Ten miles northwest of Washington, D.C., the Potomac River roars and rumbles as it drops, brown and bubbling, over Great Falls. In the entire course of the river, this rapid stands out as the largest, both in volume and height. Every year one or two people drown there, even though signs make it clear enough (in both English and Spanish) that swimming and wading are dangerous – and therefore prohibited. The prohibition applies to boats too: on occasion an expert kayaker is arrested and fined for paddling down the 40-foot falls. Jagged cliffs – popular with rock climbers – line both sides of the narrow gorge through which the river flows. In the summer, lizards scurry around on those cliffs. Herons soar up the little canyon. A pair of Bald Eagles nests there.

When the river floods, logs tumble downstream like toothpicks, and become lodged in the most precarious positions above the cliffs. Lots of people come to watch the torrents, and inevitably one of them gets in trouble. While Great Falls is dangerous any day of the year, it’s even worse during flood stage. Every few years there’s a story (with dramatic footage) about some dopey hiker on the news: stranded hiker is rescued by helicopter as river level rises rapidly around him. It’s a regular story by now.

Above the falls, on dry ground, there’s an old telephone pole which serves as a gauge to measure record flood levels. The highest mark painted on the pole, labeled 1930-something, is way up out of my reach. I’ve gaped up at that mark again and again, and every time it baffles me.

Great Falls lies on the geographical feature known as the fall line. It’s the boundary between the rolling inland foothills and the flat coastal plain. If you’re heading inland, it’s the first real bump in the terrain; if you’re aiming towards the coast, it’s the last little descent. Washington, D.C. (at only eight feet above sea level) lies just south of the fall line, on the tidewater side. (In fact, 300 years ago, it was a swamp. That’s why the Washington Monument sinks 1/4 inch every year.) I live just north of the fall line, on the hilly side.

The fall line has always been a neat place for me. It’s where rivers release the last of their energy, where the earth’s innards are still visible. It’s where two landscapes blend into one.

I used to (and still do) spend a lot of time poking around the woods in the park near my house. I was lucky – dreadful suburbs extend outward from Washington, D.C. for miles and miles in all directions, except in one spot, where there’s a large park. As a kid I wandered through every valley and up every hill in that park. I remember finding puddles brimming with tadpoles, a Pileated Woodpecker’s nest 60 feet up in a giant Oak tree, and rabbit tracks in the snow. I quickly discovered that in all of the forest, the area near the fall line was the most interesting. The hillsides there were steeper, the valleys deeper, and the creeks faster.

Whenever the largest creek flooded, though, I noticed a change on the lower side of the fall line. All sorts of junk showed up. A disturbing amount of junk.

Wherever the water was calm – around every gentle bend, or in front of fallen trees – garbage amassed like a floating landfill. I found tennis balls, tires, aluminum cans, plastic bags, candy wrappers, old sneakers, baseballs, styrofoam containers and more. And at every little ripple on the surface, bubbles collected – smeary, oily, thick bubbles that didn’t look right in a creek flowing through a forest.

I don’t remember exactly when I was first offended by all the garbage that came floating downstream with the high waters. I recall that at first I had a blast digging through all those things, searching for interesting artifacts. But I also remember wondering where all that garbage came from, and how much more there was behind it. I imagined the Potomac River, 20 times as large as that creek, carrying 20 times as much junk downstream, right over Great Falls – the very focus of all that was wild. The idea did not intrigue me.

I have since begun to think of the fall line as more than the physical divide between the hills and the coastal plains. For me, near my home, it separates wild from wasted, clean from destroyed. To the north of the fall line are sprawling, affluent, manicured, green suburbs. On the other side: a compact, hectic, dirty city. The flow of junk, at least in the creek, is from one side to the other. Water-borne pollution, that is, flows downhill.

As far as I’m concerned – I’m headed for higher ground.