Swelling and dispersion of clays are the primary processes responsible for the degradation of soil hydraulic properties in the presence of exchangeable Na. The relative importance of these processes was evaluated by studying the response of low and high swelling smectites to sodic conditions. Smectite from Ariake bay sediments in Japan represented the low swelling smectite, and clay from the Kamenose landslide in Japan represented the high swelling smectite. Changes in hydraulic conductivity (HC) and clay dispersivity of sediment-sand mixtures (10, 20, and 30 g of sediment mixed with 90, 80, and 70 g of sand, respectively) as a function of total electrolyte concentration (TEC) (0.05 and 0.01 M Cl− and distilled water, (DW)) and sodium adsorption ratio ((SAR) of 0, 10, and 20) of the percolating solutions were measured. In the low swelling smectite, no changes in HC were measured in the electrolyte solutions (TEC ≥ 0.01 M Cl−) at the three SAR values. When the low swelling smectites were leached with DW, the HC of the Ca-smectite increased, whereas the HC of the SAR 10 and 20 treatments decreased. Clay dispersion and migration out of the 10% soil column was substantial. The increase in HC in Ca-mixture of low swelling smectite leached with dilute solutions was attributable to the collapse of the open microstructure that prevails in electrolyte solutions. In low swelling smectite, clay dispersion was the main process responsible for HC deterioration under sodic conditions, and smectite dispersion was prevented when TEC exceeded the flocculation value of the clay. Clay dispersion increased with an increase in exchangeable sodium percentage (ESP), and it affected the HC of the porous media only when the pores were fine and the dispersed clay plugged the conducting pores. In the high swelling smectite mixtures, a gradual decrease in the HC was measured as the TEC decreased and the SAR increased. In these mixtures, swelling was the main process responsible for HC deterioration in electrolyte solutions with TEC ≥ 0.01 M Cl−. Swelling increased with an increase in clay percentage, ESP, and decreasing TEC.