River Delta Channel Structure

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River Deltas have a unique channel structure that reflects the integration of tidal flow and alluvial flow in their unique Hydrodynamics. The channel structure represents interactions between these hydrodynamics Wood in Streams, and Vegetated Tidal Wetlands. This page organizes evidence to describe what we know about Channel Form in River Deltas and how it relates to Hydrodynamics. Hydrodynamics are affected by Tidal Range and river basin Hydrology and Sediment Budget and Routing. All these dynamics are affected by Climate Change.

River Distributary Networks[edit source]

The number of distributaries in a river delta is best predicted by the slope of the delta (from the ‘hinge point’ or first bifurcation of the channel, to the outlet). Long rivers, with low gradients deltas have the greatest number of distributaries. Delta gradient and river length are better predictors than discharge, but are still weakly correlated (r² = 0.56 and 0.47 respectively; Syvitsky et al 2005).
Distributaries are expected split when deposition at the distributary mouth forms a bars that splits flow (Edmonds & Slingerland 2007).
Distributary cross section expands and contract slowly to accommodate flows of water that follow the path of least resistance to the sea. Sudden avulsion may occur, but is infrequent, and likely requires a very significant shortcut, with more gradual switching of distributaries more common, at least in low gradient systems like the Skagit Delta (Hood 2010).
Distributary networks are remarkably stable, similar to those river channels in low gradient, sediment poor floodplains. Distributaries may also decline and fill, converting to a blind tidal channel (Hood 2006).

Large Wood and Hydrodynamics[edit source]

At several points in the process of delta plain formation we suspect large woody debris can jam distributaries, initiate channel splitting, and speed island formation by slowing flows and increasing accretion of suspended sediments and organic flotsam.
While wood dynamics have been studied in river floodplains (Collins et al 2002; Collins & Montgomery 2002) with a reduced overall volume, coupled with a small size of individual pieces, the dynamics of large wood routing are poorly understood in delta landscapes.
We suspect that in the unvegetated delta flats wood jams are transient and unstable, but can linger long enough to trap sufficient sediment to initiate marsh development and create new channels, thereby altering hydrodynamics over a larger area (Stillaguamish Wood Debris Monitoring|Fuller & Heatwole, unpublished data.)

Confined Flow and Sheet Flow[edit source]

Delta hydrodynamics occur in two stages. At lower tides, flows stay in channels, behaving like other streams.
When tides rise above the banks, water spreads across the delta plain as sheet flow. This is a key pathway for material transport (French & Stoddart 1992), and its velocity is influenced by vegetation roughness (Leonard and Luther 1995).
Lower-elevation areas experience sheet flow every tidal cycle, while higher areas depend on river height, with more frequent sheet flow during high flows. Remnant dikes and levees from restoration affect these flows.
Sheet flow deposits sands and coarse silts near channel edges, building up island margins. Finer materials enter during floods or resuspend and settle with tides, influenced by currents and waves. As water drains, it maintains drainage channels.
Tidal channels may meander due to natural fluvial processes, possibly influenced by patchy vegetation (Eilers 1974). Sheet flow slows when it meets vegetation, trapping sediment, while in open areas, faster flow can reduce sedimentation or cause erosion, forming channels (Temmerman et al. 2005).
Channel meandering may also result from bar formation at the outlet, similar to distributary splitting (Hood 2010).
Over time, tidal channel geometry stabilizes, strongly linked to island or tidal basin size Hood 2012 tidal channel reference model.
Once established, tidal channels appear to remain stable unless major flow changes alter their structure (Hood 2004).

