Erosion monitoring in Elkhorn Slough
- Eric Van Dyke
Elkhorn Slough National Estuarine Research Reserve
- Kerstin Wasson
Elkhorn Slough National Estuarine Research Reserve
Hydrological manipulations Ė changes in water flow Ė are widespread in California estuaries and wetlands. Diking and draining lands for agricultural and other uses, or dredging to keep channels open for boat traffic, have dramatically altered the flow of water through these fragile habitats. Elkhorn Slough, a large estuary on the central Monterey Bay coast, has experienced drastic changes in its hydrology. However, it is likely that the 1947 opening of a large artificial mouth to the slough has had the most dramatic and lasting impact.
Prior to 1947, Elkhorn Slough was an estuary with sluggish tidal flow entering from a mouth a few miles to the north of the main channel, shared with the Salinas river, and open seasonally. In 1946, the Army Corps of Engineers built jetties directly west of the main channel of the slough, and in 1947, they breached the shoreline dunes and dredged a wide, deep boating channel to newly created Moss Landing Harbor.
The opening vastly altered the movement of water in and out of the slough. The tidal prism was increased dramatically and a new regime of water and sediment movement began. Tidal scour has steadily removed soft sediments on the bottom, particularly in the lower reaches of the main channel in the slough. There has also been increased loss of materials from the banks along the shore, as soft sediment banks supporting marsh plants have been undercut and eroded. An equilibrium has not yet been reached, and it remains unclear how long the slough will remain in this erosive state.
Monitoring of the erosion within Elkhorn Slough utilizes multiple approaches. One approach takes advantage of aerial images of the slough and uses GIS to track spatial changes. By digitizing, rectifying, assembling, and analyzing a chronological sequence of historical maps and aerial photographs, ESNEER staff can quantify patterns of change to the slough's wetland habitats and correlate them with historic events. Specifically, these data can provide rates of tidal erosion, changes in tidal creek morphology, and loss of vegetated salt marsh.
A second method is to physically measure rates of bank retreat along primary and secondary channels within the slough. Using fixed benchmarks, annual surveys measure how much of the soft-sediment bank has eroded each year and how far back the edge of marsh vegetation has retreated.
Summary to DateFor extensive analyses through time, click on the "Documents" tab and download the PDF entitled Wasson et al. (2103).
- Bank erosion has been occurring at relatively high but constant rates in Elkhorn Slough. Go to the "Documents" tab to access the latest data, which can be found at the Wasson et al. (2013) link.
- Rates of bank erosion and vegetation retreat are comparable.
- Rates of erosion are highest in the upper Slough, and higher in natural vs. artificially reinforced banks.
- The field rates we are observing are very similar to what Eric Van Dyke has been obtaining from measurements of aerial photographs.
DiscussionAt the level of regions within the slough, significantly higher rates of bank erosion occur in Parsonís entrance and the upper slough (>80 cm/yr) than in the other regions (20 cm/yr or less). There are less pronounced and less clear patterns of vegetation erosion by region; vegetation is farther back from the edge at Parsonís entrance and upper slough. Bank height is similar in most regions except in muted flow sites where there are no banks. Undercuts of the banks are fairly similar across regions, highest at Parsonís entrance and lowest in Parsonís complex. Holes made by the burrowing isopod Sphaeroma are lowest in muted areas (no cliff habitat) and Parsonís complex (unknown reason), but highest at the Parsonís Slough entrance. Crab holes are lowest in muted areas and Parsonís complex, but highest at Parsonís Slough entrance and the upper slough (cause or consequence of high erosion in these areas?).
There are two types of banks in Elkhorn Slough: natural and those built by humans (e.g., dikes and levees). There were non-significantly higher rates of bank erosion in natural (40 cm/yr) vs. artificial (25 cm/yr) banks. Vegetation is closer to the edge of the bank for artificial vs. natural structures. Artificial banks are taller than natural ones. Artificial banks have similar densities of isopod and crab holes as natural banks. Erosion rates are marginally significantly higher in naturally high (60 cm/hr) vs. low banks (20 cm/yr). Rates of undercutting are significantly greater in natural high vs. low banks. There are significantly higher densities of small and large holes in naturally high vs. low banks. Naturally high cliffs host high marsh vegetation while low ones have low marsh vegetation. There is a marginally significant correlation between bank height and bank erosion for natural banks along the main channel (higher erosion in areas of higher banks).
In addition to the erosion monitoring, we do rapid biological assessments at each of the sites to document presence and relative abundance of large invertebrate and algal species and monitor how these communities are changing over time.
- Non-indigenous species
- Habitat association
- Substrate characterization
Study MethodsMeasurements in the field:
A team of ESNERR & MBNMS scientists visits about 30 permanent sites in the slough to assess bank erosion rates. At each site we measure:
Measurements based on aerial imagery and using GIS:
Figures and Images
Figure 1. Aerial view of Elkhorn Slough. The main channel thalweg (taken from CSUMB Seafloor Mapping Lab data) is depicted as a central red line broken into numbered, 100 m long segments. From the center of each segment (red dot), a perpendicular cross-section line extends to the west and east banks.
Figure 2. Westside cross-section widths: Measurements progress up the slough from mouth (0) to head (96) and forward in time from cool colors to warm colors on the chart. Widths measured from the 1854 Coast Survey map (15 sections near the mouth) are shown in black. Recent large width increases at the upper slough (red and orange) are due to vegetation loss on the banks and across the former marsh plain.
Figure 3. Eastside cross-section widths: Measurements progress up the slough from mouth (0) to head (96) and forward in time from cool colors to warm colors. Widths measured from the 1854 Coast Survey map (15 sections near the mouth) are shown in black. Recent large width increases at the middle and upper slough (red and orange) are due to deterioration of Sargentís island and to vegetation loss on banks and former marsh.
Figure 4. Average annualized change for all 96 eastside cross-sections, for all 96 westside cross-sections, and for a subset of 78 cross-sections encompassing the westside region that has never been protected with levees. This figure paints a dramatic picture of accelerating marsh loss, but is misleading with regard to bank retreat since the largest increases are at locations where the marsh has deteriorated and measurements were made across the former banks and marsh plain.