Through measurements at Semeru Volcano, East Java, we define the conditions under which bulking (entrainment of sediment and pore water) and debulking (dilution and sedimentation) occur in rain-triggered volcanic floods (lahars). Two observation sites were installed 510 m apart, along the Curah Lengkong River, 11.5 km southeast of Semeru's summit. This 30-m-wide box valley, with a gravel and lava base, represents a real-world flume analogy. Pore-pressure sensors provided stage measurements, a broad-band seismograph gave insight into sediment content and frictional-collisional behavior, video cameras were used to measure surface velocities, and direct bucket samples were taken. Eight rainfall-induced lahars were recorded, lasting 1–3 h with heights of 0.5–2 m, peak velocities of 3–7 m/s, and discharges of 25–250 m3/s. Flows ranged from typical (<40 wt% sediment) to coarse and dense hyperconcentrated flows (50–60 wt% sediment). Multiple distinct flow “packets” occurred within the complex lahars, and were used to determine internal changes between sites. From the multiparameter data set at each site, volumetric bulking and wave shortening, due to portions of the lahar accelerating toward the flow front, are identified. Initial debulking of lahars between sites may reflect drainage into the dry substrate. Estimates of discharge and volume at each site lead to the quantification of bulking and debulking by these actively flowing lahars along the channel reach. From this, we observe that bulking can be localized to certain parts of lahars, resulting in intraevent increases in peak discharge that are greater than what would occur if bulking was evenly distributed throughout the flow. Such data are essential for the development of numerical descriptions and hazard models for mass flows.