Alluvial fans are cone-shaped, soil-rich geomorphological features common to mountainous regions where they offer relatively convenient, though frequently hazardous, settings for habitation and infrastructure. Debris flows (massive, muddy, rocky flows) contribute significantly to these hazards as they carry focused, collisional, fast-moving materials across alluvial fans, unpredictably in size, speed, and direction. We research how particle sizes contribute to flow and depositional dynamics toward greater predictive capabilities, particularly needed under climatic conditions of increasing high-magnitude storm events. Toward this, we analyzed field data from two neighboring alluvial fans in the White Mountains (California, USA) that exhibit dramatically different topographies despite their proximity and associated similar long-term climates. Informed by these measurements, we performed long-term and incremental alluvial fan experiments using debris-flows whose contents we varied systematically. We found (1) increasing fine particle content correlates with diversity of fan slopes and associated channelization dynamics; (2) long-term alluvial fan experiments form more uniformly complex surface channelizations than repeated flows for the same total time, indicating the importance of both particle sizes and timescales on alluvial fan dynamics and associated hazards.