Niagara

Niagara is a river going over a cliff on the border of New York and Ontario, and how much goes over is managed. Under a treaty of 1950, at least 100,000 cubic feet of water a second must pass over the falls during daytime in the tourist season, April through October, and at least 50,000 at night and in the off-season. Water above the minimum can go to power plants.

At its peak the Horseshoe Falls has carried 225,000 cubic feet a second. The falls have been moving upstream since the last glaciers retreated: about 10,900 years ago they were near Queenston, Ontario, and Lewiston, New York, and they have backed up nearly seven miles since. The green at the brink comes from the water itself, which carries an estimated 60 tonnes a minute of dissolved salts and rock flour.

The picture stands at the lip. A line of dark, choppy river runs across the top under a stormy sky, and at the brink the water turns glassy teal before breaking into long vertical streaks of white. Mist rises in a lavender haze below, and a faint rainbow arcs across the lower right. The surface is built from thick, scraped strokes, so the water looks as though it is still moving.