Small stream

Usually just a small stream runs along here. The marsh around it remaining damp throughout most of the year. When flooding happens and rivers rise then the stream becomes quite wide, in other parts of the streams coarse it narrows but deepens. Most of the year just a trickle is visible as it meanders on it's way.
This old bridge in the foreground is in fact an old railway bridge. The line was quite a busy once so I understand. How do they become redundant?