Tidal Erosion at Elkhorn Slough

Figure 1: August 1946 aerial photo showing construction of the new artificial Slough mouth and a smaller natural mouth to the north [ESNERR collection]

Virtually all of California’s coastal wetlands have been dramatically altered by hydrological manipulations. Either flow has been decreased by diking and draining lands for agricultural uses, or it has been increased by dredging of deep channels for boat traffic. Both of these hydrological alterations have shaped wetlands at Elkhorn Slough. The most extensive diking occurred in the 1870s when an embankment was constructed to carry the railroad line right through the Slough. Tidal flow to marshes south and east of the main channel was restricted, and most of these wetlands were subsequently drained and converted to agricultural use. While dikes decreased tidal flow to some parts of the Slough, another project radically increased tidal flow to the undiked main channel and marshes: the 1947 opening of a large artificial mouth to the Slough.

Prior to 1947 the Slough mouth was a few miles to the north of the main channel. The opening was small and sometimes obscured by a sandbar, so tidal flow to the Slough was very muted. In 1947 an artificial mouth was dredged through shoreline dunes directly west of the Slough’s main channel (Figure 1) to accommodate the newly constructed Moss Landing Harbor. Plans for tide gates under the Highway 1 bridge, which would have maintained the sluggish tidal character of the Slough, were never implemented. Since 1947, therefore, Elkhorn Slough’s shallow, meandering channels have been exposed to unprecedented tidal flows.

Studying Tidal Erosion
What have been the consequences for Slough wetlands of the opening of the artificial mouth to Monterey Bay? This question was first addressed in the 1980s by the seminal work of John Oliver and colleagues at Moss Landing Marine Laboratories (MLML) and subsequently in the 1990s by various graduate students. Currently habitat changes continue to be studied by Rikk Kvitek and his students at California State University Monterey Bay and by researchers at Elkhorn Slough National Estuarine Research Reserve (ESNERR). The goal of the research is not only to document past changes but moreover to predict their future trajectory. After 1947 there was a gross mismatch between the size of the new mouth and the shallow Slough channels. Tidal scour will carve out the channels until a new equilibrium is reached. But when will this happen? If such an equilibrium were reached soon, Slough habitats might persist indefinitely with the current diverse mix of habitat types. However, if an equilibrium is only reached many decades from now, the Slough may resemble a muddy fiord, due to loss of most marsh and intertidal mudflat habitats. Since most of California’s tidal marshes have already been lost to human uses, and since Elkhorn Slough boasts the second largest remaining area of salt marsh in the state, a better understanding of these habitat changes is critical for making wise conservation decisions.

Main Channel Bathymetry
Depth of the main channel was measured manually using poles at several cross sections by National Oceanic and Atmospheric Administration researchers in the 1940s, Oliver and colleagues in the 1980s, and University of California Santa Cruz student Todd Crampton in the 1990s. Also in the 1990s Kvitek and MLML student Chris Malzone assessed depth contours at these cross-sections using a single-beam echosounder. Comparison of data from these studies revealed a dramatic increase in main channel subtidal area between the 1940s and the 1990s. Depth at the mouth increased from about 1.5 meters to greater than seven meters. Crampton estimated that 1.6 million cubic meters of sediments had eroded since 1947.

In 2001 Jeremiah Brantner and Kvitek initiated a new phase of bathymetric studies by using the research vessel MacGinitie, equipped with a multi-beam echosounder and global positioning system. In one day of surveying they obtained soundings at roughly 0.25-meter intervals throughout the whole subtidal portion of the main channel, yielding by far the highest resolution bathymetric map to date. Comparison of their data with those of Malzone demonstrates that erosion rates remain high, with channel depth increasing on average 0.5 meters in less than a decade. Erosion rates were greatest at the mouth (24 percent increase in depth), and at Seal Bend (30 percent) where the channel turns northward. Brantner estimates that the main channel increased in volume by about 15 percent in just seven years, translating into a loss of 58,000 cubic meters of sediment per year.

Changes in subtidal bathymetry are a key piece of evidence that will help answer whether erosion rates are tending towards equilibrium. Therefore Kvitek plans to resurvey main channel bathymetry on a regular basis and to expand the surveys to include shallower intertidal mudflats. Such detailed long-term monitoring would not have been feasible before the advent of sophisticated mapping technology.

