Draft 9/6/05
Elkhorn Slough Tidal Wetland Plan
Summary of Planning Process to Date
BACKGROUND DOCUMENTS
We have characterized what is known about the historical changes, tidal habitats,
physical processes, and causes of tidal erosion in Elkhorn Slough through as series of
documents.

A Review of the Geology, Geomorphology, Hydrodynamics and Inlet Stability of
Elkhorn Slough, Draft 3-31-05

Elkhorn Slough Tidal Habitat Descriptions, Draft 4-4-05

Evolution of Elkhorn Slough and Associated Wetlands 20,000 years before
present (ypb) to 1880 A.D., 9-6-05

150 Years of Human Alterations and Tidal Habitat Change (1870-Present) in
Elkhorn Slough, Draft 1-3-05

Key Physical Processes Causing Tidal Erosion in Elkhorn Slough, 2-10-05

Groundwater Information for Elkhorn Slough, Draft 1-7-05
TIDAL HABITAT CHANGES
We have a shared understanding of the key physical processes contributing to subtidal
erosion and intertidal marsh loss/conversion.
 The modification of the Elkhorn Slough mouth for the creation of a harbor in 1947,
permanently fixing a deeper opening to Monterey Bay, is the main cause of subtidal
erosion and more recent marsh loss/conversion. Contributing factors include the
decreases in sediment supply (such as the diversion of the Salinas River), dike/levee
failure and removal, the presence of the Monterey Canyon, sea level rise, subsidence
of marsh areas, wave action, and other biogeochemical processes.
–
We acknowledge that the causes of marsh loss/conversion are complex, but can
be mainly attributed to flooding due to subsidence (largely from diking) and
increased tidal range, erosion due to increased tidal velocities, and lack of
sediment supply.
SELECTED TIDAL HABITAT TRENDS
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 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
Prepared by Barb Peichel, Elkhorn Slough National Estuarine Research Reserve
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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).

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 (Van
Dyke and Wasson 2005).

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 (Wasson et al. unpublished).

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).

It has been estimated that the tidal prism of Elkhorn Slough has increased from
approximately 4,700,000 m3 in the 1970s to 6,400,000 m3 in 2005 (Smith 1973,
Broenkow and Breaker unpublished, Josh Sampey personal communication).
PREDICTIONS OF FUTURE TIDAL HABITAT TRENDS
We have agreement that the Elkhorn Slough system is not at equilibrium. The
predictions for tidal habitats for the next 50 years if no actions are taken include the
continued increase in the cross-sectional area of the main channel and tidal creeks, bank
erosion, salt marsh conversion to mudflat and tidal creeks, and erosion of sediments in
soft-bottom areas. These predictions have been summarized in a consensus statement,
50-Year Tidal Habitat Predictions for Elkhorn Slough, 5/16/05.
 There is currently a positive feedback loop between the increasing tidal prism,
increasing velocities, and increasing depth and width of the channel and tidal creeks.
 There is agreement that the 50-year trends are not acceptable and that new
management actions are necessary.
VISION, GOALS, AND PRINCIPLES
We have consensus on the vision, goals, objectives, and principles for the Tidal Wetland
Plan. The consensus statements are summarized in the document Vision, Goals,
Objectives, and Strategic Planning Principles for the Elkhorn Slough Tidal Wetland
Plan, 7/29/05. We are currently in the process of developing potential management
alternatives towards achieving the goals and vision using the principles as guidance.
 Vision: “We envision a mosaic of estuarine communities of historic precedence that
are sustained by natural tidal, fluvial, sedimentary, and biological processes in the
Elkhorn Slough Watershed as a legacy for future generations.” – Strategic Planning
Team, Elkhorn Slough Tidal Wetland Plan
Prepared by Barb Peichel, Elkhorn Slough National Estuarine Research Reserve
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POSSIBLE NEXT STEPS
We are still exploring the feasibility of large, medium, and small-scale alternatives.
Please note that I have not listed all of the potential management options, but have only
provided examples. Potential management alternatives aim to reduce the tidal prism
through a combination of activities such as restricting tidal flow and adding sediment to
raise the marsh elevation.
 Unless we implement a large-scale alternative, such as a fix at the mouth (i.e. sill
under Hwy 1 or restore channel mouth/dam), we will not be able to stop or reverse
the trend of tidal erosion and marsh loss/conversion in the next 100 years.
–
We may not eventually choose this action, but we need to keep pursuing this
option to fully understand the feasibility and tradeoffs (habitats and species, water
quality, human uses, etc.) of these alternatives.
–
As a first step, we need additional funding to support quantitative
designs/modeling (estimates of changes to the tidal prism, tidal currents,
suspended sediments, etc.) for possible outcomes of each option compared with a
no action alternative.
 We may be able to slow the rate of tidal erosion and marsh loss/conversion with
other, medium-scale alternatives.
–
In order to have a system-wide effect, projects would need to be at least the size
of the Parsons Slough/South Marsh complex. We would need to obtain funding
for designs/permitting/modeling for these options.
 We will not be able to fully restore subsided marsh habitats unless we build up
areas with sediment (areas east of the railroad tracks are severely subsided by 1-2
meters).
–
We may want to artificially augment these areas with sediment. Because of the
energy in the system, we may have to add sediment behind water control
structures (water control structures alone would not fix the intertidal areas).
–
In the meantime, we need to determine out what the accretion rates of sediments
on the marsh plain and mudflats, as well as estimates of sub-watershed sediment
inputs. We also need to better understand the correlations between tide range,
inundation frequency, and vegetation cover.
 Although there would likely only be local effects, we may want to pursue smallerscale alternatives that could be implemented in a shorter time-frame, would meet the
goals, and make sense whether or not there is a fix at the mouth.
 For any possible project, we need to set up a very thoughtful integrated monitoring
system that will acquire the needed baseline information to help us adaptively manage
the system (tide stations, sediment elevation tables, tidal currents, water quality,
biological indicators, wetland functions, etc.). Again, we need to create quantifiable
objectives that can be measured.
 Ideally, once the elevation of subsided marsh/creek areas is corrected we could
reconnect them to the main channel, but without a fix at the mouth that might not be
an option.
Prepared by Barb Peichel, Elkhorn Slough National Estuarine Research Reserve
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