002
Schwartz 1986
Hornberger 1991
18,000 ybp
Ice sheets begin melting and seawater rises rapidly. This pattern continues for ~
12,000 years, with some variation in sea level rise over time.
Lambeck and Chappell 2001
Lambeck at al 2002
10,000 ybp
Ocean water enters river channels on the California coast, including Elkhorn Slough.
Rate of sea-level rise exceeds rate of sediment accumulation in these tidal inlets.
Oysters colonize mouth of Elkhorn Slough.
Atwater et al 1977
Schwartz 1986
Hornberger 1991
8,000 ybp
Elkhorn Slough is a high-energy tidal inlet.
Tidal water reaches mouth of Moro Cojo.
Schwartz 1986
Hornberger 1991
Hornberger 1991
Jones 2003 (CA-MNT-228, 229, -234, -1570)
Burton 2000
Jones 2003 (CA-MNT-234,
229)
Clams appear in Bennett, Elkhorn Slough, Moro Cojo and Tembladero Sloughs.
Native Americans arrive in Elkhorn Slough region. Archaeological site includes
fossils of northern seal pups, indicating a local rookery, perhaps on the Moss Landing
spit. Humans may have driven this species to local extinction.
7,000 ybp
Northern ice sheets have largely melted, and ocean volumes approach their present day
levels - but a small increase in ocean volume appears continue until recent times.
Rates of sediment accumulation begin to equal or exceed sea level rise – giving rise to
development of tidal salt marshes on California coast.
Invertebrate fossils indicate estuarine environment in Tembladero Slough.
Atwater and Hedel 1976
Lambeck and Chappell 2001
Lambeck et al. 2002
Jones 2003 (CA-MNT-1570)
Evolution of Elkhorn Slough and Associated Wetlands 20,000 years before present (ybp) to 1880 A.D., Elkhorn Slough Tidal
Wetland Plan, 9/7/05
2
In Elkhorn Slough, salt marshes first develop on landward edges of wetland, and
progressively move inward toward center of the channel as deposition continues;
mudflats extensive throughout Elkhorn.
Near Hudson’s Landing, where Schwartz found salt marsh beginning to colonize
landward edge, West found ditchgrass (a pondweed which thrives in salinities between
9 and 20 ppt), willows, cattails, and pickleweed appearing to coexist; suggests a
brackish environment in the upper slough.
Mouth of Elkhorn Slough becoming narrower, based on data from oyster fossils.
Oysters and clams colonize Bennett Slough.
Schwartz 1986
Freshwater marsh develops in northeastern end of Elkhorn Slough (Porter
Marsh/Carneros Creek) region.
But nearby, below Hudson’s Landing, pollen cores show an environment dominated by
pickleweed/salt marsh.
Oysters common at mouth of Moro Cojo.
Hornberger 1991
4,500 ybp
Elkhorn Slough salt marsh approximately half the extent of what it will be by early
1900s; benthic foraminifers found in this marsh layer indicate assemblage resembling
ones found today in Elkhorn Slough. Broad mudflats flank main channel.
Schwartz 1986
4,000 ybp
Freshwater peat horizon in Bennett Slough points to the emergence of a freshwater
spring in the area.
But presence of Assiminea in North Marsh (mid-Elkhorn) indicates a salt marsh
environment in the mid- to lower-Elkhorn Slough.
Mussel Mytilus appears at Moro Cojo’s mouth.
Hornberger 1991
Pickleweed disappears near Hudson’s Landing and is replaced by sedges, and cattails –
indicating a freshwater environment in upper slough.
West 1988
6,000 ybp
5,000 ybp
3,500 ybp
West 1988
Hornberger 1991
West 1988
Hornberger 1991
Hornberger 1991
Hornberger 1991
Evolution of Elkhorn Slough and Associated Wetlands 20,000 years before present (ybp) to 1880 A.D., Elkhorn Slough Tidal
Wetland Plan, 9/7/05
3
3,500 ybp
continued
Mid-Moro Cojo brackish marsh, with ditchgrass and Assiminea (although this genus
also common in more saline conditions throughout California today); and upper Moro
Cojo develops a freshwater marsh.
