Delta sediment dynamics and vegetation: Difference between revisions

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===Erosion===   
===Erosion===   
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. Winter storms and floods often cause erosion since they deliver greater hydraulic energy to the delta. including the carving of channels by river and tidal flow,
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 may be related to the vertical distance between high and low tides, with the maximum distance generally occurring during "spring" tides, or the bimonthly tides that occur near the full and new moons. At this time, the gravitational forces of the sun and moon are aligned, resulting in large tidal swings. Erosion is often seasonal since winter storms and floods deliver greater hydraulic energy to the delta.  
channel development. wave action. effect of vegetation. effect of system engineers like snow geese.  
 
wave action. effect of vegetation. effect of system engineers like snow geese.  





Revision as of 00:50, 18 July 2013


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Overview[edit]

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:

Sub-topics Efforts


Processes (things that drive habitat development over time)[edit]

Sediment Distribution[edit]

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. 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.

Sediment Budget[edit]

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[edit]

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 locally 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 to 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.

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 the Stillaguamish delta, the density and height of vegetation in meadows of Schoenoplectus americanus (three-square bulrush) varies substantially across the estuary, depending on environmental conditions (Fuller, unpublished data). As a result different areas within the same vegetation type in an estuary may experience very different levels of accretion. The seasonality of above-ground biomass may also play a significant factor. For example, in parts of the Stillaguamish low marsh vegetation zone, there is a sharp elevation change between the S. americanus and Bulboschoenus maritimus (maritime bulrush) meadows (Fuller, unpublished data). Both species are perennial and above-ground biomass senesces each fall. However B. maritimus produces more robust biomass that persists all winter, in contrast to S. americanus whose biomass quickly degrades and is washed away in the fall. 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. 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[edit]

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 may be related to the vertical distance between high and low tides, with the maximum distance generally occurring during "spring" tides, or the bimonthly tides that occur near the full and new moons. At this time, the gravitational forces of the sun and moon are aligned, resulting in large tidal swings. Erosion is often seasonal since winter storms and floods deliver greater hydraulic energy to the delta.

wave action. effect of vegetation. effect of system engineers like snow geese.


Effects of Climate Change[edit]

Hamlet and Grossman unpubl. on sediment. effects of changes in flows and sea level on sediment dynamics


Structures (the physical and biological characteristics of habitat that result from the processes)[edit]

Marsh Plain[edit]

development. big floods/sediment pulses. sheet flow.

Natural Patterns of Tidal Wetland Vegetation[edit]

Jefferson (1974) and Eilers (1974) provide detailed observations of plant communities and development patterns in Oregon Coast salt marsh. The ability of vascular plants to establish on the delta plain is largely a function of relative tidal elevation and salinity. Delta vegetation occurs primarily above Mean Tide (MT), with low marsh typically well established at Mean High Water (MHW). High marsh plant communities become predominant above Mean Higher High Water (MHHW). 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.

Vegetation Recruitment[edit]

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 vegetative 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.

low salinity favors recruitment of all species coincide with high river flow. Plants sort based on varying abilities to compete vs. tolerate stress. Higher salinities are a stress that sorts species by reducing survival, productivity and reproduction. interannual salinity can vary among wet and drought years, with drought years often causing large shifts in vegetation distribution that have long term effects. The effects of salinity on vegetation loop back on sediment via the effects on above ground biomass.


Channels[edit]

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 Interuption of Sediment Processes[edit]

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.

Map showing the fate of Skagit Delta sediment lost offshore since 1890 largely in response to river-delta channelization

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[edit]

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.