Delta sediment dynamics and vegetation
- Recent Topic Edits
- Effects of relict levees on sediment and debris deposition in delta systems
- Delta plain accretion rate among systems compared to sea level rise
- Fringing marsh resilience, wave attenuation and flood management
- Distributary configuration effects on delta sediment deposition
- Development of tidal channels following restoration
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As a river enters its delta, it slows down and spreads out in response to the more gentle topography and the counteracting force of the tide. In response, much of the sediment recruited from throughout the upper basin is deposited. The pattern of sediment distribution and deposition depends on many factors and these dynamics affect habitat elevation relative to tide, as well as the sediment texture in any given point. Along with salinity (see delta hydrodynamics and channels), these two factors largely control vegetation composition and structure, which in turn affects habitat services provided by the delta.
Vegetation, in turn, affects sedimentation patterns by increasing the capture of suspended sediment at rates related to inundation periods (elevation) and the amount, complexity and seasonality of above ground vegetation structure. Given sea level rise, and subsidence behind dikes, understanding sediment deposition patterns and its effects on future vegetation is critical to restoration of Salish Sea deltas.
The following pages are associated with Delta sediment dynamics and vegetation:
|High Uncertainty Topics||Efforts|
Processes (things that drive habitat development over time)
Sediment accumulates in different parts of the delta at different rates, depending to a large degree on elevation, hydrodynamics of river and tide, the particular configuration of river channels, and vegetation structure. Mineral sediment can be divided into two major size categories that roughly fall into "fines" that are carried as suspended load in the water column and can be transported far, and coarser sands and gravels that are usually transported as bedload, pushed and tumbled along the river bed. As a river slows down and spreads out, the heavier particles of the bedload are the first to begin to drop out and accumulate. As such, habitats near major distributary channels may be more likely to support coarse substrates with a high fraction of sand. Areas farther from major distributaries may be more likely to support finer substrates dominated by silt and clay particles. This difference in grain size affects other sediment qualities such as soil salinity, redox, and nutrient processing, and to some degree determines which plant and invertebrate species will colonize an area.
At the seaward edge of the delta, tides and longshore currents move sediment into new areas, pushing it up into blind tidal channels in marshes that may not be directly connected to the river channel, moving the sediment along the shore to other areas farther from the river channels, and transporting it offshore. Sediment that is not trapped by the vegetated delta and intertidal flats migrates to the edge of the delta which is typically a subtidal bluff that drops steeply to the bottom of the receiving bay. With sufficient sediment, this bluff builds and pushes farther seaward over time. Suspended sediment may be carried much farther by tides and currents, ending up on beaches miles away or settling on the bottom of Puget Sound.
As delta islands become established between distributaries, natural levees begin to form during overbank deposition events. Over time the sediment deposition on delta islands shifts from bedload of sand and gravels to suspended load, and the texture of island deposition shifts from sand to silt or clay. Vegetation creates a positive feedback loop with sediment deposition, with increased vegetation slowing velocity (Leonard & Luther 1995, Temmerman et al. 2005) and increasing deposition. Vegetation productivity often increases with increasing elevation (Ewing 1983) thereby further slowing flood water velocity. However as elevation increases, the frequency of flood inundation decreases, and so deposition rate is anticipated to follow a bell curve, with maximum deposition rate occurring in frequently flooded and vegetated systems.
A sediment budget describes how much sediment is recruited by a river from its basin and delivered to the delta, and how this load varies seasonally. Most sediment load is transported during flood flow and the largest sediment loads are delivered by rivers with glaciated volcanos at the headwaters (i.e. Skagit, Nooksack and Puyallup; Czuba et al 2011). Since most floods occur during late fall, winter and early spring, sediment delivery has a seasonal signal. Rivers with substantial glacier cover in their basin also see a spike in suspended sediment during late summer when melting glaciers cloud the river with glacial flour. Globally, only a small portion of sediment delivered to the sea by rivers is found in the bed load, with the majority delivered as suspended load (Syvitski et al 2005). Precise estimates of river sediment composition are not available for the Salish Sea.
Sediment budgets change over time in response to changes in land use and climate. For example heavy logging in the early 1900's substantially increased sediment recruitment and delta accretion. Increases in human population and resulting changes in development patterns have also affected hydrology and sediment recruitment.
