Reservoir-river systems in desert environments may provide the optimal combination of environmental conditions (e.g. light, nutrients, temperature, and flow) that maximizes primary production in downstream reaches. Stream metabolism was measured using an open-system approach each month during spring-summer in a semi-arid tailwater (South Fork Humboldt River) in the central Great Basin, USA. Spatial and temporal differences in metabolic rates were evident despite tailwater reaches sustaining comparable standing stocks of periphyton (>10 µg chla cm−2) during this growing season. Primary productivity was highest (15 to 36 g O2 m−2 day−1) in July, supporting previous studies that have described arid regulated/unregulated streams as ultra-productive. Substrate availability when combined with self-shading and hypoxic conditions created a system that was likely near the maximal productivity that stream systems can achieve because of the self-regulating attributes that thick periphyton mats impose upon themselves as they reach high biomass and maximal production rates. Copyright © 2011 John Wiley & Sons, Ltd.