The work investigates the influence of seasonal changes in the composition of fresh water on the rheological properties of guar‑borate hydraulic fracturing fluids. Between October 2024 and August 2025, monthly samples were taken from three sources in the Republic of Tatarstan, with analysis of pH (7.3-7.9), alkalinity (73-275 mg/l HCO^-₃), total hardness (180-520 mg/l Ca²⁺+ Mg²⁺), chlorides (42-284 mg/l), sulphates (61-146 mg/l), and iron ions (0.1-0.3 mg/l). Fracturing fluids were prepared using these waters and tested on a Brookfield PVS high‑pressure rheometer at 32 °C in accordance with ISO 3219:1993. The target viscosity range was approximately 400-700 mPa·s. We found that in water with high salinity (source N 2, average hardness ~419 mg/l, Cl^–>180 mg/l), gel viscosity decreased by 10-15 %, the time to recover to operating viscosity increased, and breakdown accelerated, which raised the risk of premature proppant settling. Correlation analysis showed strong positive correlations between hardness, chlorides, sulphates, and alkalinity (r  =  0.53-0.90) for “soft” sources N 1 and 3, whereas in the mineralized water of source N 2 these correlations are weakened (r  =  0.17-0.51). The results demonstrate that the seasonal increase in salinity (winter‑spring period) significantly impairs the rheological stability of hydraulic fracturing fluids and emphasize the need for mandatory monitoring of water composition and adaptation of formulations during periods of peak salinity.