Elkhorn Slough Tidal Wetland Plan
2/20/05
150 Years of Human Alterations and Tidal Habitat Change (1870 – Present)
Barb Peichel, Kerstin Wasson, Andrea Woolfolk, Eric Van Dyke
Elkhorn Slough National Estuarine Research Reserve
1870
2000
Figure 1. Changes to tidal habitat composition in Elkhorn Slough from 1870 to 2000 (Van
Dyke and Wasson 2005)
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
1
TABLE OF CONTENTS
Purpose and Summary…………………………………………………………………3
Overview of Human Alterations…….………………………………………………3-4
Specific Human Alterations and Tidal Habitat Changes
Tidal Volume Decreases……………………….………………………...……4-5
Tidal Volume Increases…………………………………………….…………6-7
Freshwater Decreases…………………………………………………………7-8
Sediment Decreases….………………………………………………………....8-9
Sediment Increases….………………………………………………..……….9-10
Other Factors/Reviewers..……………………………………………………..10
Annotated Bibliography………….………………………………………………..11-14
Figures……………………………………………………………………………..…...15-22
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
2
PURPOSE
The purpose of this document is to summarize the major human actions which have altered
the amount and characteristics of tidal volume, freshwater, and sediment exchange in the
Elkhorn Slough watershed from 1870 to the present and to describe the resulting changes to
tidal habitats.
SUMMARY
Human actions have drastically altered hydrogeomorphic processes such as the volume of
tidal exchange, extent of area under tidal influence, speed of tidal currents, amount of
sediment in the main channel, and inputs of freshwater and sediment from the surrounding
Elkhorn Slough watershed. As a consequence, the distribution of tidal habitat types has
changed dramatically over the past century. The increase in tidal influence has resulted in the
deepening and widening of the Elkhorn Slough main channel due to severe rates of bank and
channel erosion resulting in habitat loss and conversion. The decrease of tidal influence in
diked areas has led to the loss and decreased quality of salt marsh habitats. The reduction of
freshwater inputs has resulted in the loss of brackish marsh habitats.
MAJOR HUMAN ALTERATIONS
Beginning in the 1870s, the construction of railroads and roads isolated marshes, and culverts
and levees restricted tidal exchange along the eastern edge of Elkhorn Slough and in areas of
Moro Cojo Slough. Ranchers and farmers ditched and diked the lands to remove tidal flow
over low-lying marshes and create pasture for cattle grazing between the 1880s and 1940s.
The clearing of land in the late 1880s also resulted in substantial sediment inputs. Around
1900, waterfowl hunters built dams and levees to impound freshwater and a 150-ha wetland
near the slough mouth was blocked off to create salt ponds. The Salinas River mouth was
diverted seven kilometers south of the present-day harbor entrance in 1908, decreasing
freshwater and sediment inputs. Tide gates by Hudson’s landing, Bennett Slough, Moro Cojo
Slough, and the old Salinas River channel, which were first installed in the 1930s and 1940s,
continue to restrict tidal flow to these areas. As land use in the watershed converted to row
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
3
crops in the 1940s, there were significant decreases in freshwater surface and groundwater
resources and increases in sediment inputs in the watershed. In 1947, the Slough’s
hydrography changed dramatically when the Army Corps of Engineers dredged a channel
across the sand spit at Moss Landing to form the present-day entrance to the Moss Landing
Harbor. As land use starts converting to conservation in the 1980s, sediment inputs from the
watershed decrease but in some agricultural areas, erosion rates still remain higher than
natural levels. The California State Department of Fish and Game returned 160 ha of diked
pasture to tidal exchange in 1983. This action, supplemented by other levee breaches led to
an increase in tidal volume by approximately 40 percent from 1983 to 1993 (Malzone 1999).
Major Human Alterations to Tidal Habitats in the Elkhorn Slough Watershed
Clearing of land
for agriculture
Tide gates constructed to
regulate flow
Significant diking and draining of
wetlands for agriculture and cattle
grazing
Beginning of
railroad and
road
construction
Dams and levees built for
waterfowl hunting
Salt ponds created
Harbor mouth
constructed
Salinas River
diverted
1880
1900
Land use converted to
row crops
1920
1940
1960
Land use starts
converting to
conservation
Levees breached for
wetland management
activities
1980
2000
TIDAL VOLUME DECREASES - HUMAN ALTERATIONS

