The retreating glaciers left the eastern plains dimpled with shallow wetlands — the prairie potholes — and those countless small marshes have made North Dakota one of the most important duck-breeding grounds on the continent. Biologists call the region a duck factory, and in a wet spring the sloughs ring with the courtship of mallards, pintails, teal and canvasbacks.

A Duck Factory Left by the Ice

The potholes' fortunes rise and fall with the water. A run of wet years fills the basins and floods the country with ducks; a drought shrinks the marshes and scatters the nesting hens. Grassland cover around the water is nearly as vital as the water itself, for a hen needs undisturbed grass to hide her nest.

From the almanac's reckoning:

• Glacial wetlands make the eastern plains a premier duck-breeding ground.
• Duck numbers swing with wet and dry cycles in the potholes.
• Undisturbed grassland cover is essential for nesting hens.

Hunters, birders and conservationists all have a stake in the potholes, and much effort goes into keeping grass on the land and water in the basins. The almanac reckons the pothole country among the plains' greatest gifts to the wider continent — a nursery for the flights that fill a dozen states' autumn skies.