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Inverted River DeltaAn inverted river delta is special category of river delta in which the narrow end of the delta is located at the point of outflow of the delta, rather than the point of inflow. In a typical river delta, the narrow end of the delta is located at the point where a rivers enter a delta, with a wide fan-shaped sedimentation region extending outward into the body of water into which the river empties. In some rare cases, most notably in the case of the Sacramento River Delta, at the confluence of the Sacramento and San Joaquin Rivers in California, the delta is located at the waterway's exit from a large, flat valley, yet still inside the valley. 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 this exit, a configuration said to be inverted from that usually seen. Another example is the delta of Tagus river, in Portugal, although due to the sedimentation this delta is now only very partially inverted, since the valley is now mostly filled with sediments. It's still about 15km wide and 25km long, compared with the 2km wide exit into the sea, and forms a large lagoon with large very shallow sand banks which go uncovered during low tides. It used to be even bigger thousands of years ago. Inverted deltas typically do not last long in geological terms, since they tend to fill up with sediments rather quickly and eventually become normal deltas.
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