[Rebelbase]

I doubt it's "eco-terrorists". Instead is most likely a feud of sorts. On some of the busier rivers there is enmity on the kayaker's side against rafters.

The kayakers like to hang out on certain rapids and surf their boats in the holes. When a raft comes along the kayaks have to move out of the way or they get run over.

I know of one case where a kayaker took out his survival knife and slashed a raft after it pinned him against a rock.

[Nuzcruizer]

I doubt it would be Kayakers placing spikes. Those things would be just as dangerous for them.