Geological mapping has confirmed the presence of swelling clays in the area of the proposed water transport canal between Korkluce and Erbaa, Turkey. A swelling pressure distribution map was prepared for this part of the route, based mainly on the analyses of soils in the Erbaa and Niksar basins. It is concluded that the potential swelling pressures will exceed the overburden weight of the reinforced concrete canal, not only when empty but also when full. In addition, variations in the characteristics of the soil mean differential movement is likely along the alignment, particularly important as the canal is planned to be a rigid structure. Recommendations are made to minimise the effect.