I digitized the boundaries of the subregions based on a graphic of a conceptual model developed by the Delta Regional Ecosystem Restoration Implementation Plan (DRERIP) team (J. Burau, U. S. Geological Survey, unpublished data). The foundation of the subregions layer was a GIS layer of the boundary of the legal Delta from the Department of Water Resources. I used highways and other geographic features to guide to digitize both the interior boundaries between subregions as well as the three downstream subregions of North Bay, Suisun Marsh, and Suisun Bay. Note that the interior boundaries are approximate, given the simplified boundaries provided in the conceptual model. Note too that the boundary between the Western Delta and the Suisun Bay subregions is not static, but is dynamically defined by the position of the 2-psu isohaline (termed “X2”) that shifts in response to freshwater flows. I assumed for this exploratory analysis that the boundary is fixed as digitized.