A pricing-based mechanism that implements the optimal water policy while accounting for environmental externalities is developed. The analysis is presented in the context of a comprehensive water economy, stressing the tradeoffs between water use in the provision of ecosystem services vs. other uses. A distinction is made between conveyed and instream environmental water, which turns out to have important policy implications. It is shown that the allocation of instream water can be implemented by properly incorporating the (marginal) instream value of water within the shadow (in situ) price of natural water. The regulation of conveyed environmental water requires a quota-price combination. An example based on Israel's water economy is presented.