River Delta Adaptive Management Strategy/3. Delta Components and Processes
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Delta Components and Processes[edit]
To make decisions while working in delta ecosystems we make assumptions about how delta ecosystems work. Ideally, these assumptions are based on repeated and comprehensive measurements of the physical, chemical, and biological dynamics of the systems in which we are working, along with a range of comparable systems in different states. These models would be at our fingertips, with which we would predict the outcome of alternative treatments.
Unfortunately we have no complete empirical model of delta function. Instead we primarily work with a few isolated models, connected by conceptual diagrams, built from limited data points when available, and held together with best professional judgment. Our understanding is often extrapolated from similar systems or even other ecological disciplines. We mix data and professional intuition to make good predictions. This is fitting--most large-scale and significant human ventures are conducted in this way. Science is a useful way-finding tool, but rarely provides us a complete map.
To manage risk, it is useful to acknowledge, document, and revisit our current working assumptions, including the data and experiences upon which they are founded. By ruthlessly considering the imprecision and inaccuracy of our current map, we remain cautious as we commit resources to explore uncharted territory. We are more likely to anticipate problems, and avoid pitfalls. This effort, and the use of this interactive on-line tool, are intended to support that process.
There are diverse approaches for describing an ecosystem for planning. The Open Standards for Conservation (Open Standards) describes ecosystem ‘targets’, and defines a logical chain of human impacts on those targets (CMP). The European Environmental Agency, with its Driver-Pressure-State-Impact-Response (DPSIR) framework, is similar to The Open Standards, in that the ecosystem itself is described as being in ‘a state’, around which human forces, pressures, impacts and responses revolve (EEA). The [Salmon] Recovery Implementation Technical Team (RITT) draft adaptive management model elaborates on the Open Standards approach by including ‘drivers’, recognizing that ecosystems are shaped by intrinsic dynamics other than human impacts.
PSNERPs Nearshore Conceptual Model (Simenstad et al 2004) provides a more elaborate description of ‘ecosystem state’, wherein the functions of nearshore ecosystems emerge from interaction between structures and processes. Structures are the current arrangements of the physical and biological components. Processes are the physical and biological forces that cause structure to change over time (specifically, fluxes, energy transfers and transformations). The description of ecosystems both in terms of their structure or state, but also in terms of their processes or drivers, has been embraced by a range of regional authors (Clancey 2009; Hood 2006; Fuller in draft, and Simenstad 2011a; Roni et al. 2002; and Beechie 2003). We distinguish between processes and structures to organize our delta ecosystem conceptual model. The strong influence of physiographic processes in the nearshore lead toward a consideration of ecosystem processes in restoration planning (Goetz et al 2004), a call that has been repeated in other dynamic systems (as withBeechie et al 2010 for river restoration).
Delta Processes, Drivers, and Regimes[edit]
River deltas are deformable, and are continuously fluctuating and evolving, so developing an explicit understanding of processes is central to any assessment of delta ecosystems. Our impacts on the delta environment are amplified (or muted) to the extent that our labored manipulations of structure, in turn affect ecosystem processes. For example, we may remove a dike over a small footprint, but the vast impact is realized through the restoration of tidal flows. In deltas, the effectiveness of our restoration efforts depend on our restoration of processes (Goetz et al 2004; Simenstad et al 2006; Shipman 2008; Bartz et al 2013).
Shipman (2008) and Simenstad et al (2011) each provide a slightly different list of physical processes responsible for forming delta structure. These process lists represent an attempt to organize our thinking in the face of a complex system full of feedbacks and non-linear relationships. While these lists of processes are based on observable phenomena, each processes does not function in isolation. There are linkages and interactions among processes, such that if we envision or observe each process in isolation, we are likely to fail to predict the behavior of delta ecosystems.
