The study looked at how flooding in upstream areas can impact the frequency of extreme floods downstream. By combining a flood-inundation model with a rainfall-based flood frequency model, researchers found that river overflow from upstream areas has a bigger effect on downstream flood frequencies than dam operations. This means that when assessing flood risk, it's important to consider the potential for river overflow in upstream areas to avoid overestimating the risk of flooding in downstream areas. The study also provided a way to estimate the risk of flooding in upstream areas using a cumulative distribution function of flood area.