Ghost of trees past

Went down to the Rock Snot after work again today - think it could become a summer afternoon habit (currently Red Barn shuts too early on a week day!) I didn't take my zoom today so stood at the waters edge to capture these pieces of wood sticking out of the river. The river is much higher than it was last Friday but lower and clearer than it was on Monday!
Anyway I took a good few shots, nice one of a Kowhai tree, some of the rocks facing upstream, even one of the overall vista from the cafe's steps. but all were cast to the side when I saw this shot - why? because of the tree ghost on the right hand side of the picture. Not sure how I managed it really, maybe someone can tell me?