These process lists consistently identify three strong drivers: tidal flow, freshwater inputs, and wind-driven waves. Tides penetrate the delta front en masse with a dissipation of force as tidal flow moves uphill against the resistance of freshwater inputs and increasingly confined topography. Freshwater flows pulse with weather events, and are distributed downhill through a system of alluvial distributaries. Wind waves primarily work the delta face, perhaps shaping tidal flow through the formation of bars and shoals. These driving processes result in a pattern of local process regime, which varies systematically over the length of the delta landscape from tidal flats and through the delta to river floodplain.
If we reconsider Table 1 in light of these three drivers, we can simplify understanding of delta dyanamics by grouping some of these processes into networks. We propose two major physical dynamics within river delta that fundamentally describe delta structure. First are delta hydrodynamics and channels by which freshwater inputs, either muted or reinforced by tidal flows maintain a system of channels that convey and mix moving water. Within the flux of delta hydrodynamics, sediment and flotsam are deposited, building the delta plain in a feedback loop with vegetation, with vegetation both increasing accretion (Leonard & Luther 1996), and shifting composition and structure in response to increasing elevation and salinity. Wood entrained in the sediment-vegetation system may have similar roles as in floodplain systems (see Roni et al 2002). The local behaviors of these two dynamics largely drive the structure and evolution any particular location in the delta—a local process regime.
The behavior of this two part system is played out through local geochemical and ecological processes, each dependent on the local process regime. Local process regime can be observed in most of the biotic structures we see every day (Table 2; Simenstad et al. 2011).
Ironically, while human endeavors and concerns drive our interest in ecosystem processes (even prompting the writing of this report), the work of human populations in deltas are typically not included as an 'ecosystem process'. The Puget Sound Partnership has attempted to capture human effects in management models, dramatically increasing their complexity, while achieving a very limited command of the full range of human drivers and feedbacks. We include human social dynamics as a component of our delta ecosystem model because of the importance of our communities as a component of delta ecosystems, and perhaps the most poorly analyzed ecosystem driver. In particular, the management of surface water to limit flooding and improve drainage is critical. The local and informal understanding of motives, concerns, and landscape dynamics within human communities must become exposed and resolved for us to solve the delta management challenge.
Delta Structures[edit]
Structures are easier to measure than processes because matter is easier to measure than energy. Current landscape structure provides a cumulative snapshot of past processes. By comparing structure over time, we can describe the work of processes, and attempt to predict future conditions. Structures are used to organize sampling design, and in turn, for better or for worse, structures are then used to infer habitat functions. Restoration practice commonly attempts to restore historical structures, sometimes assuming that historical ecosystem functions will follow.
Our ability to perceive ecosystem dynamics is limited by what we can see and measure. Ultimately, an investigator decides what structural measures are most appropriate, given the resources and technologies available, and designs an investigation in the context of current theory.
The last decade has brought a surge of interest the spatial description of delta structure. [[Shipman 2008|Shipman (2008)] differentiates among six components of delta systems. Simenstad et al. (2011) repeats these components only adding a distinction between 'tidal floodplain’ and ‘alluvial floodplain’. In a contrasting approach McBride et al (2009) classifies seven kinds of linear shoreline segments occurring within deltas (Table 3), and in appendices suggests a relationship between these segments and Shipman's ‘landform’ classification, different than components.
The crosswalk proposed by McBride et al. may add some confusion to a description of delta structure by neglecting the different intent between Shipman’s conceptual classification of whole delta 'landforms', and McBride et al’s practical classification of shoreline segments. For example, you are more likely to have a McBride shoreline called ‘delta lagoon’ within a shipman ‘wave-dominated delta’, however this increased potential for a delta lagoon to be present in a whole system dominated by waves, does not indicate that the two classes are equivalent. The McBride et al. framework does add some additional components to Shipman’s list of components, more fully encompassing structures like lagoons, formed through the interaction of deltas and littoral cells, and the presence of drowned stream channels within many delta systems.
