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Fish habitats and river ecosystems are threatened by climatic warming, with some endangered species such as the Brown trout being particularly vulnerable. Heat exchange models, such as the SSTEMP model used in the present study, are useful tools for analyzing the thermal habitat of rivers with marginal field measurements. The brown trout is a native and valuable fish species in the southern Caspian Sea basin, and the Absefid, Delichai and Elarm streams are main habitats for the brown trout in the Lar national park. The aim of the present study was to estimate the effect of climatic warming on thermal habitats in recent years in the Lar national park and use the results to improve environmental management. Our results indicate that the habitat downstream of the Absefid is in a critical condition, with existing environmental stresses being identified for four years of the five-year simulation period. Other stretches of the Delichai are not in a critical condition, although in some years, the environmental stress was as high as 40% downstream; therefore, the habitat is likely to be unsuitable only at the downstream section. The Elarm stream has the lowest thermal stress along its length. The dissolved oxygen level is sufficient in all of the habitats, and it is not significantly altered by any increase in temperature observed downstream. Thermal habitat modeling appears to be a very useful tool for the assessment of climate warming effects on fish habitats in rivers of natural parks. (C) 2018 European Regional Centre for Ecohydrology of the Polish Academy of Sciences. Published by Elsevier B.V. All rights reserved.