Maybe we have found their weak point. :)
From Sun Tzu’s “The Art of War: circa 500 BC...
3. After crossing a river, you should get far away
from it.
4. When an invading force crosses a river in its
onward march, do not advance to meet it in mid-stream.
It will be best to let half the army get across,
and then deliver your attack.
5. If you are anxious to fight, you should not go
to meet the invader near a river which he has to cross.
6. Moor your craft higher up than the enemy, and facing
the sun. Do not move up-stream to meet the enemy.
So much for river warfare.