Delta Islands and Tidal Channels[edit source]

Delta islands form between distributary channels.
Depending on elevation and salinity, islands may contain tidal swamp, scrub-shrub, perennial herbaceous vegetation, or may be unvegetated tidal flats.
Within these islands, blind tidal channels occur that convey tidal ebb and flow. These channels are anticipated to form by deposition between flow pathways, rather than erosion, and tidal channels may be relic distributaries. (Hood 2006).
Tidal channel configuration follows a pattern relative to river flow, where natural levees form at the upstream island edge, and typically a single large channel drains the island to the downstream side (Eilers 1974). Events during delta plain formation may produce many variations on this theme.
Natural tidal channel systems have a predictable geometry (Hood 2002; Williams et al 2002). Puget Sound tidal channels are particularly well studied (see the Delta tidal channel reference model). While there is considerable variation, the number of channel entrances, channel network complexity, and channel area and cross section can be predicted using island area (Hood 2006). Tidal range and exposure to wind waves appear to affect channel formation (Hood 2015).
Tidal channels allow water, sediment, fish, wood, organic matter and other materials into and out of the habitat island. Tidal channel geometry is likely to have effects on many ecological functions, goods, and services including delta utilization by salmon and other fish, by providing passages for movement (Greene & Beamer 2005), and by concentrating prey availability (Hood 2002b). The level of functions provided by a delta island-channel complex is related to the area of the island, the size and degree of channel development and the landscape position within the delta.
Identifying marsh island units can support analysis of tidal channel potential, informing both project design and monitoring strategy. Tidal channel development is a important element of restoration planning (Hood 2002, Hood 2007, Hood 2015) because these relationships will effect restoration outcomes both within and surrounding a levee or dike removal site.

Tidal Prism[edit source]

The volume of water contained in a delta between low and high tide is called the tidal prism. Since this volume is a function of the tidal area and tidal range of an estuary, restoration increases the tidal prism. Changing the volume of water moving in and out of an area changes the hydraulic dynamics which will result in either increased or decreased channel area (Hood 2004).
The effects of changing tidal prism, whether reducing it by building dikes or increasing it by removing dikes, are seen not just on the footprint of the action, but well beyond it. For example, removing a dike will result in development of new channel habitat on the new site, but it will also increase the size of channels near the site due to the increase in local tidal prism.
In addition, restoration may affect the size and sinuosity of nearby distributary channels by providing greater floodplain area to dissipate flood energy (Hood 2004). Modification of delta tidal prism through restoration is likely to affect the distribution and character of the salinity regime, which has a cascading effect on vegetation, fish and likely other biota. Given sufficient hydrodynamic modeling, this regime can be predicted (Yang et al. 2010b) and observed through the distribution of vegetation (Crain et al. 2004, Hutchinson 1988, Ewing 1986).
The relative function of the delta salinity mosaic in providing services for a diverse and resilient biota is poorly understood.

Salinity Dynamics[edit source]

The salinity at any one point in an estuary varies from near zero to 30+ ppt, and is the result of river flows mixing with the salt water delivered by the tides.
The distribution of salinities across the estuary is strongly affected by the configuration of both distributaries and levees which constrain and direct river flows.
High tides force salt water up into the delta with variable intensity, driven by lunar (monthly) and diurnal (twice daily) effects. The highest high tides of the month are called spring tides and occur about twice per month at the full and new moons, when the sun and moon align their gravitational pull on Earth. Tidal salt intrusion up the delta is maximized at this time.
River flow also affects salt intrusion, with greater upstream intrusion of salt water when river flow is lower, and so salt water intrusion follows a seasonal pattern.
Salt water is more dense than freshwater, and can intrude either as a wedge along the channel bottom, or may be mixed, depending perhaps on local bathymetry.
Salinity in receiving bays may vary due to river inputs. For example, Skagit Bay surface water salinity is as low as 22 ppt (Ewing 1986), compared to a global average of 35 ppt. However.
River flows have the bigger effect than tides in terms of driving the longer term salinity regime that determines the vegetation composition and productivity at any given point. Flows define the average salinity and the range of variability across months, seasons and years. During winter high flows, salinity at a point may be very low while during the summer growing season, when low flows occur, salinity may be considerably higher.
While soil salinity fluctuates with water column salinity (e.g. Burdick et al. 2001, de Leeuw et al. 1991) soil pore water salinity is typically less variable, and is the driver of plant distribution (cite).
Soil salinities in tidal marshes may be higher than water salinities due to salt accumulation through evapotranspiration. Higher winter river flows may flush these salts, but the spatial distribution of soil salinities likely relates strongly to the distance from a river distributary and the elevation of a site. The lower salinities that come in winter and spring favor recruitment of many plant species not just of fresh marshes but also brackish and salt marshes (Noe and Zedler 2001a). However the higher salinities of summer and fall introduce stress that may restrict species distribution by reducing survival, productivity and reproduction (Callaway and Sabraw 1994, Noe and Zedler 2001b).
In the lower delta, marshes near distributaries are likely to have lower salinities than marshes farther from distributaries (Yang and Khangaonkar 2009), which can drive different vegetation patterns even at the same elevation.
In estuaries like the Skagit and Stillaguamish where levee systems constrain distributaries and separate them broadly in space, brackish tidal marshes on the mid delta front may have higher salinity regimes and be closer to the vegetation salinity tolerance threshold than marshes closer to major distributaries (Dunwiddie et al. 2009). This may be one cause of recent marsh losses seen in the Stillaguamish estuary. For restoration, the landscape position of a project relative to the salinity regime may significantly affect what habitat develops on site and how the site affects salinity patterns in the larger system.

