A weir

The white thing is a weir to control the level of the water. The ships pass a lock that is behind the weir and can't be seen from here. Most days in the year the weir is closed, but if the water level is high it will open and ships can use this passage in stead. For this weir that's approx. 25 days a year.

Fish going upstream (like trout and salmon) can't do it because of the waterworks, so every weir in the Netherlands has to provide a Fish ladder. You can see the start of the ladder at the front. Fish can move up bit by bit that way.

You can see more of the weir and the fishladder in my album on Flickr, 35 photos of todays walk.