Distinguishing single-thread and braided channel patterns and reconstructing paleo-water discharge from the sedimentary rock record has proven to be difficult. This is because only remnants of the river channels are preserved, often accounting for a small fraction of the overall stratigraphy. Instead, channel belt deposits—the amalgamation of deposits from many individual channels—are observed more often. Identifying channel patterns from geologic records is important for testing a range of hypotheses, including whether single-thread rivers were rare prior to the arrival of land plants in the Silurian. Here, we develop new quantitative metrics using distributions of channel geometry and channel belt properties with the ultimate goal of distinguishing channel types in the rock record. Metrics measured include width, sinuosity, radius of curvature, wavelength, and amplitude, and are measured from modern fluvial systems in the US via remote sensing techniques. An entropy-based braided index is used to quantify river branch number, which is directly related to a conventional braided index. Preliminary finding suggest that channel belt width and wavelength are well correlated with river branch number and water discharge via power-law relations, respectively. Specifically, mean river branch number is inversely related to channel belt wavelength, normalized by mean total channel width. In addition, mean channel belt width is directly related to water discharge at 2% exceedance flow event. This could imply that extreme flow events may contribute significantly to channel belt formation, more so than typical bankfull flow events (5–25% exceedance flows) based on threshold channel theory. Analyses presented here will aid improved interpretation of sedimentary deposits on Earth and other planets to infer past surface conditions.

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.