Evolution of Elkhorn Slough and Associated Wetlands 20,000 years before present (ypb) to 1880 A.D.

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 R

Evolution of Elkhorn Slough and Associated Wetlands 20,000 years before present (ypb) to 1880 A.D.

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

Time Event Source

20,000 ybp Last glacial maximum Ice covers large land areas in northern latitudes, and sea levels

stands 120-130 m lower than today

Lambeck and Chappell 2001 Lambeck at al 2002

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

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

Clams appear in Bennett, Elkhorn Slough, Moro Cojo and Tembladero Sloughs

Hornberger 1991 Jones 2003 (CA-MNT-228, -229, -234, -1570)

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

Burton 2000 Jones 2003 (CA-MNT-234, 229)

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

Atwater and Hedel 1976 Lambeck and Chappell 2001 Lambeck et al 2002

Invertebrate fossils indicate estuarine environment in Tembladero Slough Jones 2003 (CA-MNT-1570)

6,000 ybp 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

Schwartz 1986

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

West 1988

Mouth of Elkhorn Slough becoming narrower, based on data from oyster fossils

Oysters and clams colonize Bennett Slough

Hornberger 1991

5,000 ybp Freshwater marsh develops in northeastern end of Elkhorn Slough (Porter

But nearby, below Hudson’s Landing, pollen cores show an environment dominated by

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

Hornberger 1991

But presence of Assiminea in North Marsh (mid-Elkhorn) indicates a salt marsh

environment in the mid- to lower-Elkhorn Slough

Hornberger 1991

3,500 ybp Pickleweed disappears near Hudson’s Landing and is replaced by sedges, and cattails –

indicating a freshwater environment in upper slough

West 1988

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

Dietz et al 1988 (CA-MNT-229)

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

2,000 ybp McClusky Slough dominated by salt marsh, but beginning to diminish in size Schwartz 1986

Broad salt marshes exist throughout Elkhorn Slough Schwartz 1986 Sedges and cattails disappear at Hudson’s Landing and are replaced with combination

of ditchgrass and pickleweed, indicating a brackish environment in upper slough West 1988 Clay deposited at mouth of Elkhorn Slough – indicating a relatively quiet water estuary

with, perhaps, an indirect opening to the sea Schwartz 1986

~1000 A.D.

Precise

Upper Slough, near Hudson’s, returns to pickleweed dominated marsh West 1988 McClusky Slough evolves into a freshwater marsh Schwartz 1986

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’

with salt ponds They share a mouth with the Salinas, and the location of this mouth is variable

Diseños

The Salinas River “runs almost dry” in the summer, but becomes “impassible” during the rainy season, and it often overflows most of the lower Salinas plains in winter One resident on the lower Tembladero has to be rescued by boat from the roof of his adobe – 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.”

Hartweg 1846 Swan 1943 Duflot de Mofras 2004

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

Antisell 1853

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.”

Day 1854

1854

continued 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.”

Johnson 1854a and b Johnson and Rodgers 1854

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.”

Trask 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

1875 Reclamation of local tide lands begins In October 1875, “a levee has been thrown up

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 .”

Castroville Argus 1875

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

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