Emi Fergus et al. published a recent paper that describes the complex features of the freshwater landscape. These maps are very compelling in that they show that there are very different patterns between freshwater ABUNDANCE versus CONNECTIVITY.
FIGURE DESCRIPTION (Figure and text from Fergus et al. 2017): Freshwater abundance and connectivity maps by system type. Freshwater abundance is quantified as the total proportion area or stream density within the Hydrologic Unit (HU) 12 spatial unit for lakes (a), wetlands (b), and streams (c). Abundance values are binned as quantiles. Freshwater connectivity for lakes (d), wetlands (e), and streams (f) is represented by connectivity cluster groups determined by k‐means cluster group using principal components analysis (PCA) scores from lake, wetland, and stream connectivity metrics at the HU12 spatial scale. Dominated is in reference to where spatial units plotted on the PCA axes using relative proportion connectivity metric values. The solid black line represents the estimated boundary of the Wisconsin glacial period—north of the line is glaciated area and south of the line is unglaciated area.