California is spending more than $200 million to keep an unfolding ecological crisis from getting worse. The state wants to stabilize habitat along the southern bank of the Salton Sea, the state’s largest lake. That is good news for nearby residents concerned about their health, but the restoration could also affect everyone who draws water from the Colorado River. At issue is the wide swaths of exposed lakebed that have been uncovered as the thirsty lake’s water evaporates in the desert air. The lake bottom is typically a deep layer of fine silt. When covered by water, it poses no risk. But once exposed to the air, and whipped up by the region’s strong winds, the dust becomes a major health risk.