Science: Geomorphology and Sedimentation
Nisqually Surface Elevation Table Analysis: September 2009 to July 2010
Changes in elevation, whether slight or severe, have the potential to alter sediment composition within a given ecosystem. This can affect benthic invertebrate abundance and plant species presence alike. Avian and fish distributions, as well as soil biogeochemistry can also be influenced by such changes in elevation. The 762 acres of the Nisqually Delta, now tidally influenced after the removal of the Brown Farm dike in 2009, are no exception.
A Surface Elevation Table (SET) is a method selected by scientists to mechanically measure such changes in sediment elevation within a wetland ecosystem (Cahoon and Lynch 2003). With the support of Glenn Guntenspergen and James Lynch (USGS Patuxent Wildlife Research Center) we installed fourteen SETs at three study sites throughout the estuary: Nisqually National Wildlife Refuge (Refuge), Phase II Tribal Restoration Site (Phase II) and Reference marsh (Figure 1).
Monitoring of these SETS began in the latter part of 2009 and will continue along with other associated post-restoration monitoring projects. Below is an analysis and comparison of SET data from the 2009 and 2010 field seasons.
Surface Elevation Table Results
SET data indicate that the dike removal resulted in elevation increase throughout the delta (Figure 2). The Refuge yielded the largest amount of increase from pre-restoration 2009 to the post-restoration 2010 measurements followed by the Phase II and Reference marshes.
Feldspar Marker Horizon Results
Feldspar clay marker horizons were also placed within proximity to the SET sites. When placed on the sediment surface, this bright, white clay serves as a clear soil horizon marker in a sediment core and is used to quantify sediment deposition and erosion. The absence of a feldspar layer in a sediment core indicates erosion. Paired with a SET, feldspar marker horizons can be used to help determine if changes in elevation are due to sedimentation or other processes.
Overall, the Phase II, Refuge and Reference sites indicate the occurrence sediment accretion (Figure 2). Sediment accretion values at each study site were very similar to elevation change values (Figures 2 & 3), suggesting that changes in elevation observed from the SET data are likely due to sediment deposition.
Comparing Restored Tidal Channels
Tidal channels are the primary mechanism for sediment transport throughout tidal marshes. Within the Refuge site, SETs were located on the banks of four restored channels. One SET was placed near the mouth of the channel and a second at the upper reaches of the channel.
We compared the difference in elevation change between the mouth and upper reaches of each of the four restored channels (Figure 3). The average difference in elevation between Aug/Sept 2009 and June/July 2010 was higher at the channel mouth. This may be due to the proximity of the channel mouths to the surrounding tidal flats. Feldspar marker horizons show sediment deposition was greater at the channel mouth.
Discussion
Higher sediment deposition rates measured at the Refuge may be the result of multiple factors. The Refuge restoration area is generally lower in initial elevation than the surrounding study sites due to a lack of sediment inputs from the Nisqually River and Puget Sound and subsidence over the 100 years it was diked. This means that this area is now inundated more often and for longer periods of time during a tidal cycle, thus exposing it more frequently to sediment rich waters. Prior to dike removal in October 2009, the Refuge restoration area was also vegetated with freshwater species, primarily the invasive Reed Canarygrass (Phalaris arundinacea). This tall grass died back with the introduction of saltwater, either falling over in the same area in which it was rooted, or breaking off and resettling with the movement of the tides. Sediment has accumulated on top of this decaying vegetation creating a new sediment surface and is contributing to the increased surface elevation we measured.
Sources Cited
Cahoon D.R., and J.C. Lynch. 2003. Surface Elevation Table Website, Patuxent Wildlife Research Center, Laurel, MD. U.S.A. http://www.pwrc.usgs.gov/set/
Tidal Channels
Tidal creeks and channels are an important drainage and habitat feature of tidal salt marshes. They influence biological and physical characteristics such as plant and avian distributions, fish and invertebrate establishment, and soil biogeochemistry (Callaway et al. 2001, Zedler 2001, Hood 2006, Hood 2007). High-resolution low tide aerial photography, along with field surveys will be used to map changes in creek and channel development. We will use our integrated bathymetric system (Takekawa et al., submitted; Athearn et al., in press) consisting of a variable frequency acoustic profiler (Navisound 210; Reson, Inc., Slangerup, Denmark), real time kinematic global positioning system unit (RTK GPS; Leica Smartpole 1200), and laptop computer mounted on a shallow-draft, flat-bottom boat (Bass Hunter; Cabelas, Sidney, NE) to determine geomorphic characteristics. The echosounder determines water depth, while the RTK GPS determines location and elevation as the boat travels over inundated surfaces. Data are integrated and processed in SAS (SAS Institute 1999) and a bathymetric coverage will be generated in Geostatistical Analyst (ArcGIS; ESRI, Inc.). Annual sedimentation differences can be analyzed over surfaces using bathymetry maps and differences in elevations can be calculated in Geostatistical Analyst.
Sediment Pins
Another method we are using to measure sediment accretion and erosion are permanent depth poles or sediment pins (Takekawa et al. 2002, Woo et al. 2007). Sediment pins were placed across the marsh plain to measure sediment accretion or loss over time. Sedimentation pins (2” PVC pipe, schedule 40) will be installed throughout the project to capture subtidal (channel), mudflat, marsh, and upland habitat types. Sediment pins will be measured annually to examine the sedimentation rate. The pole will be surveyed to NAVD88 so that elevation of the sediment surface can be calculated by the pole height. As sediment accretes, the length of the exposed pole will decrease and vice versa. In addition, the characterization of sediment properties is critical for colonizing benthic invertebrates. We will gather soil samples annually and analyze for soil particle size, soil pH, organic matter content, and nutrients.
