Prairie Potholes Are Shallow And Continentally Important

Across the northern plains, the retreating ice sheet left millions of shallow depressions. These small wetlands produce a disproportionate share of the continent's waterfowl.

The landform is glacial

Blocks of ice buried in glacial debris melted out and left pits, which fill with snowmelt and rain and have no connection to any stream.

The result is a scatter of basins across otherwise flat terrain, varying from puddles to ponds and differing in how long each holds water.

That variability is the point. A region containing wetlands of many different depths offers suitable conditions somewhere regardless of how wet the year is.

Temporary water is more productive than permanent water

Basins that dry periodically do not support fish, and without fish the invertebrate populations that ducklings depend on become abundant.

Drying also oxidizes accumulated organic matter, releasing nutrients that drive a burst of productivity when the basin refills.

A wetland that never dries becomes less productive over time, which is why permanence is not the conservation goal it appears to be.

Breeding pairs space themselves across many basins

Nesting waterfowl defend small territories, so many separate ponds support more pairs than one large water body of the same total area.

Broods move overland between basins as conditions change, which makes the density of wetlands within a short distance more important than the size of any one.

Upland grass around the basins is where most nests are placed, so the surrounding cover is as much a part of the habitat as the water.

Drainage is the main pressure

Basins in cropland are commonly drained with tile or ditches to make fields workable, which removes them permanently rather than temporarily.

Drainage also sends water and nutrients downstream that previously stayed in place, affecting river systems far from the plains.

Easement programs that pay landowners to leave wetlands and adjacent grass intact are the principal counterweight, and their reach depends on funding cycles.

Wet and dry years both matter

Populations swing widely with regional moisture, rising in wet cycles when many basins hold water and falling in droughts when few do.

Surveys flown annually across the region track pond counts alongside bird numbers, which is how the relationship between water and production is established.

Conservation therefore aims at keeping the basins available for the wet years rather than at maintaining constant conditions, since the dry years are part of how the system works.