Which is most likely cause of springtime flooding in a river valley?

Rapid snow melt in regions upstream.
hope this helps!

Answer barren with few trees, in the northern hemisphere;

When a lake, river or other aquatic  system  becomes  eutrophic, it can have serious negative effects on other organisms like fish, birds and even people.if an algal bloom is so bad that it causes wide-spread death  in  the  water, the organisms that die will all sink to the bottom and start to decompose.