Did you know that the Chattahoochee River, which flows southwesterly through Atlanta, has one of the oldest and most stable river channels in the United States? It has been “locked in place” along the Brevard Fault Zone for at least 185 million years.
By contrast, the Colorado River, which drains seven states, is relatively young; geologists say that plateau uplift, resulting in the river that we know today, took place just five to six million years ago. There is some evidence that tectonic activities which occurred 70 million years ago also contributed to the river’s creation. Even assuming this earlier “birth,” the Colorado is still a youngster, when compared to our Chattahoochee.
I have always been fascinated by streams and rivers: where they begin, how they formed and the type of landscape that makes up their drainage basins, or watersheds. Not surprisingly, my vacations often revolve around rivers.
In May, I traveled to southeastern Utah on a Sierra Club trip to paddle the Colorado and Green Rivers and hike in several national parks. The buttes, bluffs and mesas towering over these rivers proved to be mesmerizing: massive, eroding rock shapes built of layer upon layer of sandstone, limestone, volcanic rock, salt and shale over millions of years. In this stone fantasyland, I envisioned the profile of iconic desert writer Ed Abbey in one of the formations.
The ancestral Colorado River formed after the landscape uplifted from an inland sea; water from the higher elevations then drained to the northeast, the opposite direction of the flow of today’s river. When the mountain range that fed the original Colorado was destroyed by faulting about 25 million years ago, the river reversed its course. New plateaus rose and the sediment-laden river “played the part of a stationary band saw”, cutting through two million year old rock to create abysses like the Grand Canyon.
Closer to home, the Brevard Fault Zone, a prominent geologic feature of the southeastern U.S., extends for nearly a thousand miles across the southern Appalachians. Its parallel ridges are responsible for the Chattahoochee River’s northeast-to-southwest course through north Georgia and its long, linear character.
Millions of years ago, there was more water in the Chattahoochee River system, until a geological phenomenon called “stream capture” occurred. Eroding rivers battled each other for supremacy and ultimately the top of the Chattahoochee watershed was cut off, its water diverted into the Savannah River basin. Prior to this event, the ancestral headwaters of the Chattahoochee extended up into what is now North Carolina, incorporating a much larger land area and collecting more rainwater, which increased the river volume downstream.
The Tugaloo River, at the top of the Savannah basin, had eroded through a ridge over thousands of years and captured the Chattooga and Tallulah Rivers, formerly tributaries to the Chattahoochee. Their waters began to crash dramatically over what is now Tallulah Falls, flowing to the Atlantic Ocean instead of the Gulf of Mexico. Atlanta’s loss was Augusta’s and Savannah’s gain.
Today, the watershed upstream of Atlanta – about a thousand square miles – is one of the smallest drainages in the country that supplies a major metropolitan area with river water. When rain fails to fall over this small watershed for months, especially in the summer, the water level in the Chattahoochee system begins to drop. Our water supply challenges in Atlanta are largely a function of our geological history – and a decision made nearly 200 years ago to locate our metropolis near the top of a river basin on the banks of a small river.
Our country is blessed with nearly three million miles of rivers. Each one of them has a geologic story to tell with chapters that will continue to unfold through the millenia.