Yeah I know exactly what he's referring to: policies in place that protect the Sacramento-San Joaquin River Delta
(
https://en.wikipedia.org/wiki/Sacramento–San_Joaquin_River_Delta)
It's one of if not the most diverse ecosystem in North America and what Trump fails to understand is the ramifications if we stopped "diverting water to the Pacific Ocean". First and foremost, we can't divert water to a place it has been going naturally for 10,000 years. The Delta is a river system that spans hundreds of miles of waterways and is home to many different species of plants, animals, fish, and birds. It acts as a natural air purifier and air conditioner, passively cooling cities nearby and providing the famous "Delta Breeze" as far inland as Modesto and Lodi. As well as provides endless recreation and local activity- the Delta was a huge part of my upbringing and the impending destruction of it to please foreign businesses and politicians who live hundreds of miles away has me pissed the fuck off.
Jerry Brown, a cohort of sleazy politicians(both Dem and Repub), and the scummiest, greediest "investors" on Earth are hellbent on diverting all water out of the Delta. The plan is to install massive 33' in diameter tunnels that pump all of the snowmelt from the American River straight down south to water almond orchards and millions of lawns in the desert. Foreign and SoCal investment has been buying up dozens of islands in the Delta in order to drill for oil once the Delta has completely dried up.
If you want an idea of what their plan will result in Google 'Owens Lake/Valley'. Now consider that millions of people live within 1hr of the Delta while nobody lived within 3 hours of Owens Lake.