Accretion of sediment occurs as the result of two primary processes, the accumulation of mostly mineral sediment delivered by the river as either suspended or bed load, and the accumulation of organic material mostly produced on site by the vegetation growing at the site. Organic accretion results from the production of above-ground and below-ground biomass by plants, which eventually becomes incorporated into the soil. Slow decomposition leads to the buildup of organic material over time. Since much of the above-ground biomass may be carried away by tidal action, it is likely that most organic accretion results from below-ground tissues (roots and rhizomes) [reference needed].
The rate of mineral accretion at a particular point depends on several factors including sediment budget, distance to a distributary, inundation period (elevation), distance to a blind tidal channel, vegetation structural complexity and seasonality, currents and the wind-wave environment. Generally, a point that is close or directly connected to a sediment source (distributary channel), has access to a larger supply of riverine sediment. Areas that are more distant may rely on tidal action to push suspended river sediment back up into blind tidal channels and across tidal flats.
Vegetation and Accretion
Above ground structure such as marsh vegetation or large wood will slow water velocities and allow suspended sediment to settle out. The more tall, dense or complex the structure, the greater the capacity to trap sediment and enhance accretion. For this reason, anything that affects plant diversity, productivity or root:shoot resource allocation, such as salinity, water depth, soil particle size and soil chemistry, will affect above-ground biomass and capacity to capture sediment. For example, in South Carolina saltmarshes, as elevation decreased and inundation increased, the root:shoot ratio in plants decreased, due to taller aboveground biomass and smaller root systems (Morris et al. 2002, Mudd et al. 2009). This increase in aboveground biomass results in an increase in plant-mediated sedimentation (Fagherazzi et al. 2012).
Even within the same vegetation type, the density and height of vegetation can vary substantially across an estuary, resulting in similarly variable rates of accretion. The seasonality of above-ground biomass may also play a significant factor. For example, most of the dominant emergent tidal wetland plants in the northwest are perennial and above-ground biomass senesces each fall. However the senescent biomass of different species varies in terms of persistence through winter. The biomass of some species such as Schoenoplectus americanus (three-square bulrush) quickly degrades and is washed away during the fall, whereas the biomass of species such as Bulboschoenus maritimus (maritime bulrush) are robust and often remain in place until the next growing season. As a result, during the winter flood and storm season, when sediment availability may be at a maximum, B. maritimus meadows are still able to capture sediment. In contrast, not only do S. americanus meadows lack the capacity to trap sediment during winter, but they may also be exposed to the erosive forces of higher wind-wave energy since there is no above ground structure to attenuate waves. These two species sometimes grow in adjacent meadows in the low marsh, and in parts of the Stillaguamish estuary, there is a sharp elevation step between these meadows (Fuller, unpublished data). Early data from paired sediment elevation tables (SETs) suggest that there is a difference in accretion rates between these two zones within the low marsh (Rybczyk, unpublished data).
Erosion moves sediment from one place to another. In the delta, erosion happens when the hydraulic energy of moving water is sufficient to move sediment previously deposited. River flows, particularly flood flows, often keep channels open by eroding recent deposits and can widen channels and initiate new channels. The back and forth of tidal flows carve and maintain tidal channels. Tidal hydraulic energy varies across both space and time. As a result, different estuaries experience different erosional dynamics, and different areas within an estuary can vary substantially.
Floods and storms occur most frequently during late fall, winter and early spring, resulting in a strong seasonal influence on erosion. Winter storms deliver higher wave heights and energy which can erode the delta front. Orientation of a delta with respect to winter storm paths, and the length of fetch, or distance of open water in the direction of the wind, affect the level of wave energy that a delta may experience. Some deltas are therefore more exposed to winter storm erosion.
