The coastal landform has been shaped by the advance and retreat of massive glaciers, which covered the southern New England landscape as recently as 20,000 years ago. Glaciers carved the many estuaries along the coastline, embayments where freshwater from the uplands mixes with tidal coastal waters. With the exception of the Connecticut River, there are few large riverine sources of freshwater in these estuaries. Because the underlying glacial soils are coarse, unconsolidated, and permeable, much of the uplands are drained by below-ground water flow. Freshwater flowing into an estuary from groundwater, streams, and surface runoff follows certain pathways that are constrained by landform geography and subsurface geology. The total area drained by these freshwater flows describes a watershed.