Starting in the early 1880s, the previously interconnected estuarine network of Bennett,
Elkhorn, Tembladero, and Moro Cojo Sloughs and the Salinas River were separated by
roads and railroads with culverts or tide gates, dams, and levees restricting tidal flow to
these areas. Figure 2 illustrates this interconnected estuarine network.
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
4

During the period between 1870 and 1956, more than 60 km of levees and embankments
were constructed, reducing the range of unobstructed tidal influence by 59% in Elkhorn
Slough (Van Dyke and Wasson 2005). Figures 4 – 9 show tidal restrictions from levees
from 1913 to 2000. Figure 11 illustrates tidal restrictions by tide gates, culverts, and
levees in Elkhorn Slough in 2004.
TIDAL VOLUME DECREASES - HABITAT CHANGES

There has been a decrease in the extent of tidal wetland areas that receive restricted tidal
flow including Bennett, Tembladero, Moro Cojo, and portions of Elkhorn Slough.

The human activities described above that dramatically decreased the tidal prism, likely
contributed to shoaling of the mouth and persistence of a sandbar documented as early as
1913 at the natural opening of Elkhorn Slough until 1947 (Van Dyke pers com.). Field
notes examining the city lands of Monterey in 1879 by A.T. Hermann, Deputy Surveyor,
attribute the lack of a persistent sandbar to the immense volume of the networked system
of Sloughs.

During the period between 1870 and 1956, the extent of intact salt marsh habitat (defined
as vegetation cover > 75%) decreased by 66% in Elkhorn Slough (Van Dyke and Wasson
2005) as shown in Figures 4-6.

Salt marsh habitats that were diked to impound freshwater marsh or drained for
agricultural land subsided through a process of organic matter decomposition and
compaction. In areas of Elkhorn Slough that have been returned to tidal flow, the
substrate was too low to support the marsh plants that once existed there, and resulted in a
conversion to mudflat or open water habitat (Van Dyke and Wasson 2005). Parson’s
Slough shown in the bottom right of Figures 7 and 8 (Parson’s Slough) is one example of
this trend.
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
5
TIDAL VOLUME INCREASES - HUMAN ALTERATIONS

In 1947, Army Corps of Engineers dredged a channel across the sand spit at Moss
Landing to form the present-day entrance to the Moss Landing Harbor. The opening of
the harbor entrance and maintained dredging results in a dramatically larger tidal prism
volume and higher tidal velocities than formerly occurred with a smaller mouth resulting
in rapid erosion in undiked regions of Elkhorn Slough (Philip Williams and Associates
1992).

Unintentional and intentional levee breaches in the 1980s led to an increase in tidal
volume by approximately 40 percent from 1983 to 1993 in Elkhorn Slough (Malzone
1999). As part of an intentional levee breech, the California State Department of Fish and
Game returned 160 ha of diked pasture to tidal exchange in 1983. Although the levee
breaches have contributed to tidal erosion in Elkhorn Slough, the extensive erosion caused
by the 1947 Harbor opening would still be occurring without the levee failures (Philip
Williams and Associates 1992). There is ongoing levee failure in Elkhorn Slough.
TIDAL VOLUME INCREASES - HABITAT CHANGES TO TIDAL CHANNELS AND
CREEKS

The main channel of Elkhorn Slough has been getting deeper and wider since 1947. The
mean cross-sectional area of Elkhorn Slough has increased by 24 percent in just eight
years (1993-2001) with areas near the mouth of Parson’s Slough increasing almost 3
meters in depth in that short time (Dean 2003, Malzone 1999). Figure 12 shows the
percent change in the cross sectional area of the main channel areas of Elkhorn Slough
from 1993 to 2001 (Dean 2003, Malzone 1999).

