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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 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.

This study introduces a new method for estimating hydraulic conductivity based on the concept of effective groundwater drainage length and DuPuit-Forchheimer assumptions. The effective groundwater drainage length is related to the surface drainage dissection patterns (as expressed in drainage density) forming over long periods of time. Application of the new method to the Oregon Cascades yielded hydraulic conductivity values similar to those documented in the literature. This method represents an effective and effi cient way of estimating hydraulic conductivity for regions where the interplay among surface drainage, groundwater, and topography has established a steady-state dynamic equilibrium. It also provides a theoretically sound approach for extrapolating limited local measurements to a large region and revealing the spatial variation of hydraulic conductivity.

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