I've had the opposite experience. I started out working in both hydrology/hydraulics & structures, but over time I gradually spent more and more effort on the H & H. As the technology changes, you just learn and keep up. One thing I reckon which helps is that it's always going to rain, so there's always going to be H & H work to do. Doing the same things over and over is never boring because each situation is unique. I've been doing this for over 20 years now and love it. It helps that I'm in business for myself.
Whether or not you get promoted through the ranks, and often ultimately to the top, seems to me to depend more upon your personality and work methods than it does your knowledge base. An engineer should pursue whatever path suits them best.