The use of stable isotope ratio data is increasingly important in deciphering the characteristics of community structure and niche width of community members. Because stable isotope ratio values measured in tissue are closely linked to those in the diet and habitat, thus providing both bionomic (resource) and scenopoetic (habitat) ecological information, the isotopic niche concept, defined as an area (in δ-space) with isotopic values (δ-values) as coordinates, has been recently formalized. Isotopic data, which are routinely presented as δ¹³C-δ¹⁵N bi-plots, delineates an organism's isotopic niche and ultimately provides representation of an organism's trophic niche. Additionally, several quantitative metrics have also been proposed to quantitatively characterize community-wide aspects of trophic structure. In the present study, 24 fish species from two sampling sites, Wushan and Dachang, downstream of the Daning River, were collected in May and October, representing the period of low- and high-water level of the Three Gorges Reservoir, respectively. Six quantitative metrics were calculated for each community assembled by the same species in food-web space using mean δ¹³C and δ¹⁵N values of each species as the x-y coordinates in bi-plots. The results showed that δ¹³C values of particulate organic matter (POM) decreased with distance upstream from the outlet, yet δ¹⁵N values were not significantly correlated with distance. There were no significant differences in δ¹³C signatures among communities of the same species either for the different sampling sites or for the different water-level periods. However, δ¹⁵N exhibited significant differences between sites and water-level periods, except for across-sites in May. Fish communities of the same species at each water-level period varied markedly across sites. In May, range of δ¹³C (CR), total area (TA) and mean distance to centroid (CD) values of the Dachang communities were greater than those of Wushan, suggesting that isotope niche width and trophic diversity were greater in Dachang. Conversely, these values for the Dachang communities were lower in October, indicating a decrease in the trophic diversity of the Dachang communities. At each site, fish communities of the same species exhibited totally opposite variations in trophic structure over time. In Wushan, the increase in isotope niche width and trophic diversity from May to October was reflected by an increase in δ¹⁵N range (NR), CR, TA and CD values. Conversely, NR, CR, TA and CD values declined in Dachang over time, thereby decreasing the total extent of trophic diversity. Additionally, at Dachang, smaller nearest-neighbor distance (NND) and standard deviation of nearest-neighbor distance (SDNND) values of the fish communities in October suggested increased trophic redundancy and more even distribution of trophic niches compared with May. The above results show that temporal and spatial variations in the trophic structure in the Daning River exhibited distinctly different tendencies, and that food abundance could be the main explanation. Other possible reasons could be that quantitative metrics were influenced by sampling time, fish species and/or sample size. Although the study was limited by these factors, the metrics derived from stable isotope ratios provided a quantitative description of the food-web structure in the Daning River and could be applied to further study and understanding the range of structural variations in food webs in any future ecological restoration of the Daning River.