Stream obstacles, naturally formed like boulders or engineered like weirs, are the major source of flow resistance; however, to quantify their flow resistance, a resistance formula needs to be selected in accordance with the specific obstacle type, i.e. obstacle type dependency. So far, whether a unified resistance formula that adequately characterizes the roughness of distinctive obstacle types is elusive. Here, we conduct flume experiments with various natural and engineered submerged obstacles, including boulders, weirs, log jams, and transverse stones. We combine them with existing datasets containing rigid vegetation, step-pool, and riffle-pool to identify a unified metric for a general resistance relation. We test three roughness metrics, the widely used roughness metrics D84 (84th percentile of bed grain size distribution), a bathymetric-line-based metric σz,centerline (the standard deviation of bed centerline elevation), and the new metric σz,bed (the standard deviation of elevation of the entire bed) as bed roughness, respectively. σz,bed is proposed to incorporate the roughness inhomogeneity in the transverse direction which widely exists in both natural and engineered channels, complementing the insufficiency of line-based metric σz,centerline. We show that the resistance equation based on σz,bed demonstrated a more consistent and superior velocity prediction capacity than D84 and σz,centerline throughout almost all types of obstacles. Interestingly, when applied to vegetated channels, the resistance formula based on only σz,bed can compare with those based on multiple parameters related to vegetation characteristics. This study shows the viability of unifying the flow resistance formula in open-channels with submerged obstacles, avoiding obstacle-type dependency.

Delta shoreline structure has long been hypothesized to encode information on the relative influence of fluvial, wave, and tidal processes on delta formation and evolution. We introduce here a novel multiscale characterization of shorelines by defining three process-informed morphological metrics. We show that this characterization yields self-emerging classes of morphologically similar deltas, i.e., delta morphotypes, and also predicts the dominant forcing of each morphotype. Then we show that the dominant forcings inferred from shoreline structure generally align with those estimated via relative sediment fluxes, while positing that misalignments arise from spatiotemporal heterogeneity in deltaic sediment fluxes not captured in their estimates. The proposed framework for shoreline characterization advances our quantitative understanding of how shoreline features reflect delta forcings, and may aid in deciphering paleoclimate from images of ancient deposits and projecting delta morphologic response to changes in sediment fluxes.

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.