Many years ago, my parents were driving cross-country from Arizona to the panhandle of Florida along Interstate Highway 10. For curiosity, my father decided to count the number of natural channels he crossed that contained flowing water. From Phoenix to San Antonio, he counted only three. Somewhere between San Antonio and the Texas-Louisiana border, he quit counting after the total grew near 100.
San Antonio lies just east of the 100th meridian, a line that holds significance to North American hydrologists, naturalists, farmers, and ranchers. This line was recognized by early American Explorers such as John Wesley Powell, causes fundamental differences in culture and politics, plays a role in economics, and greatly influenced Unites States history. For those who have driven across the USA or Canada, pull out your atlas and remember where the eastern forests ended and the prairies began. My guess is it was around the 100th meridian. I don’t like to post photos or maps from other readily available sources, but if you type “100th meridian” into a search engine, you will be rewarded with many discussions and explanations (and a song) about the significance of this line.
West of this line, arid and semi-arid weather dominates. Agriculture depends upon irrigation either from wells or a network of canals. Grassland and desert vegetation dominate. Availability of water plays a heavy influence on the locations of cities and towns. Humans exploit every drop of water from once perennial streams, turning them into sandy ditches. Legal battles over water rights are highly contested issues.
East of the 100th meridian, agriculture survives with minimal irrigation. Most channels contain at least a trickle of flowing water. Except where cleared by man, the forest canopy is nearly continuous. Kids spend summers playing in rivers, creeks, and lakes, and the nearest fishing or swimming hole is never more than a few miles away.
Weather patterns create this division. Topography and geography influence the weather patterns, and geology influences topography and geography. It’s all related. Weather patterns in the mid-latitudes of the Northern Hemisphere move West to East. They pick up moisture from the Pacific Ocean, but lose that moisture as rain and snow over high mountain ranges along the west coast of North America. The moisture-starved storms continue eastward, until they mix with Gulf of Mexico moisture near the 100th meridian, and precipitation resumes.
Moisture from the Gulf of Mexico travels far inland because it is unimpeded by the flat topography of central and eastern North America. The land is flat because it is old. The east coast of North America is called a “passive” continental margin because of it lack of plate tectonic activity. North America is moving away from Europe and Africa, and forces that erode the landscape have long dominated over those that raise the landscape.
The western coast of North America is an active continental margin because just offshore, the Pacific Plate is colliding with the North American Plate, causing uplift, earthquakes, and volcanoes. Over the past many million years, forces that uplift the landscape have dominated those that erode the landscape. This creates topography that depletes rainfall from lands between the coastal mountain ranges and the 100th meridian. Like I said, it’s all related.
Next week I will take a fast-paced road trip from Arizona to east of the Mississippi River and return. I will cross the 100th meridian. I won’t attempt to count rivers and streams like my dad, but I will look out the car window and get a few quick photos of important North American hydrologic features, and post them here.