150 YEARS OF HUMAN ALTERATIONS AND TIDAL HABITAT CHANGE (1870- Present)

ELKHORNSLOUGH TIDAL WETLAND PLAN

DRAFT DOCUMENT 1-3-05

Figure 1. Changes to tidal habitat composition in Elkhorn Slough from 1870 to 2000 (Van Dyke and Wasson in press)

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…………………………………………….…………5-7

Freshwater Decreases…………………………………………………………7-8

Sediment Decreases….………………………………………………………....8-9

Sediment Increases….………………………………………………..……….9-10

Other Factors……………………………………………………………………..10

Annotated Bibliography………….………………………………………………..11-14

Figures……………………………………………………………………………..…...15-22

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 erosionresulting 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 hasresulted 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 SalinasRiver 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 SalinasRiver 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 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 MossLandingHarbor. 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 ElkhornSlough Watershed

188019001920194019601980 2000

TIDAL VOLUME DECREASES -HUMAN ALTERATIONS

  • Starting in the early 1880s, the previously interconnected estuarine network of Bennett, Elkhorn, Tembladero, and Moro Cojo Sloughsand the SalinasRiverwere separated by roads and railroads with culverts or tide gates, dams, and leveesrestricting tidal flow to these areas. Figure 2 illustrates this interconnected estuarine network.
  • 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 in press). Figures4 – 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 receiverestricted tidal flow includingBennett, 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 Sloughuntil 1947 (Van Dyke pers com.). Field notes examining the city lands of Montereyin 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 in press) 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 in press). Parson’s Slough shown in the bottom right of Figures 7 and 8 (Parson’s Slough) is one example of this trend.

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 MossLandingHarbor. 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 Sloughhas 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).
  • There have been significant increases in the depth and width of tidal creeks in Elkhorn Slough (Kvitek unpublished, Van Dyke and Wasson in press). 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 in press) 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 clamsin 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 in press).
  • 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 in press).

FRESHWATER DECREASES- HUMAN ALTERATIONS

  • As land use in the watershed converted to row crops in the 1940s, there has beena significant decrease in freshwater surface and groundwater resources in the watershed.
  • The diversion of the SalinasRiverin 1908decreasedseasonal inputs of freshwater to Elkhorn Slough. Presently, water comprised mostly of agricultural runoff from Tembladero Slough and the SalinasRiver enters the old Salinas river channel through tide gates during low discharge times. This water flows out into the MossLandingHarbor 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 ratesfrom 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 in press).
  • Groundwater decreases may have contributed to the subsidence of marsh elevations.

SEDIMENT DECREASES- HUMAN ALTERATIONS

  • Tide gates at the old SalinasRiverchannel 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 MontereyCanyon which acts asa sediment sink.
  • 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 ofsediment inputs to Elkhorn Slough, then there may be a slower rate of accretion whichwould 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 inputsto 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 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. in press).

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.

Annotated BIBLIOGRAPHY

Byrd, K., Kelly, M., and E. Van Dyke. In Press. Decadal Changes in a Pacific Estuary: A Multi-Source Remote Sensing Approach for Historical Ecology. GIScience and Remote Sensing.

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 ElkhornSlough, California: A Bathymetric Analysis. B.S. Capstone Project, CaliforniaStateUniversity, Monterey Bay, California.

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 (CaliforniaStateUniversityMontereyBay). 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 ElkhornSlough continues to deepen and widen as a result of tidal erosion.

Lowe, P. 1999. Marsh Loss in ElkhornSlough, 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 MossLandingHarbor (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 ElkhornSloughdocuments 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 ElkhornSlough. 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, ElkhornSlough will evolve into a shallow-water tidal embayment consisting mostly of subtidal channel and lower-intertidal mudflat in 40-120 years.