Inverted river delta

An inverted river delta is a river delta turned back to front: the narrow end, or apex, sits at the seafront and the wide end lies further inland. A normal delta forms on a flat coastal floodplain, where a river fans out and drops sediment into the body of water it empties into. An inverted delta instead lies inside a large, flat valley at the point where the waterway leaves it. The sediment is dropped within the valley, and the clear water then exits into a bay or the ocean, so the apex of the delta is at that exit. Because the valley tends to fill with sediment quickly, inverted deltas do not last long in geological terms and eventually become ordinary deltas. The classic example is the Sacramento-San Joaquin River Delta in California, where the rivers draining the Central Valley leave through the narrow Carquinez Strait in the Coast Range. The Tagus delta in Portugal is another, now only partly inverted because sedimentation has filled most of the valley.