Potential mechanisms for the salt marsh recession on Sturgeon Bank

Abstract

South of Vancouver, Canada, at the western front of the Fraser Delta lies Sturgeon Bank. The marsh on this intertidal area supports migratory birds and salmon, and it is part of the coastal protection of the city of Richmond. The marsh has faced severe recession of up to 500 m between 1989 – 2011 after having been stable for over 50 years. This paper presents the results of our study investigating potential mechanisms for this recession. Using satellite imagery, aerial photos, field observations, water level records, wind records and sediment samples the system was analyzed utilizing expertise from both local Canadian agencies and the Netherlands. The study focused on defining the complex interactions between hydraulic forcing, tidal channels, sandy bed forms, elevation, and the marsh itself. Results showed that tidal channels were lost, bed forms migrated and inundation increased in conjunction with the marsh recession. Hypothesized causes like sea-level rise, a sediment deficit, migrating bed forms and pond formation could not singularly support the observed changes, and results leaned towards a combination of these causes based on the complex feedback mechanisms that inevitably are present in such a system. These feedback mechanisms (e.g. anoxic conditions, soil compaction, ponding) reduce the potential for marsh growth further after recession is initiated. Fundamental understanding in the complex interactions driving the dynamics of salt marshes is a prerequisite for successful development of future restoration strategies.