Influence of steady base‐level rise on channel mobility, shoreline migration, and scaling properties of a cohesive experimental delta

[1] Here we describe results from an alluvial delta physical experiment with and without steady base-level rise. Using a unique cohesive sediment mixture that promotes the development of persistent channels and a rugose shoreline, we quantitatively characterize channel network properties as a function of base-level rise in a distributary system that reproduces many aspects of the geometry of natural deltas. Analysis of the experimental data shows clear dynamical differences between the predominantly progadational and aggradational phases of the experiment. The experiment was conducted in two phases: a first in which the delta prograded into standing water of constant depth in the absence of base-level rise and a second during which steady base-level rise was imposed on the system, forcing a twofold increase in topset aggradation due to greater sediment retention in the fluvial reach. The shift in sediment mass balance to enhanced fluvial deposition in the second phase caused channel network mobility to increase, reducing the autogenic channel time scale from 23.5 to 12.5 h and supporting a positive correlation between deposition and channel avulsion frequency. An independent shoreline time scale that characterizes the dominant time over which shoreline regression is persistent closely correlates with measurements of the channel network (28.3 h during fan progradation and 9.5 h during fan aggradation). These metrics suggest a strong coupling between channel network and shoreline kinematics and a more active fluvial surface during fan aggradation by a factor of 2 to 3, similar to the increase in aggradation rate. Spatial scaling of shoreline roughness reveals that maximum shoreline variability is set by the scale of distributary lobes. Strong coupling exists between delta growth and shoreline geometry during progradation. During aggradation, however, shoreline variability is not solely due to distributary lobe growth but is also set by shoreline transgression over inactive portions of the delta, illustrating a decoupling between fan kinematics and fan geometry. This decoupling, together with the matched increase in channel network mobility and fluvial aggradation, suggest the stratigraphic architecture may not be a strong geometric signal of different fluvial surface conditions either.