Vegetation and Erosion
Tidal wetlands that have persistent above ground biomass during winter will attenuate wave height and energy by increasing drag resistance to passing waves. This may reduce the effects of erosion compared to areas that are bare during winter. For this reason, factors that reduce above-ground biomass density or height at a particular point may increase the incident wave height and energy, reduce sedimentation and lead to marsh erosion. In this manner, snow geese appear to have become ecosystem engineers on the Fraser delta, and likely on the Skagit and Stillaguamish as well. Snow geese, which excavate and eat bulrush rhizomes, have doubled or tripled their local wintering populations since 1990. In areas where these populations have expanded their grazing pressure, bulrush densities have declined by over half (S. Boyd presentation, Salish Sea Ecosystem Conference 2011). When geese were excluded, stem densities increased and marshes accreted faster than sea level rise, but with geese grazing, reduced stem densities led to higher wave energy and the marshes eroded (Kirwan et al. 2008). Under current trends of decreasing stem densitites, some bulrush marshes in these deltas may be functionally extinct by the mid 2020's (S. Boyd presentation, Salish Sea Ecosystem Conference 2011).
Effects of Climate Change
There are many likely impact pathways of climate change on sediment dynamics and vegetation. Since sediment dynamics and vegetation are both strongly influenced by hydrodynamics, changes in river flow and sea level will affect everything from the recruitment and distribution of sediment, to the productivity and sediment capture efficiency of wetlands. Since the late 1950's, as global temperatures have risen, the average winter elevation of the freezing level in the North Cascades has risen by about 200m or 650 feet (J. Riedel, Skagit Flood workshop presentation 2012). As the freezing level rises, more of the basin receives winter precipitation as rain rather than snow. As a result of this dynamic, the UW Climate Impacts Group projects increasing high flows during winter in coming decades for Puget Sound rivers (http://warm.atmos.washington.edu/2860/report/). High river flows generally carry proportionally higher levels of suspended sediment (Czuba et al 2011), which can be quantified in a sediment transport rating curve (Curran et al. In Review). Combining a river's sediment rating curve with it's projected future hydrology regime, it is possible to obtain projections for future sediment loads. In this manner, the change in hydrology projected for the Skagit River could double winter suspended sediment loading rates by the 2040's Hamlet and Grossman (In Prep).
As described in Delta hydrodynamics and channels, changes in river hydrology and sea level will likely lead to increased salinity penetration in river deltas. Marsh productivity (including stem density and height, and root mass) generally declines with increasing salinity, which would reduce rates for both mineral sediment capture and organic sediment production. Reduced biomass could also lead to higher rates of erosion, as occurred in the snow geese example above. However if sediment loading rates increase due to more frequent floods, the net effect of climate change on sediment accretion is unclear and likely to be variable across the landscape.
Structures (the physical and biological characteristics of habitat that result from the processes)
The ability of vascular plants to establish on the delta plain is largely a function of relative tidal elevation. Delta vegetation occurs primarily above Mean Sea Level (Kirwan and Guntenspergen 2010), with low marsh typically well established at Mean High Water (MHW). High marsh plant communities become predominant above Mean Higher High Water (MHHW). Although sea level varies over time, marsh vegetation regulates the elevation of its habitat by accreting sediment at rates that vary with elevation and tend to maintain the marsh surface at an equilibrium with mean sea level (Morris et al. 2002). Primary production and root:shoot ratios vary with changes in inundation periods, with greater inundation leading to increased above-ground biomass and greater accretion rates (Mudd et al. 2009). The resulting increase in accretion increases the surface elevation until inundation period declines, which in turn slows accretion. This feedback loop tends to keep tidal marshes in equilibrium with mean sea level, at least at low to moderate rates of relative sea level rise (Morris et al. 2002). There is an upper limit above which marshes can not keep up, and this limit is related to sediment loading rates.
Natural Patterns of Tidal Wetland Vegetation
Jefferson (1974) and Eilers (1974) provide detailed observations of plant communities and development patterns in Oregon Coast salt marsh. Many investigators in different systems have repeatedly observed the importance of tidal inundation regime on vegetation composition and productivity, particularly thorough the effect of inundation of sediment redox levels and pore water salinity (Ewing 1986; Castillo et al 2000; Crooks et al 2002). Sediment texture and the presence of tidal creeks both support drainage and higher redox levels, and so community composition can change systematically between better drained creek and distributary edges and finer textured and poorly drained panes and interiors (Bradfield & Porter 1982; Ewing 1986). Soil pore water salinity is suspected to most directly control plant composition and productivity, and soil pore water salinity may fluctuate over the year based on the changing salinities of surrounding waters (Ewing 1986). Increasing salinity and low soil redox decreases productivity (Ewing 1986; Ewing et al 1989).
