A balance of applied hydrodynamic (T_H) and resisting adhesive (T_A) torques was conducted over a chemically heterogeneous porous medium that contained random roughness of height h_r to determine the fraction of the solid surface area that contributes to colloid immobilization (S_f^*) under unfavorable attachment conditions.  This model considers resistance due to deformation and the horizontal component of the adhesive force (F_AT), spatial variations in the pore scale velocity distribution, and the influence of h_r on lever arms for T_H and T_A.  Values of S_f^* were calculated for a wide range of physicochemical properties to gain insight on mechanisms and factors influencing colloid immobilization.  Colloid attachment processes were demonstrated to depend on solution ionic strength (IS), the colloid radius (r_c), the Young’s modulus (K), the amount of chemical heterogeneity (P₊), and the Darcy velocity (q).  Colloid immobilization was also demonstrated to occur on a rough surface in the absence of attachment.  In this case, S_f^* depended on IS, r_c, the roughness fraction (f_o), h_r, and q.  Roughness tended to enhance T_A and diminish T_H.  Consequently, the effect of IS on S_f^* was enhanced by h_r relative to attachment.  In contrast, the effects of r_c and q on S_f^* were diminished by h_r in comparison to attachment.  Colloid immobilization adjacent to macroscopic roughness locations shares many similarities to grain-grain contact points and may be viewed as a type of straining process.  In general, attachment was more important for higher IS and variance in the secondary minimum, and for smaller r_c, q, and K, but diffusion decreased these values.  Conversely, straining was dominant for the opposite conditions.  Discrepancies in the literature on mechanisms of colloid retention are likely due to a lack of consideration of all of these factors.