American Eels are different from other fish which migrate down our rivers. Fish such as herring, shad and salmon will pass downstream over the tops of dams or through specially designed surface bypass structures.
American eels, by nature move along the river bottom and seek out a route downstream at the bottom of dams rather than at the top. These bottom migration patterns of the American eel have developed over thousands of years, it is an involuntary survival mechanism. We would not expect to see a woodchuck leaping through the tree tops like a squirrel. We should not expect to see eels migrating like a salmon, alewives or shad.
How does it happen?
Below is a cross section diagram of a hydro dam. Looking at it shows the eels predicament. As the eel's approach the base of the dam they find what appears to be a route downstream through the intake and into the penstock.
What the eels don't know is that at the end of the penstock is a large fan like mechanism which when driven by water turns at several hundred revolutions per minute. This fan is contained within a tube or shroud which forces the water and anything else coming down the penstock through the fan blades.
There are two basic types. The first looks like the one below. It looks and functions much like a paddle wheel on a boat.
The second looks like the one below. Similar to a large fan blade.
Most eels which attempt to pass through these structures will die if the dam and turbine are generating.
As an example
Get yourself a straw, a piece of paper towel and a household fan. Turn the fan on. Put a small piece of the paper towel in your mouth and lather up a fine spitball like you did in fourth grade. Now, take carefull aim and try to shoot it through the spinning fan blades.
This is the challenge the American Eel faces throughout New England and the Atlantic coast.
The Results are predictable.
Those are the eels gills hanging out of her mouth. The force of the turbine blade strike blew them out of her throat. Picture yourself getting hit in the abdomen with enough force to spit your lungs up.