In this talk, I aim to convince you that:
- The size of distribution of sediment supplied by hillslopes to rivers is a key control on river morphodynamics, from channel response to human land use, to landscape response to changes in tectonic and climatic forcing.
- Predicting hillslope sediment size distributions is a major unsolved problem in geomorphology, requiring advances in our understanding of how lithology, climate, life, erosion rate, and topography determine the suite of geomorphic processes that produce, weather and transport sediments on hillslopes.
- New theory offers a path forward for understanding the physical and chemical mechanisms that determine the initial size distribution of rock fragments supplied to the hillslope weathering system, and the duration and intensity of weathering experienced by particles on their journey from bed- rock to the channel.
- New field data suggest that hillslope particle size may vary systematically with climate and topography for a given rock type, and that the initial size produced from bedrock is set by the spacing of pre-existing fractures.
- Incorporating spatial variation in hillslope particle size into landscape evolu- tion models offers a stronger mechanistic basis for exploring landscape response to climatic, tectonic and lithologic boundary conditions.