How big is a boulder? Evaluating fixed and process-based definitions for boulders

Richard Mason , Lina E. Polvi

Boulders are globally widespread and influence landscape processes across hillslopes, coasts, rivers and extra-terrestrial settings. Boulders are described as particles, sufficiently large, that the movement of an individual grain promotes substantial geomorphic change. Moving beyond this conceptual definition, however, requires a somewhat arbitrary decision of how to define a minimum boulder size. Furthermore, the implications of boulder definition on study findings are rarely considered. We compare two lower thresholds for boulder size; a fixed boulder minimum diameter (> 1 m) and a variable diameter relative to the surface grain size distribution (> 84th percentile). We consider the impact of definition on measured boulder metrics, and their association with channel and catchment characteristics across 20 boulder-bed streams in northern Sweden. We also surveyed the river managers responsible for restoring these rivers, to gain a practitioner insight on boulder size definition. Definition choice resulted in fundamental differences in boulder metrics; metrics describing the number or density of boulders were negatively correlated. Using these two studies, we explore boulder definition in earth sciences, including the application of fixed definitions and those relative to grain size or system power. We emphasise the importance of evaluating the implications of the chosen boulder size definition, and communicating the reasoning behind boulder definitions and these implications. We discuss the implication of boulder size definition choice and provide guidelines for future studies seeking a process-based definition of boulders.