Dinosaur Lake by Kathryn Meyer Griffith...first in a seven book series.
Ex-cop Henry Shore has been Chief Park Ranger at Crater Lake National Park for eight years and he likes his park and his life the way it's been. Safe. Tranquil. Predictable. But he's about to be tested in so many ways. First the earthquakes begin...people begin to go missing...then there's some mysterious water creature that's taken up residence in the caves below Crater Lake and it's not only growing in size, it's aggressive and cunning...and very hungry.
And it's decided it likes human beings. To eat.
And it can come up onto land.
So Henry, with the help of his wife, Ann; a young paleontologist named Justin; and a band of brave men, must not only protect his park and his people from the monster but somehow find where it lives and destroy it...before it can kill again. *** Now seven books in the series! Dinosaur Lake, Dinosaur Lake: Dinosaurs Arising, Dinosaur Lake: Infestation and Dinosaur Lake IV: Dinosaur Wars, Dinosaur Lake V: Survivors, Dinosaur Lake VI: The Alien Connection, Dinosaur Lake VII: The Aliens Return.