Linda Proud, age 13, of Foremost, Alberta, Canada, for her question:
What are oxbow lakes?
In pioneer days, sturdy oxen were used to pull wagons, plows and to perform other muscular farm duties. Their broad shoulders were fitted with yokes under large, U shaped wooden frames called oxbows. Oxbow lakes were named for these wooden oxen yokes. Many of them are U shaped. But others are curved crescents.
Whatever its shape, an oxbow lake is the abandoned child of a lazy river. It is born when a great river reaches a large, level plain, covered with deposits of crumbly soil. The land tips gently toward the sea, but only the flowing water knows that it is tilted at all. Steep slopes make a stream run faster. Here it takes its own sweet time, lazily swishing its muddy petticoats from side to side in curving loops.
Several things happen to this lazy river. The water in the channel travels at different speeds, somewhat like long hair blowing in the breezes. At it swerves around a bend, the stream flows faster on the outside of the curve and lags behind on the inside. Along the outer. bank, the faster stream has more strength to erode muddy gravel and dig a deeper channel. This swirling water swishes its eroded debris to the center of the channel and across to the opposite bank.
Alone the inside of the curve, the river is shallower and the bank is clogged with soggy mud. In time, the muddy deposits in the channel. become as high as the river's banks. Then they form a dam in the path of the flowing water. But the great river must continue forward. It is pushed by the current from behind and coaxed from the gentle slope toward the sea. It solves the problem by using its flowing energy to dig itself a new channel.
When the river is choked in this way, the main stream usually flows around the muddy dam and digs a shortcut across the curve. The, new channel may be a gentler curve. The muddy deposits are left behind the old curve and the new channel. The old bend in the river is sealed off from the main stream.
This abandoned child of the meandering river becomes an oxbow lake. It is a narrow lake, perhaps a mile or ten miles long. If the old curve swung around is almost a circle, the lake will be a ring with a bite on one side. If the curve was gentle, the lake will be shaped like a thin, crescent moon. Many of these severed bends in the river are U shaped, somewhat like the oxbow yokes worn by draft oxen.
Like all lakes, an oxbow is a body of fresh water surrounded by land. Like all lakes, it lives only as long as supply of water lasts. An oxbow is not refilled by springs and streams, so its water soon evaporates. Mud and choking weeds gradually turn it into a swampy hollow. Finally all its water disappears and the old oxbow becomes a patch of fertile soil.