The Ewaso Ng'iro rises on the Mau Escarpment, where it drains the south part of the Mau Forest. The forest, which plays an important role in regulating and filtering the inflow to the river, is under threat from logging and land clearance for farming. Destruction would increase sediment loads in the river and cause greater seasonal variance in the volume of water. The river flows south through the rift valley to the east of the Nguruman Escarpment. It crosses the border into Tanzania, where it empties into Lake Natron. The river, which runs all year round, is the main inflow to the lake.