The Rio Grande almost disappears into the broad expanse of its delta as the river heads eastward toward the Gulf of Mexico. The water channel of the Rio Grande is discernible by evaluating color differences on either side of it. Visible are the linear, dredged Brownsville Ship Channel and the southern end of Padre Island, part of a chain of sandy barrier islands found along this coastline that creates large, muddy Laguna Madre. Large quantities of sand are transported by air and water, generally northwest across the delta and coastal areas. Much of the fertile land north of Brownsville is used for agriculture; whereas, the coastline consists of tidal flats, sandy soils, sand dunes, and small lagoons.