As a physical geographer who has conducted research on river landscapes on four different continents, Philip Marren's research has encompassed a wide range of physical landscapes. His primary research interest is on the impacts of extreme floods and environmental change (especially glacier retreat) on rivers in glaciated environments, particularly in Iceland. He has been carrying out research in these environments since 1996, and this continues to be his main research focus to the present day. Postdoctoral work in South Africa added a new dimension to his research, focusing on the geomorphology of low-energy meandering and anabranching rivers in dryland environments. This work on dryland, low-energy rivers continued when he was based in Australia, where he also investigated landscape change in catchments impacted by extreme floods, bushfires, and extensive anthropogenic modification. More recently, he has investigated the effectiveness and longevity of natural flood management interventions in the UK.