Morphodynamics of a gravel-dominated macrotidal estuary: Rio Grande, Tierra del Fuego

Abstract

Rio Grande city (Tierra del Fuego) is located on two attached beach systems, one of Upper Pleistocene (Sangamonian) and the other of Holocene age. Both gravel spits grew from north to south modifying the inlet of the Rio Grande estuary. The present estuary is constrained by the modern and recurved spit Popper Spit. The main characteristic of this macrotidal estuary is that both margins and the bottom are mainly composed of rounded gravel. Expansion of the city is limited by oceanic and estuarine coasts, and is taking place towards salt marshes taking up more than 30 hectares in the last 20 years. The alteration of the tidal prism induced by marsh reclamation and the construction of a bridge may be affecting the inlet dynamics. The area of salt marsh and gravel banks were calculated by means of supervised classifications derived from a Landsat TM image. The inlet morphology changes in response to cycles dominated by longshore drift, wave refraction and ebb-tidal delta configuration. Oceanic beaches are characterised by large disc-shape boulders at the storm berm, spherical pebbles and sand runs at the foreshore, and fine sand on the low-tide terrace. Although tidal effects are very significant in the dynamics of the estuary, wind can prevail during some days or during slack water.