We estimated aboveground and belowground net primary productivity (NPP) for two reaches of a montane riparian ecosystem in the eastern Sierra Nevada Mountains of California with differing stream flow regimes resulting from varying degrees of stream flow diversion for hydroelectric power generation. Total understory productivity (herbaceous and shrub) was 2.5 times higher (P < 0.001) at the high-flow site than at the low-flow site (22.4 and 8.9 g C m⁻² yr⁻¹, respectively). Annual litterfall was also higher (P = 0.03) at the high-flow (235 g C m⁻² yr⁻¹) than at the low-flow site (180 g C m⁻² yr⁻¹). However, tree and total aboveground NPP, as well as annual soil respiration and belowground NPP, were all statistically similar between sites. Furthermore, total (above- plus below-ground) NPP was statistically similar between sites (903 and 643 g C m⁻² yr⁻¹ for the high- and low-flow sites, respectively). These productivity estimates are, to our knowledge, the first ever reported for a western montane riparian ecosystem. Our results suggest that NPP of montane riparian ecosystems located along gaining stream reaches is only loosely coupled to stream flow, and understory aboveground NPP may be the most sensitive productivity measure to altered stream flow.