Ridge and swale (also referred to as dune and swale in dunal areas) is a landform characterized by a series of regular, parallel ridges alternating with marshy depressions known as swales. This topographic pattern is most commonly formed by the gradual movement of a beach—for example, as a result of gradually fluctuating water levels—or by the shifting meanders of a river.
Formation
In riverine environments, ridge-and-swale landscapes are commonly formed by scroll bars, which are arcuate ridges of sediment deposited on the inside bends of meandering channels. As the river migrates laterally, these scroll bars are incorporated into the floodplain, creating a series of parallel ridges (the former bars) and swales (the intervening low-flow channels). The resulting topography often records the historical migration paths, curvature, and widths of the paleo-channel.
In coastal settings, ridge-and-swale topography develops along shorelines where wave action, longshore drift, and fluctuating water levels deposit successive beach ridges. The ridges are composed of coarser sediment (e.g., sand or gravel), while the swales are lower-lying areas that may become wetlands or marshes.
Great Lakes ridge and swale
Freshwater ridge-and-swale ecosystems are globally rare and found primarily in parts of the Great Lakes region of North America. They formed as a result of the gradual retreat of beaches due to falling water levels and post-glacial rebound (isostatic uplift). In these systems, the swales and the adjoining lake or river commonly form a single hydrological unit, so that rising or falling lake levels cause corresponding changes in water levels within the swales.
Many ridge-and-swale landscapes have been destroyed by industrial development, particularly in areas such as the Calumet Region of Northwest Indiana. However, extensive dune-and-swale complexes persist in Michigan (e.g., at Wilderness State Park) and in protected areas such as the Indiana Dunes National Park. These remaining complexes often host rare plants and animals, including the endangered Karner Blue butterfly (Plebejus melissa samuelis).
Occurrence
- River floodplains: Ridge-and-swale topography is common on the floodplains of meandering rivers, where it records the lateral accretion of point bars.
- Coastal environments: Found along ocean coasts, for example on the Outer Banks of North Carolina, and along the shores of the Great Lakes.
- Continental shelves: Ridge-and-swale topography also occurs on continental shelves, where it forms a gently rolling seafloor landscape.
