Creature Crossing the Embankment Road

It's less of a scary story and more of a slightly mysterious one... I guess. This is a story my mom told me. Sorry if it's hard to read 🙇 My mom is from a certain city in Osaka Prefecture. Decades ago, she was on her way to work on her scooter one day, just like usual. She was riding on the embankment road of a Class 1 river when she saw a "giant, worm-like creature" right in the middle of the road. On the right side was the river, and on the left was a pond. The creature apparently came out of the pond, moved like an inchworm, and started crossing the road. Its size was about the same as a standard body pillow, or maybe a little smaller. The road wasn't very wide, and the path was blocked by this worm(?), so my mom stopped and waited for it to pass. According to her, a truck came up from behind and also stopped, so the truck driver must have seen it too. Eventually, the worm just slid down into the river, and my mom continued on her way to work. It was an era before smartphones, even feature phones weren't common, so she couldn't take a picture. Even now, I've searched online extensively, but I haven't found anything that quite fits.