River Delta Morphotypes Emerge From Multiscale Characterization of Shorelines.

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, that is, 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. Plain Language Summary: It has long been hypothesized that delta morphology and its evolution are primarily dictated by the interplay of three primary forcings: riverine, wave, and tidal. The rich diversity of delta morphologies observed across the globe is attributed to the relative dominance of those forcings. This study aims to retrieve the forcing information archived in delta morphology, specifically in the delta's shoreline, to establish a morphologic classification of deltas. To accomplish that, we propose a set of morphologic metrics targeting the specific locations and scales of the shoreline reworked by riverine, wave, and tidal processes. We show that the projection of the complex shoreline geometries into the low‐dimensional space defined by the proposed morphologic metrics leads to clusters of deltas with similar features, denoted here as morphotypes. Guided by the values of the metrics, we can infer the dominant forcings underlying each morphotype, which generally align with those obtained by estimating the sediment fluxes transported by each forcing. Misalignments are also observed and are mostly due to space‐time heterogeneities and uncertainties in the quantified sediment fluxes. Our methodology relies on readily‐available remote sensing images and is shown to have low sensitivity to its defining parameters, offering insight into predicting deltaic geomorphic response to changing forcings. Key Points: Three process‐informed, geometric, and spectral metrics are introduced to characterize multiscale shoreline featuresUnsupervised clustering of the shoreline metrics reveals five distinct delta morphotypes which correspond to distinct forcingsMorphotypes can be robustly estimated from readily available satellite imagery [ABSTRACT FROM AUTHOR]