Coastal erosion in river deltas is often caused by rising sea levels and human activities. The Parnaíba River Delta in Brazil is a natural delta still forming without much human interference. By using satellite images, researchers found that the river supplies sediment to the coast, helping stabilize and expand the delta. Sediment from the river is carried along the coast by ocean currents. The river's discharge pushes sediment offshore during low tide and towards the coast during high tide, creating a barrier that prevents sediment from moving along the shoreline. The eastern part of the delta is growing, while the western part is eroding due to sediment transport from the river mouth.