Bank erosion rates along the main channel of Elkhorn Slough are between 0.4 and 0.6
m/yr in the upper slough and average 0.3 m/yr in lower slough, with some areas that
approach rates of 2.0 m/yr as shown in Figure 13 (Wasson et al. unpublished).
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
6

There have been significant increases in the depth and width of tidal creeks in Elkhorn
Slough (Kvitek unpublished, Van Dyke and Wasson 2005). The mean width of 196 tidal
creeks in undiked areas of Elkhorn Slough increased from 2.5 m in 1931 to 12.4 m in
2003 (Van Dyke and Wasson 2005) as illustrated by Figure 14.

There has been a loss of fine sediments from various intertidal and subtidal mudflats
causing a shift from gaper clams to boring clams in the Elkhorn main channel between
1970s and 1990s (Oliver et al. unpublished).

There has been a shift in the composition of the lower intertidal invertebrate communities
attributed to the change in sediment size in the main channel of Elkhorn Slough from the
1970s to 1990s (Fenn 2003).
TIDAL VOLUME INCREASES - HABITAT CHANGES TO MARSH HABITATS

There has been an overall decrease in vegetated salt marsh along the Elkhorn Slough main
channel which has been attributed mainly to tidal volume increases, but may also be due
to other factors such as subsidence (Lowe 1999, Van Dyke and Wasson 2005).

The percent cover of pickleweed marsh in Elkhorn Slough decreased significantly and the
percent cover of bare mud increased significantly from 1931 to 1997, with the most
significant decline between 1949 and 1956 (Lowe 1999). The mean percent cover of salt
marsh vegetation in undiked marshlands in Elkhorn Slough decreased from approximately
90 percent in 1931 to 46 percent in 2003 as shown in Figure 15 (Van Dyke and Wasson
2005).
FRESHWATER DECREASES - HUMAN ALTERATIONS

As land use in the watershed converted to row crops in the 1940s, there has been a
significant decrease in freshwater surface and groundwater resources in the watershed.
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
7

The diversion of the Salinas River in 1908 decreased seasonal inputs of freshwater to
Elkhorn Slough. Presently, water comprised mostly of agricultural runoff from
Tembladero Slough and the Salinas River enters the old Salinas river channel through tide
gates during low discharge times. This water flows out into the Moss Landing Harbor at
the mouth of Elkhorn Slough seasonally with a flow between 0 and 10 m3/s (Caffrey et al.
2002).

The contribution of seasonal freshwater from Carneros Creek has likely decreased in the
past 100 years due to land uses. Current discharge rates from Caneros Creek flows into
Elkhorn Slough are between 0.2 and 38 m3/s from December to April (Caffrey et al.
2002).
FRESHWATER DECREASES - HABITAT CHANGES

There has been a significant decrease in extent of brackish marshes that would have been
common along the margins of entire estuarine network.

Levees and dams constructed to either impound freshwater or decrease saltwater intrusion
have decreased the extent of important transitional habitat areas in the Elkhorn Slough
watershed that would have naturally graded from higher salinity areas to a more of a
brackish salinity (Van Dyke and Wasson 2005).

Groundwater decreases may have contributed to the subsidence of marsh elevations.
SEDIMENT DECREASES - HUMAN ALTERATIONS

Tide gates at the old Salinas River channel decreased the amount of sediments entering
the lower Elkhorn Slough from this source.

The amount of sediment entering Elkhorn Slough from long-shore currents has likely also
decreased because the current location of the Elkhorn Slough mouth is in line with the
head of the Monterey Canyon which acts as a sediment sink.
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
8

The amount of sediment entering Elkhorn Slough from the watershed has decreased in the
past ten years due to improved agricultural practices and land acquisition activities, but
levels still remain much higher than natural erosion rates.
SEDIMENT DECREASES - HABITAT CHANGES

If there has been an overall decrease of sediment inputs to Elkhorn Slough, then there may
be a slower rate of accretion which would decrease the ability to build up the elevation
and extent of tidal marsh in some areas of Elkhorn Slough. The higher present-day tidal
velocities in Elkhorn Slough may also preclude much sediment deposition.
SEDIMENT INCREASES- HUMAN ALTERATIONS

Clearing of land for agriculture (approximately 1850-1900) resulted in a substantial loss
of topsoil and an increase in sediment inputs to adjacent wetlands. This trend continued
as land use shifted to row crops in the 1940s.

