ABSTRACTTunnel erosion has been noted in many areas, but few attempts have been made to measure development rates or to assess geomorphic or hydrologic significance. This paper reports measurements of two tunnel systems in the Dinosaur Badlands, Alberta, carried out as part of a major study of water and sediment budgets. These were instrumented and monitored through summer storm events and detailed water, sediment and solute discharge patterns were established. Tunnel hydrographs were similar to those for surface sheetwash and channel flow with short lag times and rapid recession. Total discharges were low compared with the main channels, but water yields per unit area drained were similar. Row rates indicated contributions to channel storm hydrographs. The results, together with an inventory of tunnel inlets and outlets, suggest that about 10 per cent of the total water flow in the catchment may be routed through tunnel systems. Suspended sediment concentrations were high, reaching 97.1 g 1-1in a moderate rainstorm, substantially above peak channel concentrations. Minimal estimates of total sediment yield during a single storm event ranged up to 0.42 kg m-2, equivalent to a denudation rate of about 0.3 mm. Solute concentrations were also higher than channel flow with peak electrical conductivities reaching 1200μS cm-1at 25°C. The solute load was dominated by sodium, reflecting high concentrations found in bordering lithologic units.A substantial portion of the catchment discharge passes through tunnels but a clear distinction cannot be made between surface and subsurface contributions in the storm hydrograph. Nevertheless solute and sediment yields indicate that tunnel erosion is a significant geomorphic process in this area.