Stable Not Equal Sustainable: Delta Dynamics Versus The Human Need For Stability

Arising from the non-uniform dispersal of sediment and water that build deltaic landscapes, morphological change is a fundamental characteristic of river delta behavior. Thus, sustainable deltas require mobility of their channel networks and attendant shifts in landforms. Both behaviors can be misrepresented as degradation, particularly in context of the "stability" that is generally necessitated by human infrastructure and economies. Taking the Ganges-Brahmaputra-Meghna Delta as an example, contrary to public perception, this delta system appears to be sustainable at a system scale with high sediment delivery and long-term net gain in land area. However, many areas of the delta exhibit local dynamics and instability at the scale at which households and communities experience environmental change. Such local landscape "instability" is often cited as evidence that the delta is in decline, whereas much of this change simply reflects the morphodynamics typical of an energetic fluvial-delta system and do not provide an accurate reflection of overall system health. Here we argue that this disparity between unit-scale sustainability and local morphodynamic change may be typical of deltaic systems with well-developed distributary networks and strong spatial gradients in sediment supply and transport energy. Such non-uniformity and the important connections between network sub-units (i.e., fluvial, tidal, shelf) suggest that delta risk assessments must integrate local dynamics and sub-unit connections with unit-scale behaviors. Structure and dynamics of an integrated deltaic network control the dispersal of water, solids, and solutes to the delta sub-environment and thus the local to unit-scale sustainability of the system over time.