Geology of the Lagoons, the Barra and the Beach

Lagoons are formed when water from rivers and streams flows towards the ocean, washing sand and sediment with it. As the streams hit the sea, the momentum of the water slows and the sand and sediment settle to the bottom, forming shallows and eventually sand banks and beaches. In some places, particularly where the land is flat and the build up of sand is great, the ocean currents and winds build a network of sand dunes and bars near the shore that block the fresh water from reaching the sea. The resulting backup is a lagoon.

Since this area normally has a rainy season from July until October, those are the months when fresh water from rivers builds behind the beach dunes raising the water level in the lagoon. Eventually the level builds to overflowing and the water erupts and gouges a channel through the sandbar to the sea where the raging outflow dumps tens of thousands of tons of sand that has been washed down from the mountains. This provides the sand that forms Playa Blanca and the beach is replenished every year by this process. Once the lagoons and the ocean have reconnected, there is a twice-daily tidal flow that brings sea water into the lagoon and then, as the tide turns, washes it and fresh water into the sea. This tidal action stimulates enormous amounts of sea life. Small crabs, shrimps and other creatures are washed from the lagoon into the ocean and the hungry mouths of waiting fish. The inflowing saltwater cleans and purges buildups of sediment and debris in the lagoon. After several months of tidal flow, currents and winds cause the beach sand to reseal and a sand bar separates the lagoon from the ocean once again. The entire ecosystem relies on this seasonal pattern.

The lagoon at Barra de Potosi is one of the best places to see this phenomenon. The village is named for the sand bar or “barra” that seasonally shuts off the flow from the lagoon to the sea. The lagoon itself is fed by three rivers, and from late summer until late winter, the channel between the lagoon and the sea is normally open. After the rains end, the fresh water flow stops, the sand rebuilds the barra and the channel closes until the next rainy season.