Stream Piracy Revisited: A Groundwater-Sapping Solution

Darryll T. Pederson
2001 GSA Today  
Stream piracy describes a water-diversion event during which water from one stream is captured by another stream with a lower base level. Its past occurrence is recognized by unusual patterns of drainage, changes in accumulating sediment, and cyclic patterns of sediment deposition. Stream piracy has been reported on all time and size scales, but its mechanisms are controversial. Some researchers conclude that stream piracy is a rare event and happens only on small scales; this is based on a
more » ... s is based on a recognition that surface-water energy decreases near divides and the belief that groundwater-sapping processes decrease in effectiveness near divides and are not effective in rock and cohesive sediment. In contrast, numerous studies show that groundwater-sapping is effective in rock and cohesive sediment, focused by the intersection of the extending channel with the water table, and effective in hillslope processes. Further, destruction of evidence by surface water is the reason for the general lack of recognition of groundwater-sapping effects. I argue that the persistence of groundwater-flow systems, coupled with the evolving geometry as a pirating stream approaches a divide, can sustain breaching by groundwater-sapping processes. The principal determinant of the maintenance of energy is the position of the groundwater divide as compared to the topographic divide where streams in adjacent drainage basins are at different elevations. Wetter climatic periods can add energy to the system as increased recharge causes groundwater levels to rise, accelerating stream piracy.
doi:10.1130/1052-5173(2001)011<0004:sprags>;2 fatcat:orgmkw5hnfeabffs4glqa2h3ym