Subterranean treatment fluids comprising: an aqueous base fluid and a reaction product of a gelling agent comprising a copolymer or salt thereof comprising N-[tris(hydroxymethyl)methyl]acrylamide, 2-acrylamido-2-methylpropanesulfonic acid or a salt thereof, acrylamide; and a crosslinking agent. Such treatment fluids are particularly well-suited for high temperature applications. In particular, they are suited for subterranean temperatures up to about 400° F.