We use in situ measurements of suspended mud to assess the flocculation state of the lowermost freshwater reaches of the Mississippi River. The goal of the study was to assess the flocculation state of the mud in the absence of seawater, the spatial distribution of floc sizes within the river, and to look for seasonal differences between summer and winter. The data was also used to examine whether measured floc sizes could explain observed vertical distributions of suspended sediment concentration through a Rouse profile analysis. The surveys were conducted at the same location during summer and winter at similar discharges and suspended sediment concentrations, and in situ measures of the size distribution of the mud over the longitudinal, transverse, and vertical directions within the river were obtained using a specially developed underwater imaging system. These novel observations show that mud in the Mississippi is flocculated with median floc sizes ranging from 50 to 200 microns depending on location and season. On average flocs were found to be 40 microns larger during summer than in winter and to slightly increase in size moving downriver from the Bonnet Carré Spillway to Venice, LA. Floc size statistics varied little over the depth or laterally across the river at a given station. Bulk settling velocities calculated from size measurements matched values obtained from a Rouse profile analysis at stations with sandy beds, but underestimated settling velocities using the same equation parameters for measurements made during winter over muddy beds.

River delta avulsions are a primary mechanism to distribute sediment and build coastal land. Experiments show that an avulsion can generate a new delta lobe, and subsequent avulsions yield multiple lobes that amalgamate to produce a semi-circular fan deposit. For channels that are actively building lobes, a condition of sediment transport equilibrium develops, termed alluvial grade, which is characterized by material bypassing the delta topset and dispersing to the delta foreset. Previous studies have examined alluvial grade under conditions of steady subsidence and uniform basin depth. However, on tectonically active margins, deltas are affected by punctuated subsidence and lobes prograde into basins with variable depth. Both conditions disrupt alluvial grade, which in turn affects avulsion timescales and thus delta morphology. We explore these interrelated processes using measurements of delta and basin morphology based on field surveys and remote sensing collected from the Selenga Delta, which is located along the Baikal Rift Zone. Major earthquakes, affiliated with normal faulting and possessing recurrence intervals of several millennia, lower large portions of the subaerial delta several meters below mean lake level. This results in an increased regional gradient that triggers lobe-scale avulsions. Moreover, the timescale for these events is shorter than that predicted via autogenic lobe switching. Additionally, during periods tectonic quiescence, smaller channel-scale avulsions occur every 10--90 yrs, which produces sedimentation that compensationally fills embayments located between distributary channels. This process gives rise to the delta's fan-shape morphology. Stratigraphically, tectonically driven subsidence events are expected to preserve discrete sedimentary units that represent deposition and reworking associated with short-term channel avulsions. Understanding the interplay between discrete, tectonically driven subsidence events and autogenic sediment accumulation patterns of a delta prograding into a tectonically active basin will improve interpretations of stratigraphy of ancient systems.

Channel avulsions on river deltas are the primary means to distribute sediment and build land at the coastline. Many studies have detailed how avulsions generate delta lobes, whereby multiple lobes amalgamate to form a fan-shaped deposit. Physical experiments demonstrated that a condition of sediment transport equilibrium can develop on the topset, characterized by neither deposition nor erosion of sediment, and material is dispersed to the foreset. This alluvial grade condition assumes steady subsidence and uniform basin depth. In nature, however, alluvial grade is disrupted by variable subsidence, and progradation of lobes into basins with variable depth: conditions that are prevalent for tectonically active margins. We explore sediment dispersal and deposition patterns across scales using measurements of delta and basin morphology compiled from field surveys and remote sensing, collected over 150 years, from the Selenga Delta (Baikal Rift Zone), Russia. Tectonic subsidence events, associated with earthquakes on normal faults crossing the delta, displace portions of the topset several meters below mean lake level. This allogenic process increases regional river gradient and triggers lobe-switching avulsions. The timescale for these episodes is shorter than the predicted autogenic lobe avulsion timescale. During quiescent periods between subsidence events, channel-scale avulsions occur relatively frequently because of in-channel sediment aggradation, dispersing sediment to regional lows of the delta. The hierarchical avulsion processes, arise for the Selenga Delta, preserves discrete stratal packages that contain predominately deep channels. Exploring the interplay between discrete subsidence and sediment accumulation patterns will improve interpretations of stratigraphy from active margins and basin models.