Roads affect the lie of the landscape and how water moves within it. Hydrological impacts of roads affect the fundamentals of ecosystems: moisture availability, soil and nutrients, and stream flow and function. Geomorphic impacts can have large footprints and affect landforms and their stability, as well as compounding hydrological impacts. Impacts include landslides, large-scale erosion, fish extinction due to stream fragmentation, stream rerouting, and water-starvation of downslope landscapes; they may have synergistic interactions with other ecological changes. The hydrological and geomorphological impacts of roads are poorly studied relative to their potential scale and significance. Principles for mitigation include considering these impacts in planning and impact assessment processes, identifying impact drivers and compounding factors to inform mitigation, and implementing hydrologically considerate road design. Future research should better quantify and predict impacts; improve best practice protocols for avoiding, managing, and offsetting these impacts and rehabilitating roads; and further develop hydrologically considerate road design.