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[1] Alluvial river channels are intrinsically mobile. We mapped channel planform extent in a series of experiments to measure instantaneous rates of channel motion, loss of planform overlap with the original positions of the channels, and reworking of the fluvial surface over which the channels moved. These experiments comprise two aggrading deltas, one subsiding delta that underwent cyclical base level changes, and one braided channel system that was seeded with vegetation. We find that the amounts of channel planform overlap and remaining unreworked fluvial surface area both decay exponentially with time, and that these metrics and the instantaneously-measured rates of channel motion scale predictably with one another in spite of the different time scales of the processes they record. Rates of channel planform change increase with increasing sediment flux and bed and planform irregularity, and decrease with the establishment of riparian vegetation. Aggradation does not noticeably affect channel mobility, but induces avulsions that allow the channels to more rapidly rework the fluvial surface. Additional findings include that: (1) sediment flux in the braided experiment equals its rate of bar migration, (2) channel widths are normally distributed with time, and (3) we can use our channel mobility metrics to connect surface processes with the resultant fluvial stratigraphy.

[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.Citation: Martin, J., B. Sheets, C. Paola, and D. Hoyal (2009), Influence of steady base-level rise on channel mobility, shoreline migration, and scaling properties of a cohesive experimental delta,

Sequence stratigraphy has been applied from reservoir to continental scales, providing a scale-independent model for predicting the spatial arrangement of depositional elements. We examine experimental strata deposited in the Experimental EarthScape facility at St. Anthony Falls Laboratory, focusing on stratigraphic surfaces defined by discordant contact geometries, surfaces analogous to those delineated in the original work on seismic sequence stratigraphy. In this controlled setting, we directly evaluate critical sequence-stratigraphic issues, such as stratigraphic horizon development and time significance, as well as the internal geometry and migration of the bounded strata against the known boundary conditions and depositional history. Four key stratigraphic disconformities defined by marine downlap, marine onlap, fluvial erosion, and fluvial onlap are mapped and vary greatly in their relative degree of time transgression. Marine onlap and downlap contacts closely parallel topographic surfaces (time surfaces) and, prior to burial, approximate the instantaneous offshore topography. These stratalbounding surfaces are also robust stratigraphic signals of relative base-level fall and rise, respectively. Marine onlap surfaces are of special interest. They tend to be the best preserved discordance, where widespread, allogenic-based onlap surfaces subdivide otherwise amalgamated depositional cycles amidst cryptic stacks of marine foresets; however, local, autogenic-based marine

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[1] Under projected scenarios of sea-level rise, subsidence, and sediment starvation many deltas around the world are expected to drown. Delta growth dynamics, which determine the ability of a delta to adapt to these changes, are poorly understood due to the difficulty of measuring change in slowly evolving landscapes. We use timeseries imagery of experimental, numerical, and field-scale deltas to derive four laws that govern the growth of riverdominated deltas. Land area grows at a constant rate in the absence of relative sea level change, while wetted area keeps pace, maintaining a constant wetted fraction over the delta surface. Scaling of edge-lengths versus areas suggests delta shorelines are nonfractal, even though the channel network is fractal. Consequently channel-edge length, which provides critical habitat, grows more rapidly than delta area. These laws provide a blueprint for delta growth that will aid in delta restoration and help predict how existing deltas will evolve. Citation: Wolinsky, M.

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