Since its first appearance in Lake St. Clair in 1988, the zebra mussel has become the scourge of the Great Lakes. If unchecked, some estimate that the mussels could spread to two-thirds of the nation's waterways and cause $5 billion in damage by the end of the century.

One Canadian water plant on Lake Ontario removed 30 tons of mussels clogging a 2 1/2-mile intake pipe. But the imported shellfish have another side. Researchers have found the shellfish to be adept at filtering toxic substances from the water. A single zebra mussel filters as much as 2 quarts of water a day.

The irony of a plague that purifies is also reflected in the Psalms. Over and over the writer cried out to God from the deep waters of pain and loss (119:153-154). Yet it was those same troubled waters that caused him to keep running back to the Lord.

Impurities in our lakes are nothing compared to the misbeliefs that invade our thoughts. Water pollution hardly compares to the poisons of the lies we tell ourselves. Nothing is more toxic than pride and self-reliance.

So we can say, "It is good for me that I have been afflicted" (v.71). The pain that stops us can also purify us, for in our problems we can learn God's ways.  — Mart De Haan