The Natural Resource Conservation Service estimates that natural erosion rates occur at
an approximate rate of 1-5 tons per acre per year depending on soil type in Elkhorn
Slough (Soil Conservation Service, 1984). They also found that the rates of erosion from
strawberry farming using minimal to no erosion control practices combined with natural
erosion are estimated to be as high as 33 tons per acre per year. These levels may be less
today with increasing erosion control practices in the Elkhorn Slough watershed.
SEDIMENT INCREASES- HABITAT CHANGES

Prior to 1946, Elkhorn Slough had been a depositional system with muted tides. However
the dramatic increase in sediment load in the 1800s rapidly decreased the depth of the
main channel. As late as 1880, the main channel was regularly navigated by steamships
between the mouth as far up as Hudson Landing, and maps showed that the channel
extended miles into what is now Carneros creek, navigable by smaller vessels. Then
within decades the channel disappeared above Hudson Landing, and the main channel
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
9
below this area filled in so that only shallow draft vessels could navigate, and shoaling
near the mouth was indicated on maps and photographs. During this period the natural
mouth was sometimes closed by a sandbar. The shoaling and sandbar closing was also
likely due in part to the decreased tidal volume resulting from tide gates across Moro
Cojo, Bennett, and Tembladero Sloughs, which decreased the water exchanged on each
tide (Van Dyke pers com.).