Bank Erosion
In the early 1990s Crampton and Malzone set up markers along the main channel and tidal creeks of the Slough, returning after months or years to measure how the distance from the markers to the bank edge had changed. These simple techniques revealed astoundingly high rates of erosion. The markers were abandoned in the late 1990s but were reinstated in the summer of 2000 by San Jose State University student Shannon Bane in cooperation with the Monterey Bay National Marine Sanctuary and ESNERR. The first year of data suggests that bank erosion rates remain high, averaging forty centimeters per year and approaching two meters per year at some locations. Erosion at these markers will continue to be monitored annually.

Figures 2a and 2b: Diagrammatic representation of change in tidal creeks over time in the northwestern Slough [E. Van Dyke]

Tidal Creek Structure
Analysis of aerial photographs complements field measurements of tidal erosion. Oliver and colleagues used visual interpretation to estimate a 70 percent average increase in tidal creek width in the first four decades following harbor opening. Eric Van Dyke at ESNERR is expanding these studies, using dozens of photographs taken at five- to ten-year intervals between the 1930s and the present and employing computer-based image analysis techniques to delineate vegetated marsh boundaries. This work is confirming trends reported by Oliver and has also revealed that tidal creek systems have become more extensive and reticulated at their margins, bringing tidal flow deeper into the marshes (Figures 2a and 2b).

Loss of Vegetated Marsh
Although bank erosion and tidal creek widening have caused marsh vegetation to be lost from the edges of the Slough, a more serious trend appears to be thinning from the interior. Historic aerial photographs clearly reveal that many areas that were covered with dense pickleweed prior to 1947 now have only sparse, patchy vegetation. This pattern was first described by Oliver and his co-workers. More recent work by MLML student Patricia Lowe demonstrated that percent cover of marsh vegetation declined sharply in the decade following the harbor opening and then declined again in the 1980s with more stable periods in between. While the initial decline is likely the direct result of increased tidal flow, the subsequent decline may be due to marsh subsidence.

Crampton documented a decrease in marsh plain elevation averaging twelve-centimeters since 1947. In transplant experiments, Lowe found that a twelve centimeter decrease in elevation significantly affected the growth of pickleweed: nearly all of the lower plants died, while the higher ones grew vigorously. Tidal scour of surface sediments, ground-water overdraft, and earthquakes may all contribute to subsidence. In any case, erosion now clearly outweighs deposition in the Slough’s marshes, so subsidence can no longer be countered by sediment trapping by pickleweed. As long as this trend of subsidence persists, vegetated marshes will continue to be lost.

Restoring and Managing Slough Wetlands
The research described above indicates that Elkhorn Slough’s channels have deepened and widened, banks have eroded, and salt marsh has been lost—all as a result of tidal erosion. Our new monitoring programs, carried out at higher resolution in both time and space, should soon reveal whether rates of tidal erosion are decreasing over time, but so far there is no evidence to support that this is occurring or that an equilibrium will soon be approached. Prediction of the trajectory of habitat change also requires that hydrology and sediment transport processes in the Slough be modeled; such studies will soon be undertaken as part of the Sanctuary Integrated Monitoring Program (SIMoN).

It appears likely that erosion at Elkhorn Slough will continue at high rates for decades to come, with substantial loss of 5,000-year-old salt marsh and intertidal mudflat habitats. These habitats play a critical role at the base of estuarine food webs; provide shelter for birds, crabs, and other animals; and serve as a filter between the land and the sea. Furthermore, if tidal creeks continue to widen and become more similar to the main channel, their characteristic fish and invertebrate assemblages may disappear. Continued erosion of marshes, mudflats, and tidal creeks threaten these rare natural communities and may also contribute to salt water intrusion in the area and pose a danger to the rail line that bisects the Slough. Coastal decision makers in the region may need to consider intervention to mitigate for the effects of the creation and maintenance of the large artificial mouth to the Slough. In this unusual system, wetland restoration may best be accomplished by maintaining dikes, rather than removing them. Muting tidal flow to some of the Slough’s wetlands may be the best way of preserving salt marshes and intertidal wetlands and the diverse invertebrates, fishes, and shorebirds they sustain.

Kerstin Wasson(1), Eric Van Dyke1, Rikk Kvitek(2), Jeremiah Brantner(2), and Shannon Bane(3)
(1) Elkhorn Slough National Estuarine Research Reserve
(2) Earth Systems Science and Policy, California State University
Monterey Bay
(3) San Jose State University