When a dam is built, the flow of the river is changed, usually keeping a somewhat constant torrent all over the year (contrarily to what happens in naturally flowing rivers).
(a dam; is built; ) (the flow of the river; is changed; usually keeping a somewhat constant torrent all over the year (contrarily to what happens in naturally flowing rivers; T:When a dam is built)
a dam the flow of the river
is built is changed
usually keeping a somewhat constant torrent all over the year (contrarily to what happens in naturally flowing rivers T:When a dam is built