Smooth to the shelving brink a copious flood 
Rolls fair and placid: where collected all, 
In one impetuous torrent down the steep 
It thundering shoots, and shakes the country round. 
At first, an azure sheet, it rushes broad; 
Then whitening by degrees, as prone it falls, 
And from the loud-resounding rocks below, 
Dash'd in a cloud of foam, it sends aloft 
A hoary mist, and forms a ceaseless shower. 
Nor can the tortured wave here find repose: 
But, raging still amid the shaggy rocks, 
Now flashes o'er the scatter'd fragments, now 
Aslant the hollow channel rapid darts; 
And falling fast from gradual slope to slope, 
With wild infracted course, and lessen'd roar, 
It gains a safer bed, and steals, at last, 
Along the mazes of a quiet vale.
James Thomson
http://www.poemhunter.com/poem/waterfall-29/