Hard to believe they didn’t know this would happen.
They knew, they just didn’t care.
The question has always been how much sediment would it trap.
Apparently the authors of this article haven't figured out that answer either since in one place they say the dam traps 2/3 of the sediment and in another they say the sediment leaving the mouth of the river has been decreased by 31% (as opposed to being decreased by 2/3.
There are a lot of environmentalists that are opposed to any kind of man made dam, or interfering with those made by beavers which are ok because they were not made by people.
Putting in the dam destroyed huge amounts of one type of habitat, and created huge amounts of other types of habitats.
However all you will hear from these people are bad things.
If a beaver builds a dam and floods someone's property they will fight to protect these new wetlands and to protect the beavers new habitat, but when mankind changes their environment to better suit their needs all they have to say are bad things.
They are simply anti-human.