By now, it’s old news that the Colorado River – notwithstanding the current abundant water year – is generally shrinking, and is expected to shrink more in coming decades. The states that share the river completed a drought plan earlier this year that brings them closer to living within currently available supplies, and a new round of negotiations on long-term management of the river is due to begin next year.

However, a new report warns that planning for gradually declining water suppl...