In Arkansas v. Tennessee, 246 U. S. 158, 176 (1918), the Supreme Court held that federal law governed the question of how far into the river channel a State held title. Only then did this Court turn to state law to determine whether riparian owners had been accorded any rights in that land. But even the State's disposition of its submerged land vis-?-vis private owners was to be "in each case limited by the interstate boundary," a matter determined by federal law. 246 U. S., at 176.