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Satellite remote sensing imagery and numerical modelling were used for thestudy of river bulge evolution and dynamics in a non-tidal sea, the Gulf ofRiga (GoR) in the Baltic Sea. Total suspended matter (TSM) images showed aclearly formed anti-cyclonically rotating river bulge from Daugava Riverdischarge during the studied low wind period. In about 7–8 days the bulgegrew up to 20 km in diameter, before being diluted. A high-resolution(horizontal grid step of 125 m) General Estuarine Transport Model (GETM) wasused for detailed description of the development of the river plume in thesouthern GoR over the period when satellite images were acquired. In themodel simulation, the bulge growth rate was estimated as rb ∼ t0.5 ± 0.04 (R2 = 0.90). Both the model simulation and the satelliteimages showed that river water was mainly contained in the bulge and therewere numerous intrusions at the outer perimeter of the bulge. We performednumerical sensitivity tests with actual bathymetry and measured river runoffwithout wind forcing (1) having an initial three-dimensional densitydistribution, and (2) using initially a homogeneous ambient density field. Inthe first case, the anti-cyclonic bulge did not develop within the course ofthe model simulation and the coastal current was kept offshore due to ambientdensity-driven circulation. In the second case, the river plume developedsteadily into an anti-cyclonically recirculating bulge, with rb ∼ t0.28 ± 0.01 (R2 = 0.98), and a coastal current. Additionalsimulations with constant cross-shore and alongshore winds showed asignificant effect of the wind in the evolution of the river bulge, even ifthe wind speed was moderate (3–4 m s−1). While previous studiesconclude that the mid-field bulge region is governed by a balance betweencentrifugal, Coriolis and pressure gradient terms, our study showed thatgeostrophic balance is valid for the entire mid-field of the bulge, exceptduring the 1–1.5 rotation period at the beginning of the bulge formation. Inaddition, while there is discharge into the homogenous GoR in the case of ahigh inflow Rossby number, the river inflow might split into two jets, withstrong mixing zone in-between, in the plume near-field region

River bulge evolution and dynamics in a non-tidal sea – Daugava River plume in the Gulf of Riga, Baltic Sea