Geomorphology shapes the way we perceive the world around us, making certain ways of understanding and interacting with landscapes more possible than others. Despite this, geomorphologists have a reputation for not engaging with philosophical questions regarding their work, particularly the role of theory, framing and language. Recent calls for a critical physical geography present significant opportunities for physical geographers to engage with questions that often go unasked, and unanswered, in the Earth sciences. Here we discuss what a critical physical geography might bring to geomorphology, examining the implications of what we measure in our efforts to classify and understand river form and process. While geomorphology benefits from using the approaches and methods of science, it struggles with the fundamental problem of closure. Through the example of river diversity we explore how our decisions, tools and knowledge frameworks shape environmental outcomes. We challenge the assumption that once a landscape is measured it is ‘known’, and argue for geomorphologists to actively explore alternative ways of knowing the landscape towards a discipline which is both more just and more scientific. We imagine an inclusive critical physical geography, drawing on disparate theoretical approaches to constructively critique the practice of geomorphology. We argue that this engagement will be most productive if it is framed in a readily accessible manner. Negotiating such a project is undoubtedly challenging, but there is more to be gained by building bridges over the science/humanities divide than by shouting across it.