Mudflats are often dissected by nearly-parallel runnels separated by ridges. High resolution data of tidal velocity and sediment concentration measured in a mudflat runnel in Willapa Bay, Washington State, USA, indicate that very shallow flows draining the mudflat platform are concentrated in the runnels. These flows, with water depths of few tens of centimeters, are characterized by velocities close to the flood and ebb maxima associated to the regional flow when the mudflat is submerged. The corresponding shear stresses are higher than the critical stress for erosion, thus suggesting possible remobilization of surface sediments. Moreover, suspended sediment concentrations in the runnel are greater than those associated with the regional tidal flow, and comparable to concentrations during energetic wave conditions. All this evidence indicates that shallow flows in runnels are important morphodynamic agents producing noteworthy geomorphic work.