Mount Kenya serves as a natural water tower in the savanna-dominated Upper Ewaso Ng'iro River Basin in the Kenyan highlands. Major water users in the upper reaches of the river include medium- and large-scale commercial horticulture farms that produce flowers and vegetables for export using perennial irrigation schemes. These farms first appeared in the region in 1991 and gradually became powerful stakeholders that compete with small-scale farmers and urban centers over seasonally scarce water, increasing the potential for conflict. A comprehensive survey of commercial horticulture farms in the study area, including expert interviews with managers, enabled detailed analysis of the sector's development and its impact on local river water resources. Calculation of the horticulture sector's dry season water use revealed an increase from zero liters per second (L/s) in 1991 to 357 L/s in 2003 and 663 L/s in 2013, far exceeding minimum river flows. Despite this absolute increase in total water use, reliance on river water has decreased by roughly 30% since 2003, with a dramatic absolute increase in the use of alternative sources such as water stored in dams and groundwater. At the same time, the share of river water used varies greatly between specific rivers (2.2-32.5%), depending on the local availability of alternative water sources. Overall, to mitigate water conflicts, long-term monitoring and local stakeholder engagement must accompany practices and policies of efficiency. © 2018 Lanari et al.