The rapid advance of remote sensing technology during the last few decades provides a new opportunity for measuring detectable estuarine spatial change. Although estuarine surface area and convergence are important hydraulic parameters often used to predict long-term estuarine evolution, the majority of automated analyses of channel plan view dynamics have been specifically written for riverine systems and have limited applicability to most of the estuaries in the world. This study presents MorphEst, a MATLAB-based collection of analysis tools that automatically measure estuarine planform geometry. MorphEst uses channel masks to extract estuarine length, convergence length, estuarine shape, and areal gain and loss of estuarine surface area due to natural or human factors. Comparisons indicated that MorphEst estimates closely matched with independent measurements of estuarine surface area (r = 0.99) and channel width (r = 0.92) of 39 estuaries along the South Korean coast. Overall, this toolbox will help to improve the ability to solve research questions commonly associated with estuarine evolution as it introduces a tool to automatically measure planform geometric features from remotely sensed imagery.

Estuaries are regions where rivers flow into the coastal ocean. The physical functioning of estuaries varies greatly and depends on the estuary size, shape, sedimentary characteristics, and external physical forcing by the river, tides, waves, and wind (Wolanski & Elliot, 2016). The net result of these features is the overall water circulation pattern. Sediments are important in estuaries because sediment flux gradients contribute to the depth and texture of the bed and the overall turbidity of the water column which in turn affects estuarine fisheries and navigability. At present, sediment fluxes are changing in estuaries, as humans have modified the shape of estuaries worldwide through activities such as land reclamation, dredging, and the construction of hard engineering structures (Jung et al., 2021;Wang et al., 2015). Land reclamation and dredging are often carried out to create land area and improve navigability, respectively (Winterwerp & Wang, 2013), while hard engineering structures such as breakwaters, riprap, jetties, and seawalls have been designed to absorb wave energy and stabilize the coast (Davis &

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