What do river stages mean?

River stages are calculated from an arbitrary "gage zero" point. The reason that there are sometimes negative levels is because these points were established decades ago and, over time, the river has shifted. The reason that this is not adjusted is because the people who live around the river are familiar with the flood stage and how the current stage compares to it. It takes a vast amount of planning and education in order to change a "gage zero," which would also change the flood stage.

SOURCE: National Weather Service