Whether due to damming, drought, or pollution, sometimes when you don't treat your bodies of water right they just up and leave. They're not particularly hostile, but moving uphill is tough for them so they're gonna take the easiest route to where they're going, and if you're in the way it's your responsibility to figure out what to do about it.
On that note, they need to concentrate while on a slope to keep from rolling downhill, which I find very cute.
The Fluffy Folio has done a million adorable monsters like this; I've been using them in my own game for years and my players have loved every single one of them. This is the enrichment you need in your life today; go follow them on Instagram, Tumblr, or Twitter.
They've got a Patreon as well where you can get all these lil' guys wrapped up in a Foundry module with high-res art, lovely little tokens, beautifully illustrated magic item cards, and patron-exclusive Huge monsters.