Ice covered delta dynamics

Tuesday, December 10, 2019 - 13:40
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River delta dynamics and depositional patterns are strongly influenced by a suite of upstream and downstream boundary conditions. In the Arctic, ice and permafrost also play a large role in governing delta dynamics. Here, we present results from remote sensing analyses and numerical modeling of Arctic deltas aimed at characterizing Arctic delta morphologies and understanding how ice influences the form and function of river deltas. We show that thick ice cover limits channel mobility and encourages preservation of channels by limiting in-channel sedimentation. Ice also enhances overbank flooding and aggradation, as well as incision of channels under the ice that transport more sediment offshore. In natural systems, these effects may result in a shift in the population of island sizes towards smaller islands, owing to the preservation of small channels in ice-covered environments. Deltas with higher channel density tend to be more resilient to sea level rise, as the small land-channel distances make for increased overbank sediment delivery to all areas. Thus, ice cover may have an amplified effect in that it both encourages overbank flooding directly by increasing water surface elevations upstream of ice and helps maintain higher channel density that increases ease of aggradation on delta islands. However, this also indicates that decreased ice thickness in a warming Arctic is likely to have a drastic effect on delta aggradation, making Arctic deltas especially vulnerable to rising sea levels. Finally, we show that Arctic deltas are characterized by an uneven distribution of channels nearshore, such that they convey riverine fluxes to only a small portion of their coastlines. We suggest that ice cover is responsible for the large variability in channel size due to a feedback between channel size, river discharge, and ice retreat that maintains small channels and widens large channels over long timescales. These results indicate that ice cover affects delta morphology and dynamics that influence the transport of water, sediment, nutrients, and heat, as well as delta resilience in a warming Arctic.

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