The lowlands of North America
Irregular ridges of coarse morainic deposits mark the outer limits of the advance of the Cordilleran ice sheets, which swept down preexisting river valleys in the Rockies. Broad, low moraines also mark the southern limit of the larger, thicker continental sheets, which advanced south out of Canada. The long tonguing loops of end moraines of the continental ice sheet are prominent features north of the Missouri and Ohio rivers—some running for hundreds of miles and standing 300 or 400 feet (90 or 120 metres) above the plains. Several glacial advances—at least three major ones but perhaps as many as eight or more—successively covered the lowlands with glacial drift.
Glaciers began advancing into the mid-continent as early as two million years ago. Deposits left by some of these early ice sheets now lie buried in Iowa and Minnesota and crop out as old low-relief landscapes in eastern Kansas and Nebraska. Later, the “superglaciation” known as the Illinoian advanced as far south as southern Illinois about 135,000 years ago. Finally, the last major advance of the ice—the Wisconsin glaciation—began about 115,000 years ago and left behind most of the hills, valleys, and surficial glacial deposits in the upper Great Lakes region. By about 7,000 years ago it was essentially gone. Meltwaters from this ice sheet occasionally ponded in front of the wasting ice mass, leading to vast, flat lake plains that have been drained and are now intensively farmed—for example, the plains west of Toledo, Ohio, or the vast flatlands of Lake Agassiz in eastern North Dakota and southern Manitoba. Major river valleys that carried summertime meltwaters, such as the Missouri, Illinois, Wabash, and Mississippi, dried up in winter, allowing the cold winds to blow silty river sediment onto the uplands. Today much of the Midwest’s fertile soils have formed in deep deposits of this windblown glacial silt, known as loess.
In the ice-free areas, lowlands formed because of the continued action of rivers. Streams debouching from the Rockies have spread sands, occasionally whipped up into sand hills, well beyond their banks; those funneling into the Mississippi River have created a vast plain that is known as the Mississippi delta. In addition, the Coastal Plain is marked by lines of sand hills, which are the relics of stranded beaches that eroded as the plains were lifted slowly out of the seas in postglacial times.
Drainage
Drainage conditions and water supply are affected markedly by climate, though they also reflect topography. North America has one of the longest rivers in the world (the Mississippi) and also a drainage system with one of the greatest water capacities (the Great Lakes–St. Lawrence system). It is a continent of immense rivers—largely because of their vast drainage area in the long and broad plains between the central shield and the marginal mountains. Rivers that rise in the Canadian Shield, the Appalachians, or the Cordilleras and flow into the Interior Lowlands must travel a great distance to reach the sea. The Great Lakes–St. Lawrence system on the east and the Mackenzie River on the west drain much of the outer edges of the shield. The Nelson River takes advantage of the low saddle in the shield to carry out the waters of the Saskatchewan River and the Red River of the North by way of Lake Winnipeg to Hudson Bay, which also has fairly long rivers draining into it from the uptilted edges of the shield. The vast Missouri-Mississippi-Ohio system receives waters from the Cordilleras, the Canadian Shield, and the Appalachians to unite the Central and Gulf lowlands in the heartland of the continent. From the Rocky Mountains, long rivers such as the Colorado, Columbia, Fraser, and Yukon flow west to the Pacific. Fed by perpetual snowpacks and ice, they are particularly valuable as sources of water in the arid Southwest.
There is a marked east-west asymmetry to North America’s drainage: the chief continental divide, along the Rockies, is well to the west, thus shedding the longest rivers to the east. The longest tributaries—the Peace into the Mackenzie, the Saskatchewan into the Nelson, and the Missouri into the Mississippi—are all west of the main streams. The chief gulfs—Hudson Bay, the Gulf of St. Lawrence, and the Gulf of Mexico—taking the discharge of many of the rivers, are to the east. All these factors facilitated travel by European settlers from their Atlantic bases into the heart of North America. Considerable north-south disparity also exists in North America’s drainage pattern: there are no rivers or lakes south of the Rio Grande that approach the size of those found to the more humid north.