Is The Willows a Science Fiction story?

 Once I read that The Willows by Algernon Blackwood scared cosmic horror writer, H. P. Lovecraft, I knew I had to try it out. 

The story follows two men canoeing on the Danube River aiming to reach the Black Sea. They reach an area between present-day Bratislava and Budapest devoid of human civilization and decide to stop to camp on a sand bar while the river swells because of flooding. There, they have a creepy experience with something not of our world, something hiding in the willow trees. Is it something supernatural or something that can be explained by science? It is more horrifying if it has a scientific explanation. 

There is a mystical explanation for the creatures. Blackwood was into the occult and theosophy and believed in other realms. He believed that sometimes the line between our realm and the realm of the beyond is thin and things can seep through. That is why we see ghosts, etc. 

However, if they are creatures from another dimension, a fourth dimensional monster peering into our own, one that only knows of our existence if it knows it is being observed, is something so terrifying that it could cause an existential crisis. If creatures like this exist, then we are like ants to them, annoying pests that they want to swat away. 

This is why I think that though this story was intended to by a mystical horror story, ended up being a science fiction story with themes that we later see in harder science fiction novels like Solaris and Blindsight.