While Simenstad et al. follows Shipman in the identification of delta landforms, they do not use a geomorphic analysis within the delta unit, but rather use mapped vegetation to identify zones based on salinity regime. This approach is consistent with the vegetation-based wetland classification proposed by Cowardin which informs Collins and Sheik (2005). Collins and Sheik provide the historical reconstructions of delta landscapes critical for the change analysis completed by PSNERP. In practice however, the use of general vegetation zones to define delta composition blurs the distinction between channel and floodplain components proposed through Shipman's components. Braided river distributaries, large tidal channels, tidal flats and lagoons become equivalent due to their lack of vegetation. Griffith (2005) provides an example of a purely structural assessment of the Stillaguamish flats--appropriate for its own purposes--considering sediment texture and vegetation, regardless of the presence of fluvial processes.
A structural analysis of deltas for the purpose of restoration would be at a disadvantage if it did not consider the relationship between channel networks and the delta plain. Somewhat parallel to Puget Sound change analysis work, a UW-USGS team developed a nested six scale estuarine ecosystem classification strategy for the Columbia River Estuary (Simenstad et al 2011b). This approach attempts to provide a more comprehensive multi-scale method for describing the current conditions of large estuarine landscapes. Four of those levels may have potential application to the description of Puget Sound deltas in order of decreasing scale:
- Hydrogeomorphic Reach divides the landscape longitudinally, defined by a variety of hydrogeomorphic thresholds including: extent of salt intrusion, extent of estuarine turbidity, mean extent of current reversal, from the extent of tidal water level fluctuation to the mouth, and all areas between, which have the historical or current potential to be estuary. These zones are not adequately defined in Puget Sound deltas by existing data, but offer a conceptual alternative to using historical vegetation patterns to divide major zones within a delta. Jay & Simenstad (1996) describes the effect of shifting hydrogeomorphic boundaries on sediment dynamics in the Skokomish Delta.
- Ecosystem Complex defines major units within a Hydrogeomorphic Reach. An initial partition is made between channel features and floodplain features. Delineation of channel features uses vegetation and bathymetry, while delineation of floodplain features relies more heavily on interpretation, of geologic and soil mapping, wetland mapping, and vegetation observable in aerial photography.
- Geomorphic Catena – this level involves extensive interpretation of remote sensing data to define discrete biological units within ecosystem complexes.
- Primary Cover Class – A final quantification of structure is provided by a land cover classification using aerial photo and LANDSAT interpretation.
The Columbia River Delta is exceptionally large. Four levels of sub-division may be excessive within Puget Sound deltas. However defining hydrogeomorphic reaches based on salinity and hydraulic phenomena may provide a more robust zonation than the use of historical vegetation maps that reflect hydrodynamic environments that no longer exist. The definition of ecosystem complexes based largely on the presence of tidal barriers has been developed in the Snohomish (Stanley et al 1997).
However, current conditions differ from the regular structural patterns of historical deltas. Channelized rivers create new proto-deltas, hydrodynamics have been altered, and diked wetlands have subsided at different rates. Current deltas contain a very irregular maze of fluvial 'chutes and pools' not found in unaltered deltas. Restoration may ultimately alter 'hydrogeomorphic reach' by altering tidal prism and site hydrodynamics.
It is unclear how a particular approach of structural measurement actually adds value to management decision making within an individual delta. In ecosystem management, accounting systems can easily precede a clear understanding of their scientific value. Accountability systems have sociopolitical value independent of their scientific rigor. These concepts are explored further in the Delta metrics project, where the ESRP program is working to develop an restoration accounting approach that integrates current aerial photography and digital elevation models with PSNERP change analysis data to quantify delta structure before and after restoration, and can integrate our evolving understanding of what makes a delta 'restored'.
Delta Ecosystem Functions, Goods and Services[edit]
Twice a day, the delta becomes a vegetated lake intersected by deeper channels with complex patterns of salinity and turbidity. Twice a day the delta is a wet meadow and shrubland complex intersected by muddy trenches and streams. Over a day, over a season, over a year, this complex system provides a range of different services to people and wildlife. Through diking and drainage of delta landscapes, we obtain a different set of ecosystem services. It is because of how we value these various services that we are concerned about the structure and processes of deltas (Leschine and Peterson, 2007).
