Signatures of glaciation on river channel long profiles: changes in slope and concavity

Abstract:

Glaciers in river headwaters supply water and sediment to rivers, and therefore drive fluvial aggradation, incision, and channel long profile evolution. In contrast, water and sediment inputs to unglaciated rivers are distributed throughout the catchment via precipitation and hillslope processes. As a result, both sediment and water supply in unglaciated drainage basins increase with drainage-basin area, though the former can be attenuated by weathering and downstream fining. This increase in water discharge with drainage-basin area underpins the drainage-area ("m") exponent in the stream-power law, which produces concave detachment-limited river long profiles. Likewise, systematically varying water and sediment supply in transport-limited rivers can produce characteristic channel slopes and concavities following Lane's Balance. However, for headwater glaciated rivers, glacial inputs are not spatially distributed. If this glacial signal dominates over precipitation and hillslope sediment inputs, then increasing water input moving downstream should be negligible, as most water will be received directly from the headwaters. If this is true, then river slope should evolve independently of drainage area and, assuming uniform erodibility, be nearly constant and result in minimal channel concavity.

We analyze the channel long profiles of rivers that have experienced headwater glaciation to evaluate how glaciation has directly shaped their slopes, concavities, and long-profile morphologies. We have completed preliminary work on the Santa Cruz River in southern Patagonia, Argentina, which is actively receiving meltwater from the Southern Patagonian Icefield, and the Zumbro River, in Minnesota, USA, which was glaciated during the Last Glacial Maximum. Profiles for both of these rivers are close to linear, in line with our hypothesis. Our full analysis will provide a systematic understanding of the impact of glaciation on channel long profile evolution and improve our understanding of how glaciation, both past and current, shapes fluvial systems.