The hydrology of the subducting plate and its dewatering behavior through the shallow subduction zone is linked to the structure and deformation of the forearc prism, the nature of the seismogenic zone, the composition of seawater for selected elements, and the composition of the residual slab subducted to depths of magma generation at the volcanic arc. Two locally independent systems of fluid flow govern the transport of heat and chemistry through the Costa Rica subduction complex, a dominantly nonaccretionary subduction zone. One fluid system is the margin wedge, décollement, and underthrust sediment section. Fluid sources include local sediment compaction and mineral dehydration at depth. A second flow system occurs in basement, beneath the sedimentary sequence on the incoming plate. This region is characterized by extremely low conductive heat flow, and the sediment overlying basement has pore-water geochemistry similar to that of seawater. Flow nearly parallel to the trench could be directed by permeability associated with faults and driven by a combination of differential heating and earthquake strain cycling.