We use meteoric 10Be measured in 24 fluvial sand samples collected along the mainstem and from prominent tributaries within the tectonically active Waipaoa River basin, New Zealand, to identify sediment sources and monitor the mixing of sediment as it travels from headwater basins to the sea. Deforestation for agriculture beginning in the early 1900s resulted in severe, but nonuniformly distributed erosion. Tributaries in the northern headwaters, where large amphitheater gullies that continually feed prodigious amounts of deeply sourced sediment to the mainstem are prevalent, yield exceptionally low concentrations of meteoric 10Be (∼1.5 × 106 atoms g−1). In the more stable eastern and western tributaries, concentrations of meteoric 10Be are nearly an order of magnitude greater (∼14 × 106 atoms g−1). Meteoric 10Be concentrations in samples collected along the mainstem above and below tributary confluences steadily and regularly increase downstream (R2 = 0.92) as large amounts of low-concentration gully-derived sediments are augmented with higher concentration sediment from more stable tributaries, providing strong evidence that meteoric 10Be concentrations reflect sediment sourcing in this fluvial network. A two-component mixing model indicates that the gullied northern region of the Waipaoa basin produces sediment at a rate ∼20 times that of the eastern and western regions. These results suggest that meteoric 10Be, in contrast to the widely applied in situ technique that is limited by the availability and distribution of quartz, is an effective tool for the rapid assessment of sediment dynamics and movement within a wide range of fluvial networks.