Will a tunnel solve these problems?

One proposed solution to the Delta’s water woes is to build a 35-to-40-mile-long tunnel that starts upstream of the Delta, near Sacramento, and routes water around the Delta before reconnecting with the southbound canal system. 

Former Gov. Jerry Brown promoted a two-tunnel project for eight years without success. Newsom has downsized the plan to one tunnel and hopes to begin construction within three years. 

Currently, major Delta pumps pull water from the heart of the estuary. This causes obstacles for migrating fish. In addition, the pumps are vulnerable to saltwater during low river flow and high ocean tides. A tunnel would shift water diversion miles upstream, which might alleviate these problems. Intakes would be several feet above present-day sea level, and the rivers would flow through the Delta and out to sea, which helps migrating fish. 

But the tunnel plan is extremely controversial. Many opponents worry it could harm the ecosystem by diverting too much water before it reaches the estuary. Excessive diversions could also allow saltwater to threaten farms and communities in the Delta area that rely on freshwater. Moreover, moving the diversion point upstream to avoid rising seas simply kicks that can down the road a few decades. 

The tunnel will cost an estimated $11 billion. Water suppliers, like the Metropolitan Water District of Southern California, which supports the project, would likely cover the cost, although details remain unsettled.

The proposed Delta tunnel could take one of two different paths — the Central Tunnel Corridor or the Eastern Tunnel Corridor.
Source: California Department of Water Resources