Here in France, we usually see this kind of sign when the road crosses a river - not systematically, but rather often :
Isere is the largest river in my area
At this place, it is about 100 m wide, 150 miles away from its source (one of Alps glaciers)
A few minutes later, the same day, I found this other one: Not the same size indeed! But sun was back again...
Click here for map