Periodic flash flooding, Stilly. It also washes away some of the riff-raff who make their homes by cuddling up to the (overflowing) spitoons every night. Shoots 'em right down to the stamping mill where, if they're lucky, they shoot through when a stamping bar missing them. If they're not lucky, well, the Legion holds a ceremony and gets a 50% cut on any gold in their teeth.