Restoration of river and riparian ecosystems has become one of the most intense areas of research as human demands for freshwater from streams and rivers increase worldwide. Rivers are not only vital sources of water for agriculture and human consumption, but key areas of biodiversity. The middle Rio Grande river (MRG), which flows through central New Mexico, contains one of the most extensive riparian forests of native cottonwood remaining in the southwestern United States. The objectives of the proposed study are to examine the role of flooding on cottonwood nutrient allocation and riparian food web dynamics, and the role nutrient availability has on the physiological constraints of a key detritivore along the MRG. The effects of flooding on cottonwoods and food web structure will be measured at eight study sites along a 250 km reach of the MRG. Samples of cottonwood leaves, litter, soil, and soil arthropods will be collected over a two year period and analyzed for carbon (C), nitrogen (N), phosphorus (P), and natural abundance of 13C and 15N stable isotopes. C and N stable isotopes used in concert with C/N, C/P, and N/P ratios are important tools to assess nutrient cycling and food web structure and function in ecosystem studies. The effects of nutrient availability on terrestrial isopod nutrient allocation, growth, and reproduction will be measured during a series of experiments with treatments of low and high diet quality and different 13C and 15N stable isotope signatures. Although diet quality C/N has been studied in many organisms, the role of P is largely unknown. This research will contribute to better general understanding of ecosystem processes along riparian ecosystems in arid and semiarid regions and improved river management for water quality, biodiversity, and water use.