When a river or stream from one drainage system erodes through a land divide and acquires the ﬂow from another drainage system, the ﬁrst is said to have captured the second in an act of stream piracy. The headwaters of the captured river are diverted to the captor, and at this juncture, the two become one. For example, the Rio Grande once ﬂowed into a closed basin called Lake Cabeza de Vaca; after it was captured it ﬂowed into the Gulf of Mexico. A related term, elbow of capture, is the bend where the pirated channel is forced to change direction and ﬂow into the waters of the aggressor. Wind gaps are the result of dry valleys left behind when the captured river departs. The most common form of capture, or river piracy, is called abstraction. It occurs when the stream that ﬂows at a lower level cuts through the land dividing it from another stream ﬂowing at a higher level, causing that higher-level stream to rechannel itself. This is how, in geography, the low takes the high.