ICE-RELATED LONGITUDINAL AND LATERAL CONNECTION AND DISCONNECTION OF RIVERS AND FLOODPLAINS, SOME EXAMPLES FROM THE NIOBRARA AND BELLE FOURCHE RIVERS: GREAT PLAINS, UNITED STATES

In North America, ice is a powerful fluvial geomorphic agent in rivers north of the 40^th parallel. Fluvial geomorphic processes commonly associated with ice include changes in river stage, velocities, and sediment concentration per unit discharge, jam-induced backwater flooding, jam-breakup flood pulses, bank erosion, bed scour, floodplain deposition, and riparian vegetation removal. We present field data and observations from two ice-affected Great Plains Rivers, the Niobrara River in Nebraska, and the Belle Fourche River in South Dakota. In both rivers, ice is a primary driver of floodplain connection and disconnection. Intermittent floodplain connections result from jam-induced backwater flooding. This seasonal phenomenon eventually results in long-term, multi-reach floodplain disconnection through deposition at higher stages per unit discharge, lower-level floodplain destruction, and/or channel incision. In the Niobrara River, ice-jam induced flooding has also caused longitudinal disconnections of long river segments by triggering bedrock knickpoint incision and propagation, forming waterfalls and rapids which may persist for decades. Lateral disconnection has occurred through ice-induced channel avulsion, plugging of side channels, and channel cutoffs. These modern ice effects are also apparent in the continuum of fluvial landform elevations and the internal stratigraphy of floodplain deposits of both rivers. In the Belle Fouche River, these processes complicate the dynamics of a river system that has experienced large discharges of mine-tailings for nearly 100 years.