Distributary configuration effects on delta sediment deposition
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Distributary connections are necessary to spread sediment laden river water to sustain marsh function and delta resilience in the face of sea level rise. Distributary systems are commonly simplified or constrained by agricultural and industrial development.
- Distributary configuration affects the routing of sediment, large woody debris and freshwater, and the connectivity of estuarine habitats for fish.
- While some projects may result in distributary reconnection (Stillaguamish Old Channel, Red Salmon Slough Restoration, Port Susan Restoration) and distributary reconstruction projects have been discussed for the Skagit Delta and Snohomish Delta and Nisqually Delta, we have limited ability to predict the effects of distributary configuration on sediment deposition.
- Fir Island Farm Restoration is an example of a project that is more removed from distributary nourishment than other projects in the Skagit Delta
- Some initial work associated with the Nisqually Refuge Restoration suggests that low sediment delivery by the river, combined with a weak connection between the river and restoration site is resulting in limited delta accretion.
- Aerial photo observations associated with the Port Susan Restoration show loss of marsh area in areas with limited distributary connections, and expansion of marsh area in areas receiving river flood waters. Ongoing monitoring is describing patterns of sediment deposition across the face of Florence Island.
- The landscape location and the design of levee and dike removal projects may affect wave energy, freshwater and suspended sediment routing, and patterns of salinity intrusion. These may substantially affect the function of existing wetlands. Thus some projects may substantially improve ecosystem resilience to climate impacts by increasing retention of river sediment in wetlands. On the other hand some actions may create unstable systems, or degrade existing marsh as a new hydrodynamic and sediment regime may result in marsh erosion.
- Existing projects are commonly located opportunistically, based on landowner willingness, and hydrodynamic design may be poorly informed or constrained by infrastructure or stakeholder interests (like trails development requiring retention of levees).
- We need exemplary projects that predict and verify the effects of site position and design on off-site marsh formation processes in a way that builds a more generalized project development policy.
- A river may only support a limited number of distributary channels. The phenomena of distributary switching has been explored in the Skagit (Need citation).
- File:Grossman 2013 delta sediment dynamics presentation.pptx summarizes observations of distributary distribution on delta sediment deposition among multiple systems.
- Czuba et al 2011 provides a summary of our current limited knowledge of sediment budgets on major rivers.
- USGS has been working to increase instrumentation that will allow estimates of annual sediment budget.
- Episodic events may radically change river sediment output, such as the effects of the Oso landslide on the Stillaguamish Delta.
Delta Strategy Analysis
The uncertainties in how this topic affects delta restoration has resulted in its inclusion in the ESRP River Delta Adaptive Management Strategy. This three criteria analysis should build off the analysis above, and supports development of learning projects.
We are not faced with a large number of urgent distributary restoration actions, however we anticipate that in the larger Snohomish, Nooksack, Stillaguamish, and Snohomish deltas, understanding of distributary connectivity is part of the restoration analysis.
There are limited opportunities for experimental verification of distributary formation and maintenance. This may be largely a modeling exercise, better allocated to individual project feasibility study within specific systems.
Definition of a target distributary state that maximizes performance and resilience of a river delta is a significant policy decision that frames planning, acquisition , and negotiation with stakeholders, as well as the location and scope of future project work.