Formation of highly impermeable assemblages on membranes in water treatment can result in significant problems in maintaining permeate flux and necessitate frequent cleaning of the membrane. In this study, we describe the hydraulic properties of montmorillonite clay particles and examine the link between microscopic particle–particle interactions and the bulk properties of the clay assemblage. In particular, we show that changes in the ionic composition of feed waters can have a dramatic impact on the fouling properties of montmorillonite particles with highly impermeable, voluminous filter cakes formed at low ionic concentrations and dramatically more porous assemblages formed at higher ionic strengths.