Garroting Why Do People Put Barbed Wire Across Rivers?

Why do people put barbed wire across rivers? - garroting

It is dangerous and stupid - rivers are public, even if the banks do not. And tense, with an almost invisible barrier of barbed wire across the river does not keep kayakers / etc out, because nobody can see until they are hanging around / d or destruction 'stick to one.

Please enlighten me, why do people.