The mountains of eastern Tennessee form the rooftop of the Tennessee River watershed. This is the highest ground upon which rainwater falls, ultimately making its way down to the river. The town of Sewanee is on that part of the rooftop known as the Cumberland Plateau. Water falling here has only one desire – to move downhill.
The sublime ease of living at the top of a watershed.
And the journey off the rooftop is a quick one, provided water molecules aren’t caught by thirsty tree roots and evaporated back into the sky. It is estimated that the forests on the Plateau can catch and transpire up to 75% of rain that falls during summer months, thereby creating more clouds and afternoon thunderstorms. The dense sandstones of the plateau make the mountain flat-topped and prevent most water from infiltrating the ground. The water is quickly whisked along small creeks, finally jumping and skipping over cliffs that line the Plateau’s edge.
Water leaving the Cumberland Plateau.
After a brief journey down the steep upper slopes of the Plateau, the water now plunges deep into caverns and passageways of rock formed in the limestones of the lower reaches of the Plateau. The water has changed chemically and physically. When it first made its way across the Plateau top it was little-altered chemically, save for excess silicon picked up from the quartz grains in the sandstone. Its pH also remained like that of the rain, slight acidic with values around 4 to 5. The journey through limestone caverns has raised its pH to around 8 and large amounts of calcium and magnesium have been released into the water from the dissolving limestones.
Water making its way out of Buggytop Cave near Sewanee (source: Nooga.com).
Towns atop the Plateau also change the chemistry of the water. Winter road salting increases sodium. Rain washing through metal gutters and across automobiles picks up excess copper, lead, nickel, aluminum and other metals. Overloaded streams leaving the hardscape of roads, parking lots and roofs run more turbid than their counterparts in forested ecosystems, having picked up larger quantities of silt and clay. These surfaces also contribute oil, bacteria, grease, litter and plastics of all sizes – even the microscopic. Effluent from sewage treatment plants add their bit to the chemistry of streams through pharmaceuticals, hormones, nitrates and phosphates.
Silty urban runoff during a storm.
Many of these contaminants will make their way all the way down to the Tennessee River via tributaries like the Elk and Duck Rivers. There is a sublime ease in living on a rooftop, unaffected by pollutants found further downstream, but our actions here impact the quality of water others drink downhill. The chemical character of a river is given to it by the sum of what goes on in its watershed.
Wendell Berry once said:
“Do unto those downstream as you would have those upstream do unto you.”