Xiaolangdi Reservoir is located on the lower end of the middle reaches of the Yellow River, downstream of the major water and sediment source areas of the basin. At this point the catchment area is 92.3% of the basin total, runoff is 91.5% of the basin total, and sediment load is 98% of the basin total. Xiaolangdi Reservoir has a total capacity of 12.6×109 m3 and a capacity to trap 7.5×109 m3 of sediment. Thus, the reservoir is effective in regulating water and sediment regimes. This regulation has a major impact on channel morphology, sediment transport processes, and flow regime, with consequences for biota that are dependent on these river processes. Changes to flow and sediment regimes affect all trophic levels, with some direct and some indirect impacts on fish. The Yellow River Carp (Cyprinus (Cyprinus) carpio haematopterus Temminck et Schlegel) is an iconic fish in the lower Yellow River, and a suitable ecological indicator of river health impacts from flow regulation. Although operation of Xiaolangdi Dam is thought to have affected the breeding habitat of the Yellow River Carp, this paper quantifies the impact. The habitat and flow requirements of the Yellow River Carp are defined through reference to historical information and field ecological surveys. The paper also determines suitable eco-hydrological indicators of the impacts of regulation, with a focus on the reach downstream from the Yellow River Bridge in Huayuankou from 200 m downstream to 1 km downstream. A Yellow River Carp habitat simulation model was established, based on the known habitat and flow component requirements. Then, the impact of operation of the Xiaolangdi Dam on the ability of Yellow River Carp to survive and multiply was determined by running the simulation model for pre- and post-reservoir scenarios. The study showed that: (1) The flow components identified in the unregulated river regime are closely correlated with the known ecological habitat requirements of the Yellow River Carp. The operation of Xiaolangdi Dam significantly altered the pattern of flow pulses, small floods and large floods. Elimination or reduction in the frequency of flow pulses compromised the availability of spawning habitat for the Yellow River Carp. Declining frequency of small floods reduced the availability of food sources and rearing habitat for the Yellow River Carp. Elimination of floodplain inundation events caused loss of access to food and habitat opportunities on the floodplain. (2) A River2D hydraulic model was used to predict the area of suitable hydraulic habitat available to Yellow River Carp over the range of flows known to be relevant to the important life stages of the fish. The model showed that, following construction of Xiaolangdi Dam, significant reduction in frequency and duration of flows exceeding 1400 m3/s had a negative effect on Yellow River carp eggs. The pattern of low flows did not change significantly after Xiaolangdi Dam construction, but this range of flows was found to be less important in terms of habitat provision for juvenile and adult fish compared to flow pulses and floods. (3) In order to restore the habitat for Yellow River Carp, it will be necessary to operate the flow and sediment releases from Xiaolangdi Dam to be sympathetic to the ecological requirements of the fish.