Less intensive observations have been made of now rare oligohaline and freshwater swamps. Large wood platforms are important for woody plant recruitment in some systems (Hood 2007). Modification of composition by introduced species can be high even in sites that have not been cleared, diked, grazed or drained (Elliot 2004; Cereghino 2007). Some description of surge plain vegetation has been completed for the Columbia River Delta and outer coast (Kunze 1994), but strong documentation of the controls of freshwater tidal vegetation are largely absent.
More subtle attributes of sediment permeability are likely to affect water retention between tidal cycles, thereby lowering redox and affecting composition (Crooks et al 2002) and productivity. Crooks et al (2002) also observed that higher calcium carbonate levels appeared to increase subsurface drainage, increase soil redox levels, and thereby affected vegetation. Conversely, Callaway et al (1997) observed that sandy sediments increased sub-surface drainage reducing inundation effects. Other factors that reduce sub-surface permeability, such as loss of organic content at depth, subsidence, and compacted layers may also affect subsurface drainage, soil redox levels, and thus marsh composition and productivity; however these dynamics have been poorly studied.
Less is known about the mechanics of vegetation establishment than factors controlling mature vegetation. An intensive investigation of British marsh found that seed bank strongly reflected standing vegetation and was not persistent, and that higher elevation marsh tended to more successfully produce seedlings (Hutchings & Russell 1989). Casual observation of restoration sites suggests that dominant perennial vegetation often establishes unevenly from local seed, with strong establishment associated with wrack accumulation, expanding downslope vegetatively over time (Cereghino 2007). Current practices rely on natural colonization of salt marsh vegetation following restoration of hydrology where seed sources are present. Where seed sources are absent, such as where deltas have been extensively filled to create industrial ports, planting may be necessary for seedling recruitment. Adjacent plantings may contribute to local seed availability and initiate natural regeneration (NOAA, unpublished data).
In general, vegetation is strongly predicted by the abiotic setting and changes quickly when hydrology and salinity are altered, but the historical processes by which sediments and debris are delivered and deposited on the delta are not necessarily present in all systems. Due to subsidence, restoration efforts depend on marsh accretion to restore historical patterns of topography and vegetation. However, the mechanisms that provide for accretion in a restoring delta are different than found in natural systems. Therefore we recommend a focus on improving our ability to predict sediment dynamics as the critical precursor to resilient vegetation.
sediment distribution relative to channels. sediment grain size higher near major distributary mouths. distributaries deliver sediment directly as tide pushes river overbank. blind tidal - sediment pushed down to bay front by distributaries then tide pushes back up into marsh via blind tidal channels.
Degradation from Modification of Sediment Processes
The erosion of marsh along the delta face has been observed where river sediment inputs are reduced (Fuller, personal communications). Puget Sound deltas may contain areas actively subject to river inputs, as well as other areas where river inputs are more remote (which Shipman (2008) describes as the active or inactive delta). The confinement of river flows by levees is anticipated to change the pattern of sediment deposition as sediment is not distributed on the delta plain and is instead pushed onto tidal flats (Grossman et al2011; Fuller). Sediment movement on the delta flat is complex, with suspension and landward transport likely during flood tides and under wave action, and seaward transport pronounced during ebb and low tides driven by river flow (Ralston et al 2012).
Exclusion of tidal flow with dike results in subsidence affecting potential vegetation (Thom et al. 2002; Appendix A) and subsided soils have reduced levels of organic matter (MacClellan 2011). Boumans et al (2002) predicts that survival of some east coast marshes are dependent on sediment inputs, and that some restored sites are likely to deteriorate into open water following restoration of tidal flow. Recovery of historical elevations following subsidence may take tens or even over 100 years (Thom et al 2002), depending on sediment deposition rate. Following restoration, organic matter and nutrient pools increased rapidly in marsh, and most quickly among infrequently flooded marsh. While shallow organic matter pools were anticipated to recovery in 15-30 years, nutrient pools were slower to build (Craft et al 1988). Deeper organic matter may not recovery following restoration, however the effects of slow recovery of deep organic pools are unknown and anticipated to be less than for shallow organic matter (Craft et al 2002).
