There are several reasons that this might be done:

They have drained the lake across from me for dredging and increasing the
depth. I too thought that this soil might make a good garden additive. I
checked with the local extension agent and he explained that the soil from this
lake (because it is in the city and gets run off from streets at times) would
be toxic to my garden. It has salts, heavy metals, pesticides, anti-freeze,
etc. The dredging is to remove these deposits and make the environment
healthier for plant and animal life.
sed5555