It has been widely accepted that channel geometry on alluvial fans is predominantly controlled by upstream boundary conditions and remains stable if those conditions do not change. This study challenges that notion by examining how channel width on alluvial fans varies in space and time under constant upstream conditions. Experiments using a sediment mixture (sand and crushed walnut) with constant sediment and water discharge rates reveal distinct patterns in channel width. In lower water discharge runs, total channel width decreases over time, while in higher discharge runs, it increases. Theoretical principles suggest that channel width is inversely proportional to grain size. Sand results in a wider channel width compared to walnut sediment, making its proportion along the channel a key factor in controlling the average channel width. The disproportional advancement of sand reaches relative to the fan margin, which varies across different discharge runs, drives changes in slope and channel width over time. This study highlights that surface grain-size distribution can change as the fan grows, even when external conditions remain constant, leading to variations in channel geometry. Sedimentary records often attribute signal changes to upstream boundary variations. However, our findings highlight the critical role of intrinsic fan width dynamics, underscoring the need to consider this factor in fan evolution studies.