Fluid-structure interaction problem occurs in many engineering structures like dams, gates, tanks, and etc. Analysis of such interaction problems is important for safe and economic design of future structures and performance evaluation of present ones. In this study, reservoir water level variation effects on nonlinear seismic response of high concrete arch dams are investigated. For this purpose, 203m DEZ dam in Iran is selected as case study in which contraction and perimetral joints are modeled using node-to-node contact elements with ability of partially opening and closing and tangential movement associated with discrete crack method (DCM) approach. The reservoir is assumed compressible (Eulerian formulation) and the foundation rock is modeled for simulating its flexibility on the dam body. The provided FE model is excited using the MCE earthquake record. It is found that by increasing the reservoir water level, almost all structural responses of the dam body such as crest displacement, velocity and acceleration decrease until a special level, then growth significantly for reservoir maximum level in linear model. Also principal stresses on the US and DS faces of the dam body increase significantly associated with reservoir lessening. On the other hand responses of dam in NL-DCM have special intricacies, but the percentage of cracked nodes and also joint openings in both US and DS faces develop meaningfully with decreasing reservoir level. Based on the results, in the considered concrete arch dam, operating the reservoir in low levels can lead to lower margin of seismic safety for the structure.