Finance, Property, Contracts
(1) Unless the President provides otherwise, a pre-Constitution state law cannot tax water/electricity stored/generated/consumed/distributed/sold by an inter-State river/river-valley regulatory authority. (2) A state may pass a new law imposing such a tax, but only with the President's assent (after reservation), and any rate-setting rules under it also need the President's prior consent.
This protects inter-state river/water-management authorities (which coordinate water and electricity resources shared across multiple states) from being taxed by any one individual state, unless that state gets specific presidential approval - preventing one state from unilaterally undermining a shared, multi-state resource-management arrangement through its own taxation power.
This is why a state through which a major inter-state river authority's hydroelectric project passes cannot simply impose its own tax on that authority's electricity generation without first getting the President's specific approval for that tax.