#gulf of papua – @earthstory on Tumblr

Tidally dominated deltas

This is a satellite photo of part of the Gulf of Papua captured by the USGS Landsat program in 1989. It shows two major river deltas – the large river on the western side is the Fly River and the river that splits into many channels in the center of the image is the Kikori River. Several other smaller stream channels sit in-between these two larger features.

These features are river deltas but they don’t look like the Nile River where it meets the Mediterranean Sea (origin of the term “delta). These rivers have many different channels and islands with a pattern that is almost perpendicular to the coastline.

The Gulf of Papua is blocked on several sides, isolating it from stronger waves in the open ocean, and it is a rainforest so the rivers don’t carry a huge sediment load. Consequently, the shapes of these deltas are dominated by the tides.

Tidally dominated deltas form when tidal energy is the main process that redistributes sediment. Sediment comes down the river channels, but not enough to overwhelm the force of the ocean. Every day, the tide goes out and comes back in, creating a motion path in and out from the shoreline. That in and out motion redistributes sediment, creating channels and islands that generally run perpendicular to the coastline as seen here.

When wave action is stronger, sediment can be redistributed alongshore, creating a wave-dominated delta. On the other hand, when sediment loads are so high that the waves and tides can’t redistribute all the dirt, a river-dominated delta is formed.

-JBB

Image credit: USGS/Landsat http://landsatlook.usgs.gov/viewer.html