River deltas provide essential habitat to many terrestrial, avian, estuarine, and marine species of ecological, cultural, and commercial importance. Sediment and large woody debris supplied by the many rivers in Puget Sound form a large proportion of the sound's nearshore geologic framework for important estuarine ecosystemsincluding wetlands, marsh channels, and eelgrass meadows. Such environments in Puget Sound have undergone rapid deterioration and loss of habitat because of alterations to streamflow, sediment delivery, and water quality through damming, land clearing and drainage, and shoreline hardening (construction of dikes, seawalls) for agricultural development.
Of all the rivers flowing into Puget Sound, the Skagit River contributes the greatest amount of freshwater. Despite sustaining an approximately 70-percent loss of salmon-rearing habitat since the 1880s, the Skagit River and Delta complex supports the largest salmon run in Puget Sound and is the only river system in the lower 48 States that is home to all five species of Pacific salmon.
In March and September 2004, U.S. Geological Survey (USGS) scientists partnered with Federal, State, academic, and non-governmental-organization scientists and resource managers to conduct surveys in the Skagit Delta and the San Juan Islands. The goals of this research, which is part of the USGS Coastal Habitats in Puget Sound Project, are to characterize (1) the geologic framework for deltaic ecosystem structure and (2) the hydrodynamic, sedimentary, and biogeochemical processes that influence ecosystem functions in deltaic and coastal environments. The Skagit Delta and San Juan Island areas were selected for two reasons: (1) they lie along a gradient in sediment inputs and human land-use activities that are adversely affecting nearshore habitats, and (2) they have ongoing and planned restoration projects that require scientific understanding of the natural processes that shape estuarine habitat. This work supports many needs of the Puget Sound science community and specifically the goals of the Puget Sound Restoration Program, a cooperative effort among government organizations, Native American tribes, and industry and environmental organizations to preserve and restore the health of Puget Sound's nearshore habitat.
USGS research in the Skagit Delta is focusing on Fir Island and the delta front, where historical changes to habitat structure are suspected to be limiting salmon survival. These settings continue to respond to landscape alterations, including restoration projects and the largest dike-removal project in North America, completed at Deepwater Slough in 2000. Research in the San Juan Islands is targeting sites on San Juan, Shaw, and Orcas Islands, where extensive eelgrass declines and die-offs that have been documented in recent years are poorly understood. Here, nearshore environments are fed sediment and nutrients from small, semiarid, and lightly developed watersheds.
Field Activities and Methods
In March 2004, Eric Grossman, Larry Kooker, Mike Boyle, Andy Stevenson, and Bill Danforth collected approximately 40 km of high-resolution bathymetric and backscatter data along the North Fork Skagit River and delta front, using a 234-kHz Submetrix interferometer swath-sonar system in water depths of 2 to 20 m aboard the USGS research vessel Karluk. To ground-truth this data and map eelgrass coverage, ship-tow video was collected along the delta front from a 17-ft Boston Whaler with the assistance of Greg Hood of the Skagit River System Cooperative.
We collected samples of surface sediment and eelgrass along a gradient between high and low eelgrass coverage for sedimentologic and geochemical analyses to explore possible sediment controls on eelgrass distribution and to test for the presence of eelgrass biomarkers-chemical compounds whose abundance in the sediment may vary in proportion to eelgrass coverage. If such compounds are identified, they will be measured in older, deeper sedimentary layers to determine where and how abundantly eelgrass grew in the past. Vibracores ranging from 3 to 4 m in length were collected from the North Fork Skagit River marsh and tidal flats, and sediment and detrital wood were subsampled from three 10-m-deep auger holes in the South Fork Skagit River marsh. We imaged the morphology and distribution of old marsh and tidal channels buried below agricultural fields by using ground-penetrating radar. Surface-sediment grain-size measurements were made at 160 sites across the tidal flats to establish baseline data for examining sediment transport.
