Nutrient availability strongly affects the species composition and productivity of most upland ecosystems, but the importance of nutrient availability is largely undefined for riparian ecosystems in semi-arid regions of the western United States. The establishment and persistence of riparian cottonwood (Populus spp.) seedlings depends largely on water availability, but this does not preclude an important role for nutrient availability. To investigate how nitrogen availability may influence the composition and productivity of riparian communities, we tested the hypothesis that the growth and survival of first-year Fremont cottonwood seedlings is limited by the availability of both water and nitrogen. Plots of naturally germinated cottonwood seedlings along the Yampa River in Northwest Colorado were randomly assigned one of four treatments: control, water, nitrogen, or water plus nitrogen. Additions of nitrogen or water doubled total (root plus shoot) seedling and shoot length. Water additions did not increase root growth, while N addition doubled the root extension of first-year cottonwood seedlings. The water-plus-nitrogen treatment doubled total seedling and root length, and tripled shoot length. Additions of water or nitrogen also more than doubled cottonwood seedling survival through the first growing season. This co-limitation of cottonwood germinants by both water and nitrogen suggests that the productivity and species composition of riparian vegetation may need to be examined in relation to supplies of resources other than water.