Look at a nighttime photo of the earth and the pattern jumps out: humanity is not spread across the planet, it is clumped. Great glowing ribbons along certain rivers and coasts, vast dark stretches of desert, mountain, tundra, and forest. Most people live on a small share of the land, and the pattern is not random. It answers to three things: water, food, and reach.
Water first, because nothing negotiates with thirst. Settlements grow where fresh water is — rivers, lakes, springs — and the first great civilizations all rose in river valleys: the Nile, the Tigris and Euphrates, the Indus, the Yellow River. Second, food: people gather where soil and climate will grow it — plains and valleys, not ice and sand. Third, and least obvious: reach. Before highways, water was the highway. A river moves grain a hundred times cheaper than a road; a harbor opens a settlement to the whole world’s trade. Cities are, above all, creatures of routes — they grow where routes cross, where cargo changes hands, where a harbor meets a river’s mouth.
Take the city many of you are sitting in. New York holds one of the finest natural harbors on earth at the mouth of a navigable river — and when the Erie Canal opened in 1825, that river reached clear through to the Great Lakes and the continent’s interior. Suddenly the produce of half a continent funneled through one port. The city’s size is not an accident of pride; it is geography plus one canal, doing exactly what routes do.
Once a city starts, it feeds itself. Jobs pull people; people attract employers; ports attract factories, factories attract railroads, railroads attract more of everything. Which is why cities outlive their original reasons — the canal traffic is long gone, the city is not.
The pattern can shift when technology changes what “reach” means. Railroads let cities bloom far from harbors; highways and air conditioning turned the hot, dry American Sunbelt into a boom region in a single lifetime. But the oldest rule still holds veto power: water. The great cities of the desert West live on rivers stretched to their limits, and the fights over that water — who gets how much of the Colorado — are among the sharpest in the country. Geography does not repeal itself; it just waits.
So when the GED shows you a population map, you now know how to read the clumps and the gaps: look for the water, the growing land, and the routes. Where people are is almost never a mystery. It is a map of what the land offers.