Climate change is likely to have a major impact on future hydrological regimes, impacting numerous sectors reliant on surface waters. We use the Soil and Water Assessment Tool (SWAT) to model future (2021-2080) streamflow and water quality variables (nitrogen, phosphorus, suspended sediment, and dissolved oxygen), in five catchments in Wales, under a worst-case scenario of future greenhouse gas concentrations (RCP8.5). Results show a decline in annual average flows (-4% to -13%) but larger changes seasonally (spring, up to 41% increase; autumn, up to 52% reduction). The magnitude and frequency of high flow events increases in spring (magnitude: Sen’s slope range 0.165-0.589, p <0.01), with more low flows in autumn (Sen’s slope range 0.064-0.090, p <0.01). Water quality declines, with higher nitrogen, phosphorus, and sediment concentrations and lower dissolved oxygen levels. The findings have economic and environmental implications for abstractors, as water resources could become more unreliable, seasonal, and polluted.