In September, Renee Takesue and Eric Grossman sampled water, surface sediment, eelgrass, and marsh and aquatic vegetation for geochemical analyses. They recovered more than 50 m of material in 26 vibracores in marsh and nearshore areas to develop histories of change in sedimentation, fauna, contaminants, and eelgrass biomarkers. With the help of Rob Kayen, Guy Gelfenbaum, Jodi Eshleman, and Greg Hood, they used a portable Riegl laser scanner to acquire baseline high-resolution topographic data in areas undergoing rapid erosion. They also mapped and sampled a recent flood deposit within the recently restored marsh of Deepwater Slough.
Aerial photography of the Skagit Delta illustrates the staggering change occurring to marsh-channel and eelgrass environments as a result of the shoreline hardening of Fir Island. Historically, many distributaries flowed across Fir Island and distributed flow and sediment evenly along its shore. Diking has focused flow into fewer channels, leading to an increase in sediment delivery to the delta front at the mouths of the North and South Forks of the Skagit River and a decrease along the central part of Fir Island. In aerial photographs and in ship-tow video, this focusing of sediment can be seen to be burying eelgrass and fragmenting it into patches.
Samples from vibracores obtained at the delta front also show significantly coarser sediment overlying much finer material below, consistent with a recent change in sediment source. These samples are being analyzed and radiometrically dated to determine whether this lithologic transition represents an abrupt change in depositional history and whether the timing and new sediment source are associated with human land-use activities (for example, increased sediment runoff, focusing of flow) or natural climatic or geologic processes.
Ground-penetrating-radar imagery collected in the marsh near the mouth of the South Fork Skagit River reveals evidence of marsh and tidal channels buried below modern agricultural fields and shows sedimentary facies internal to the delta. Analyses of sediment composition from cores of these sedimentary facies will help to determine their origin, the environment that they supported, and the flow conditions that deposited them.
Radiometric ages of wood debris ranging from 150 to 400 calendar years before present (cal yr BP) from the base of three auger holes across the marsh reveal that, over several kilometers of southern Fir Island, more than 10 m of marsh sediment and vegetation has accumulated since logging and land clearing began around 1850. This thickness is equivalent to a vertical accretion rate of between 2 and 6 cm/yr, which is 10 to 100 times higher than long-term rates spanning the Holocene. The thickness of this historical deposit in the South Fork Skagit River region rivals that of lahar runout deposits (debris flows from the slopes of a volcano) that are known to radically alter the landscape, suggesting that human modifications of the Puget Sound lowlands may match catastrophic geologic agents of environmental change.
In contrast, sediment cores in the San Juan Islands reveal dry, stiff, basal green clay across three separate embayments; this clay is interpreted to be a glacial deposit lying only 1 to 2 m below the modern sea floor (approx 4 m below the marsh surfaces). It suggests that sedimentation is significantly lower in the islands than on the mainland and likely not a major factor in the massive eelgrass die-offs observed since the year 2000. These sediment samples are currently being analyzed to determine their age and geochemical composition.
Geophysical mapping, using seismic reflection and ground-penetrating radar, is targeting the depth and expanse of lahar and basal glacial deposits and the internal structure of the delta to establish the stratigraphic framework of the Holocene deposits. Analyses of the sedimentary facies will be integrated with geophysical mapping to develop a model of recent marsh and delta development and to quantify the variation in accumulation history associated with climatic, geologic, and land-use change. Geochemical analyses are examining contaminant and nutrient loading, eelgrass biomarkers, compound-specific stable isotopes, and other proxies of eelgrass presence and abundance in sediment cores to reconstruct a history of change in eelgrass distribution over time. A sediment budget for specific subenvironments of the Skagit Delta is being constructed to analyze changes in accumulation rates, sources, and the ultimate fate of sediment reaching the sound as a result of natural climatic and geologic agents versus changes caused by human land-use activities.
These field efforts benefited greatly from the wonderful support of Sandy Wyllie-Echeverria and his adventurous family crew, Captain Martin Sampson and Todd Mitchell of the Swinomish Indian Tribal Community's Office of Planning and Community Development, Doug Bulthuis and Paula Margerum of the Padilla Bay National Estuarine Research Reserve, and Greg Hood and Steve Hinton of the Skagit River System Cooperative.
in this issue:
Deltaic Habitats in Puget Sound