Delta hydrodynamics and channels
- Recent Topic Edits
- Salish Sea References
- Wiki Rules
- What Links To This Page?
River deltas are the extension of river floodplains into marine waters (Shipman 2008). The movement of water (hydrodynamics) is the fundamental force that shapes the delta. The topography of the delta affects how river flow, tides, and waves affect the movement of water. Water movement in turn turn affects erosion, deposition, inundation period, soil particle size, and salinity as well as water quality. Channel networks, both river distributaries, and tidal channels, provide pathways for the movement of water, sediment, debris and aquatic organisms. These channel systems are formed and maintained by river and tidal flows.
The following pages are associated with Delta Hydrodynamics and Channels:
River Flow Dynamics and their Structures
The river delivers freshwater to the delta, following a characteristic regime that varies over time. Depending on whether a basin is snow dominant, rain dominant, or transient snow (Hamlet et al. 2001), the river’s hydrological regime may have its highest flows in either late spring or winter, or it may show two peaks with one each in winter and late spring. All rivers in the northwest experience their lowest flows in late summer, though the range of difference between mean high and mean low is basin specific and also affected by anthropogenic regulation with dams, irrigation, etc. During high flows, the water has the hydraulic energy to erode channels, carry high levels of sediment, wood and other materials, and spread freshwater to the far corners of the estuary. During summer low flows, the lower energy and volume allows high salinity marine water to intrude farther up the delta. Salt water is heavier than freshwater, and in confined distributaries the incoming tide can push a saltwater wedge far up river underneath the lens of freshwater.
River Distributary Networks
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
At several points in the process of delta plain formation we suspect large woody debris can play an influential role on hydrodynamics, jamming distributaries, initiating channel splitting, and speeding island formation by slowing flows and increasing accretion of suspended sediments. 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.)
Tidal Ebb and Flow
Within the delta hydrodynamics occur in two distinct stages. At lower tides, flows are confined to channels and are similar to other fluvial systems. Above a bank-full tidal stage, flows leave the channels and wander the delta plain. This sheet flow is anticipated to be a significant pathway for material transport (French & Stoddart 1992) and velocity is affected by the roughness of vegetation growing on the delta plain (Leonard and Luther 1995). At lower elevations in the delta, sheet flow occurs every tidal cycle. At higher elevations sheet flow frequency becomes affected by river height, with greater frequency at higher flows. Remnant dikes and levees, left in the landscape following restoration treatments, are expected to affect sheet flows.
During sheet flow, sands and coarse silts deposit near channel margins, building up the island edge. Finer material may enter the island during flood stage during sheet flow, or may enter, re-suspended and deposited by tidal floods (and affected by currents and waves). During tidal ebb, water draining off the island surface maintains a system of drainage channels.
The meander of tidal channels may be a result of natural fluvial meander formation, perhaps induced by patchy vegetation establishment (Eilers 1974). As sheet flow crosses any unvegetated flat, velocity drops as soon as any vegetation structure is encountered, and sediment trapping rapidly increases. In contrast, in unvegetated areas between vegetation patches, velocity is enhanced, reducing sedimentation or causing erosion, which can lead to channel development. (Temmerman et al. 2005).
Tidal channel meander may also be driven by bar formation at the channel outlet, not unlike the distributary splitting observed at a distributary mouth (Hood 2010). Over time tidal channel geometry within an island reaches an equilibrium state strongly correlated with island or tidal basin area (Hood in press). Once embedded in the island surface, tidal channel position is relatively stable over time unless a substantial change in flow drives a change in channel structure (Hood 2004).
Delta Islands and Tidal Channels
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.
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.
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, in terms of driving the longer term salinity regime that determines the vegetation composition and productivity at any given point, it is the river flows that have the bigger effect than tides. 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.
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
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
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 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. In addition, 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 even 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
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
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
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).