Hornberger 1991
Talley and Levin 1999
3,000 ybp
Native Americans occupy a site near the mouth of Elkhorn Slough (this site had been
occupied in the past, from 8,000 to 6,500 ybp). Archaeological data reveal this group
regularly hunts sea otters, fur seals, and harbor seals. Less frequently, Stellar sea lions
and California sea lion bones appear in midden. Otters and seals may have been found
in the local sloughs or in Monterey Bay. Sea lions were probably restricted to the bay.
At the same site, commonly gathered shellfish include littleneck clam (Protothaca
staminea), mussel (Mytilus sp.), basket cockle (Clinocardium nuttallii), oyster
(Ostreola conchaphila) and gaper clam (Tresus nuttallii). These are estuarine species
that were probably collected in Elkhorn Slough, and they are still found there today.
Cerithidea californica, is also found, and it persists in Elkhorn until the 1940s.
Fish at the archaeological site include a range of fresh, estuarine, and marine species.
The report’s authors believe these people caught freshwater fish in the Salinas and
Pajaro rivers, surfperch on the outer coast, and sharks, rays, silversides, herrings, and
hake in the local estuaries.
Dietz et al 1988 (CA-MNT229)
2,000 ybp
McClusky Slough dominated by salt marsh, but beginning to diminish in size.
Broad salt marshes exist throughout Elkhorn Slough.
Sedges and cattails disappear at Hudson’s Landing and are replaced with combination
of ditchgrass and pickleweed, indicating a brackish environment in upper slough.
Clay deposited at mouth of Elkhorn Slough – indicating a relatively quiet water estuary
with, perhaps, an indirect opening to the sea.
Schwartz 1986
Schwartz 1986
West 1988
Upper Slough, near Hudson’s, returns to pickleweed dominated marsh
McClusky Slough evolves into a freshwater marsh.
West 1988
Schwartz 1986
~1000 A.D.
Precise
dates
unknown
Schwartz 1986
Evolution of Elkhorn Slough and Associated Wetlands 20,000 years before present (ybp) to 1880 A.D., Elkhorn Slough Tidal
Wetland Plan, 9/7/05
4
1769 A. D.
First European account of north Monterey Co wetlands. Father Crespi and Miguel
Costanso place the mouth of the Salinas River near Moss Landing, and describe the
river as an estuary that is tidally influenced and brackish to almost the Blanco
Crossing, approximately 9 miles southeast of Moss Landing. Near its mouth this
estuary is “very full” and deep
Crespi 1769 (translated in
Brown 2002)
Costanso 1769 (translated in
Teggart 1911).
1774 A. D.
Father Palou describes Native American living near Elkhorn Slough, who collect fish
and mussels from a nearby estuary (maybe Elkhorn Slough)
Palou 1774 (translated in
Bolton 1930)
1830s - 40s
Maps made of Mexican land grants show Elkhorn Slough and Moro Cojo as “esteros’
Diseños
with salt ponds. They share a mouth with the Salinas, and the location of this mouth is
variable.
The Salinas River “runs almost dry” in the summer, but becomes “impassible” during
Hartweg 1846
the rainy season, and it often overflows most of the lower Salinas plains in winter. One Swan 1943
resident on the lower Tembladero has to be rescued by boat from the roof of his adobe
Duflot de Mofras 2004
– he “disliked saltwater . . .[and he feared he] might have to take a cruise on [the
adobe’s roof] in Monterey bay, by way of the Salinas river against his will.”
1853
A geologist with the U.S. government’s Railroad Exploration reports that “the lower
portion of the [Pajaro] valley is swamp land from the influx and infiltration of sea
water, forming lagoons near the bay.” He also predicts that Americans will someday
remove the sand bars from the mouth of the Salinas River and “narrow the area
covered by the lagoons and marsh in the lower 15 miles” of the Salinas Valley
1854
Deputy Surveyor in California conducts survey of public lands in Elkhorn watershed.
Describes North Marsh (mid-Elkhorn Slough) as salt marsh with many tidal creeks.
Describes Elkhorn Slough as a “tide slough” and explains that it is “said to be about 8
feet deep at high tide in channel.”
