A method for recovering a hydrocarbon fluid from a subterranean formation, comprising: introducing into the formation an aqueous flooding fluid that improves the sweep efficiency of the formation, the fluid comprising water and a water-soluble polymer having hydrolysable cross-linked monomer units, the polymer comprising 1 mol% to 100 mol% acrylamide monomers, wherein, after introducing the aqueous flooding fluid into the formation, the hydrolysable cross-linked monomer units are hydrolysed to produce an aqueous flooding fluid after hydrolysis having a viscosity that is the same or higher than a viscosity of the aqueous fluid prior to injection, wherein the hydrolysable cross-linked monomer units are covalently cross-linked and the covalently cross-linked monomer units have the following formula: wherein: each X is selected from the group consisting of O, S and NR b; each R b is independently selected from the group consisting of -H, optionally substituted C 1 -C 24 alkyl, optionally substituted C 2 -C 24 alkenyl and optionally substituted C 2 -C 24 alkynyl; each R is independently selected from the group consisting of-H, optionally substituted C 1 -C 24 alkyl, optionally substituted C 2 -C 24 alkenyl and optionally substituted C 2 -C 24 alkynyl; Y is selected from a group consisting of a bond and a linker comprising 1 to 1000 member atoms; and each represents a point of attachment to a first polymer backbone, and each represents a point of attachment to the first polymer backbone or to a second polymer backbone. introducing into the formation an aqueous flooding fluid that improves the sweep efficiency of the formation, the fluid comprising water and a water-soluble polymer having hydrolysable cross-linked monomer units, the polymer comprising 1 mol% to 100 mol% acrylamide monomers, wherein, after introducing the aqueous flooding fluid into the formation, the hydrolysable cross-linked monomer units are hydrolysed to produce an aqueous flooding fluid after hydrolysis having a viscosity that is the same or higher than a viscosity of the aqueous fluid prior to injection, wherein the hydrolysable cross-linked monomer units are covalently cross-linked and the covalently cross-linked monomer units have the following formula: wherein: each X is selected from the group consisting of O, S and NR b; each R b is independently selected from the group consisting of -H, optionally substituted C 1 -C 24 alkyl, optionally substituted C 2 -C 24 alkenyl and optionally substituted C 2 -C 24 alkynyl; each R is independently selected from the group consisting of-H, optionally substituted C 1 -C 24 alkyl, optionally substituted C 2 -C 24 alkenyl and optionally substituted C 2 -C 24 alkynyl; Y is selected from a group consisting of a bond and a linker comprising 1 to 1000 member atoms; and each represents a point of attachment to a first polymer backbone, and each represents a point of attachment to the first polymer backbone or to a second polymer backbone. each X is selected from the group consisting of O, S and NR b; each R b is independently selected from the group consisting of -H, optionally substituted C 1 -C 24 alkyl, optionally substituted C 2 -C 24 alkenyl and optionally substituted C 2 -C 24 alkynyl; each R is independently selected from the group consisting of-H, optionally substituted C 1 -C 24 alkyl, optionally substituted C 2 -C 24 alkenyl and optionally substituted C 2 -C 24 alkynyl; Y is selected from a group consisting of a bond and a linker comprising 1 to 1000 member atoms; and each represents a point of attachment to a first polymer backbone, and each represents a point of attachment to the first polymer backbone or to a second polymer backbone.