The acreage and number of sediment fans have increased greatly on the upper margins of
salt marshes since the early 1900s, resulting in the conversion of some high marsh to
willow woodlands. Such habitat conversion due to elevation increases from agricultural
sedimentation has continued over the past decades at Elkhorn Slough (Byrd et al. 2004).
OTHER FACTORS
A number of other factors such as the subsidence of the marsh plain, sea level rise, invasive
species, pollution, and disease may also be contributing to tidal habitat changes in Elkhorn
Slough. Subsidence of the marsh plain from groundwater overdraft, drying of marsh
sediments during agricultural reclamation activities, and tectonic events such as the 1989
earthquake can all result in an ecological drowning effect of marsh vegetation (Orr et al.
2003). According to the Intergovernmental Panel on Climate Change, the average rate of
global sea level is rising and estimated to be approximately 3 mm/yr over the next 50 years,
although the local contribution to habitat changes is unknown.
REVIEWERS
Strategic Planning Team and Science Panel members of the Elkhorn Slough Tidal Wetland
Plan who provided reviews of this document include John Callaway, University of San
Francisco, Mike Foster, Moss Landing Marine Laboratories, Edwin Dean, Consultant, Greg
Shellenbarger, U.S. Geological Survey, and Becky Christensen, Elkhorn Slough National
Estuarine Research Reserve.
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
10
ANNOTATED BIBLIOGRAPHY
Byrd, K., Kelly, M., and E. Van Dyke. 2004. Decadal Changes in a Pacific Estuary: A MultiSource Remote Sensing Approach for Historical Ecology. GIScience and Remote Sensing 41:
285-308.
Analysis of historic aerial photographs documenting decadal-scale changes to salt marshes in
Elkhorn Slough, California caused by off-farm-sedimentation from 1971-2001. Change
detection identified a process of plant succession that led to arroyo willow encroachment into
pickleweed marsh and the continued growth of sediment fans.
Dean, E. 2003. Tidal Scour in Elkhorn Slough, California: A Bathymetric Analysis. B.S.
Capstone Project, California State University, Monterey Bay, California.
http://seafloor.csumb.edu/publications/capstones/ewdean3_Capstone_2003_Archive.pdf.
Investigation of main channel bathymetry in 2001 with multi-beam technology, and
comparison of results to those of Malzone and Kvitek from 1993. Documents deepening and
widening of lower main channel during these seven years.
Caffrey J., M. Brown, W. B. Tyler, and M. Silberstein, (eds.), Changes in a California
estuary: A profile of Elkhorn Slough. Elkhorn Slough Foundation, Moss Landing, California.
Synthesis of existing knowledge (up to about 2000) about Slough ecosystems, history, and
management.
Fenn, K.H. 2003. Spatial and temporal variability of benthic infaunal communities in the
soft-substrates of the Elkhorn Slough, Moss Landing, California. M.S. Thesis, Miami
University, Ohio.
Investigation of benthic invertebrate assemblage of lower intertidal (sampled with coffee can
cores) at four sites along main channel in 2001, with comparison to data from 1970s. She
found significant shift in assemblages and increase in sediment size over these 25 years, and
suggests tidal erosion is most likely cause.
Hermann, A.T. 1879. Field Notes of The Reservation and Examination made in the City
Lands of Monterey. Submitted to the U.S. Surveyor General’s Office on July 18, 1879.
A. T. Hermann, Deputy Surveyor, commenced the survey on September 8, 1879 and
completed it on September 23, 1879.
Kvitek, R (California State University Monterey Bay). Unpublished data.
Kvitek has investigated Slough bathymetry with student teams since 1993; results from early
studies are found in Malzone 1993 and Dean 2003; results from later studies have not yet
been written up and were orally reported on at 13 October 2004 Tidal Wetland Planning
meeting. All these studies suggest that the lower main channel of Elkhorn Slough continues to
deepen and widen as a result of tidal erosion.
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
11
Lowe, P. 1999. Marsh Loss in Elkhorn Slough, CA: Patterns, Mechanisms, and Impact on
Shorebirds. M.S. Thesis, Moss Landing Marine Laboratories, San Jose State University,
California.
Analysis of aerial photographs from 1931 to 1997 showed significant decreases in pickleweed
cover after the opening of the Moss Landing Harbor (1947) and the Loma Prieta Earthquake
(1989). Bird surveys suggest that well-vegetated marsh may provide important habitat at
extreme high tides. Transplant experiments suggested that increased elevation decreased
pickleweed mortality.
Malzone, C.M. and R. Kvitek. 1994. Tidal scour, erosion, and habitat loss in Elkhorn Slough,
California. A report of the Elkhorn Slough Foundation pursuant to National Oceanic and
Atmospheric Administration Award #NA370M0523, 15 pp.
Detailed bathymetry of Elkhorn Slough documents channel deepening and widening and