Antisell 1853
Day 1854
Evolution of Elkhorn Slough and Associated Wetlands 20,000 years before present (ybp) to 1880 A.D., Elkhorn Slough Tidal
Wetland Plan, 9/7/05
5
U.S. Coast and Geodetic survey map show the mouths of Elkhorn Slough and Moro
Cojo as tidal environments, flanked by narrow mudflats, broad salt marsh, and salt
ponds. Bennett and Tembladero Sloughs are also shown as tidal wetlands with
flanking salt marsh. Tide lines appear at the mouth of the Pajaro River and up the
Salinas River to the inland turn. Salt marsh extends into today’s Moss Landing Harbor
District parking lot and south along the Salinas River in areas that are crop fields today.
The accompanying report states that the mouths of the Salinas and Pajaro rivers move
frequently, and that winter rains cause the Salinas River to overflow and deposit “on
the low lands sand and gravel, frequently to the depth of two and a half feet.”
The sloughs near the mouth of the Salinas River are deep enough “to float a medium
sized vessel to the Bay of Monterey,” although sand bars at the river’s mouth are a
major obstacle in 1854. A State contracted geologist suggests building a breakwater at
the Salinas river mouth to improve the area and make these waters navigable,
especially since the State will end up with jurisdiction of these “over-flowed and saline
lands.”
Johnson 1854a and b
Johnson and Rodgers 1854
1870
Lower Moro Cojo Slough is 8 to 9 feet deep, and it is “alive with waterfowl,” including
gulls, pelicans, and geese. Castroville Slough, a tributary of Moro Cojo is also 8 feet
deep, and tidally influenced almost as far as the town of Castroville. Tidal mudflats are
also found near the head of Castroville Slough.
Castroville Argus 1870
1872
Railroad constructed in Elkhorn Slough, restricting tidal flow to many of the eastern
wetlands.
1873
Marsh at Parsons Slough (Elkhorn Slough tidal creek) is labeled “Salt lands”
Map of Turnpike Road 1873
1876
A passenger on the train through Elkhorn Slough remarks on its “worthless salt marsh
lands” and its hundreds of pelicans, and great flocks of ducks, gulls, and other birds.
Philips 1877
1854
continued
Trask 1854
Evolution of Elkhorn Slough and Associated Wetlands 20,000 years before present (ybp) to 1880 A.D., Elkhorn Slough Tidal
Wetland Plan, 9/7/05
6
1875
Reclamation of local tide lands begins. In October 1875, “a levee has been thrown up
Castroville Argus 1875
to a height of five feet [at mid-Moro Cojo]. . .extending from the higher land lying west
of the slough to the railroad bridge, a distance of half a mile. . . We think [it will]
effectually protect from the tide about 120 acres of land that otherwise would be
profitless for any purpose. . . Mr. Castro intends doing further work of this kind where
practicable. . .”
1879
Elkhorn Slough, Bennett Slough, Moro Cojo, and Tembladero are described by deputy
surveyor as tidal environments – Elkhorn receives tidal influence as far inland as
Hudson’s Landing. Herrmann reports that the Salinas River mouth is kept open “due
to the four large sloughs further north…The Turnbladers [Tembladero], the Castroville
[Moro Cojo], the Elkhorn and the Northfork of the Elkhorn [Bennett Slough]. All
these are filled brimful at high tide (which as I have made sure to observe acts in the
Elkhorn… up to the upper Landing near Watsonville and to a similar distance in the
others), and at Ebbtide they all must discharge the immense volume of water taken in,
through the narrow Bar north of Moss Landing. . .”
Herrmann 1879
Reviewers
Historical Ecology Working Group members for the Elkhorn Slough Tidal Wetland Plan who provided reviews of this
document during meetings or in written format include Dave Schwartz, Cabrillo College, Robert Curry, California State
University Monterey Bay, John Oliver, Moss Landing Marine Laboratories, Bryan Largay, Resource Conservation District
Monterey County, and Eric Van Dyke and Kerstin Wasson, Elkhorn Slough National Estuarine Research Reserve.
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Wetland Plan, 9/7/05
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Wetland Plan, 9/7/05
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Evolution of Elkhorn Slough and Associated Wetlands 20,000 years before present (ybp) to 1880 A.D., Elkhorn Slough Tidal
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