ASTORIA — Saturday, April 6, is Sea Monster Day at the Columbia River Maritime Museum. David Gordon, the author of “The Sasquatch Seeker’s Field Manual” and 19 other titles, presents “River Monsters, Sea Monsters and the search for ‘Hidden’ Animals” from 11 a.m. to noon.

Cryptozoology, the systematic study of so-called “hidden animals,” owes its origins to freethinking Bernard Heuvelmans, a French-Belgian zoologist in the late 1950s. Since then, Heuvelmans’ followers and peers have studied numerous unexplained creatures, including Scotland’s Loch Ness Monster, the Abominable Snowman, or Yeti, of Tibet and Mbolel Mkembe, a miniature long-necked dinosaur, reportedly from the central African swamps.

This presentation describes the state of our understanding of these strange entities and more, including Colossal Claude, a purported inhabitant of the Columbia River, and reports of sea monsters off the Washington Coast.

Then, from noon to 2 p.m., participants will have the opportunity to build their very own monster of the sea with driftwood, rocks and fun materials. The museum will provide the necessary materials for the activity.

The museum is at 1792 Marine Drive.