Alien Thunder

July 2nd, 1947: An alien space craft crashed just outside of Roswell, New Mexico. This was an alien research vessel collecting rare and exotic life forms from around the galaxy, not all perished in the crash. Two exotic aquatic life forms survived and fled off into the night and flourished in our freshwater lakes and rivers.
The US Army air force collected the debris and bodies, rushed them into hiding and accurately reported a UFO had crashed. Then the government stepped in and the report changed to a crashed weather balloon.
The most hideous and vile creature known to man was accidentally set loose in the United States and this story is about the locating and eradication of those beasts with the help of the aliens that caused this. It culminates in the Skagit Valley of northwest Washington State.