Ah, sandbars! I was thinking the same thing. However if the Ianna river is very deep, sandbars may not be a concern.

The part of the river I lived on was a few miles below a dam and therefore very low at times. There were tons of sandbars, but I'd still see people out there on boats at night with flashlights. Crazy people! XD Above the dam, where it was deeper, there weren't any sandbars, therefore it was much safer to boat at night.

BTW, on Lake Superior there's an infamous sandbar that has wrecked multiply ships that were trying to harbor at night. It's so easy to run into a sandbar in the dark!