Differences in the thermal state of subducting crust along the trench of a subduction zone cause differences in subduction zone temperature that persist to tens of kilometers down-dip of the trench. The resulting differences in fluid viscosity, permeability, and hydraulic conductivity can lead to trench-parallel variations in fluid pressure on the plate boundary fault. Temperature differences in locations with low décollement temperature (<75°C) at the trench result in large differences in fluid viscosity and fluid flow from the cold to the hot side of the system. Margins characterized by high décollement temperature at the trench are probably dominated by temperature-controlled differences in intrinsic permeability, resulting in fluid flow from the hot to the cold side of the system. Margins with large trench-parallel temperature differences support considerable trench-parallel fluid flow, and the effect is accentuated for cases where compaction driven dewatering is concentrated near the trench. Type locations for along-strike hydrologic differences include margins with subducting crust with differences in plate age along the trench or patchy hydrothermal circulation. In such cases, the three-dimensional pattern of subduction zone fluid flow should be considered when inferring the location of the source of fluids sampled from boreholes or seeps. Additionally, trench-parallel differences in fluid pressure control effective stress on the plate boundary fault. Reduced effective stress on the plate boundary on the low hydraulic conductivity side of a subduction zone reduces the effective friction coefficient, which both reduces frictional heating and may delay the onset of frictional instability.