For water (actor umi- & inst; s actor mangi- & inst) to carry something (theme i-) downstream.
Passive theme focus is signalled by either mi- or ma-.
Na'ānud (or niyānud) din babāih nan bad'āngan hi wangwang at inyuynah ampāna.
That woman was carried downstream at the river crossing and so (the river) took her below.