Mud bedforms in a natural ice flume

An ephemeral, ice-based flume developed in Medway Creek (London, Ontario) during a February thaw when water at ~1C flowed over the ice surface forming a < 10 cm deep, ~ 3 m wide channel. Eroded muddy bank sediment, composed of silt to medium-sand sized aggregates, formed linear streamers that revealed streaks in the boundary layer. In water 6-8 cm deep with a flow velocity of ~ 8-12 cm/s, mud aggregates were molded into lunate, transverse, and ovoid ripples a few mm high. Clear water allowed mud aggregates in streamers to be observed accreting to, and migrating over mud ripples. Downstream of larger ripples, mud streamers were swept clear of the bed, perhaps due to vortices shed by the ripple. Where flow exceeded ~ 12 cm/s, mud ripples were gradually replaced by mud aggregate streamers which in turn were washed out in an area of faster (undetermined rate) flow. The flow conditions and bedforms in this ephemeral, natural flume are closely comparable to those described from laboratory flumes at 25C; however the increased viscosity of water at 1C may alter the stability field of mud ripples.