Anchor ice formation and release in small regulated and unregulated streams

Anchor ice is a type of river ice that occurs on river beds in supercooled, turbulent water. Its formation and release can affect the water level, discharge, bed roughness, and morphology of rivers in cold regions. Despite these important effects, the number of anchor ice events documented in the literature has historically remained small because of the ephemeral nature of anchor ice. The purpose of this study was to determine the hydrometeorological factors that control anchor ice formation, release, and morphology, as well as to determine the impact of hydropower regulation on the anchor ice regime of small streams. In total, 161 anchor ice formation and release events were examined in 2 regulated streams and 1 unregulated stream in north-central New Brunswick, Canada, using hourly or half-hourly ice observations and near-continuously measured environmental variables. The day after formation, anchor ice accumulations either completely released, stayed in place to form multi-day accumulations, or were incorporated into the surface ice cover of the stream. 98% of anchor ice accumulations completely released on days when there was a net heat gain to the water surface and the air temperature was >− 15 °C, indicating a strong thermal control on anchor ice release. The release of one accumulation could not be attributed to either thermal effects or to the 'plucking' of the substrate, suggesting the need for further study of the strength of ice-pebble bonds. Finally, the regulated and unregulated streams had different ice regimes: the regulated stream experienced a greater number of anchor ice events, which occurred with a different seasonal pattern compared to the unregulated streams.