I’ve taken an interest in water. Drought conditions mean less water for the greater Los Angeles metropolitan area and the one way to reclaim water during rainstorms is to funnel water into tunnels or cisterns and pump it into storage tanks where it can be saved and cleaned for usage. I’ve made some pictures with screenshots of Google maps and embedded them into a cad program to give a rough idea of what might be possible. The yellow line depicts a tunnel under the river bottom. The water is then dropped into a cavity where pumps take the water to storage tanks, water treatment plants and reservoirs. The storage tanks are 100 feet diameter by 100 feet deep and are 50 feet into the ground.
I believe that a twelve mile tunnel should be constructed under the Los Angeles River from Slauson Ave. to Wardlow Road. At fifty feet in diameter it would hold 2,856 acre feet of water or 930,631,680 gallons. Recent rains in the spring of 2019 show the viability of such a project as enough rain fell to fill half of Lake Tahoe. Depending of the total usage of water in the Los Angeles area there might be enough left over for drier periods. The tunnel could act as a cistern by holding water until it is pumped to local water treatment plants. The water treatment plants themselves could build extra tanks.
In addition, adjacent to the tunnel, a large cistern could be built that would hold huge amounts of water. See links for an example: https://inhabitat.com/new-submission-33/
There are plenty of tunnel building companies in the United States and around the world that could build tunnels.
Furthermore, a miles long tunnel could be shortened by simply building a cistern directly underneath the river. Think of a cistern as a large building, as in LA Convention Center and Las Vegas Convention center. The roof of which would be near the level of the floor of the river. The water could just drop down into the cistern.
Construction with the finest concrete and stainless steel rebar is a Planetary Edger MUST DO.
What to do with all the leftover dirt? On the leeward side of the Long Beach breakwater could be built a water park for fishing, diving, boating and picnicking.