On a career ladder, you might "lose" a rung or have to start climbing again at a new organization. It feels like a step back. A river's curves and changes in direction are all part of the journey. What came before informs what flows after. 

This takes some of the pressure off job-hunting decisions, too. While a career ladder has only one direction, a career river can create a delta of many different paths, all equally valid to pursue. Not only is it ok to shift directions, it's expected. We need to give ourselves the freedom to explore what matters to us instead of locking ourselves to one inflexible path.