estimating large sediment loss rates. The report concludes that the tidal scour process has
not reached a steady state and is still increasing. A series of recommendations to slow tidal
currents was also proposed.
Malzone, C.M. 1999. Tidal Scour and its Relation to Erosion and Sediment Transport in
Elkhorn Slough. M. S. Thesis, Moss Landing Marine Laboratories, San Jose State University,
California.
Field measurements of bathymetry, bank erosion, tidal and hydrologic characteristics, and
sediment transport were taken from 1993 to 1996. The results indicate that the increased
water volume caused by the larger mouth created in 1947 led to high current velocities and
high rates of sediment loss. It is proposed that if the conditions continue, Elkhorn Slough will
evolve into a shallow-water tidal embayment consisting mostly of subtidal channel and lowerintertidal mudflat in 40-120 years.
Oliver, J. (Moss Landing Marine Laboratories), Kim, S., Zimmerman, R., Kvitek, R.,
Welschmeyer, N., Foster, M., Cailliet, G., and J. Harvey. Unpublished Data. Characterization
of the Benthic and Planktonic Communities of Elkhorn Slough.
These researchers are compiling historical data on benthic and planktonic communities of
Elkhorn Slough and carrying out new sampling particularly of benthic invertebrates, funded
in part by the Monterey Sanctuary’s Integrated Monitoring Network. A report is forthcoming,
but initial results have been presented at various meetings, and include the observation that
subtidal communities along the main channel that were formerly dominated by soft-sediment
species such as gaper clams now harbor species adapted for hard substrates, such as boring
clams.
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
12
Orr, M. Crooks, S., and P. Williams. 2003. Will Restored Tidal Marshes Be Sustainable? In:
Larry R. Brown, editor. Issues in San Francisco Estuary Tidal Wetlands Restoration. San
Francisco Estuary and Watershed Science. Vol. 1, Issue 1 (October 2003), Article 5.
Assessment of whether or not restored marshes in the San Francisco Estuary are expected to
be sustainable in light of future landscape scale geomorphic processes give typical restored
marsh conditions. Vertical accretion modeling suggests that salt marshes in San Pablo Bay
will be sustainable for moderate relative sea level rise (3 to 5 mm/yr) and average sediment
supply (c. 100 mg/L).
Philip Williams and Associates, 1992. Elkhorn Slough Tidal Hydraulics Erosion Study,
Technical report prepared for the U.S. Army Corps of Engineers, San Francisco District, 34 p.
The Elkhorn Slough Tidal Hydraulics Erosion Study (1992) was prepared for the U.S. Army
Corps of Engineers by Philip Williams and Associates, Ltd. and Moffatt & Nichol Engineers.
The study determined that the Corps' Moss Landing Harbor Navigation Project has caused
and is causing the rapid erosion of the channel and banks in the lower channel of Elkhorn
Slough. The study also examined other possible causes that may have led to erosion and
evaluated and recommended solutions to the erosion problems in Elkhorn Slough.
Soil Conservation Service, 1984. Strawberry Hills Target Area Watershed Area Technical
Study Report, Monterey County, California. Prepared by the USDA Soil Conservation
Service River Basin Planning Staff and Target Area Team. Davis, California.
This document describes a plan to reduce erosion and sediment damage and non-point source
pollution in Elkhorn and Moro Cojo Sloughs and their watersheds.
Van Dyke, E. and K. Wasson. 2005. Historical Ecology of a Central California Estuary: 150
Years of Habitat Change. Estuaries 28:173-189.
Documentation and quantification of habitat changes in Elkhorn Slough wetlands between
1850-2000. Overall distribution and extent of habitat types was examined over the whole
period, revealing in particular loss of salt marsh and gain of mudflat, attributed both to
diking and to opening of the harbor mouth. Hundreds of fixed plots were also examined in
photos between 1931-2000 in undiked areas, revealing dramatic increases in tidal creek
width and decrease in salt marsh cover, likely resulting from opening of harbor mouth.
Wasson, K (Elkhorn Slough National Estuarine Research Reserve)., De Vogelaere, A., Van
Dyke, E., Lonhart, S., and Fork, S. Unpublished Data.
Investigation of bank erosion in Elkhorn Slough at 30 permanent sites (mostly reoccupied
from Malzone 1996) between 2001-2004 has revealed about .5m/year loss; data available
from research@elkhornslough.org on request.
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
13
Wasson, K. (Elkhorn Slough National Estuarine Research Reserve) and A. Woolfolk.
Unpublished Data.
Monitoring of about 80 permanent sites at the marsh-upland transition around Elkhorn
Slough has revealed that ecotone boundary has moved down between 2001-3 at diked Porter
Marsh as upland weeds invade former marsh; Poison Hemlock in particular is moving down
