Following the recent floods, people with property next to waterways may have found that the stream bed has moved or perhaps a gravel bar has developed along one bank.
For many people, the logical solution would be to remove the gravel bar and straighten out the stream bed.
But during a forum Wednesday at Lycoming College, Larry Brannaka, of the U.S. Fish and Wildlife Service, warned that changing a stream's channel without fully understanding how sediment moves through rivers could cause serious harm to the environment.
"It seems like we've seen the 100-year flood quite a bit in the last nine years," said Brannaka.
Brannaka explained that Mother Nature is set up to balance out. If left alone, streams would settle themselves down a balanced path to carry sediment downstream. But when people start attempting to direct the path of a stream without properly understanding how sediment flows through water, they can cause further degradation of the stream banks, which makes it easier for serious damage to occur during a flood.
"There is a natural energy balance where the resistant forces balance out the forces that create movement of sediment," Brannaka said. He explained that sediment build-up is a primary cause of stream movement and degradation.
Fact Box
(EDITOR'S NOTE: This is the second story in a series of three days of coverage of this week's conservation issues forum at Lycoming College. Watch tomorrow's edition for a final report on the condition of the Susquehanna River and the fish population.)
In a cross section of river, there is a column of fast-moving water that flows over a bed filled with sediment of various sizes. Small pieces of sediment, such as sand, can move easily through the current. Larger pieces of sediment, such as rocks, move through a process called saltation, which Brannaka described as "a combination of billiards and tiddly winks."
As force builds up behind a rock in a stream bed, it will hop up into the water above it, move, and then fall back to the bottom of the river. On its way down, it may hit another rock, which will jump that up into the column of water above, and the process repeats.
Gravel bars are formed when flow through the stream is not strong enough to move larger pieces of sediment. Brannaka explained, when a river moves through a curve, the force of the current on the outside of the curve is stronger than that on the inside. Because of this, gravel bars tend to form on the bank inside the curve, while the opposite bank begins to be eaten away by the current.
Removing a gravel bar without restructuring the flow of the stream will have only a temporary effect.
"We need to change hydraulics of stream to have a lasting effect on gravel bars, otherwise we're just doing maintenance. The gravel bar will reform," he said.
Widening a stream bed, through dredging, has equally degenerative effects, according to Brannaka. A narrowed stream limits what types of species can thrive in a river and weakens the force of the current. In an attempt to balance itself, the stream will continue to degrade until it is narrow enough to transport the proper amount of sediment downstream.
For proper stream maintenance, experts need to measure proper channel dimensions that allow sediment to keep moving through the stream.
"We can go in and create a bank full-width cannel, put in structures to try and keep the banks from eroding, and ensure that sediment can properly move through the system," Brannaka said.
"If you don't direct the flow of the water properly, you might not like where the river decides to put the gravel," he added.
