A dowsing rod aligns with a fault or dike as a compass needle aligns with the Earth's magnetic field.
Consider as an example, a gabbro (which is a coarse crystalline basalt) and obsidian (which is a fine crystalline basalt) These are two of the types of rock in recrystallized fault zones in basalt, where no new magma is introduced. They are chemically and mineralogically identical to basalt, but with slower and coarser cooling rates from molten rock (magma) to give larger and finer crystals of feldspar, amphiboles, pyroxenes and olivine. Small amounts of ilmenite and magnetite are also present, giving the rock its magnetic nature. These different fault zones can be filled in by liquid magma of the same country rock and the minerals and small amounts of iron within the shearing stressed fault or dyke, align in the direction of that dike or fault to create a magnetic field that the dowser or magnetometer picks up. This is the feature that I find when dowsing.This fault or dike may be more or less porous to water than the surrounding basalt.