IAIowa Geography

No other state has water on both its eastern and western sides for the entire length of each. The Mississippi handles the east, from the Minnesota line down to the Missouri corner. The Missouri River and then the Big Sioux handle the west. Only the north and south edges are surveyed straight lines, the northern one following 43 degrees 30 minutes.

Rivers make unreliable fences. In 1877 the Missouri jumped its channel during a flood and stranded an Iowa neighbourhood on the Nebraska bank. Carter Lake is still Iowa soil, wraps around an oxbow of the abandoned channel, and can be reached by road only through Omaha. The resulting lawsuit produced the rule American courts still use for shifting rivers: gradual accretion moves a boundary, a sudden avulsion does not.

The Loess Hills, and where the wind put them

Along the Missouri River in western Iowa runs a ridge of hills made not of rock but of dust. As the last glaciers melted, meltwater rivers dumped enormous quantities of silt on their floodplains, and prevailing winds picked it up and piled it against the eastern valley wall. In places the loess is 200 feet deep.

The formation runs roughly 200 miles from Sioux City south to the Missouri state line, and deposits of that thickness are found in only one other region on Earth, along the Yellow River in China. The material stands in near vertical bluffs when dry and slumps dramatically when wet, so the hills carry a landscape of narrow ridges, steep prairie faces and yucca growing far from any desert.

Seven landforms, one flat reputation

Iowa's reputation for uniformity does not survive a soil map. The state geological survey recognises seven landform regions, and they were built at different times by different processes.

• Des Moines Lobe. A tongue of Wisconsin age ice pushed down the middle of the state and melted back only about 12,000 to 14,000 years ago. It left the youngest, least eroded ground in Iowa, so poorly drained that the prairie potholes had to be tiled before crops would grow.
• Southern Iowa Drift Plain. Much older till, long enough exposed for streams to carve it into rolling hills and broad valleys.
• Iowan Surface. Stripped and levelled by freeze and thaw during the last glacial period without ice actually covering it.
• Northwest Iowa Plains. Gently sloping ground with the state's driest climate.
• Loess Hills. The wind built ridge above the Missouri.
• Paleozoic Plateau. The northeast corner, part of the Driftless Area, never buried by the last ice. Limestone bluffs, cold water trout streams, deep valleys and the burial mounds preserved at Effigy Mounds National Monument.
• Alluvial Plains. The flood built bottoms along the two boundary rivers.

Hawkeye Point to Keokuk

The high ground is Hawkeye Point in Osceola County at 1,670 feet, marked in a former farmyard near Sibley with a mosaic and a silo. The low ground is 480 feet where the Des Moines River joins the Mississippi in Lee County. The 1,190 foot spread is modest, and it slopes almost uniformly from the northwest corner toward the southeast.

At the bottom of that slope stands Lock and Dam 19 at Keokuk. Its 38 foot lift is the largest on the Mississippi, built with the hydroelectric dam completed in 1913 to drown the rapids that had blocked steamboats for a century.

Everything drains to the Mississippi eventually

A low divide runs diagonally across Iowa. Water on the northeast side reaches the Mississippi directly through the Turkey, Maquoketa, Wapsipinicon, Cedar, Iowa, Skunk and Des Moines rivers. Water on the southwest side reaches the Missouri through the Little Sioux, Boyer and Nishnabotna. Both halves end up in the same basin, which makes Iowa one of the few states draining entirely into a single river system.

The Des Moines River is the longest inside the state at about 525 miles and the largest tributary of the upper Mississippi. Natural lakes cluster only in the north, on the Des Moines Lobe, where blocks of buried ice melted into kettles. Spirit Lake in Dickinson County is the largest at roughly 5,684 acres and West Okoboji next door is the deepest at about 136 feet. The biggest water in the state overall is a reservoir, Lake Red Rock on the Des Moines River.

The Honey War and the line to Missouri

Iowa's southern boundary took a decade of argument. Missouri claimed a strip roughly nine miles wide based on a different reading of the Des Moines rapids, and in 1839 both states called out militia. Nobody fired a shot. The only casualties were three hollow trees cut down for their honey, which gave the episode its name. The Supreme Court settled it in 1849 by upholding the 1816 Sullivan Line.

What sits between all these boundaries is farmland at a proportion no other state matches, roughly 85 percent of the land area. The mollisols under it formed beneath about 30 million acres of tallgrass prairie, of which a fraction of one percent survives untouched. Iowa keeps a single time zone, Central, and its temperature range is wide for a state with so little topography: 118 degrees at Keokuk in July 1934 and 47 below zero at Elkader in February 1996.

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