In [12], the authors provide an evaluation of combinations of hierarchical watersheds, showing that some combinations outperform individual hierarchical watersheds. In this article, we aim to achieve a deeper understanding of those combinations. We study which of the four combinations evaluated in [12] always result in flattened (simplified) hierarchical watersheds. Then, we provide a sufficient condition for a combination to always output flattened hierarchical watersheds, and a new combining function that outputs flattened hierarchical watersheds. We conclude that, among the combinations assessed in [12], only a particular case of combinations with infimum always result in flattened hierarchical watersheds.