A viscosified aqueous solution, optionally for use in a subterranean wellbore, is made by reacting in aqueous solution (i) an initial polymer with either carboxylic acid or amino groups on its polymer chain, (ii) a second compound with an amino group or carboxylic acid group, and (iii) a coupling agent, so that molecules of the second compound join to the polymer chain through amide groups, thereby forming an aqueous solution of a modified polymer which, without separation from the aqueous solution, participates in cross-linking thereby enhancing viscosity of the solution. The second compound may include hydrophobic groups so that the modified polymer contains hydrophobic groups and is able to cross-link with itself or a viscoelastic surfactant through association of hydrophobic groups (without covalent bond formation) in aqueous solution. Such cross-links can reform after the fluid has been subjected to shear.