The title tells you these are riddles, so the poem asks you to guess before it explains. The first image, a "dark waterfall of fur" slipping down rocks, suggests something moss-like or mossy-dark clinging at the water's edge, though the poem never says. The "soft yoke / no neck will ever wear" is a lovely twist: a yoke is a burden for an animal, but this one is gentle and empty. The second riddle, with its "two kingdoms" and "no ruler," sounds a lot like lichen, a green thing that isn't quite a plant, settling where little else survives. The last two lines shift tone. Hemlock trees are dying, marked by tiny grayish white clots, which, judging by the imagery, could be some kind of infestation.
The speaker mostly observes rather than declares feelings. Still, the mood moves from playful curiosity to quiet unease, as the small, soft, odd life of the creek gives way to slow, nearly invisible death.