Inundation Period[edit source]

Patterns of vegetation distribution, productivity and sediment capture are related to inundation periods (Morris 2007, Morris et al. 2002).
Plant species differ in their tolerance for inundation, and morphological response to increasing inundation.
The daily and seasonal frequency and duration of inundation is a function of the elevation of a particular point and the combined effects of river and tidal flooding.
Inundation periods also interact with above ground biomass to affect sedimentation, with longer inundation providing more time for suspended sediment to be captured. In this way tidal wetlands can keep up with certain levels of sea level rise, because sediment capture rates increase as the inundation period increases (Kirwan and Murray 2007).
Inundation periods are often estimated by intersecting a digital elevation model (DEM) with a tidal datum. A tidal datum is a standard elevation defined by a certain phase of the tide. Common tidal datums include Mean Higher High Water (MHHW), Mean High Water (MHW), Mean Tide Level (MTL), and Mean Lower Low Water (MLLW) (http://tidesandcurrents.noaa.gov/datum_options.html).
Tides and tidal datums vary substantially within Puget Sound (Mofjeld et al. 2002), so it is important that individual deltas understand their tidal range and are able to reference elevations accurately for their site.

Winds and Waves[edit source]

Waves have an influential role, particularly on the outer margin of the delta where they affect currents, sediment transport, erosion and salinity dynamics. Winds vary seasonally in terms of both direction and intensity. Higher winds occur during winter, which is also when there is less above ground biomass in wetlands to brake the erosive effects of wind.
The degree of exposure of any marsh area to wind-driven waves varies from system to system depending on delta orientation and fetch (length of open water in the direction of the wind). Even within deltas, exposure can vary substantially. For example the northern Skagit Delta front is exposed to a greater fetch for winter winds from the south, whereas the southern delta front is protected by Leque and Camano Islands.
The distribution pattern of salinity may be affected by the dominant wind conditions at an estuary. Yang and Khangaonkar (2009) show that including wind conditions in a hydrodynamic model of the Skagit can significantly change the position of the river plume and the resulting salinity pattern.
Although the mechanisms have not been explored, tidal channel networks appear to vary systematically based on wind exposure, with channel area reduced with increasing wind exposure (Hood 2015).

Climate Change and Sea Level Rise Effects[edit source]