Sediment transport and deposition are key drivers of habitat development in estuaries. Erosion and accretion determine the elevation at different places, which determines the inundation regime and the plant species capable of inhabiting those places. Vegetation structure creates a biophysical feedback by increasing the capture of suspended sediment and thereby increasing elevation. Sediment texture, or particle size distribution, also affects the plant and invertebrate species that will colonize, and how productive they will be.
Sediment dynamics at any location are context sensitive, meaning that the conditions at any particular place will depend on where that place occurs relative to the channel system, other habitats, and infrastructure in the system. Predicting both the short and long term outcomes of restoration depend on our understanding of sediment dynamics at a site, as well as the larger system context of the site. In the context of climate change, the effect of sediment dynamics on elevation is central to understanding how a site will respond to sea level rise, changes in freshwater inputs and other impacts.
Sediment sources include the delivery of sediment from offsite sources upstream or offshore, as well as the onsite production of organic sediments largely by rooted plants whose belowground and aboveground parts contribute to the organic component of soil. The relative importance of each source is site dependant. Hydraulic energy both delivers and removes sediment. Whether a site accretes or erodes sediment therefore depends on the energy environment and the sediment sources. How habitats and functions at a site evolve in the future depends on changes to the energy and source factors.
Predicting the factors that drive wetland elevation leads to a better understanding of the composition and structure of delta vegetation and in turn the habitat services provided by vegetation. Wetland elevation is also linked to the formation and complexity of channel systems, so understanding elevation dynamics also informs our understanding of channel evolution over time.
The ecological outcomes of a restoration project depends on the broader sediment dynamics of the system within which the project is located. Similarly, many projects result in changes in landscape scale sediment dynamics such as altering the distribution of sediment or changing the volume of suspended sediment delivered to other areas. Our understanding of these scale-dependant relationships between sediment and restoration is limited, particularly how individual projects cumulatively affect system dynamics. Increasing our understanding would improve our ability to design projects that have the biggest effect on improving whole system function. We would also be able to better predict the ecological importance of individual projects and their longer term trajectory in the face of climate change.
A regional predictive model of sediment dynamics based on observation of sediment loads, sediment distribution, hydraulics, elevation, landscape configuration and biophysical feedbacks, would allow for better project design and prediction of short- and long-term outcomes. Even so, it will be important to consider watershed-specific controls on sediment fate as gradients in sediment supply, hydrologic modifications, and biophysical processes result in significantly different estuarine responses.
For example, although the Skagit River has dams that trap sediment, the majority of sediment production in the watershed is sourced from undammed tributaries like the Sauk River (Paulson, 1997). Recent studies of bathymetric change and sediment cores across the Skagit Delta indicate that since 1890 more than 60,000,000 m3 of sediment accumulated offshore of the delta representing a 300-400% increase over natural rates (Grossman et al. 2011a; Grossman et al. in review). The increase in sediment delivery to Skagit Bay is largely due to river-delta channelization that eliminated floodplain connectivity and sedimentation across the historic floodplain connecting the river to Samish, Padilla and Skagit Bays (Collins 2000). The 0.5 km seaward progradation of the delta tidal flats was accommodated by sediments bypassing the delta. This bypassing has disturbed nearshore habitat structure by fragmenting seagrass and burying a once mud-dominated tidal flat with sand. The sediment disturbance has reduced benthic invertebrate biomass and diversity that are important food-prey for valued salmon, forage fish, and birds. The bypassing sediment represents a lost resource for delta marshes to accrete in light of future sea-level rise. In contrast, delta and marsh accretion in Nisqually Delta are strongly affected by sediment trapping in Alder Lake which reduces the annual load of sediment to the delta and its marshes by 90-95% (Grossman et al. 2011b).
Sediment and Vegetation in the Context of Restoration
landscape position: effects on the site, and effects of the site on off site dynamics. subsidence and accretion rates. subsidence and erosion of adjacent marsh plain. sediment budget context. accretion rates and sea level rise. treatments to accelerate accretion. design elements to address sedimentation issues (elevations, soil treatments, channel creation...) marsh nourishment. re-use of dredged sediment.
persistence of vegetation post-restoration at elevations lower than 'normal' - effects of recruitment vs. adult niches. cf Hood 2013.