The Millenium Ecosystem Assessment framework has been broadly embraced as a tool for describing this range of goods and services (MEA 2005), considering not only the goods we extract for provisioning, but also the cultural services we derive, and the supporting and regulating feebacks critical to ecosystem functions and resilience. Simenstad et al (2011) combined the MEA framework with a best professional ranking of relative service provision, to suggest that delta ecosystems provide a greater diversity of ecosystem services than any other nearshore landform in Puget Sound. Batker et al. (2010) proposes some generalized quantification of ecosystem services, many of which originate in delta wetlands.
A meeting of delta restoration practitioners in 2008 was asked what aspects of delta restoration were most compelling to the communities within which they work (see our development notes). They indicated that flood and drainage services were valued by members of surrounding communities, while juvenile salmonid rearing was responsible for the financing of much of their work, with salmon not only representing an economic commodity, but also a cross-cultural heritage and way of life. Further that we risk not only loosing services, but loosing the opportunity to recover service resilience at a landscape scale.
Ultimately we are concerned with how deltas provide goods and services. Accurately measuring the provision of goods and services is more difficult and therefore more expensive than either measurement of structure or process (see delta topics associated with this strategy). However, defining factors that drive service provision and resilience may be among the most important components for defining restoration policy.
Differences among Puget Sound Deltas[edit]
Not all Puget Sound deltas are alike in the relative dominance of drivers, local process regime, their resulting structure, or their provision of ecosystem services. Post glacial geomorphology, river basin character, and development patterns all conspire to create variation in deltas. Shipman (2008) suggests distinguishing between whole delta systems based on the relative influence of river flow, wave energy, and tidal processes in affecting hydrodynamics. The geologic setting of each delta may affect the degree to which each of these forcing processes drives the evolution of delta form.
Simenstad et al. (2011) did not choose to differentiate between delta system types as suggested by Shipman. Sixteen sites were delineated where major river floodplains enter marine waters. The differences between these systems and slightly smaller systems like the Union Estuary or the Snow-Salmon Watershed Ecosystem are a matter of professional judgment.
Collins and Sheik (2005) delineated and categorized coastal wetland complexes based on a geomorphic approach, and so integrate well with Shipman. Steep fan-type deltas, deltas that fill glacial troughs, and the extraordinary Skagit-Stillaguamish Complex are placed in distinct groups. Collins and Sheik both precede and likely inform the conclusions of Simenstad et al. (2011) in that they differentiate between ‘large deltas’ and other estuaries located in coastal embayments.
Cereghino et al (2012) uses a cluster analysis to differentiate between the 16 delta process units identified by Simenstad et al. based on watershed size, historical wetland size and character, and delta shoreline length--grouping all Olympic Deltas, differentiating Olympic from Cascade Deltas, and separating the Skagit and Snohomish deltas as uniquely large systems, accounting for approximately half of all delta wetlands in Puget Sound. The magnitude of current river discharge may be altered by human diversion (in the case of the Duwamish Delta) or by geologic history (as with the Samish Delta), making the current river more or less fitted to the current delta plain. The character of river discharge may vary among systems as well, based on the origin of river flow in glaciers, and how rivers entrain and deposit sediments (Czuba et al 2011). Cereghino et al. (2012) further identifies a set of 7 coastal inlets where relatively large watersheds enter large tidal wetland systems.
The impact of human development also varies dramatically among systems. Different deltas vary dramatically in how the damming of rivers affects delta evolution, and how future population growth within Puget Sound is anticipated to impact individual delta sites (Cereghino et al 2012).
Due to the limited number of delta sites, all these different sources of information are generally complementary in that they, in combination, provide a more complete understanding of each delta ecosystem, and are easily integrated into a cohesive and unique story for each delta site. These differences among deltas have not been articulated as affecting delta restoration strategy, adaptive management, or monitoring. As we evolve an increasingly robust river delta strategy, we will likely find that each delta presents a unique set of constraints and opportunities.
