The compositional continuum observed in primitive calc-alkaline lavas erupted from small volcanoes across the southernmost Cascade arc is produced by the introduction of a variable proportion of slab-derived fluid into the superjacent peridotite layer of the mantle wedge. Magmas derived from fluid-rich sources are erupted primarily in the forearc and are characterized by Sr and Pb enrichment (primitive mantle-normalized Sr/P > 5.5), depletions of Ta and Nb, low incompatible-element abundances, and MORB-like Sr and Pb isotopic ratios. Magmas derived from fluid-poor sources are erupted primarily in the arc axis and behind the arc, and are characterized by weak enrichment in Sr [1.0 < (Sr/P)N < 1.3], weak depletions in Ta and Nb, higher incompatible-element abundances, and OIB-like Sr, Nd, and Pb isotopic ratios. Fluxing the mantle wedge above the subducting slab with H2O-rich fluid stabilizes amphibole and enriches the wedge peridotites in incompatible elements, particularly unradiogenic Sr and Pb. The hydrated amphibole-bearing portion of the mantle wedge is downdragged beneath the forearc, where its solidus is exceeded, yielding melts that are enriched in Sr and Pb, and depleted in Ta and Nb (reflecting both high Sr and Pb relative to Ta and Nb in the fluid, and the greater compatibility of Ta and Nb in amphibole compared to other silicate phases in the wedge). A steady decrease of the fluid-contributed geochemical signature away from the trench is produced by the progressive dehydration of the downdragged portion of the mantle wedge with depth, resulting from melt extraction and increased temperature at the slab-wedge interface. Inverse correlation between incompatible-element abundances and the size of the fluid-contributed geochemical signature is generated by melting of more depleted peridotites, rather than by significant differences in the degree of melting. High-(Sr/P)N lavas of the forearc are generated by melting of a MORB-source-like peridotite that has been fluxed with a greater proportion of slab-derived fluid, and low (Sr/P)N lavas of the arc axis are produced by melting of an OIB-source-like peridotite in the presence of a smaller proportion of slab-derived fluid. This study documents the control that a slab-derived fluid can have on incompatible element and isotopic systematics of arc magmas by 1) the addition of incompatible elements to the wedge, 2) the stabilization of hydrous phases in the wedge, and 3) the lowering of peridotite solidi.