Climate change is anticipated to have substantial effects on key drivers of Hydrodynamics, including river hydrology, sea level, water temperatures and storms.
Future projections differ by basin, depending on how much a basin's flow is affected by snow, glaciers and geology.
In general, winter flood frequency and size is increasing as more precipitation falls as snow and less as rain. Summer flows are decreasing as less snow in the mountains means less water release during summer.
Projections for both high and low flows are available from the UW Climate Impacts Group at http://warm.atmos.washington.edu/2860/products/sites.
The rate of change differs by basin, as is being described by the Coastal Resilience to Climate Change effort. For example, by the 2020's the Skagit 20 year flood volume is projected to increase by over 20%, while the Skokomish is only expected to increase by a little more than 5%. Similarly for summer low flow projections, the Snohomish is projected to decrease by 18% by the 2020's (7Q10 low flow parameter), and by 8% for the Nisqually. These changes will have a wide range of effects including sediment mobilization and fate, instream habitat, redd survival, channel migration, off channel habitat formation and dewatering, water temperature, tidal wetland accretion, salinity intrusion in the estuary during the growing season, eelgrass dynamics and Puget Sound water quality.
Sea level is projected by the National Academy of Sciences 2012 to rise 62 cm (24 in) by 2100, however local vertical land motion (VLM) may be quite variable around Puget Sound and the relative sea level rise seen at any individual delta could vary substantially from other deltas. For example, Mote et al. 2008 found one source that estimated subsidence of 2.0mm/yr for southern Puget Sound while other sources found little VLM in that region.
A higher rate of subsidence would substantially increase the local rate of sea level rise for southern Puget Sound.
The biggest impact of sea level rise, at least in the near term, is not likely to be the slow gradual increase in sea level, but the effect of small changes in sea level on the intensity and size of storm waves, which could change rates and patterns of sediment erosion and accretion on deltas.
Changes in salinity distribution in the estuary will happen as a result from changes in both river flow and sea level. Substantially lower low flows in the summer will increase salinity stress during the growing season on tidal vegetation in the delta, particularly for the lower elevation freshwater tidal habitats (Callaway et al. 2007).
A modeling exercise in the Stillaguamish estuary suggested that up to about the 2040's, when projections suggest a 22% drop in the 7Q10 low flow statistic, the change in summer salinity may have a larger effect on tidal marsh vegetation than projected changes in sea level (Fuller, unpublished data). After that time, sea level rise may affect vegetation distribution more strongly, depending on whether accretion rates are sufficient to allow marshes to stay in place. These dynamics have implications for management strategies, since management of forest lands and floodplains may have a bigger effect on mitigating low flows and thereby reduce a major potential climate change risk to estuaries.
The spatial position of a delta restoration project may have variable effects on system salinity patterns, ranging from substantially improving freshwater distribution to negligible effects. In the context of climate change, it may become even more important to strategically select new restoration projects by geographic location, rather than rely on the usual opportunistic approach to project selection (Dunwiddie et al. 2009).
It is possible for a delta project that has limited connectivity with a river distributary to increase salinity intrusion into nearby marshes, thereby reducing the resilience of those marshes to climate driven changes in salinity.

Human Modification of Delta Hydraulic Systems[edit source]

Reduced sediment input reduces the rate of recovery from subsidence following restoration, and creates vulnerability to sea level rise (See Delta sediment dynamics and vegetation).

Dike and Levee Effects[edit source]

Construction of dikes and levees in the delta plain decreases the volume of water entering and leaving the delta plain during a tidal exchange.

Discharge during ebb flow is the flow anticipated to maintain blind tidal channels. When flows are not sufficient to scour new deposition, tidal channels water-ward of the diked site are anticipated to shrink through sediment accumulation. Thus while loss of habitat within diked land is obvious, habitat functions may be lost seaward of dikes, particularly for species dependent on channel volume (Hood 2004).
Dikes and levees also constrain river flows. When river flood is confined to channels during flood, distributaries must convey all flood flow, potentially increasing scour and reducing sinuosity, and associated habitat features downstream.
Thus by allowing over-bank flow across marshes, and increasing flood storage, dike and levee removal may modify downstream habitat structure increasing sinuosity including pool and point bar structures (Hood 2004).
Confining river flow to channels may also reduce sediment retention in the delta plain, forcing river sediments into tidal flats and offshore (Grossman et al. 2011).

Relationship between Hydraulic Change and Change in Sediment Retention[edit source]

Construction of causeways, training dike, and other delta floodplain structures can reduce the rate of sediment retention, by reducing sediment rich river inputs.
Reduced sediment input may increase the size of channels, perhaps by reducing the work tidal forces must expend on sediment transport. Channel widening has been observed in locations where sediment supplies have likely been reduced (Hood 2015).