The U.S. Supreme Court case of Iowa v. Illinois in 1905 was a dispute over the boundary line between the two states, specifically regarding the Mississippi River's main channel (known as thalweg). The controversy arose when both states claimed jurisdiction over certain islands and parts of the river for taxation purposes. The court ruled that according to common law principles, when a navigable river serves as a state border, the boundary is typically located along thalweg - which is not necessarily at equal distance from each bank but rather follows deepest part of stream where most water flows or where navigation usually takes place. Therefore, it held that all islands and waters eastwardly thereof belonged to Illinois while those westwardly were under Iowa’s jurisdiction unless there was evidence showing contrary intention by Congress during admission process into Union.

The dissenting opinion in the case of Iowa v. Illinois, 1905 argued that the boundary line between Iowa and Illinois should not be determined by the middle of the main channel of navigation on Mississippi River but rather by a fixed geographical point as established during their admission into statehood. The dissenters believed that any changes to this original boundary due to natural shifts or alterations in river's course should not affect this predetermined border. They contended that such an approach would ensure stability and certainty for both states regarding jurisdictional matters, property rights, taxation issues etc., which could otherwise become contentious with changing river courses over time. This view however was contrary to majority opinion which held that boundary is indeed defined by mid-channel principle thus subjecting it to potential modifications with shifting navigational channels.