Geomorphology and Sedimentation


Surface Elevation Tables & Sediment Pins

Post-restoration shifts in elevation can lead to changes in inundation patterns and soil geochemistry and affect plant and invertebrate communities.  Changes in elevation also alter the amount of time the marsh is accessible for foraging fish, birds, and other wildlife.

Field Set

USGS scientists measure elevation on the Nisqually Delta using Sediment Elevation Tables (SET). This location is in the reference marsh.

Surface Elevation Tables (SET) are a method to measure 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) the Refuge installed 14 SETs at three study sites throughout the estuary: Nisqually National Wildlife Refuge (Refuge; restored in 2009), Phase II Tribal Restoration Site (Phase II; restored in 2006) and Reference marsh (Reference; Figure 1).

Monitoring of these SETS began in 2009 and has continued along with other associated post-restoration monitoring projects.

Figure 1

Location of Surface Elelvation Tables (SET) and Feldspar Marker Horizons in the Nisqually River Delta.

Soon after the restoration construction, there was a large influx of sedimentation. During the first year following dike removal, the Refuge averaged a 37.3 mm increase in elevation, largely because of initial sediment gains resulting from soil stemming from the dike removal. Since then, there has been a net loss of sediment, especially on the east side of the refuge. Meanwhile Phase II has a steady rate of sedimentation through time. Based on sediment accumulation measurements around each SET, the primary driver of elevation increase in the estuary has been from sedimentation.

SET_mean_change

The Refuge area has show more sedimentation than either Phase II or the reference sites. This may be because the land in the refuge sites subsided prior to the restoration and were at the lowest elevations.

Sediment pins are also located throughout the Nisqually Delta to measure the annual gain or loss of sediment. The sediment pins are located 5m, 20 m and 40 m from channel edges so that scientists can understand the rate of erosion or sedimentation across a broader area. When reading these data, sediment pin height gain represents a loss of sediment, while a heigh loss represents sediment gain.

Sediment Pin

This graph shows the accumulation or erosion of sediments at increasing distances from channel edges. When reading the graph, an increase in pin height represents a loss in sediment; a decrease in height represents an increase in sediment.

Feldspar Marker Horizon Results 

Field Set

Feldspar marker horizon is shown by the white layer of clay, giving a clear soil horizon marker. Scientists can use the soil core to determine sediment deposition or erosion.

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.

Assessing the feldspar marker horizons on the refuge has been problematic in areas that were submerged and marker horizons were unclear or not found.  Regardless scientists found that sedimentation patterns with feldspar marker horizon were consistent with the SET data.  The Reference and Phase II sites have consistently exhibited minimal accretion of sediment.

Accretion_mean_change

Feldspar marker horizon data produced show similar sediment accretion rates as the SET data does.

Major Channel Morphology

This project aims to restore historic channels and sloughs across the Nisqually Delta. Not surprisingly, when the tides returned to the restoration area after 100 years of exclusion, erosion occurred, changing channel morphology. Channel cross sections were measured annually from 2009-2014. Additional remote sensing data have been used to measure channel area, length, perimeter and sinuosity within each slough, enabling scientists to compare changes in channels from historic conditions, pre-restoration conditions and post-restoration conditions.

Channel morphology and accessibility have changed considerably as a result of the restoration. Channels in the restoration area are now almost 6 times longer and are growing about 6 cm deeper per year. (Ellings, C., Grossman, E., Davis, M., Hodgson, S., Turner, K., Woo, I., Takekawa, J., 2015). The expansion of this tidal network has opened up habitat in which juvenile salmon and waterfowl can forage and find refuge.

Field Set

Sediment Elevation Table (SET) measures changes in elevation. SET data complements data collected through sediment pins and feldspar horizon markers.

We found that the SET and feldspar marker horizon data showed similar patterns of sediment accumulation.  Higher sediment deposition rates measured at the Refuge may be the result of multiple factors.  The Refuge restoration area is generally lower in elevation than the surrounding study sites due to subsidence during a century of being disconnected from tidal flow.  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 restoration and dike removal in November 2009, the Refuge restoration area was 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.  Long-term monitoring throughout the Nisqually Estuary will enable scientists to relate wildlife responses to changes in elevation, hydrology, and geomorphology and to better understand if the estuary will be able to keep pace with sea level rise.

In addition, increases in channel length and depth has resulted in increase habitat available for Chinook salmon. Because Chinook are thought to be the most estuary-dependent salmon species, this increase in available habitat may be important in the continued salmon recovery efforts. Likewise, the changes in channel morphology may relate to improved water quality, like decreased temperature and salinity. For more information on the impacts of restoration on Chinook salmon, please visit the Fish page.

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/

Ellings, C.S., E. Grossman, M. Davis, S. Hodgson, K. Turner, I. Woo, J. Takekawa, G. Nakai. 2015. Post-Restoration Changes in Salmonid Opportunity [In Revision]. Olympia, Washington