into Porter Marsh and also the diked Estrada Marsh; data available from
research@elkhornslough.org on request.
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
14
FIGURES
Figure 2. 1879 map showing the interconnected estuarine network of Bennett, Elkhorn,
Tembladero, and Moro Cojo Sloughs and the Salinas River from an official examination of
the City Lands of Monterey by A. T. Hermann, Deputy Surveyor.
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
15
1870
Figure 3. Tidal flow and habitat mapping of Elkhorn Slough from historic maps in 1870 (Van
Dyke and Wasson 2005).
1913
Figure 4. Tidal flow and habitat mapping of Elkhorn Slough from aerial photographs in 1913
(Van Dyke and Wasson 2005).
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
16
1931
Figure 5. Tidal flow and habitat mapping of Elkhorn Slough from aerial photographs in 1913
(Van Dyke and Wasson 2005).
1956
Figure 6. Tidal flow and habitat mapping of Elkhorn Slough from aerial photographs in 1956
(Van Dyke and Wasson 2005).
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
17
1980
Figure7. Tidal flow and habitat mapping of Elkhorn Slough from aerial photographs in 1980
(Van Dyke and Wasson 2005).
2000
Figure 8. Tidal flow and habitat mapping of Elkhorn Slough from aerial photographs in 2000
(Van Dyke and Wasson 2005).
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
18
Figure 9. Changes to the extent of acreage (ha) receiving tidal flow in Elkhorn Slough from
1870 to 2000 (Van Dyke and Wasson 2005).
Figure 10. Changes to the extent of acreage (ha) of tidal habitats in Elkhorn Slough from 1870
to 2000 (Van Dyke and Wasson 2005).
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
19
Restrictions to tidal flow Elkhorn Slough in 2004
•about 30 cm/year at lower sites
in the last years
•about 50 cm/year at the upper
sites
Figure 11. The white dots illustrate tidal restrictions by tide gates or culverts and white and
gray lines indicate levees and berms in Elkhorn Slough in 2004. Warmer colors (red) show
lower elevations based on uncorrected LIDAR data.
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
20
Percent Change in Cross Section Area of Channel Areas in Elkhorn
Slough from 1911 to 2001
0.00
•about 30 cm/year at lower sites in the last
Elkhorn Slough Thalweg
1911-2003
years
•about 50 cm/year at the upper sites
0.00
-1.00
1000.00
2000.00
3000.00
4000.00
5000.00
6000.00
7000.00
8000.00
9000.00
10000.00
Subtidal Depth (meters from MLLW)
-2.00
-3.00
-4.00
ANOVA
Table for change in d to edge
-5.00
Row exclusion: 4-04 stats.svd
-6.00
region
DF
Sum of Squares
Mean Square
F-Value
P-Value
Lambda
Pow er
2
13713.925
6856.963
2.374
.1103
4.749
.431
30
86637.711
(Depths Not Shown)
2887.924
Parson's Mouth
-7.00
Subject(Group)
Category for change in d to edge
1
-8.00
Foreslough
Seal
Bend * regionSB to Parson's
Category for
change in d to
edge
2
00-09
10-19
Category for change in d to edge * Subje...
-9.00
20-29
30
130.153
130.153
.080
.7797
.080
.058
1724.386
Midslough
862.193
30-49
.527
.5955
Backslough
1.055
50-66
.126
49039.228
1634.641
Distance Along Baseline (meters)
1911 Data
1940 Data
1993 Depths
2001 Depths
2003 Data
Means Table for change in d to edge
Effect: Category for change in d to edge * region
Row exclusion: 4-04 stats.svd
Count
Mean Std. Dev. Std. Err.
Figure 12. Percent change in the cross sectional area of the main channel areas of Elkhorn
Slough from 1993 to 2001 (Dean 2003, Malzone 1999).
low er main, 2002
14 -29.445
24.199
low er main, 2003
14 -29.472
27.211
7.273
20.453
10.226
south marsh, 2002
south marsh, 2003
upper main, 2002
6.467
4
-9.945
4
-1.840
6.373
3.186
15 -40.195
45.400
11.722
Bank Erosion 15(cm)
in Elkhorn
Slough from 2002-2004
-58.324
79.375
20.494
upper main, 2003
0
-20
-40
-60
•about 30 cm/year at lower sites in
the last years
•about 50 cm/year at the upper
sites
-80
low er main
south marsh
upper main
-100
-120
-140
-160
2002-3
2002
2003-4
2003
Figure 13. Bank erosion in the main channel of Elkhorn Slough measured by the change in the
distance to the edge in centimeters from 2002 to 2004 (Wasson et al. unpublished).
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
21
Change in Tidal Creek Width (m) in Elkhorn Slough
from 1930 to 2003
•about 30 cm/year at lower sites in
the last years
•about 50 cm/year at the upper
sites
Figure 14. Increase in tidal creek width in Elkhorn Slough from 1930 to 2003 at 196 creeks
followed over time in undiked areas (Van Dyke and Wasson 2005).
Change in the Percent Cover of Marsh Vegetation in
Elkhorn Slough from 1930 to 2003
•about 30 cm/year at lower sites in
the last years
•about 50 cm/year at the upper
sites
Figure 15. Decrease in the percent cover of marsh vegetation in Elkhorn Slough from 1930 to
2003 in 196 quadrats (100 m x 100 m) followed over time in undiked areas (Van Dyke and
Wasson 2005).
150 Years of Human Alterations and Habitat Change (1870 – Present), Elkhorn Slough Tidal
Wetland Plan, 2/20/05
22