How do deltas experiencing forced regression select their path to the shore? A natural example from the Salton Sea, CA, USA.

Global relative sea level rise threatens the sustainability of deltaic landscapes. Understanding the rich diversity of shoreline responses that can occur as a result of sea level change depends on knowing how the balance between sediment supply and available accommodation manifests in the landscape at all points in the sea-level cycle. A sequence stratigraphy approach provides a geometric view of how deltas respond under cycles of sea level rise and fall. In stratigraphic studies, all parts of the sea level cycle have been identified and studied via seismic and outcrop stratigraphy. In the modern landscape, examples of deltas experiencing transgression and regression under sea level rise are common, and deltas experiencing regression due to tectonic uplift are also prevalent. However, there are few extant examples of deltas experiencing forced regression due to eustatic sea level fall. Forced regression classically causes rivers to incise into their deltas, but existing models that explain how rivers build terraces and incise valleys remain untested in a field setting. To better understand how deltas behave under forced regression, we documented the evolution of three river deltas in an endorheic desert playa lake (the Salton Sea, California) in the southwestern USA, since 1980, as lake level fell due to diminishing water inflows. All three rivers have similar types of sediment available, and while the discharge differs between the three, irrigation runoff is the primary source of water discharge for these river deltas. However, these deltas all have different prodelta bathymetry, and engineering interventions to suppress windblown dust on the playa have affected parts of the lake shoreline. We mapped the temporal evolution of shorelines and channel locations on these three deltas, and conducted aerial surveys to map the delta topography. We interpret the landforms, and compare the observed topography with predictions from existing models. We derive a new framework to interpret why most rivers incise their deltas, and what conditions are required for a delta to avoid incising and leaving delta plain terraces.