At a time of unprecedented aridification across the American West, the 1,450-mile-long Colorado River and its primary storage reservoirs — Lake Powell and Lake Mead — have fallen to historic lows. Fragile ecosystems that depend on the river for water, food and habitat are in peril. In fact, at least 44 of 49 freshwater fish species native to the basin are now endangered, threatened, or extinct. … Environmental water transactions — when owners of water rights are paid to leave water in the river — could provide a solution. However, states have been slow to enact laws that enable these transactions.